EXAM 2 PHYSSS

The first autonomic synapse receptor is Sympathetic receptors tend to be more sensitive to The parasympathetic division uses receptors named for their Autonomic varicosities allow coordination of

The first autonomic synapse receptor is cholinergic (ACh), 2nd may be cholinergic or adrenergic (Epi, NorEpi) norepinephrine than epinephrine toxic agonists multiple effector units

Glutamate: GABA (gamma-aminobutyric acid): Glycine also ACH slows •Alcohol decreases •Caffeine increases •PCP "angel dust" increases •Tranquilizers (Valium) increase

>50% brain synapses, excitatory 30-40% brain synapses, inhibitory inhibitory the heart glutamate activity and increases GABA glutamate and blocks GABA receptors glutamate activity GABA activity

contraction process 4 steps

ATP hydrolysis. As mentioned earlier, a myosin head includes an ATP-binding site that functions as an ATPase—an enzyme that hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group. The energy generated from this hydrolysis reaction is stored in the myosin head for later use during the contraction cycle. The myosin head is said to be energized when it contains stored energy. The energized myosin head assumes a "cocked" position, like a stretched spring. In this position, the myosin head is perpendicular (at a 90° angle) relative to the thick and thin filaments and has the proper orientation to bind to an actin molecule. Notice that the products of ATP hydrolysis—ADP and a phosphate group—are still attached to the myosin head. 2 Attachment of myosin to actin. The energized myosin head attaches to the myosin-binding site on actin and releases the previously hydrolyzed phosphate group. When a myosin head attaches to actin during the contraction cycle, the myosin head is referred to as a crossbridge. Although a single myosin molecule has a double head, only one head binds to actin at a time. 3 Power stroke. After a crossbridge forms, the myosin head pivots, changing its position from a 90° angle to a 45° angle relative to the thick and thin filaments. As the myosin head changes to its new position, it pulls the thin filament past the thick filament toward the center of the sarcomere, generating tension (force) in the process. This event is known as the power stroke. The energy required for the power stroke is derived from the energy stored in the myosin head from the hydrolysis of ATP (see step 1 ). Once the power stroke occurs, ADP is released from the myosin head. 4 Detachment of myosin from actin. At the end of the power stroke, the crossbridge remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin.

types of adrenergic receptors Effects triggered by adrenergic neurons typically are

Adrenergic receptors are metabotropic receptors that trigger second messenger pathways by coupling to G proteins. Different types of adrenergic receptors couple to different types of G proteins. alpha 1 (Most sympathetic target tissues) alpha 2 (Digestive glands and smooth muscle in certain parts of digestive tract) beta 1 (Cardiac muscle and kidneys) beta 2 (Smooth muscle in walls of airways, some blood vessels, and certain visceral organs, such as the urinary bladder.) b3 (multiply adipose tissue) longer lasting than those triggered by cholinergic neurons.

sources muscle fibers use to make atp

Anaerobic Glycolysis Produces ATP When Oxygen Levels Are Low When muscle activity continues and the supply of creatine phosphate within the muscle fiber is depleted, glucose is catabolized to generate ATP. Glucose passes from the blood into contracting muscle fibers via facilitated diffusion (see Section 5.4), and it is also produced by the breakdown of glycogen within muscle fibers. Then a series of reactions known as glycolysis quickly breaks down each glucose molecule into two molecules of pyruvic acid (Figure 11.12). Glycolysis occurs in the cytosol and produces a net gain of two molecules of ATP (see Section 4.5). Because glycolysis does not require oxygen, it can occur whether oxygen is present (aerobic conditions) or absent (anaerobic conditions). Ordinarily, the pyruvic acid formed by glycolysis in the cytosol enters mitochondria, where it undergoes a series of oxygen-requiring reactions called aerobic respiration (described next) that produce a large amount of ATP. During heavy exercise, however, not enough oxygen is available to skeletal muscle fibers. Under these anaerobic conditions, the pyruvic acid generated from glycolysis is converted to lactic acid (Figure 11.12). The entire process by which the breakdown of glucose gives rise to lactic acid when oxygen is absent or at a low concentration is referred to as anaerobic glycolysis. Each molecule of glucose catabolized via anaerobic glycolysis yields two molecules of lactic acid and two molecules of ATP. Most of the lactic acid produced by this process diffuses out of the skeletal muscle fiber into the blood. Liver cells can take up some of the lactic acid molecules from the bloodstream and convert them back to glucose. In addition to providing new glucose molecules, this conversion reduces the acidity of the blood. When produced at a rapid rate, lactic acid can accumulate in active skeletal muscle fibers and in the bloodstream. This buildup of lactic acid is thought to be responsible for the muscle soreness that is felt during strenuous exercise. Compared to aerobic respiration, anaerobic glycolysis produces fewer ATPs, but it is faster and can occur when oxygen levels are low. Anaerobic glycolysis provides enough energy for about 2 minutes of maximal muscle activity.

Neurons and muscle fibers communicate with one another using two types of electrical signals: Graded potentials and action potentials are deviations in Neurons and muscle fibers are considered to be excitable cells because they . An action potential that occurs in a neuron is called a In most neurons, an action potential causes the release of An action potential that occurs in a muscle fiber is called a When an action potential occurs in a muscle fiber,

(1) graded potentials, which are used for short-distance communication only, and (2) action potentials, which allow communication over long distances in the body. membrane potential(voltage), the difference in electrical charges that exists just across the plasma membrane. exhibit electrical excitability, the ability to respond to a stimulus and convert it into an action potential nerve action potential or nerve impulse. neurotransmitters, which allow the neuron to communicate with another neuron, a muscle fiber, or a gland cell. muscle action potential or muscle impulse. the muscle fiber contracts.

A sensory system is a component of the nervous system that consists of 3 things: Sensory receptors and the neural pathways that convey sensory input into the CNS constitute the Recall that this division provides the CNS with information about the Once this sensory information enters the CNS, the CNS The major sensory systems include the

(1) sensory receptors that detect information in the external or internal environment, (2) neural pathways that convey the sensory information from the receptors to the CNS (brain and spinal cord), and (3) the parts of the CNS that process the information. afferent division of the peripheral nervous system (PNS). somatic senses (tactile, thermal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium) (see Figure 7.1b). integrates (processes) the input by analyzing it and making decisions for appropriate responses. somatic sensory, visual, olfactory (smell), gustatory (taste), auditory (hearing), and vestibular (equilibrium) systems. In this chapter you will explore the functions of each of these sensory systems.

Nociceptors can activate two types of pathways: ( Spinal reflex pathways that are activated by nociceptors provide For example, stepping on a tack or touching a hot burner on the stove elicits the flexor reflex, which quickly withdraws the affected limb away from the painful stimulus (see Figure 12.5). Ascending pathways to the brain that are activated by nociceptors allow pain information to be One ascending pain pathway, namely the anterolateral (spinothalamic) pathway, conveys input from nociceptors to the This pathway is responsible for . Details about the anterolateral pathway are provided later in this chapter. Other ascending pain pathways convey input from nociceptors to the reticular formation, limbic system, and hypothalamus. The pathway to the reticular formation increases your level of the pathway to the limbic system causes the and the pathway to the hypothalamus elicits the l

1) spinal reflex pathways and (2) ascending pathways to the brain. unconscious protective responses when a noxious stimulus begins to damage the body (Figure 9.15a). processed by higher centers (Figure 9.15b). cerebral cortex via relays in the spinal cord and thalamus. conscious awareness of pain sensations arousal in response to a painful stimulus; emotional responses (fear, anxiety, etc.) that may occur after a painful experience; autonomic responses (increased heart rate, sweating, etc.) that may accompany a painful incident.

Sensory receptors are either ____ or ____. When the peripheral endings of sensory neurons serve as sensory receptors, they may be these two types of peripheral endings are referred to as Encapsulated nerve endings include the receptors for The presence of the connective tissue capsule Examples of free nerve endings include the receptors for

1) the peripheral endings (dendrites) of sensory neurons or (2) separate cells that synapse with sensory neurons. encapsulated (surrounded by a connective tissue capsule) or bare (not encapsulated); encapsulated nerve endings and free nerve endings, respectively (Figure 9.2a). pressure, vibration, and some touch sensations. enhances the sensitivity or specificity of the receptor. pain, thermal, itch, tickle, and some touch sensations.

Strong Pain: C fiber stimulates Weak Pain: C fiber stimulates

2nd-order neuron and inhibits Inhibitory Interneuron Pain signal reaches the brain 2nd-order neuron and inhibits Inhibitory Interneuron, Aβ mechanoreceptor stimulates Inhibitory Interneuron over threshold, II fires reduced pain signal to brain

Knee jerk

A stretch reflex causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reflex occurs via a monosynaptic reflex arc. The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints. An example of a stretch reflex is the patellar (knee jerk) reflex. This reflex involves extension of the leg at the knee joint by contraction of the quadriceps femoris muscle of the thigh in response to tapping the patellar ligament. Recall that extension is the act of straightening out a limb at a joint. Muscles such as the quadriceps femoris that cause extension of a limb are known as extensors. A stretch reflex operates as follows (Figure 12.3): 1 Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles (shown in more detail in Figure 9.19). The spindles monitor the length of the muscle (both static muscle length and changes in muscle length). 2 In response to being stretched, a muscle spindle generates a receptor potential. If the receptor potential reaches threshold, it triggers one or more action potentials that propagate along a somatic sensory neuron through the dorsal root of the spinal nerve and into the spinal cord. 3 In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with and thereby activates a motor neuron in the ventral gray horn. 4 If the excitation is strong enough, one or more action potentials arise in the motor neuron and propagate along its axon, which extends from the spinal cord into the ventral root and through peripheral nerves to the stimulated muscle. The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle. 5 Acetylcholine released by action potentials at the NMJs triggers one or more action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.

Babinski

Babinski sign (ba-BIN-skē). This reflex results from gentle stroking of the lateral outer margin of the sole. The great toe extends, with or without a lateral fanning of the other toes. This phenomenon normally occurs in children under 1½ years of age and is due to incomplete myelination of fibers in the corticospinal tract. A positive Babinski sign after age 1½ is abnormal and indicates an interruption of the corticospinal tract as the result of a lesion of the tract, usually in the upper portion. The normal response after age 1½ is the plantar flexion reflex, or negative Babinski—a curling under of all the toes.

Neuroglia (noo-RŌG-lē-a; -glia= glue) or glia make up about half the volume of the . Their name derives from the idea of early histologists that they were simply the "glue" that It is now known that neuroglia support neurons in a variety of other ways: They Generally, neuroglia are smaller than neurons, and they are 5 to 50 times more numerous. In contrast to neurons, glia do not generate or propagate . (Mature neurons are unable to divide because they remain in the G0 phase of interphase; see Section 3.6.) In cases of injury or disease, neuroglia multiply to Brain tumors derived from glia, called gliomas, tend to be highly malignant and to grow rapidly. Different types of neuroglia are present in the CNS and PNS. Glial cells are particularly important in the CNS, where they

CNS held nervous tissue together, providing physical support to neurons. nourish and protect neurons and maintain homeostasis in the interstitial fluid that bathes them. action potentials, and they can multiply and divide in the mature nervous system fill in the spaces formerly occupied by neurons. replicate the functions of other organ systems

muscarinic receptors

Cholinergic receptors that are located postsynaptically in the effector organs such as smooth muscle, cardiac muscle, and glands supplied by parasympathetic fibers.

Resting Membrane Potential Is the Voltage That To understand the concept of resting membrane potential, it is important to have a basic knowledge of electricity. The body contains many types of charged particles such as ions, proteins, and the phosphate groups of ATP. Electrical forces exist between these charged particles. Like charges repel each other, and opposite charges attract each other. In some cases, a partition may separate opposite charges. Such a separation of positive and negative charges is a form of The electrical potential difference between Most cells, including excitable cells, have a separation of positive and negative charges just across their plasma membranes. The voltage that exists across the plasma membrane of a cell is called When a cell is at rest (unstimulated), the voltage that exists across the plasma membrane is . You will soon learn, however, that the membrane potential can change when These changes in membrane potential give rise to _____ and _____, allowing neurons to communicate with other neurons, muscle fibers, or gland cells.

Exists Across the Plasma Membrane in an Excitable Cell at Rest potential energy, which can do work. opposite charges that are separated from each other is termed voltage, which is measured in units called volts or millivolts (1 mV = 0.001 V). membrane potential (Vm). specifically termed the resting membrane potential excitable cells are stimulated. graded potentials and action potentials

Leak channels (what are they and location??) Ligand gated channels Mechanically-gated channels Voltage-gated channels

Gated channels that randomly open and close. Found in nearly all cells, including dendrites, cells bodies, and axons of all types of neurons. Gated channels that open in response to the binding of a ligand (chemical) stimulus. Dendrites of some sensory neurons such as pain receptors and dendrites and cell bodies of interneurons and motor neurons. Gated channels that open in response to a mechanical stimulus (such as touch, pressure, tissue stretching, or vibration). Dendrites of some sensory neurons such as touch receptors, pressure receptors, and some pain receptors. Gated channels that open in response to a voltage stimulus (change in membrane potential). Axons of all types of neurons.

Graded is Action is The resting membrane potential is determined by three factors: 1. Unequal distribution of ions in the ECF and cytosol. The concentrations of major cations and anions are Extracellular fluid (ECF) is rich in In cytosol, however, the main cation is The symbol A− is used to refer collectively to the negatively charged (anionic) proteins and phosphate ions of the cytosol. The concentrations of selected solutes in extracellular fluid and cytosol (intracellular fluid) are listed in Table 7.2. As you will soon learn, the unequal distribution of ions in the ECF and cytosol establishes the .

Graded (additive) -Action (all-or-none) 1) unequal distribution of ions in the ECF and cytosol, (2) differences in membrane permeability to various ions, and (3) action of the Na+/K+ ATPases. different outside and inside cells (Figure 7.10). Na+ and chloride ions (Cl−). K+, and the two dominant anions are proteins and the phosphate ions attached to molecules such as the three phosphates in ATP. concentration gradients that certain ions use to help generate the resting membrane potential

smooth muscle contraction

In smooth muscle fibers, the thin filaments attach to structures called dense bodies, which are functionally similar to Z discs in striated muscle fibers (Figure 11.28). Some dense bodies are dispersed throughout the sarcoplasm; others are attached to the sarcolemma. Bundles of intermediate filaments, which are part of the cytoskeleton, also attach to dense bodies and stretch from one dense body to another. During contraction, the sliding filament mechanism involving thick and thin filaments generates tension that is transmitted to intermediate filaments. These in turn pull on the dense bodies attached to the sarcolemma, causing a lengthwise shortening of the muscle fiber (Figure 11.28). Contraction and Relaxation Occur More Slowly in Smooth Muscle Than in Striated Muscle An increase in Ca2+ concentration in the sarcoplasm of a smooth muscle fiber initiates contraction, just like in striated muscle. Ca2+ flows into smooth muscle sarcoplasm from two sources: Most comes from extracellular fluid and the rest from the sarcoplasmic reticulum (SR). To enter the sarcoplasm, calcium ions move across the sarcolemma of the smooth muscle fiber or the membrane of the SR by passing through ion channels. Because SR is present in small amounts in smooth muscle, it provides only a small portion of the Ca2+ needed for contraction. Smooth muscle and striated muscle differ in the way that an increase in Ca2+ concentration in the sarcoplasm causes contraction. As you already know, in striated muscle fibers, Ca2+ binds to troponin, causing tropomyosin to move away from myosin-binding sites on actin. Once the myosin-binding sites are exposed, myosin attaches to actin, and muscle contraction begins. In smooth muscle fibers, the thin filaments lack troponin and, although they contain tropomyosin, tropomyosin does not cover the myosin-binding sites on actin. In addition, myosin molecules in smooth muscle can bind to actin only after phosphate groups are added to light chains in the myosin heads. An increase in Ca2+ concentration in the sarcoplasm of a smooth muscle fiber causes contraction in the following way (Figure 11.29): 1 Ca2+ binds to calmodulin, a regulatory protein in the sarcoplasm that is similar in structure to troponin. 2 The Ca2+-calmodulin complex activates an enzyme called myosin light chain kinase (MLCK), which is also present in the sarcoplasm. 3 Activated MLCK in turn phosphorylates (adds a phosphate group to) light chains in the myosin heads. 4 The phosphorylated myosin heads bind to actin, and muscle contraction begins.

Diagnostic

Most autonomic reflexes are not practical diagnostic tools because it is difficult to stimulate visceral effectors, which are deep inside the body. An exception is the pupillary light reflex, in which the pupils of both eyes decrease in diameter when either eye is exposed to light. Because the reflex arc includes synapses in lower parts of the brain, the absence of a normal pupillary light reflex may indicate brain damage or injury.

alpha gamma coactivation

In the reflex arc just described, sensory input enters the spinal cord on the same side from which motor output leaves it. This arrangement is called an ipsilateral reflex (ip′-si-LAT-er-al). All monosynaptic reflexes are ipsilateral. In addition to the large-diameter motor neurons (called alpha motor neurons) that innervate typical skeletal muscle fibers, smaller-diameter motor neurons (known as gamma motor neurons) innervate the intrafusal muscle fibers associated with the muscle spindles them selves (see Figure 9.19). The brain regulates muscle spindle sensitivity through pathways to these smaller motor neurons. This regulation ensures proper muscle spindle signaling over a wide range of muscle lengths during voluntary and reflex contractions. By adjusting how vigorously a muscle spindle responds to stretching, the brain sets an overall level of muscle tone, which is the small degree of contraction present while the muscle is at rest. Because the stimulus for the stretch reflex is stretching of muscle, this reflex helps avert injury by preventing overstretching of muscles.

In a real neuron, which is more permeable to As noted above, the Na+/K+ ATPases help maintain the resting membrane potential by using This means that there is no Because there is no net charge movement across the membrane and energy is required to maintain this constant condition,

K+ ions than to Na+ ions, an inside-negative membrane potential is generated because the number of K+ ions that move out of the neuron is greater than the number of Na+ ions that move into the neuron. energy derived from ATP hydrolysis to pump out Na+ as fast as it leaks in and to bring in K+ as fast as it leaks out. net charge movement across the membrane of a resting neuron because Na+ leakage into the cell and K+ leakage out of the cell are exactly balanced by the constant activity of the Na+/K+ ATPases. the resting neuron exists in a steady state. Recall that when a steady stateexists, energy is required to keep a particular condition constant (see Section 1.4).

As you know, neurons are permeable to bothK+ and Na+ ions. Because the plasma membrane of a neuron is more permeable to In other words, there is net movement of positive charge out of the neuron, which generates an The membrane potential becomes increasingly negative with the continuous movement of more K+ out of the neuron and less Na+ into the neuron. Eventually, K+ movement out of the neuron There is still net movement of K+ ions out of the neuron, however, because the magnitude of the With the help of Na+/K+ ATPases, the resting membrane potential of the neuron stabilizes when it reaches At this point, K+ movement out of the neuron is exactly balanced by As you have already discovered, the resting membrane potential (−70 mV) is closer to the K+ equilibrium potential (−90 mV) than to the Na+ equilibrium potential (+60 mV). This is because a resting neuron is more The resting membrane potential of a neuron can be calculated by using the Goldman-Hodgkin-Katz (GHK) equation (see the Physiological Equation box on the Goldman-Hodgkin-Katz equation).

K+ ions than to Na+ ions, the number of K+ ions that move out of the neuron through K+ leak channels is greater than the number of Na+ ions that move into the neuron through Na+ leak channels. inside-negative membrane potential (Figure 7.13a). slows down and Na+ entry into the neuron speeds up because the negative membrane potential creates an electrical gradient that favors movement of both Na+ and K+ ions into the neuron (Figure 7.13b). K+ concentration gradient is greater than the magnitude of the K+ electrical gradient. about −70 mV (Figure 7.13c). Na+ movement into the neuron and there is no net movement of charge across the membrane. permeable to K+ ions than it is to Na+ ions, which allows K+ ions to have a greater influence on the resting membrane potential than Na+ ions.

what is an interneuron. two point discrimination test

Neuron located entirely within the central nervous system; it integrates sensory input and motor output. Also called an association neuron. The size of the receptive field varies inversely with the number of sensory receptors that it contains. The smaller the receptive field, the more densely packed it is with sensory receptors and the greater its acuity. Conversely, the larger the receptive field, the less densely packed it is with sensory receptors and the lower its acuity. A measure of tactile acuity, or sharpness of touch perception, is two-point discrimination—the ability to perceive two points applied to the skin as two separate points. Two-point discrimination is the ability to perceive two points applied to the skin as two separate points. Two-point discrimination can be demonstrated by applying the two points of a caliper to the skin. If the two caliper points stimulate the same receptive field, then only one point of touch is perceived (Figure 9.6a). If the two caliper points stimulate different receptive fields and this input is conveyed into the CNS along separate pathways, then two points of touch are perceived (Figure 9.6b). If the distance between the two caliper points is less than the two-point discrimination threshold, then only one point of touch will be felt. The two-point discrimination threshold varies throughout the body (Figure 9.6c). For example, the two-point discrimination threshold is high in areas such as the back and calf, where there are a small number of large receptive fields (Figure 9.6a). In addition, there is little overlap between the receptive fields, and several sensory neurons often converge on a common postsynaptic neuron, causing only one signal to be conveyed to the brain. In these areas, two points as far apart as 40 mm are perceived as just one point.

norepinephrine and epinephrine

Produced in the adrenal medulla. Associated with wakefulness and alertness. Released during fight or flight. (sympathetic)

Crossed extensor reflex

Something else may happen when you step on a tack: You may start to lose your balance as your body weight shifts to the other foot. In addition to initiating the flexor reflex that causes you to withdraw the limb, the pain impulses from stepping on the tack also initiate a crossed extensor reflex, which causes extension of the opposite limb to help you maintain balance. The crossed extensor reflex operates as follows (Figure 12.6): 1 Stepping on a tack stimulates the sensory receptor of a pain-sensitive neuron in the right foot. 2 This sensory neuron then generates action potentials, which propagate into the spinal cord. 3 Within the spinal cord (integrating center), the sensory neuron activates several interneurons that synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Thus, incoming pain signals cross to the opposite side through interneurons at that level and at several levels above and below the point of entry into the spinal cord. 4 The interneurons excite motor neurons in several spinal cord segments that innervate extensor muscles. The motor neurons in turn generate more action potentials, which propagate toward the axon terminals. 5 Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the unstimulated left limb to contract, producing extension of the left leg. In this way, weight can be placed on the foot that must now support the entire body. A comparable reflex occurs with painful stimulation of the left lower limb or either upper limb.

Pre vs Post ganglionic neurons Both sympathetic and parasympathetic neurons secrete Postganglionic sympathetic neurons secrete Postganglionic parasympathetic neurons secrete acetylcholine onto

The first neuron, called the preganglionic neuron, has its cell body in the brain or spinal cord. Its axon exits the CNS via a cranial or spinal nerve and then extends to an autonomic ganglion, where it synapses with the second neuron. (Recall that a ganglion is a cluster of neuronal cell bodies in the PNS). The second neuron, called the postganglionic neuron, lies entirely in the PNS. Its cell body is located in the autonomic ganglion, and its axon extends from the ganglion to the visceral effector. Thus, preganglionic neurons convey action potentials from the CNS to autonomic ganglia, and postganglionic neurons relay the action potentials from autonomic ganglia to visceral effectors. acetylcholine onto nicotinic receptors within the autonomic ganglion. norepinephrine onto adrenergic receptors on target cells at the neuroeffector junction. muscarinic receptors on target cells at the neuroeffector junction. There are some exceptions.

relaxation explain

The membrane of the sarcoplasmic reticulum (SR) also contains active transport proteins called Ca2+-ATPase pumps that constantly transport Ca2+ from the sarcoplasm into the SR (Figure 11.10). While muscle action potentials continue to propagate through the T tubules, the Ca2+ release channels are open. Calcium ions flow into the sarcoplasm more rapidly than they are transported back into the SR by the pumps. After the last action potential has propagated throughout the T tubules, the Ca2+ release channels close. As the pumps move Ca2+ back into the SR, the concentration of calcium ions in the sarcoplasm quickly decreases. Inside the SR, molecules of a calcium-binding protein, appropriately called calsequestrin, bind to the Ca2+, enabling even more Ca2+ to be sequestered (stored) within the SR. As a result, the concentration of Ca2+ is 10,000 times higher in the SR than in the sarcoplasm in a relaxed muscle fiber. As the Ca2+ level in the sarcoplasm drops, Ca2+ dissociates from troponin, tropomyosin covers the myosin-binding sites on actin, and the muscle fiber relaxes

What is the role of the somatosensory cortex?

The primary somatosensory cortex is located in the postcentral gyrus of the parietal lobe. The postcentral gyrus refers to the fold of parietal cortex that is just behind the central sulcus. The primary somatosensory cortex receives sensory information for touch, pressure, vibration, temperature (coldness and warmth), pain, and proprioception (muscle and joint position) and is involved in the perception of these somatic sensations. Somatic sensory input that occurs on one side of the body is conveyed to the primary somatosensory cortex of the cerebral hemisphere on the opposite side. This occurs because ascending somatic sensory pathways cross over to the opposite side of the body before they reach the primary somatosensory cortex. The primary somatosensory cortex allows you to pinpoint where somatic sensations originate so that you know exactly where on your body to swat that mosquito. The primary somatosensory cortex also permits you to determine the size, shape, texture, and weight of an object by feeling it and to sense the relationship of one body part to another.

NMJ At the NMJ, a terminal branch of the somatic motor neuron's axon divides into a Inside each synaptic vesicle are thousands of molecules of ACh has an ___ effect on The region of the muscle fiber plasma membrane opposite the synaptic end bulbs is called the Within the motor end plate are 30 to 40 million These receptors are abundant in Although the synaptic end bulbs and motor end plate are close to each other, they do not actually touch; instead, they are separated by a small space called the . A neuromuscular junction thus includes A skeletal muscle fiber has only one NMJ and it is usually located near the midpoint of the fiber.

The synapse formed between a somatic motor neuron and a skeletal muscle fiber is called the neuromuscular junction (NMJ) (Figure 10.10). cluster of synaptic end bulbs, which contain synaptic vesicles (Figure 10.10a, b). acetylcholine (ACh), the neurotransmitter released at the NMJ. excitatory effect on the NMJ, ultimately causing the skeletal muscle fiber to contract. motor end plate (Figure 10.10b, c). acetylcholine receptors (of the nicotinic type) that bind specifically to ACh. junctional folds, deep grooves in the motor end plate that provide a large surface area for ACh. synaptic cleft all of the synaptic end bulbs on one side of the synaptic cleft, the synaptic cleft itself, plus the motor end plate of the muscle fiber on the other side.

Sources muscle fibers use to make AYTP

Unlike most cells of the body, skeletal muscle fibers often switch between a low level of activity, when they are relaxed and using only a modest amount of ATP, and a high level of activity, when they are contracting and using ATP at a rapid pace. A huge amount of ATP is needed to power the contraction cycle, to pump Ca2+ into the sarcoplasmic reticulum, and for other metabolic reactions involved in muscle contraction. However, the ATP present inside muscle fibers is enough to power contraction for only a few seconds. If muscle contractions continue past that time, the muscle fibers must make more ATP. Skeletal muscle fibers have three ways to produce ATP: (1) from creatine phosphate, (2) by anaerobic glycolysis, and (3) by aerobic respiration (Figure 11.12). While muscle fibers are relaxed, they produce more ATP than they need for resting metabolism. Most of the excess ATP is used to synthesize creatine phosphate, an energy-rich molecule that is found in muscle fibers. The enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP. This reversible reaction is summarized as follows: Creatine is a small, amino acid-like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers. Creatine phosphate is three to six times more plentiful than ATP in the sarcoplasm of a relaxed muscle fiber. Based on the law of mass action, when contraction begins and the ADP level starts to rise, CK catalyzes the transfer of a high-energy phosphate group from creatine phosphate back to ADP. This direct phosphorylation reaction quickly regenerates new ATP molecules (Figure 11.12). Because the formation of ATP from creatine phosphate occurs very rapidly, creatine phosphate is the first source of energy when muscle contraction begins. The other energy-generating mechanisms in a muscle fiber (anaerobic glycolysis and aerobic respiration) take more time to produce ATP. Together, stores of creatine phosphate and ATP provide enough energy for muscles to contract maximally for about 15 seconds.

The best-studied neurotransmitter is ACh is synthesized from the precursors Neurons that release ACh, called ___ are present in the Once released, ACh binds to a cholinergic receptor in the postsynaptic membrane, causing a response in the This response is short-lived, however, because ACh is . The choline is transported back into the synaptic end bulb where it is used to synthesize another ACh molecule. The acetate diffuses out of the synaptic cleft and into the blood. There are two types of cholinergic receptors: Nicotinic acetylcholine receptors are so named because the drug nicotine is an . Nicotine, a natural substance in tobacco leaves, is not a naturally occurring substance in humans and is not normally present in nonsmokers. Muscarinic acetylcholine receptors are so named because the Note that ACh activates both types of cholinergic receptors. However, nicotine activates only nicotinic ACh receptors, and muscarine activates only muscarinic ACh receptors. Nicotinic ACh receptors are present in Two types of nicotinic ACh receptors exist. Both types are Activation of nicotinic ACh receptors causes

acetylcholine acetyl coenzyme A (acetyl CoA) and choline. cholinergic neurons (kō′-lin-ER-jik), CNS and PNS. postsynaptic cell. quickly broken down into acetate and choline by the enzyme acetylcholinesterase (AChE), which is located on the postsynaptic membrane nicotinic acetylcholine receptors and muscarinic acetylcholine receptors. agonist mushroom poison muscarine is an agonist. some neurons of the CNS, in certain autonomic neurons of the PNS, and in skeletal muscle at the NMJ. ionotropic receptors that contain two binding sites for acetylcholine and a cation channel (see Figure 7.27a). depolarization (EPSP) and thus excitation of the postsynaptic cell.

Each voltage-gated Na+ channel has two separate gates, an In the resting state of a voltage-gated Na+ channel, the inactivation gate is open, but the As a result, Na+ cannot move into the cell through these channels. At threshold, voltage-gated Na+ channels are . In the activated state of a voltage-gated Na+ channel, both the As more channels open, Na+ inflow increases, the membrane depolarizes further, and This is an example of a positive feedback mechanism. During the few ten-thousandths of a second that the voltage-gated Na+ channel is open, about 10,000 Na+ ions flow across the membrane and change the membrane potential considerably, but the concentration of Na+ hardly changes because of the millions of Na+ present in the ECF. The sodium-potassium pumps easily bail out the 10,000 or so Na+ ions that enter the cell during a single action potential and maintain the low concentration of Na+ inside the cell.

activation gate and an inactivation gate. activation gate is closed (step 1 in Figure 7.22). activated activation and inactivation gates in the channel are open and Na+ inflow begins (step 2 in Figure 7.22). more Na+ channels open.

Epinephrine

adrenaline

The PNS is divided into __ and ____divisions. The afferent division of the PNS conveys This division provides the CNS with The efferent division of the PNS conveys This division is further subdivided into a The somatic nervous system conveys output from the ___ to ___ ONLY Because its motor responses can be consciously controlled, the action of this part of the PNS is .The autonomic nervous system (ANS) conveys output from the CNS to Because its motor responses are not normally under conscious control, the action of the ANS is The ANS is comprised of two main branches, the With a few exceptions, effectors receive innervation from both of these branches, and usually the two branches have For example, neurons of the sympathetic nervous system increase ____, and neurons of the In general, the parasympathetic nervous system takes care of "______activities, and the sympathetic nervous system helps support A third branch of the autonomic nervous system is the enteric nervous system (ENS), an extensive network of The ENS helps regulate the activity of the Although the ENS can function independently, it communicates with and is regulated by the other branches of the ANS.

afferent and efferent input into the CNS from sensory receptors in the body (Figure 7.1b). sensory information about the somatic senses (tactile, thermal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium). output from the CNS to effectors (muscles and glands). somatic nervous system and an autonomic nervous system (Figure 7.1b). CNS to skeletal muscles only. voluntary smooth muscle, cardiac muscle, and glands. involuntary parasympathetic nervous system and the sympathetic nervous system. opposing actions. heart rate, and neurons of the parasympathetic nervous system slow it down. rest-and-digest" exercise or emergency actions—the so-called "fight-or-flight" responses. neurons confined to the wall of the gastrointestinal (GI) tract. smooth muscle and glands of the GI tract.

While the voltage-gated K+ channels are open, outflow of K+ may be large enough to cause an During this phase, the voltage-gated K+ channels remain open. The membrane potential becomes even more Once voltage-gated K+ channels close, the membrane potential returns to the Unlike voltage-gated Na+ channels, most voltage-gated K+ channels do not enter an The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus is called the During the absolute refractory period, even a very strong stimulus cannot initiate a This period coincides with the period of Na+ channel activation and inactivation (steps 2 and 3 in Figure 7.22). Inactivated Na+ channels cannot The relative refractory period is the period of time during which a second action potential can be initiated, but only by a . It coincides with the period when the voltage-gated K+ channels are still open after In contrast to action potentials, graded potentials do not

after-hyperpolarizing phase of the action potential (see Figure 7.20a). negative as it approaches the K+ equilibrium potential (EK) of about −90 mV. resting level of −70 mV. inactivated state. Instead, they alternate between closed (resting) and open (activated) states. refractory period (see key in Figure 7.20a). second action potential. reopen; they first must return to the resting state (step 1 in Figure 7.22). larger-than-normal stimulus inactivated Na+ channels have returned to their resting state (see Figure 7.20a). exhibit a refractory period.

The gates of leak channels randomly Typically, plasma membranes have many more potassium ion (K+) leak channels than sodium ion (Na+) leak channels, and the K+ leak channels are leakier than the Na+ leak channels. Thus, the membrane's permeability to K+ is Leak channels are important for establishing the 2. A ligand-gated channel opens or closes in response to a A wide variety of ligands—including For example, the neurotransmitter ACETYLCHOLINE ⭐️ opens 3. A mechanically-gated channel opens or closes in response to The force distorts the channel from its Examples of mechanically-gated channels are those found in Like ligand-gated channels, mechanically-gated channels are involved in the 4. A voltage-gated channel opens in response to a Examples include Voltage-gated channels are responsible for the

alternate between open and closed positions (Figure 7.8a). much higher than its permeability to Na+. resting membrane potential. specific ligand (chemical) stimulus. neurotransmitters, hormones, and chemicals in food or an odor—can open or close ligand-gated channels. cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward (Figure 7.8b). Ligand-gated channels participate in the generation of graded potentials. mechanical stimulation in the form of touch, pressure, tissue stretching, or vibration (such as sound waves) (Figure 7.8c). resting position, opening the gate. touch receptors and pressure receptors in the skin, in receptors that monitor stretching of internal organs, and in auditory receptors in the ears. formation of graded potentials. change in membrane potential (voltage). voltage-gated K+ channels (Figure 7.8d), voltage-gated Na+ channels, and voltage-gated Ca2+ channels. generation and conduction of action potentials.

To say that these electrical signals are graded means that they vary in Graded potentials are larger or smaller depending on how many The opening or closing of these ion channels alters the number of The amplitude of a graded potential can vary from less than 1 mV to more than 50 mV. After it is generated, a graded potential spreads along the The spread of a graded potential is accomplished by local current flow. Local current flow refers to the passive To understand how local current flow occurs, consider a region of membrane that is depolarized (Figure 7.17). Local current flow occurs in the cytosolas positive charges move from the depolarized membrane region to the more negative adjacent membrane regions that are still at resting membrane potential. At the same time, local current flow occurs in the ECF as positive charges move from adjacent membrane regions to the more negative depolarized membrane region.As a result of local current flow, adjacent membrane regions become depolarized. In other words, the graded potential spreads along the membrane in both directions away from the stimulus source.

amplitude (size), depending on the strength of the stimulus (Figure 7.16). ligand-gated or mechanically-gated channels have opened (or closed) and how long each remains open. specific ions that move across the plasma membrane to cause the graded potential. membrane in both directions away from the stimulus source. movement of charges from one region of membrane to adjacent regions of membrane due to differences in membrane potential in these areas.

Astrocytes (AS-trō-sī-ts) are the most numerous of the neuroglia. They have processes that wrap The walls of brain capillaries consist of endothelial cells (see Figure 8.5b) that are joined together by tight junctions. In effect, the tight junctions between the endothelial cells create a blood-brain barrier, which isolates neurons of the CNS from harmful agents and other substances in the blood. Astrocyte processes surrounding brain capillaries secrete chemicals that Astrocytes also help to maintain the appropriate For example, they regulate the concentration of important ions such as 2.Oligodendrocytes (OL-i-gō-den′-drō-sīts) are responsible for 3.Microglia (mī-KROG-lē-a) function as 4. Ependymal cells (ep-EN-de-mal) line the

around capillaries (the smallest blood vessels) in the CNS. maintain the "tightness" of these tight junctions. chemical environment for the generation of action potentials. K+, take up excess neurotransmitters, and serve as a conduit for the passage of nutrients and other substances between capillaries and neurons. In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnections among neurons in the brain. Astrocytes may also play a role in the formation of neural synapses. forming and maintaining the myelin sheath around axons of neurons in the CNS. The myelin sheath is a multilayered lipid and protein covering that will be described in more detail shortly. phagocytes. They remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue. ventricles of the brain and central canal of the spinal cord. (Ventricles and the central canal are spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord.) Functionally, ependymal cells produce and assist in the circulation of cerebrospinal fluid.

Norepinephrine (NE), also known as noradrenaline, is involved in Norepinephrine is released by certain neurons in the brain stem and by some Epinephrine, also known as ____, is released by only a small number of neurons in the brain and has the Both norepinephrine and epinephrine also serve as hormones. Cells of the adrenal medulla, the inner portion of the adrenal gland, release them into the blood. Norepinephrine and epinephrine bind to Adrenergic receptors are present in There are two main groups of adrenergic receptors: These receptors are further classified into subtypes—α1, α2, β1, β2, and β3—based on the specific responses they elicit and by their selective binding of drugs that activate or block them. All adrenergic receptors are . Norepinephrine stimulates _____; epinephrine is a potent stimulator of Activation of adrenergic receptors can cause either A large variety of drugs can activate or block specific adrenergic receptors. Examples include

arousal (awakening from sleep), attention, and regulating mood. autonomic neurons of the PNS. adrenaline lowest concentration in the brain of all of the catecholamines. adrenergic receptors (ad′-ren-ER-jik) in the postsynaptic membrane. some neurons of the CNS and in effectors (cardiac muscle, smooth muscle, and glands) innervated by certain autonomic neurons of the PNS. alpha (α) receptors and beta (β) receptors. metabotropic alpha receptors more strongly than beta receptors both alpha and beta receptors. excitation or inhibition of the postsynaptic cell, depending on which type of adrenergic receptor is activated. phenylephrine, which is an agonist at α1 receptors, and propranolol, which is a nonselective antagonist at β receptors.

Neurons that release the neurotransmitter dopamine (DA) are present in the There are several types of dopamine receptors, all of which are . Activation of dopamine receptors can either cause Dopamine is involved is generating ___ responses.

brain, especially in the substantia nigra and ventral tegmental area of the midbrain. metabotropic excitation or inhibition of the postsynaptic cell, depending on which type of dopamine receptor is activated. emotional

As its name suggests, most skeletal muscle is It is striated; that is, striations, or Skeletal muscle works mainly in a voluntary manner: Its activity can be Many skeletal muscles are also controlled subconsciously to some extent. For example, your diaphragm continues to alternately contract and relax without conscious control so that you don't stop breathing. In addition, the skeletal muscles that maintain posture or stabilize body position contract without conscious control. Together, all of the skeletal muscles of the body comprise the muscular system. Only the heart contains Like skeletal muscle, cardiac muscle is . However, unlike skeletal muscle, cardiac muscle is Instead, the heart beats because it has a pacemaker that initiates each contraction. This built-in rhythm is termed . Several neurotransmitters and hormones can adjust heart rate by speeding up or slowing down the pacemaker. Smooth muscle is located in the Under the light microscope, smooth muscle For this reason, it looks The action of smooth muscle is usually Both cardiac muscle and smooth muscle are regulated by

attached to bones and moves parts of the skeleton (Figure 11.1a). alternating light and dark bands, are visible under a light microscope. consciously controlled by motor neurons that are part of the somatic nervous system. cardiac muscle, which forms most of the heart wall (Figure 11.1b). striated involuntary: Its contractions are not under conscious control. autorhythmicity walls of hollow internal structures, such as blood vessels, the airways, stomach, intestines, and uterus (Figure 11.1c). lacks the striations that are present in skeletal and cardiac muscle. nonstriated, which is why it is referred to as smooth. involuntary, and some smooth muscle tissue, such as the muscles that propel food through your gastrointestinal tract, has autorhythmicity. motor neurons that are part of the autonomic nervous system and by hormones released by endocrine glands.

Motor or efferent neurons (EF-e-rent) convey action potentials They comprise the Most motor neurons have numerous dendrites and one main Depending on the branch of the efferent division of the PNS to which they belong, motor neurons are further classified into two groups: Somatic motor neurons are part of the somatic nervous system; they convey Autonomic motor neurons, which are components of the 3. Interneurons or association neurons are located entirely within the Interneurons are responsible for ____— they Like motor neurons, interneurons usually have numerous dendrites and one main axon extending from their cell bodies. About 99% of all neurons in the body are interneurons.

awayfrom the CNS to effectors in the periphery. efferent division of the PNS. axon extending from their cell bodies. somatic motor neurons and autonomic motor neurons. action potentials to skeletal muscles. autonomic nervous system, convey action potentials to cardiac muscle, smooth muscle, or glands. CNS between sensory and motor neurons. integration—they process incoming sensory information from sensory neurons and then may elicit a motor response by activating the appropriate motor neurons.

A large variety of drugs and natural products can selectively activate or block specific cholinergic or adrenergic receptors. An agonist is a substance that _____ is a common ingredient in cold and sinus medications. Because it constricts blood vessels in the nasal mucosa, phenylephrine . An antagonist is a substance that binds to and For example, atropine dilates the pupils, reduces glandular secretions, and relaxes smooth muscle in the gastrointestinal tract by blocking muscarinic ACh receptors. As a result, it is used to dilate the pupils during eye examinations and to treat smooth muscle disorders such as intestinal hypermotility, and as an antidote for chemical warfare agents that inactivate acetylcholinesterase. Smaller receptive fields allow for

binds to and activates a receptor, in the process mimicking the effect of a natural neurotransmitter or hormone. Phenylephrine, an adrenergic agonist at α1 receptors, reduces production of mucus, thus relieving nasal congestion blocks a receptor, thereby preventing a natural neurotransmitter or hormone from exerting its effect. greater discrimination of stimuli

The central nervous system (CNS) consists of the The brain is the part of the CNS that is located in the The spinal cord is connected to the brain and is enclosed by the The CNS processes many different kinds of incoming sensory information. It is also the source of Most signals that ___ and ____originate in the CNS. Peripheral Nervous System The peripheral nervous system (PNS) consists of all Components of the PNS include A nerve is a Twelve pairs of cranial nerves emerge from the brain and thirty-one pairs of spinal nerves emerge from the spinal cord. A sensory receptor is a structure that Examples of sensory receptors include

brain and spinal cord (Figure 7.1a). skull bones of the vertebral column. thoughts, emotions, and memories. stimulate muscles to contract and glands to secrete nervous tissue outside the CNS (Figure 7.1a). nerves and sensory receptors. bundle of axons (described shortly) that lies outside the brain and spinal cord. monitors changes in the external or internal environment. touch receptors in the skin, olfactory (smell) receptors in the nose, and stretch receptors in the stomach wall.

In the parasympathetic nervous system, preganglionic neurons have their cell bodies in the Hence, the parasympathetic nervous system is also known as the Parasympathetic preganglionic axons exit the CNS through four cranial nerves (III, VII, IX, and X) and several sacral spinal nerves (S2-S4). The axons then extend to From the terminal ganglia, parasympathetic postganglionic axons extend to cells in the Because terminal ganglia are located either close to or in the wall of the visceral effector, parasympathetic preganglionic axons are

brain stem and sacral regions of the spinal cord (Figure 10.2). craniosacral division of the ANS. parasympathetic post ganglionic neurons in terminal ganglia, which are located close to or within the wall of the visceral effector (Figure 10.2). visceral organ. long, and parasympathetic postganglionic axons are short.

In the parasympathetic nervous system, preganglionic neurons have their cell bodies in the Hence, the parasympathetic nervous system is also known as the Parasympathetic preganglionic axons exit the CNS through four cranial nerves (III, VII, IX, and X) and several sacral spinal nerves (S2-S4). The axons then extend to parasympathetic post ganglionic neurons in terminal ganglia, which are located From the terminal ganglia, parasympathetic postganglionic axons extend to cells in the visceral organ. Because terminal ganglia are located either close to or in the wall of the visceral effector,

brain stem and sacral regions of the spinal cord (Figure 10.2). craniosacral division of the ANS. close to or within the wall of the visceral effector (Figure 10.2). parasympathetic preganglionic axons are long, and parasympathetic postganglionic axons are short.

The length of an axon can vary from one neuron to another. Some neurons have axons as short as just a few microns. Other neurons have axons as long as a meter or more. Adjacent axons with similar lengths are often A nerve is a bundle of whereas a tract is a Functional Classes of Neurons Neurons are divided into three functional classes based on the direction in which the action potential is conveyed relative to the CNS (Figure 7.3): 1. Sensory or afferent neurons (AF-er-ent) convey action potentials They constitute the Most sensory neurons have only one process that This single process is an axon that has dendrites at Sensory neurons are associated with sensory receptors that detect a sensory stimulus such as Sensory receptors are either the ____of sensory neurons or separate When the peripheral endings of sensory neurons serve as sensory receptors, they may be The trigger zone for action potentials is at the Once an action potential is generated, it

bundled together with connective tissue. axons in the PNS, bundle of axons in the CNS. into the CNS. afferent division of the PNS. extends from their cell bodies. its peripheral end. touch, pressure, light, or sound. peripheral endings (dendrites), cells located close to sensory neurons. encapsulated (surrounded by a connective tissue capsule) or free (not encapsulated). junction of the dendrites and the axon of the sensory neuron. propagates along the axon into the CNS.

Throughout your life, your nervous system exhibits plasticity, the capability to At the level of individual neurons, the changes that can occur include the sprouting of new dendrites, synthesis of Undoubtedly, both ___ and ____drive the changes that occur. Despite this plasticity, mammalian neurons have very limited powers of In the PNS, an axon may undergo repair if the cell body is Schwann cells aid the repair process by Therefore, a person who injures axons of a nerve in an upper limb, for example, has a good chance of regaining nerve function.

change based on experience. new proteins, and changes in synaptic contacts with other neurons. chemical and electrical signals regeneration, the capability to replicate or repair themselves. intact and if the Schwann cells that produce myelination remain active. forming a regeneration tube that guides and stimulates regrowth of the axon.

The somatic motor division uses ___ receptors to stimulate skeletal muscles Exogenous toxins can Several chemical agents selectively alter certain events at the neuromuscular junction (NMJ). Among these substances are Botulinum toxin, produced by the As a result, acetylcholine (ACh) is not The bacteria proliferate in improperly canned foods, and their toxin is one of the most lethal chemicals known. A tiny amount can cause death by paralyzing skeletal muscles. Breathing stops because respiratory muscles (which are skeletal muscles) are paralyzed. Despite these lethal effects, botulinum toxin can be used in small, diluted amounts as a medicine (Botox®). Injections of Botox into the affected muscles can help patients who have strabismus (crossed eyes), uncontrollable blinking, or spasms of the vocal cords that interfere with speech. Botox is also used as a cosmetic treatment to relax muscles that cause facial wrinkles and to alleviate chronic back pain due to muscle spasms. The venom of the black widow spider contains α-latrotoxin, which induces This causes the release of The plant derivative curare, a poison used by South American Indians on arrows and blowgun darts, causes In the presence of curare, the cation channel of the nicotinic ACh receptor Curare-like drugs are often used during surgery to relax skeletal muscles. Organophosphates are chemicals that inhibit acetylcholinesterase (AChE). Inhibition of AChE at the NMJ initially causes uncontrolled contractions of skeletal muscles and then causes muscle paralysis as muscle fibers become refractory to further stimulation. Organophosphates include the nerve gas sarin, which is used in chemical warfare, and malathion, which is an ingredient used in certain insecticides. curare is a ___ chemical.

cholinergic interfere with acetylcholine activity at the neuromuscular junction botulinum toxin, α-latrotoxin, curare, and organophosphates. bacterium Clostridium botulinum, blocks exocytosis of synaptic vesicles at the NMJ. released, and muscle contraction does not occur. massive exocytosis of synaptic vesicles at the NMJ. excessive amounts of ACh, which leads to overstimulation of skeletal muscles. Prolonged stimulation of respiratory muscles can lead to respiratory failure and death. muscle paralysis by binding to and blocking nicotinic ACh receptors on the motor end plate. does not open. non depolarizinf

Some sympathetic preganglionic axons extend to specialized cells called The adrenal medulla develops from the same embryonic tissue as the Therefore, chromaffin cells are modified sympathetic postganglionic neurons that lack dendrites and axons. Rather than extending to another organ, however, these cells release hormones into the blood. Upon stimulation by sympathetic preganglionic neurons, the chromaffin cells of the adrenal medulla release a mixture of catecholamine hormones—about 80% epinephrine, 20% norepinephrine, and a trace amount of dopamine (Figure 10.3). These hormones circulate through the body and

chromaffin cells in the adrenal medulla (inner portion of the adrenal gland) without synapsing in either the sympathetic trunk or collateral ganglia (Figure 10.3; see also Figure 10.2). sympathetic ganglia. intensify responses elicited by the sympathetic nervous system. (activate fight or flight)

Neurotransmittersare the chemical substances that neurons use to They are divided into two classes based on size: The small-molecule neurotransmitters include The neuropeptides are larger in size; they consist of many Most small-molecule neurotransmitters cause EPSPs or IPSPs by . By contrast, some small-molecule neurotransmitters and most neuropeptides do not Instead, these neurotransmitters function as For example, a neuromodulator may act on the postsynaptic cell to alter the cell's response to a Alternatively, the neuromodulator may act on the presynaptic cell to alter the The synaptic effects of neuromodulators usually are

communicate with other neurons, muscle fibers, and gland cells. small-molecule neurotransmitters and neuropeptides (Figure 7.32). acetylcholine, amino acids, biogenic amines, purines, gases, and endocannabinoids. amino acids linked together by peptide bonds. opening or closing ion channels in the postsynaptic membrane change the membrane potential of the postsynaptic cell. neuromodulators, substances that do not generate EPSPs or IPSPs but alter the strength of a particular synaptic response. specific neurotransmitter. synthesis, release, or reuptake of a specific neurotransmitter. long-lasting, having a duration of days, months, or even years.

Large-diameter, myelinated axons called A fibers . They carry By contrast, small-diameter, unmyelinated axons known as C fibers They carry information that is less critical, such as Origin: Graded vs Action potentials Types of channels for both Type or conduction amplitude duration polarity refractory period

conduct action potentials at velocities ranging from 12-130 m/sec (27-280 mi/hr) urgent information such as sensory signals associated with touch, pressure, and position of joints, and motor signals that cause contraction of skeletal muscles. conduct action potentials at velocities ranging from 0.5-2 m/sec (1-4 mi/hr). motor signals that cause contraction of smooth muscle in digestive organs. graded (Arise mainly in dendrites and cell body.), Actuon (Arise at trigger zone and propagate along axon.) graded: (ligand gated or mechanically gated ion channels), action: (voltage gated channels for Na+ and K+) graded: (décrémental.. not propagated), permit communication over short distances.) action: Propagate and thus permit communication over longer distances. graded: Depending on stimulus strength, varies from less than 1 mV to more than 50 mV., action: All or none; typically about 100 mV. graded; Typically longer, ranging from several msec to several min) Action: Shorter, ranging from 1-2 msec. graded: May be hyperpolarizing (inhibitory to generation of an action potential) or depolarizing (excitatory to generation of an action potential). Not present; thus summation can occur. Present; thus summation cannot occur

At an electrical synapse, action potentials Each gap junction contains tubular connexons, which function as As ions flow from one cell to the next through the connexons, the action potential At many electrical synapses, the flow of ions through gap junctions is Gap junctions are common in Electrical synapses have two main advantages: 1. Because action potentials conduct directly through gap junctions, electrical synapses are At an electrical synapse, the action potential passes directly from the 2. . Electrical synapses can synchronize the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can The value of synchronized action potentials in the heart or in visceral smooth muscle is l

conduct directly between adjacent cells through gap junctions. tunnels that connect the cytosol of the two cells directly (see Figure 6.1a). spreads from cell to cell. bidirectional; at other electrical synapses, ions flow through gap junctions in one direction only. cardiac muscle and visceral smooth muscle. They also occur in the CNS. Faster communication. faster than chemical synapses. presynaptic cell to the postsynaptic cell. The events that occur at a chemical synapse take some time and delay communication slightly. Synchronization produce action potentials in unison if they are connected by gap junctions. coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract.

In its broadest definition, sensation is the The nature of the sensation and the type of reaction generated vary according to the ultimate destination of the action potentials (nerve impulses) that convey sensory information to the . Sensory information that reaches the spinal cord may serve as input for Sensory input that reaches the brain stem elicits more complex reflexes, such as changes in When sensory information reaches the cerebral cortex, you become consciously aware of the Perception is the You have no perception of some sensory information because it never reaches the For example, certain sensory receptors constantly monitor the pressure of blood in blood vessels. Because the action potentials conveying blood pressure information propagate to the

conscious or subconscious awareness of changes in the external or internal environment. CNS spinal reflexes, such as the flexor (withdrawal) reflex that moves a limb away from a painful stimulus. heart rate or breathing rate. sensory stimuli and can precisely locate and identify specific sensations such as touch, smell, hearing, or taste. conscious awareness and interpretation of sensations and is primarily a function of the cerebral cortex. cerebral cortex. cardiovascular center in the medulla oblongata rather than to the cerebral cortex, blood pressure is not consciously perceived.

There are two types of propagation: The type of action potential propagation described so far is continuous conduction, which involves In continuous conduction, ions flow through their voltage-gated channels in . Note that the action potential propagates only a relatively short distance in a few milliseconds. Continuous conduction occurs in Action potentials propagate more rapidly along If you compare parts (a) and (b) in Figure 7.24, you will see that the action potential propagates much farther along the Saltatory conduction (SAL-ta-tō-rē; saltat= leaping), the special mode of action potential propagation that occurs along Few voltage-gated channels are present in regions where a myelin sheath covers the axon plasma membrane. By contrast, at the nodes of Ranvier (where there is ___), the Hence, current carried by Na+ and K+ flows across the membrane mainly When an action potential propagates along a myelinated axon, local current flow occurs through the The action potential at the first node generates . The resulting ionic flow through the opened channels constitutes an action potential at the Then the action potential at the second node generates After it depolarizes, each node

continuous conduction and saltatory conduction. step-by-step depolarization, repolarization, and after-hyperpolarization of each adjacent segment of the plasma membrane (Figure 7.24a). each adjacent segment of the membrane unmyelinated axons and in muscle fibers. myelinated axons than along unmyelinated axons. myelinated axon in the same period of time. myelinated axons, occurs because of the uneven distribution of voltage-gated channels. no myelin sheath), the axon plasma membrane has many voltage-gated channels. at the nodes. extracellular fluid surrounding the myelin sheath and through the cytosol from one node of Ranvier to the next. local current flow in the cytosol and extracellular fluid that depolarizes the membrane to threshold, opening voltage-gated Na+ channels at the second node second node. local current flow that opens voltage-gated Na+ channels at the third node, and so on. repolarizes, hyperpolarizes, and then is restored back to resting membrane potential.

As you have already learned, sensory information is ultimately converted into action potentials before it is . If all action potentials are the same, how does the nervous system tell the difference between one stimulus and another? This is accomplished by Sensory systems encode four attributes of a stimulus: modality, location, intensity, and duration. Stimulus Modality Each unique type of sensation—such as touch, pain, vision, taste, or hearing—is called a . Modality is encoded by which sensory receptor and neural pathway are activated when a stimulus is applied.For example, the modality of touch is perceived whenever a stimulus activates a touch receptor and its neural pathway to the cerebral cortex. Likewise, the modality of taste is perceived whenever a stimulus activates a gustatory receptor and its pathway to the cortex. Different types of sensory information are conveyed to different parts of the cerebral cortex for perception. The neural pathways that convey information about modality from peripheral receptors to specific regions of the cerebral cortex are called Each labeled line consists of a Therefore, you seem to see with your eyes, hear with your ears, or feel pressure on your arm because The association of a modality with the activation of a particular labeled line is called Activation of a given labeled line always causes For example, you normally perceive light when light waves activate the visual pathway to the cerebral cortex. Recall, however, that you also perceive light in the form of "stars" when there is a blow to the eye. Although the blow to the eye is not a visual stimulus, it still causes you to see light because the visual pathway to the cortex is activated.

conveyed into the CNS for integration sensory coding, the use of organizational and functional features of the nervous system to represent specific details about a stimulus. modality labeled lines (Figure 9.5). set of neurons that transmits information for only one modality. stimulation of receptors in each of these parts of the body activates a separate labeled line to a specific region of the cerebral cortex, which perceives the modality associated with that labeled line. labeled line coding. perception of its modality, regardless of how the pathway is stimulated.

The membrane potential is like voltage stored in a battery. If you connect the positive and negative terminals of a battery with a piece of wire, electrons will flow along the wire. This flow of charged particles is called . In living cells, the flow of ions (rather than electrons) constitutes the Current flow depends on two main factors: ( Resistance is the Conductors are substances that The extracellular and intracellular fluids of the body are . Insulators are substances that The plasma membrane is a good insulator since Because the plasma membrane is a good electrical insulator, the main paths for current to flow across the membrane are The relationship among current, voltage, and resistance is expressed by an equation called Ohm's law:

current electrical current. 1) voltage (the electrical potential difference between opposite charges that are separated from each other) and (2) the type of substance through which the charges move. hindrance to the flow of charges. permit fast current flow because they have a low resistance. good conductors because they consist of ions that carry the current decrease current flow because they have a high resistance. membrane lipids have few charged groups and cannot carry current. through ion channels. I= V/R .. current = voltage / resistance

A mature skeletal muscle fiber is a long, Because each skeletal muscle fiber arises during _____, a mature skeletal muscle fiber has multiple nuclei. Once fusion has occurred, the muscle fiber The contractile elements of muscle fibers, the

cylindrical structure with a diameter that ranges from 10 to 100 μm and a length that is typically about 10 cm (4 in.), although some are as long as 30 cm (12 in.). embryonic development from the fusion of many small, undifferentiated cells called myoblasts (Figure 11.3a) loses its ability to undergo cell division. myofibrils, contain overlapping thick and thin filaments.

An increase in the extracellular K+ concentration causes a Consequently, less K+ leaves the neuron, which causes the neuron to . By contrast, a decrease in the extracellular K+ concentration causes an As a result, more K+ leaves the neuron, which causes the neuron to . .An increase in the extracellular Na+ concentration causes an . Consequently, more Na+ enters the neuron, which causes the neuron to . Conversely, a decrease in the extracellular Na+ concentration causes a As a result,

decrease in the K+ concentration gradient across the plasma membrane of a neuron. depolarize increase in the K+ concentration gradient across the plasma membrane of the neuron. hyperpolarize increase in the Na+ concentration gradient across the plasma membrane of the neuron depolarize decrease in the Na+ concentration gradient across the plasma membrane of the neuron. less Na+ enters the neuron, which causes the neuron to hyperpolarize.

During the repolarizing phase of the action potential, membrane permeability to Na+ . The repolarizing phase begins when the inactivation gates of the voltage-gated Na+ Now the voltage-gated Na+ channel is in an inactivated state. The threshold-level depolarization that opened the voltage-gated Na+ channels also Because the voltage-gated K+ channels open more slowly, their opening The slower opening of voltage-gated K+ channels and the closing of previously open voltage-gated Na+ channels produce the As the Na+ channels are inactivated, Na+ inflow slows. At the same time, the K+ channels Slowing of Na+ inflow and acceleration of K+ outflow causes the membrane potential to change from +30 mV to −70 mV.

decreases and membrane permeability to K+ increases (see Figure 7.20b) channels close (step 3 in Figure 7.22). opens voltage-gated K+ channels (steps 3 and 4 in Figure 7.22). occurs at about the same time that the voltage-gated Na+ channels are closing. repolarizing phase of the action potential. are opening, accelerating K+ outflow. As the membrane potential approaches −70 mV, the inactivated Na+ channels revert to their resting state.

An action potential occurs in the membrane of the axon when Different neurons may have different thresholds for action potential generation, but the threshold in a particular neuron is usually constant. The generation of an action potential depends on whether a particular stimulus is able to bring the An action potential does not occur in response to a However, an action potential does occur in response to a threshold stimulus, a stimulus that is Several action potentials form in response to a suprathreshold stimulus, a stimulus that is strong enough to depolarize the membrane above threshold (Figure 7.21). Each of the action potentials caused by a Therefore, once an action potential is generated, the amplitude of an action potential is Instead, the greater the stimulus strength above threshold, the greater the

depolarization reaches a certain level termed the threshold (about −55 mV in many neurons) (Figure 7.21). membrane potential to threshold. subthreshold stimulus, a stimulus that is a weak depolarization that cannot bring the membrane potential to threshold (Figure 7.21). just strong enough to depolarize the membrane to threshold (Figure 7.21). suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus. always the same and does not depend on stimulus intensity. frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period (described shortly).

A graded potential is a small When the response makes the membrane less polarized (inside less negative), it is termed a When the response makes the membrane more polarized (inside more negative), it is termed a The duration of a depolarizing or hyperpolarizing graded potential can last from several milliseconds (msec) to several minutes. A graded potential occurs when a stimulus causes Mechanically-gated channels and ligand-gated channels can be present in the Hence, graded potentials occur mainly in the

deviation from the membrane potential that makes the membrane either less polarized (inside less negative) or more polarized (inside more negative). depolarizing graded potential (Figure 7.14a). hyperpolarizing graded potential (Figure 7.14b). mechanically-gated channels or ligand-gated channels to open or close in an excitable cell's plasma membrane (Figure 7.15). dendrites of sensory neurons, and ligand-gated channels are numerous in the dendrites and cell bodies of interneurons and motor neurons (see Table 7.1). dendrites and cell body of a neuron.

Chemical Synapses Involve the Release of Neurotransmitter into a Synaptic Cleft Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they They are separated by the Action potentials cannot conduct across the In response to an action potential, the presynaptic neuron releases a . The postsynaptic neuron receives the chemical signal and, in turn, Thus, the presynaptic neuron converts an The postsynaptic neuron receives the chemical signal and, in turn, . The time required for these processes at a chemical synapse, a synaptic delay of about 0.5 msec, is the reason that chemical synapses

do not touch. synaptic cleft, a space of 20-50 nm that is filled with interstitial fluid. synaptic cleft, so an alternate, indirect form of communication occurs. neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron produces a postsynaptic potential, a type of graded potential. electrical signal (action potential) into a chemical signal (released neurotransmitter). generates an electrical signal (postsynaptic potential) relay signals more slowly than electrical synapses.

in Chapter 9 you learned how the sensory systems detect, convey, and process information from the external or internal environment. Once the central nervous system (CNS) integrates this sensory information, it Recall that the efferent division of the PNS is . The somatic nervous system conveys output from Because its motor responses can be consciously controlled, the action of the somatic nervous system is . The autonomic nervous system conveys output from the CNS to Because its motor responses are not normally under conscious control, the action of the autonomic nervous system is . In this chapter, you will learn about the functions of both the autonomic and somatic nervous systems.

elicits an appropriate response by activating the efferent division of the peripheral nervous system (PNS), which conveys motor output from the CNS to effectors (muscles and glands) in the body. further subdivided into a somatic nervous system and an autonomic nervous system (see Figure 7.1b) the CNS to skeletal muscles. voluntary smooth muscle, cardiac muscle, and glands. involuntary

The ANS is also comprised of a third branch known as the The ENS consists of millions of neurons in Its operation is . Although the neurons of the ENS can function autonomously, they can also be regulated by the other branches of the . The ENS contains Enteric sensory neurons monitor chemical changes within the Enteric interneurons integrate information from the Enteric motor neurons govern contraction of Despite the presence of sensory neurons and interneurons, the ENS is considered to be part of the

enteric nervous system (ENS). plexuses that extend most of the length of the gastrointestinal (GI) tract. involuntary ANS sensory neurons, interneurons, and motor neurons. GI tract as well as the stretching of its walls. sensory neurons and provide input to motor neurons. GI tract smooth muscle and secretion of GI tract glands. efferent (motor) division of the PNS because of its association with the ANS.

The resting membrane potential of a cell exists because of an The excess charges are located only very close to the membrane; the rest of the extracellular fluid or cytosol contains It is important to point out that only a tiny fraction of all of the charges in the ECF and cytosol must be separated across the plasma membrane in order to establish the normal resting membrane potential. By convention, membrane potential always compares the amount of . In neurons, the resting membrane potential ranges from − A typical value is . A cell that exhibits a membrane potential is said to be . Most body cells are polarized; the membrane potential varies from

excess of negative ions in the cytosol along the inside surface of the membrane and an equal excess of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane (Figure 7.9). equal numbers of positive and negative charges and is electrically neutral. net excess charge inside the cell relative to the outside -40 to −90 mV. −70 mV polarized +5 mV to -100 mV in different types of cells.

Glutamate (glutamic acid) has powerful Most excitatory neurons in the ___ and ___communicate via glutamate. Aspartate (aspartic acid) is an Aspartate activates some of the same types of neurotransmitter receptors as Gamma-aminobutyric acid (GABA) (GAM-ma am-i-nō-bū-TIR-ik) is an important GABA is found in the . As many as one-third of all brain synapses use Like GABA, the amino acid glycine is an About half of the inhibitory synapses in the ____ use glycine; the rest use GABA. Like GABAA and GABAC receptors, glycine receptors are Activation of glycine receptors causes the Cl− channels to open. As a result,

excitatory effects. CNS and perhaps half of the synapses in the brain excitatory neurotransmitter that is released by certain neurons of the CNS. glutamate (namely, the NMDA receptors). inhibitory neurotransmitter that is derived from the amino acid glutamate. CNS, where it is the most common inhibitory neurotransmitter GABA. inhibitory neurotransmitter. spinal cord ionotropic receptors that contain Cl− channels. Cl− moves from the ECF into the cytosol, and the postsynaptic membrane becomes hyperpolarized (IPSP).

A single postsynaptic neuron receives input from many presynaptic neurons, some of which release The sum of all of the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron, which may respond in the following ways: EPSP. If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an Following an EPSP, subsequent stimuli can more easily generate Action potential(s). If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, Action potentials continue to be generated as long as the EPSP is IPSP. If the total inhibitory effects are greater than the excitatory effects, the membrane The result is

excitatory neurotransmitters and some of which release inhibitory neurotransmitters (Figure 7.30). EPSP that does not reach threshold. an action potential through summation because the neuron is partially depolarized. one or more action potentials will be triggered. at or above the threshold level. hyperpolarizes (IPSP). inhibition of the postsynaptic neuron and an inability to generate an action potential.

A neurotransmitter has either an If the binding of a neurotransmitter to a neurotransmitter receptor opens or closes ion channels in the postsynaptic membrane, the neurotransmitter either A neurotransmitter that depolarizes the postsynaptic membrane is A depolarizing postsynaptic potential is called an . Although a single EPSP normally does not Because it is partially depolarized, it is more likely to reach A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is . During hyperpolarization, generation of an action potential is more . A hyperpolarizing postsynaptic potential is termed an inhibitory postsynaptic potential (IPSP). Neurotransmitters do not always change the membrane potential of the postsynaptic cell because the binding of some neurotransmitters to their receptors does not Instead, there may be another response, such as . This is often the case for those neurotransmitters that bind to receptors in ⭐️ In these situations, the neurotransmitter has an The neurotransmitter has an inhibitory effect if it

excitatory or inhibitory effect on the membrane of the postsynaptic cell. depolarizes or hyperpolarizes the postsynaptic cell. excitatory because it brings the membrane closer to threshold. excitatory postsynaptic potential (EPSP) initiate an action potential, the postsynaptic cell does become more excitable. threshold when the next EPSP occurs. inhibitory difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state result in the opening of ion channels in the postsynaptic membrane. synthesis of new proteins or changes in the intracellular Ca2+ levels effector cells (muscle fibers and gland cells). excitatory effect if it causes contraction of the muscle fiber or stimulates gland cell secretion. causes relaxation of the muscle fiber or inhibits gland cell secretion.

Sensory function. Sensory receptors detect This sensory information is then conveyed through Integrative function. The CNS processes sensory information by Motor function. Once sensory information is integrated, the CNS may For this to occur, motor information is conveyed from the Stimulation of the effectors causes muscles to The three basic functions of the nervous system occur, for example, when you answer your cell phone after hearing it ring. The sound of the ringing phone stimulates sensory receptors in your ears (sensory function). This auditory information is subsequently relayed into your brain, where it is processed and the decision to answer the phone is made (integrative function). The brain then stimulates the contraction of specific muscles that allow you to grab the phone and press the appropriate button to answer it (motor function).

external or internal stimuli, such as a raindrop landing on your arm or an increase in blood acidity. cranial and spinal nerves of the PNS into the brain and spinal cord of the CNS. analyzing it and making decisions for appropriate responses—an activity known as integration. elicit an appropriate motor response. CNS through cranial and spinal nerves of the PNS to effectors (muscles and glands). contract and glands to secrete.

Changes in the extracellular Ca2 + concentration. The voltage-gated Na+ channels of a neuron are sensitive to the This is because Ca2+ ions in the ECF Consequently, an increase in the extracellular Ca2+ concentration increases the . This decreases the The opposite situation occurs when there is a decrease in the extracellular Ca2+ concentration. Less Ca2+ in the ECF reduces the This increases the Hypocalcemia Calcium can interfere with Na+ flow through its channels. The decreased [Ca++]o provides

extracellular Ca2+ concentration. bind to the extracellular surfaces of voltage-gated Na+ channels and increase the voltage that these channels require to open. number of Ca2+ ions that bind to the voltage-gated Na+ channels excitability of the neuron because the voltage-gated Na+ channels now require a higher voltage than normal to open. number of Ca2+ ions that bind to the voltage-gated Na+ channels. excitability of the neuron because the voltage-gated Na+ channels are able to open at a lower voltage than normal. Hypocalcemia increases irritability by lowering the threshold for action potential firing less interruption for Na+, therefore increasing depolarization rate (speed, likelihood).

Recall that summation is the process by which . The greater the summation of EPSPs, the greater the chance that At threshold, one or more There are two types of summation: Spatial summation is summation of postsynaptic potentials in response to For example, spatial summation results from the buildup of Temporal summation is summation of postsynaptic potentials in response to stimuli that occur at the For example, temporal summation results from buildup of neurotransmitter released by a Because a typical EPSP lasts about 15 msec, the second (and subsequent) release of neurotransmitter must occur Summation is rather like voting on the Internet. Many people voting yes or no on an issue at the same time can be compared to spatial summation. One person voting repeatedly and rapidly is like temporal summation. Most of the time, spatial and temporal summations are acting together to influence the chance that a neuron fires an action potential.

graded potentials add together threshold will be reached. action potentials arise. spatial summation and temporal summation. stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. neurotransmitter released simultaneously by several presynaptic end bulbs (Figure 7.29a). same location in the membrane of the postsynaptic cell but at different times. single presynaptic end bulb two or more times in rapid succession (Figure 7.29b). soon after the first one if temporal summation is to occur.

The nervous system plays a major role in maintaining It keeps most controlled variables within limits that sustain life. When stimuli cause controlled variables to deviate from their set points, ____ of the ____ The ability to produce action potentials in response to stimuli is referred to as Once generated, neurons use action potentials to This, in turn, regulates the

homeostasis neurons of the nervous system respond rapidly by forming electrical signals (graded potentials and action potentials). electrical excitability. communicate with other neurons, muscle fibers, or gland cells in one or more organs of the body. organ's activity and ultimately restores homeostatic conditions.

The same neurotransmitter can be excitatory at some synapses and _____, depending on the For example, at some excitatory synapses, acetylcholine (ACh) binds to By contrast, at some inhibitory synapses ACh binds to IPSP VS EPSP Fast and Slow Responses in Postsynaptic Cells The time required for a neurotransmitter to induce a postsynaptic potential (either an EPSP or an IPSP) and the duration of that postsynaptic potential are both influenced by whether the neurotransmitter . In general, binding of a neurotransmitter to an ionotropic receptor causes a By contrast, binding of neurotransmitter to a metabotropic receptor causes a

inhibitory at others, structure of the neurotransmitter receptor to which it binds. ionotropic receptors containing cation channels that open and subsequently generate EPSPs in the postsynaptic cell (Figure 7.27a). metabotropic receptors coupled to G proteins that open K+ channels, resulting in the formation of IPSPs in the postsynaptic cell (Figure 7.27c). An EPSP is received when an excitatory presynaptic cell, connected to the dendrite, fires an action potential. ... An inhibitory postsynaptic potentials (IPSP) is a temporary hyperpolarization of postsynaptic membrane caused by the flow of negatively charged ions into the postsynaptic cell. binds to an ionotropic receptor or to a metabotropic receptor fast responsein the postsynaptic cell because activation of an ionotropic receptor quickly opens or closes ion channels in the postsynaptic membrane, and the subsequent postsynaptic potential that is generated usually lasts for only a few milliseconds or less. slow responsein the postsynaptic cell because activation of a metabotropic receptor opens or closes ion channels more slowly (because G protein and second messengers are involved) and the subsequent postsynaptic potential that is generated typically lasts for hundreds of milliseconds to several minutes or even longer.

To understand how K+ and Na+ ions influence the resting membrane potential, first consider what would happen if a neuron were permeable only to K+. The passage of K+ through K+ leak channels would allow the K+ ions to move down their concentration gradient from the cytosol into the ECF, generating an As K+ ions continue to move out of the neuron, the interior of the membrane becomes more negative, creating an Initially, however, there is net movement of K+ ions out of the neuron because the magnitude of the As the membrane potential becomes even more negative, the magnitude of the K+ Eventually, the K+ electrical gradient becomes equal in magnitude to the opposing K+ concentration gradient and there is The membrane potential that exists at this equilibrium is called the K+ equilibrium potential (EK) and it is equal to In general, an equilibrium potential is the membrane potential at which the The K+ equilibrium potential is close to, but not exactly equal to, the resting membrane potential (−70 mV), which means that K+ ions are not the

inside-negative membrane potential (Figure 7.11a). electrical gradient that favors movement of K+ ions from the ECF back into the cytosol (Figure 7.11b). K+ concentration gradient is greater than the magnitude of the K+ electrical gradient. electrical gradient increases. no net movement of K+ ions into or out of the neuron (Figure 7.11c). −90 mV. concentration gradient and electrical gradient for a particular ion are equal in magnitude but opposite in direction and there is no net movement of that ion across the plasma membrane. only ions that contribute to the resting membrane potential.

Many inhibitory neurotransmitters bind to In cells that actively transport Cl− ions into the ECF, . When Cl− channels open, there is net movement of chloride ions into the cell because the The inward flow of Cl− ions causes the inside of the postsynaptic cell to become In cells that do not actively transport Cl− ions, opening Cl− channels does not change the membrane potential because However, opening these Cl− channels does Metabotropic Receptors A metabotropic receptor is a type of neurotransmitter receptor that The G protein, in turn, either Some inhibitory neurotransmitters bind to metabotropic receptors that cause When the K+ channels open, a larger number of potassium ions The outward flow of K+ ions causes the inside of the postsynaptic cell to become

ionotropic receptors that contain chloride channels (Figure 7.27b). IPSPs result from opening these Cl− channels chloride equilibrium potential (ECl) is not equal to the resting membrane potential. more negative (hyperpolarized). ECl is equal to the resting membrane potential and there is no net movement of Cl− ions into or out of the cell. stabilize the membrane potential and makes it more difficult for excitatory neurotransmitters to depolarize the cell. contains a neurotransmitter binding site and a site that is coupled to a G protein. directly opens (or closes) an ion channel or it activates a second messenger pathway that opens (or closes) an ion channel or causes another response in the cell, such as increasing the synthesis of new proteins, modifying the activity of existing proteins, or increasing the intracellular Ca2+ levels. K+ channels to open (Figure 7.27c). diffuses outward. more negative (hyperpolarized), resulting in the formation of an IPSP.

As a graded potential spreads to adjacent regions of membrane by local current flow, it gradually dies out because This mode of travel by which graded potentials die out as they spread along the membrane is known as Figure 7.18 shows that the amplitude of a graded potential decreases as the Because they die out within a few millimeters of where they originate, graded potentials are useful for Although an individual graded potential undergoes decremental conduction, it can become stronger and last longer by Summation is the process by which graded If two depolarizing graded potentials summate, the net result is a If two hyperpolarizing graded potentials summate, If two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), they

its charges are lost across the membrane through open leak channels. decremental conduction. distance from the graded potential's point of origin increases. short-distance communication only. summating with other graded potentials. potentials add together. larger depolarizing graded potential (Figure 7.19). the net result is a larger hyperpolarizing graded potential. cancel each other out and the overall graded potential disappears.

The flow of current across the membrane only at the nodes of Ranvier has two consequences: 1. The action potential appears to " Because an action potential leaps across long segments of myelinated axon plasma membrane as current flows from one node to the next, it travels 2. Opening a smaller number of channels only at the nodes, rather than many channels in each adjacent segment of membrane, is a more . Because only small regions of the membrane depolarize and repolarize, minimal inflow of Thus, less ATP is The velocity of action potential conduction is affected by two major factors: Axon diameter. The larger the diameter of the axon, the An axon with a large diameter offers 2. Presence or absence of myelin. Conduction of action potentials is more rapid along Recall that this is because an action potential leaps across

leap" from node to node as each nodal area depolarizes to threshold, thus the name saltatory. much faster than it would in an unmyelinated axon of the same diameter. energy-efficient mode of conduction Na+ and outflow of K+ occurs each time an action potential passes by. used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+. axon diameter and presence or absence of myelin faster the action potential is conducted. less resistance to local current flow, which allows adjacent regions of membrane to be brought to threshold more quickly. myelinated axons than along unmyelinated axons. long segments of membrane of a myelinated axon, whereas it must travel through each adjacent segment of membrane of an unmyelinated axon.

For somatic sensations and vision, stimulus location is encoded by the Sensory receptors at the body surface and in the retina of the eye are organized in such a way that adjacent receptors give rise to This orderly arrangement in which the relationship between adjacent sensory receptors is maintained as information is processed in the CNS is referred to as a So when a stimulus activates one or more sensory receptors in the receptive field of a somatic sensory or visual neuron, the brain uses that input to In the auditory and olfactory systems, receptive fields do not provide information about Recall that the receptive field of an auditory sensory neuron is a particular set of sound frequencies, and the receptive field of an olfactory receptor cell is a select group of odorants. So how is stimulus location coded in the auditory and olfactory systems? The answer is based on slight timing differences in the arrival of sound waves at the two ears or odorants at the two nostrils. The brain compares the timing of receptor activation in the two ears or two sides of the nose to determine the location of sound or smell, respectively. For example, if receptors in the right ear are activated before receptors in the left ear, input from the right ear will reach the brain before input from the left ear. The brain uses the time difference of this input to determine that the sound is coming from the right. Now that you have an understanding of how stimulus location is encoded, you are ready to learn about acuity, an important property related to stimulus location. Acuity is sharpness of perception—in other words, the ability to precisely locate and distinguish one stimulus from another. In the somatic sensory and visual systems, two major factors affect acuity: (1) the size of the receptive field and (2) lateral inhibition.

location of the activated receptive field. pathways that project to adjacent regions of the cerebral cortex. topographic pattern. identify a specific topographic location for the stimulus. stimulus location because the receptive fields of auditory sensory neurons and olfactory sensory neurons are not physical areas.

Gate control theory of pain: There are two types of pain: fast and slow. Fast pain is a sharp, pricking sensation that is well localized. For example, the pain felt from a needle puncture or a knife cut to the skin is fast pain. Signals for fast pain are transmitted along Sensory neurons associated with mechanical or thermal nociceptors have Slow pain, by contrast, is a dull, aching sensation that is poorly localized. An example is the pain associated with a minor toothache. Signals for slow pain are transmitted along Sensory neurons associated with polymodal nociceptors have You can perceive the difference in onset of fast and slow pain when you injure a body part that is far from the brain because the conduction distance is long. When you stub your toe, for example, you first feel the sharp sensation of fast pain and then feel the slower, aching sensation of slow pain.

mechanical stimulus can suppress pain sensations A-delta (Aδ) fibers, small, myelinated axons with conduction velocities ranging from 12 to 30 m/sec. Aδ fibers and are the source of the signals for fast pain. C fibers, small, unmyelinated axons that have conduction velocities ranging from 0.5 to 2 m/sec. C fibers and are the source of the signals for slow pain.

Each of your skeletal muscles is composed of hundreds to thousands of muscle cells, called An individual muscle fiber is surrounded by a In addition, connective tissue surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called Connective tissue also surrounds the entire muscle itself. All of the connective tissues of a muscle are A tendon is a

muscle fibers, arranged parallel to one another (Figure 11.2). sheath of connective tissue. fascicles (FAS-i-kuls= little bundles) (Figure 11.2). continuous with its tendons. cord of connective tissue that attaches the muscle to a bone (Figure 11.2).

Nociceptors have Heat •Vanilloid receptors •Heat and capsaicin •Cold •Menthol receptors •Cation pore •Cold and menthol •Warm •Camphor

naked nerve endings and signal various types of pain

Neurons, also known as ____, are the basic (A functional unit is the smallest component of a system that can carry out the functions of that system.) Examples of functions performed by neurons include Components of a Neuron A typical neuron has three major parts: Dendrites are Because they receive signals from other neurons or from stimuli in the environment, dendrites function as the Most neurons have numerous dendrites, an aspect that substantially increases the

nerve cells, are the basic functional units of the nervous system. sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. a cell body, dendrites, and an axon (Figure 7.2). short, highly branched processes that extend from the cell body. main input portions of the neuron. receptive surface area of the cell.

The synapse between an autonomic postganglionic neuron and a visceral effector is called the The organization of the NEJ differs from a typical neuron-to-neuron synapse in two major ways: 1. The axon terminals of the postganglionic neuron lack 2. In the effector, the receptors for the neurotransmitters are not confined to a Based on the neurotransmitters they produce and release, most autonomic neurons are classified as either The receptors for the neurotransmitters are integral membrane proteins located in the plasma membrane of the postsynaptic neuron or effector cell. Cholinergic Neurons and Receptors Cholinergic neurons release the neurotransmitter In the ANS, cholinergic neurons include (1)

neuroeffector junction (NEJ) (Figure 10.4). synaptic end bulbs; instead, they exhibit swollen regions called varicosities, which contain synaptic vesicles with neurotransmitter. specific receptor region; rather, they are located along the entire surface of the cell. cholinergic or adrenergic. acetylcholine (ACh). all parasympathetic and sympathetic preganglionic neurons, (2) most parasympathetic postganglionic neurons, and (3) sympathetic postganglionic neurons that innervate most sweat glands (Figure 10.6).

Schwann cells (SCHVON or SCHWON) (Figure 7.5a) are a type of They form the Schwann cells also participate in In the peripheral nervous system, Schwann cells Axons in the CNS or PNS that have a myelin sheath are said to be Within the brain and spinal cord are regions that look white, known as white matter, and regions that appear gray, called gray matter(Figure 7.6). White matter is composed primarily of The whitish color of myelin gives white matter its name. The gray matter of the nervous system contains It appears grayish rather than white because of the absence of myelin in these areas

neuroglia found only in the PNS. myelin sheath around axons of PNS neurons. axon regeneration, which is more easily accomplished in the PNS than in the CNS. provide insulation and more effective connectivity myelinated, and those without it are said to be unmyelinated. myelinated axons. neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.

As you have just learned, neurotransmitters released from a presynaptic neuron bind to Neurotransmitter receptors are classified into two main categories: Ionotropic Receptors An ionotropic receptor is a type of neurotransmitter receptor that contains Ionotropic receptors are examples of In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor . When the correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell. Many excitatory neurotransmitters bind to EPSPs result from opening these cation channels. When cation channels open, they allow passage of the However, the amount of Na+ that enters the postsynaptic cell is In addition, more Na+ ions than Ca2+ ions Therefore, the net effect of opening cation channels in the postsynaptic cell is that Na+ inflow is

neurotransmitter receptors in the plasma membrane of a postsynaptic cell. ionotropic receptors and metabotropic receptors. both a neurotransmitter binding site and an ion channel as part of its structure. ligand-gated channels (see Figure 7.8b). is closed ionotropic receptors that contain cation channels (Figure 7.27a). three most plentiful cations (Na+, K+, and Ca2+) through the postsynaptic cell membrane. greater than the amount of K+ that leaves it because the resting membrane potential of the postsynaptic cell is closer to the potassium equilibrium potential (EK) than to the sodium equilibrium potential (ENa). enter the postsynaptic cell because the concentration of Na+ ions in the ECF is higher than the concentration of Ca2+ ions in the ECF. greater than either K+ outflow or Ca2+ inflow, and the inside of the postsynaptic cell becomes less negative (depolarized).

Adrenergic neurons release Most sympathetic postganglionic neurons are (Figure 10.6b). Once NE is released from an adrenergic neuron, it diffuses across the synaptic cleft and As you learned in Chapter 7, adrenergic receptors respond to the binding of either ___, or ___. In the ANS, norepinephrine can be released as a The two main types of adrenergic receptors are alpha (α) receptors and beta (β) receptors. In the ANS, these receptors are found on visceral effectors innervated by most sympathetic postganglionic axons (Figure 10.6b). Alpha and beta receptors are further classified into subtypes—α1, α2, β1, β2, and β3—based on the specific responses they elicit and by their selective binding of drugs that activate or block them. The α receptors have a higher affinity for norepinephrine than epinephrine. β1 receptors have nearly equal affinity for norepinephrine and epinephrine; β2 receptors have a higher affinity for epinephrine than norepinephrine; and β3 receptors have a higher affinity for norepinephrine than epinephrine. Adrenergic receptors are Different types of adrenergic receptors couple to different types of The α1 receptors activate Gq proteins that trigger the IP3/DAG second messenger pathway, which increases intracellular Ca2+ levels. The α2 receptors activate Gi proteins, which inhibit adenylyl cyclase and decrease cAMP levels. β1, β2, and β3 receptors all activate Gs proteins, which activate adenylyl cyclase and increase cAMP levels. Although there are some exceptions, activation of α1 and β1 receptors generally produces β3 receptors are present mainly on the cells of brown adipose tissue, where their activation causes thermogenesis (heat production). Cells of most effectors contain either alpha or beta receptors; some visceral effector cells contain both.

norepinephrine (NE), also known as noradrenaline. adrenergic binds to specific adrenergic receptors on the postsynaptic membrane, causing either excitation or inhibition of the effector cell. norepinephrine or epinephrine (see Section 7.5). neurotransmitter (by sympathetic postganglionic neurons) or as a hormone (by chromaffin cells of the adrenal medulla); epinephrine is released as a hormone. metabotropic receptors that trigger second messenger pathways by coupling to G proteins. G proteins. excitation, and activation of α2 and β2 receptors causes inhibition of effector tissues.

Removal of the neurotransmitter from the synaptic cleft is essential for If a neurotransmitter could linger in the synaptic cleft, it would Neurotransmitter is removed in three possible ways: 1.______. Some of the released neurotransmitter molecules _____ Once a neurotransmitter molecule is out of reach of its receptors, it can no longer Enzymatic degradation. Certain neurotransmitters are inactivated through For example, the enzyme acetylcholinesterase, which is located on the postsynaptic membrane, breaks down Uptake by cells. Many neurotransmitters are actively transported Others are transported into The neurons that release norepinephrine, for example, rapidly The membrane proteins that accomplish such uptake are called

normal synaptic function. influence the postsynaptic neuron, muscle fiber, or gland cell indefinitely. Diffusion, diffuse away from the synaptic cleft (Figure 7.28a). exert an effect. enzymatic degradation (Figure 7.28b). acetylcholine in the synaptic cleft. back into the neuron that released them (reuptake) (Figure 7.28c). neighboring neuroglia (uptake). take up the norepinephrine and recycle it into new synaptic vesicles. neurotransmitter transporters.

The cell body (soma) contains most of the Because of its ability to direct protein synthesis and other cellular activities, the cell body functions as the . Like dendrites, the cell body also serves as an Throughout the nervous system, the cell bodies of adjacent neurons are often clustered together. A cluster of neuronal cell bodies in the PNS is called a The axon is a single long, thin process that It functions as the output portion of the neuron by The axon usually connects to the cell body at a cone-shaped region called the In most neurons, action potentials arise at the The axon hillock is also known as the trigger zone because of its Along the length of an axon, side branches called . The axon and its collaterals end by dividing into

organelles, including the nucleus. control center of the neuron input portion of the neuron because it can receive signals from other neurons. ganglion (plural is ganglia); a similar arrangement of neuronal cell bodies in the CNS is known as a nucleus (plural is nuclei). extends from the cell body. generating action potentials and then propagating them toward another neuron, a muscle fiber, or a gland cell. axon hillock (Figure 7.2). axon hillock, from which they travel along the axon to their destination. role in the generation of action potentials. axon collaterals may extend off smaller processes called axon terminals.

What is a nocireceptor? Are hot receptors nociceptors? Cold?

pain receptor Transduction of noxious stimuli into electrical signals involves ion channels that are present in the membrane of the nociceptor. One example is TRPV1†, a member of the transient receptor potential (TRP) family. TRPV1 channels are found in the membrane of a polymodal nociceptor (Figure 9.14). They are cation channels that open in response to extreme heat or to capsaicin—the ingredient that makes chili peppers painfully hot. Opening these cation channels allows Na+ and Ca2+ ions to enter the nociceptor, which causes a depolarizing receptor potential to form in the nociceptor's membrane (Figure 9.14). If threshold is reached, an action potential is generated in the axon of the sensory neuron. Temperatures below 10°C and above 45°C primarily stimulate pain receptors rather than thermoreceptors, producing painful sensations. so yes

A mechanical stimulus such as touch, pressure, or vibration can suppress Within the spinal cord are interneurons that normally When part of the body experiences a painful stimulus without an accompanying mechanical stimulus, C fibers carrying information about the painful stimulus from nociceptors As a result, second-order pain neurons are If part of the body experiences a painful stimulus along with a mechanical stimulus, C fibers still activate second-order pain neurons and inhibit the activity of the inhibitory interneurons; however, the inhibitory interneurons are also Because the inhibitory interneurons receive excitatory input in addition to inhibitory input, they Consequently, second-order pain neurons are . The gate control theory of pain explains why rubbing a damaged part of the body helps to reduce pain in that area: rubbing (a mechanical stimulus) activates Aβ fibers that ultimately decrease the activity of second-order pain neurons. The gate control theory of pain is also the rationale for using transcutaneous electrical nerve stimulation (TENS) for pain relief. In TENS, current is applied to a painful area through electrodes placed on the skin. This method provides pain relief by activating nearby Aβ fibers. inhibitory neurons are

pain sensations, a concept known as the gate control theory of pain. inhibit second-order neurons of ascending pain pathways—an action that prevents transmission of pain signals to the brain. excite second-order pain neurons and completely inhibit the activity of the inhibitory interneurons (Figure 9.17a). fully activated and transmit pain signals to the brain. excited by Aβ fibers (large diameter axons) carrying information about the mechanical stimulus from mechanoreceptors (Figure 9.17b). become partially activated. partially inhibited and transmit fewer pain signals to the brain Since Ia interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing. Thus, it prevents the antagonist muscle from contracting.

The Autonomic Nervous System includes both the The autonomic nervous system (ANS) innervates These tissues are often referred to as ___ effectors bc Autonomic motor neurons regulate visceral activities by either ___,____,____are examples of autonomic responses. Unlike skeletal muscle, tissues innervated by the ANS often function to some extent even if their The heart continues to beat when it is The ANS usually operates without For example, you probably cannot voluntarily slow down your heart rate; instead, your heart rate is subconsciously regulated. For this reason, some autonomic responses are the basis for polygraph ("lie detector") tests. The ANS was so-named because it was thought to function However, as you will soon learn, centers in the brain and spinal cord do regulate The ANS consists of two main branches: the parasympathetic nervous system and the sympathetic nervous system. Most organs receive nerves from both of these branches, an arrangement known as In general, one branch stimulates the organ to For example, neurons of the sympathetic nervous system The parasympathetic nervous system enhances By contrast, the sympathetic nervous system promotes the

parasympathetic (normal) and sympathetic (stress) divisions smooth muscle, cardiac muscle, and glands. visceral effectors because they are usually associated with the viscera (internal organs) of the body. increasing (exciting) or decreasing (inhibiting) ongoing activities in their effectors. Adjustment of the rate and force of the heartbeat, dilation or constriction of the bronchial tubes of the lungs, and secretion of digestive glands nerve supply is damaged or cut. removed for transplantation into another person, smooth muscle in the lining of the gastrointestinal tract contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control. conscious control. autonomously or in a self-governing manner (autonomic = autonomous), without control by the CNS. autonomic activities. dual innervation. increase its activity (excitation), and the other branch decreases the organ's activity (inhibition). increase heart rate, and neurons of the parasympathetic nervous system slow it down. rest-and-digest activities, which conserve and restore body energy during times of rest and recovery. fight-or-flight response, which prepares the body for emergency situations.

A sensory receptor is activated by a Most stimuli are in the form of __,___ or ___. The type of stimulus to which a sensory receptor responds best is known as its Sensory receptors are classified into five major groups according to their adequate stimuli: Mechanoreceptors are sensitive to Mechanoreceptors provide sensations of They also monitor the stretching of Thermoreceptors detect changes in . Photoreceptors detect Chemoreceptors detect Nociceptors (nō′-sē-SEP-tors; noci- = harmful) respond to Although a sensory receptor is most responsive to its For example, photoreceptors of the eye are most responsive to . However, an intense mechanical stimulus such as a blow to the eye activates

particular stimulus. mechanical energy, such as pressure changes or sound waves; electromagnetic energy, such as light or heat; or chemical energy, such as in a molecule of glucose. adequate stimulus. mechanical stimuli such as the deformation, stretching, or bending of cells. touch, pressure, vibration, proprioception (muscle and joint position), and hearing and equilibrium. blood vessels and internal organs. temperature light that strikes the retina of the eye. chemicals in the mouth (taste), nose (smell), and body fluids. painful stimuli resulting from physical or chemical damage to tissues. adequate stimulus, it can also respond to other stimuli if the intensity is high enough. light photoreceptors and causes you to "see stars."

Graded potentials have different names depending on which type of stimulus causes them and where they occur. For example, when a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter, it is called a The graded potentials that occur in sensory receptors are termed When a graded potential occurs in the plasma membrane of a skeletal muscle fiber at the neuromuscular junction (NMJ), it is called an An action potential (AP) or impulse is a sequence of An action potential has two main phases: a During the depolarizing phase, or rising phase, the . The depolarizing phase reaches its peak at +30 mV. The part of the depolarizing phase between 0 mV and +30 mV is called the . During the repolarizing phase, or falling phase, the membrane potential is Following the repolarizing phase, there may be an after-hyperpolarizing phase, also called the undershoot, during which Two types of voltage-gated channels open and then close during an action potential. These channels are present mainly in the The first channels that open, the Then voltage-gated K+ channels open, allowing The after-hyperpolarizing phase occurs when the . The duration of the action potential in most neurons is about 1-2 msec.

postsynaptic potential (explained in Section 7.4). receptor potentials (explained in Chapter 9). end plate potential (EPP rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. depolarizing phase and a repolarizing phase (Figure 7.20a). negative membrane potential becomes less negative, reaches zero, and then becomes positive overshoot restored to the resting state of −70 mV. the membrane potential temporarily becomes more negative than the resting level. axon plasma membrane and axon terminals. voltage-gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. K+ to flow out, which produces the repolarizing phase. voltage-gated K+ channels remain open after the repolarizing phase ends

Through sustained contraction or alternating contraction and relaxation, muscle has four key functions: 1. Producing body movements. Movements of the whole body such as walking and running, and localized movements such as grasping a pencil or nodding the head, rely on the 2. Stabilizing body positions. Skeletal muscle contractions 3. Storing and moving substances within the body. Storage is accomplished by sustained contractions of ringlike bands of smooth muscle called sphincters, which prevent outflow of the contents of a hollow organ. Temporary storage of food in the stomach or urine in the urinary bladder is possible because smooth muscle sphincters close off the outlets of these organs. Cardiac muscle contractions of the heart pump blood through the blood vessels of the body. Contraction and relaxation of smooth muscle in the walls of blood vessels help adjust blood vessel diameter and thus regulate the rate of blood flow. Smooth muscle contractions also move food and substances such as bile and enzymes through the gastrointestinal tract, push gametes (sperm and oocytes) through the passageways of the reproductive systems, and propel urine through the urinary system. Skeletal muscle contractions promote the flow of lymph and aid the return of blood to the heart. 4. Generating heat. As muscle contracts, it .

producing body movements, stabilizing body positions, storing and moving substances within the body, and generating heat. integrated functioning of skeletal muscles, bones, and joints. stabilize joints and help maintain body positions, such as standing or sitting. Postural muscles contract continuously when you are awake; for example, sustained contractions of your neck muscles hold your head upright. produces heat, a process known as thermogenesis. Much of the heat generated by muscle is used to maintain normal body temperature. Involuntary contractions of skeletal muscles, known as shivering, can increase the rate of heat production

adrenergic receptors

receptor sites for the sympathetic neurotransmitters norepinephrine and epinephrine

When scientists examined the first electron micrographs of skeletal muscle in the mid-1950s, they were surprised to see that the lengths of the thick and thin filaments were the same in both It had been thought that muscle contraction must be a folding process, somewhat like closing an accordion. Instead, researchers discovered that skeletal muscle The model describing this process is known as the sliding filament mechanism of muscle contraction. Muscle contraction occurs because As a result, the thin filaments slide inward and meet at the center of a sarcomere. They may even move so far inward that their ends overlap (Figure 11.7c). As the thin filaments slide inward, the I band and H zone narrow and eventually disappear altogether when the muscle is maximally contracted. However, the width of the A band and the individual lengths of the thick and thin filaments remain unchanged. Since the thin filaments on each side of the sarcomere are attached to Z discs, when the thin filaments slide inward, the Z discs come closer together and the sarcomere shortens. Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle.

relaxed and contracted muscle. shortens during contraction because the thick and thin filaments slide past one another. myosin heads attach to and "walk" along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line (Figure 11.7).

In the CNS, there is little or no This seems to result from two factors: (1) Axons in the CNS are myelinated by _____ rather than ____ and this CNS myelin is one of the factors Also, after axonal damage, nearby astrocytes Thus, injury of the brain or spinal cord usually is permanent.

repair of an axon after injury. inhibitory proteins secreted by neuroglia, particularly oligodendrocytes, and (2) absence of growth-stimulating cues that were present during fetal development. oligodendrocytes, Schwann cells, inhibiting regeneration of neurons. proliferate rapidly, forming a type of scar tissue that acts as a physical barrier to regeneration.

The parasympathetic nervous system enhances The ___,___, and ___contain centers that control autonomic activities The somatic nervous system innervates the When a somatic motor neuron stimulates a skeletal muscle, it . If somatic motor neurons cease to stimulate a skeletal muscle, the result is a The somatic nervous system usually operates under Voluntary control of movement involves For example, if you want to perform a particular movement (kick a ball, turn a screwdriver, smile for a picture, etc.), neural pathways from the motor cortex activate somatic motor neurons that cause the appropriate skeletal muscles to contract. The somatic nervous system is not always under voluntary control, however. The somatic motor neurons that innervate skeletal muscles involved in

rest-and-digest activities. Parasympathetic responses support body functions that conserve and restore body energy during times of rest and recovery. hypothalamus, brain stem, and spinal cord skeletal muscles of the body. contracts paralyzed, limp muscle that has no muscle tone. voluntary (conscious) control. motor areas of the cerebral cortex that activate somatic motor neurons whenever you have a desire to move. posture, balance, breathing, and somatic reflexes (such as the flexor reflex) are involuntarily controlled by integrating centers in the brain stem and spinal cord.

The acronym SLUDD can be helpful in remembering five parasympathetic responses. It stands for Other important parasympathetic responses are "three decreases": Vagus nerve? Vagus nerve carries

salivation (S), lacrimation (L), urination (U), digestion (D), and defecation (D). All of these activities are stimulated by the parasympathetic nervous system. decreased heart rate, decreased diameter of the bronchial tubes of the lungs (bronchoconstriction), and decreased diameter of the pupils (pupillary constriction). One of the effects of increased sympathetic activity is severe vasoconstriction, which elevates blood pressure. In response, the cardiovascular center in the medulla oblongata (1) increases parasympathetic output via the vagus (X) nerve, which decreases heart rate, and (2) decreases sympathetic output, which causes dilation of blood vessels superior to the level of the injury. carries 75% of traffic in the parasympathetic nervous system ... motor heart, stomach, intestines, gallbladder

The multiple nuclei of a skeletal muscle fiber are located just beneath the Thousands of tiny invaginations of the sarcolemma, called ____, tunnel in from the surface toward the center of each muscle fiber. Because T tubules are open to the outside of the fiber, they are filled with Muscle action potentials travel along the The sarcolemma surrounds the sarcoplasm, the cytoplasm of a muscle fiber (Figure 11.3b-d). Within the sarcoplasm are mitochondria, which produce large amounts of ATP for muscle contraction. The sarcoplasm also contains glycogen, a large polysaccharide consisting of thousands of glucose molecules covalently linked together. Glycogen serves as a storage form of glucose. It can be broken down into individual glucose molecules that can be used to synthesize ATP. Also present in the sarcoplasm are molecules of myoglobin (mī-ō-GLO-B-in), a red-colored, oxygen-binding protein that is found only in muscle. Myoglobin stores oxygen until it is needed by mitochondria to generate ATP. Extending throughout the sarcoplasm are myofibrils (mī-ō-FĪ-brils), the contractile elements of the skeletal muscle fiber (Figure 11.3c, d). Within myofibrils are smaller structures called filaments, which can have either a thin or thick diameter. Thin filaments are 8 nm in diameter and 1-2 μm long, while thick filaments are 16 nm in diameter and 1-2 μm long. Both thin and thick filaments are directly involved in the contraction process.

sarcolemma, the plasma membrane of a muscle fiber (Figure 11.3b-d). transverse (T) tubules extracellular fluid. sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber. This arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant.

The mechanism of adaptation can vary, depending on the . In many cases, adaptation occurs because Recall that the depolarizing receptor potential that forms in a sensory receptor is often due to the . If these cation channels inactivate, inflow of Na+ and Ca2+ is In sensory receptors that are encapsulated, the connective tissue capsule may influence A Sensory Pathway Conveys Sensory Information A sensory pathway is a group of Each chain of the sensory pathway is a The neurons of a sensory pathway are referred to as . Integration (processing) of information occurs at l

sensory receptor cation channels in the membrane of the sensory receptor inactivate after being open for a period of time. opening of cation channels that allow Na+ and Ca2+ ions to enter the cell reduced, the depolarization is diminished, and generation of action potentials by the sensory neuron decreases or completely stops. how long the cation channels stay open. parallel chains of neurons that conveys sensory information from sensory receptors in the periphery to the cerebral cortex (Figure 9.10). labeled line that conveys sensory information about one particular modality. first-, second-, third-, fourth-, or higher-order neurons based on the order in which they occur in the chain (Figure 9.10) each synapse along the sensory pathway.

1.) As you touch the pen, a graded potential develops in a 2 The graded potential triggers the 3 The neurotransmitter stimulates the interneuron to form a 4 In response to the graded potential, the axon of the interneuron forms a 5 This process of neurotransmitter release at a synapse followed by the formation of a graded potential and then a nerve action potential occurs over and over as Once interneurons in the cerebral cortex, the outer part of the brain, are activated, As you will learn in Chapter 8, perception, the conscious awareness of a sensation, is primarily a function of the cerebral cortex.

sensory receptor in the skin of the fingers. axon of the sensory neuron to form a nerve action potential, which travels along the axon into the CNS and ultimately causes the release of neurotransmitter at a synapse with an interneuron. graded potential in its dendrites and cell body. nerve action potential. The nerve action potential travels along the axon, which results in neurotransmitter release at the next synapse with another interneuron. interneurons in higher parts of the brain (such as the thalamus and cerebral cortex) are activated. perception occurs and you are able to feel the smooth surface of the pen touch your fingers.

For a sensation to arise, sensory information must be transformed into the "language" of the nervous system—namely graded potentials and action potentials. The process of sensation typically involves the following events (Figure 9.1): 1 Stimulation of the —— The process of sensation begins in a sensory receptor, a structure of the nervous system that is associated with a A given sensory receptor responds mainly to 2 _____of the stimulus. The sensory receptor converts . Recall that a graded potential is a 3 Generation of ____. If a graded potential in a sensory neuron ____, it triggers 4 ____ of sensory input. As sensory input is relayed from one synapse to another in the CNS, the information is This means that the information is either Sensory input that reaches the level of consciousness is

sensory receptor. sensory (afferent) neuron. one particular type of stimulus. A stimulus is a change in the external or internal environment that can activate the sensory receptor. Transduction the energy in the stimulus into a graded potential, a process known as transduction change in membrane potential that causes the membrane to become either depolarized or hyperpolarized (see Section 7.3). action potentials reaches threshold, it triggers one or more action potentials, which then propagate into the CNS. Integration, integrated (processed). modified, allowed to continue on as is, or filtered out, depending on how important the information is. integrated in the cerebral cortex.

The receptors for most special senses are specialized, These include gustatory receptor cells in taste buds, photoreceptors in the retina of the eye for vision, and hair cells for hearing and equilibrium in the inner ear. The olfactory receptors for the special sense of smell are not separate cells; instead they are located in olfactory cilia, which are hairlike structures that project from the dendrite of an olfactory receptor cell (a type of neuron) (see Figure 9.23). During transduction, a sensory receptor The graded potential that forms in a sensory receptor is referred to as a A stimulus causes a receptor potential by In many sensory systems, the stimulus opens cation channels that allow In the visual system, however, cation channels In sensory receptors that are peripheral endings of sensory neurons, if the receptor potential is large enough to reach threshold, it The action potentials then In sensory receptors that are separate cells, the receptor potential triggers . The neurotransmitter molecules liberated from the synaptic vesicles diffuse across the synaptic cleft and produce a If threshold is reached, the PSP will trigger

separate cells that synapse with sensory neurons (Figure 9.2b). responds to a stimulus by generating a graded potential. receptor potential (Figure 9.2a,b). opening or closing ion channels in the membrane of the sensory receptor (either directly or indirectly by activating a second messenger pathway). Na+ and Ca2+ ions to enter the sensory receptor, resulting in depolarization of the sensory receptor's membrane. close in response to the stimulus (light) and the sensory receptors (photoreceptors) are hyperpolarized. triggers one or more action potentials in the axon of the sensory neuron (Figure 9.2a). propagate along the axon into the CNS. release of neurotransmitter through exocytosis of synaptic vesicles (Figure 9.2b) postsynaptic potential (PSP), a type of graded potential, in the sensory neuron. one or more action potentials, which propagate along the axon into the CNS.

LABEL PARTS OF SYNAPSE A synapse is the Synapses are essential for homeostasis because they Synapses are also important because some diseases and neurological disorders result from At a synapse between two neurons, the neuron sending the signal is called the Neural synapses are named based on the parts of the neurons that form the synapse and the direction of information flow. Examples include axodendritic (from axon to dendrite), axosomatic (from axon to cell body), and axoaxonic (from axon to axon) synapses (Figure 7.25). Most neural synapses are either axodendritic or axosomatic.

site of communication between two neurons or between a neuron and an effector cell. allow information to be filtered and integrated. disruptions of synaptic communication, and many therapeutic and addictive chemicals affect the body at these junctions. presynaptic neuron, and the neuron receiving the message is called the postsynaptic neuron.

Differences in membrane permeability to various ions. In neurons at rest, there are differences in membrane permeability to various ions because In general, the more permeable the plasma membrane is to a particular ion, the greater the Neurons at rest are permeable to K+ and Na+ ions because there are This permeability allows K+ and Na+ ions to . Neurons at rest are also permeable to However, for reasons that will be discussed later in this section, Cl− ions do not make a significant contribution to the resting membrane potential in most neurons. Resting neurons are essentially impermeable to the . This means that A− ions do not have a direct impact on the resting membrane potential. Considering the different types of ions just discussed, the main ions that influence the resting membrane potential of a neuron are K+ and Na+ ions do not have equal roles, however, in establishing the resting membrane potential. The plasma membrane of a neuron has In fact, resting neurons are more permeable to K+ ions than to any other type of ion in the ECF or cytosol because As a result of the high permeability to K+ and relatively lower permeability to Na+, the resting membrane potential of most neurons is influenced K+ and Na+ ions help establish the resting membrane potential in the following way: Because the plasma membrane of a neuron has more K+ leak channels than Na+ leak channels,

some ions are able to pass through the plasma membrane via specific leak channels, whereas other ions do not have transport mechanisms allowing them passage through the membrane. influence that ion has on the resting membrane potential. specific leak channels for these ions in their plasma membranes (Figure 7.10). influence the resting membrane potential Cl− ions because there are Cl− leak channels in their plasma membranes. anionic proteins and phosphates (A− ions) of the cytosol because their plasma membranes do not have transport mechanisms for these ions K+ and Na+ ions. more K+ leak channels than Na+ leak channels (Figure 7.10), which means that resting neurons are much more permeable to K+ ions than to Na+ ions. K+ leak channels are the most abundant type of leak channel in their plasma membranes. primarily by K+ ions and to a lesser extent by Na+ ions. the number of K+ ions that diffuse down their concentration gradient out of the cell into the ECF is greater than the number of Na+ ions that diffuse down their concentration gradient from the ECF into the cell. As more and more K+ ions exit, the inside of the plasma membrane becomes increasingly negative, and the outside of the membrane becomes increasingly positive.

Muscarinic ACh receptors are present in There are different types of muscarinic ACh receptors and they are all . Activation of muscarinic ACh receptors causes excitation or inhibition of the postsynaptic cell, depending on For example, one type of muscarinic ACh receptor opens K+ channels, which leads to Some chemical substances can bind to and The plant derivative curare As a result, skeletal muscle becomes . The drug atropine blocks Clinically, it is used to It can also be used as an antidote for chemical warfare agents that inhibit

some neurons of the CNS and in effectors (cardiac muscle, smooth muscle, and glands) innervated by certain autonomic neurons of the PNS. metabotropic which type of muscarinic ACh receptor is activated. hyperpolarization (IPSP) of the postsynaptic cell (see Figure 7.27c). block cholinergic receptors. blocks nicotinic ACh receptors in skeletal muscle at the NMJ. paralyzed muscarinic ACh receptors. dilate the pupils, reduce glandular secretions, and relax smooth muscle in the gastrointestinal tract. acetylcholinesterase (AChE).

The tactile sensations (TAK-tīl; tact- = touch) encompass a variety of sensations—touch, pressure, vibration, itch, and tickle. Although we perceive differences among these sensations, they arise by activation of Several types of encapsulated mechanoreceptors attached to large-diameter myelinated A fibers mediate sensations of . Other tactile sensations, such as itch and tickle sensations, are detected by Recall that larger diameter, myelinated axons propagate action potentials more

some of the same types of receptors. touch, pressure, and vibration free nerve endings attached to small-diameter, unmyelinated C fibers. rapidly than do smaller diameter, unmyelinated axons.

Somatic pain arises from Visceral pain results from stimulation of nociceptors in In many instances of visceral pain, the pain is felt at a site other than For example, the pain of a heart attack typically is felt in the skin over the heart and along the left arm. This phenomenon is called Referred pain occurs because both Since the brain is more accustomed to receiving sensory input from _____it may incorrectly Figure 9.16b shows the skin regions to which visceral pain may be referred.

stimulation of nociceptors in skin, skeletal muscles, and joints. visceral (internal) organs. the place of origin. referred pain. somatic sensory and visceral sensory neurons often converge on second-order neurons of the same ascending pathway to the brain (Figure 9.16a). somatic sensory neurons than from visceral sensory neurons, interpret pain from a visceral organ as having a somatic origin.

A typical chemical synapse transmits a signal as follows (Figure 7.26a): 1 An action potential arrives at a 2 The membrane of the synaptic end bulb contains The depolarizing phase of the action potential . Because calcium ions are more concentrated in the 3 An increase in the Ca2+ concentration inside the synaptic end bulb serves as a 4. The neurotransmitter molecules The receptor shown in Figure 7.26a is part of a ligand-gated channel; in other cases, the receptor may be coupled to a G protein that regulates the opening of an ion channel or causes another effect in the cell. 5 Binding of 6 As ions flow through the opened channels, the voltage across the membrane . This change in membrane voltage is a Depending on which ions the channels admit, the postsynaptic potential may be a For example, opening of Na+ channels allows inflow of Na+, which causes . However, opening of Cl− or K+ channels causes hyperpolarization. Opening Cl− channels permits 7 When a depolarizing postsynaptic potential reaches threshold, it triggers an

synaptic end bulb of a presynaptic axon. voltage-gated Ca2+ channels in addition to the voltage-gated Na+ and K+ channels found in other parts of the axon. opens not only the voltage-gated Na+ channels but also the voltage-gated Ca2+ channels extracellular fluid, Ca2+ flows inward through the open voltage-gated Ca2+ channels. signal that triggers exocytosis of the synaptic vesicles diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron's plasma membrane. neurotransmitter molecules to the receptor sites on ligand-gated channels opens the channels and allows particular ions to flow across the membrane. changes postsynaptic potential. depolarization or hyperpolarization. depolarization Cl− to move into the cell, while opening the K+ channels allows K+ to move out—in either event, the inside of the cell becomes more negative. action potential in the axon of the postsynaptic neuron.

A nerve action potential in a somatic motor neuron elicits a muscle action potential in a skeletal muscle fiber in the following way (Figure 10.11): 1 A nerve action potential arrives at a 2 Voltage-gated Ca2+ channels present in the membrane of the synaptic end bulb open in response to the nerve action potential. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels. 3 An increase in the Ca2+ concentration inside the synaptic end bulb serves as a signal that triggers exocytosis of the synaptic vesicles, liberating ACh into the synaptic cleft. 4 ACh diffuses across the synaptic cleft and binds to nicotinic ACh receptors on the motor end plate. Recall that a nicotinic ACh receptor is a type of ionotropic receptor that contains two binding sites for ACh and a cation channel. Binding of two ACh molecules to the nicotinic ACh receptor opens the cation channel. Opening the cation channel allows passage of cations (mainly Na+ and K+) through the end plate membrane, but Na+ inflow is greater than K+ outflow. 5 The net influx of Na+ ions into the muscle fiber through the open nicotinic ACh receptors causes the motor end plate to depolarize. This change in membrane potential is called an end plate potential (EPP). An EPP is a type of graded potential that is similar to an excitatory postsynaptic potential (EPSP), which forms at synapses between neurons (see Section 7.4). However, an EPP has a larger amplitude (size) than an EPSP because there are more neurotransmitter receptors in the motor end plate and a larger number of ion channels open in response to neurotransmitter-receptor binding. Consequently, a single EPP typically is large enough to depolarize a muscle fiber to threshold (see step 6 ), whereas a single EPSP normally is too small to depolarize a neuron to threshold. 6 The EPP spreads by local current flow to an adjacent region of plasma membrane on each side of the motor end plate and depolarizes these areas to threshold. 7 The adjacent membrane areas contain voltage-gated Na+ channels, which open in response to the threshold-level depolarization. The resultant inflow of Na+ into the muscle fiber through the open voltage-gated Na+ channels initiates a muscle action potential. 8 Since the NMJ is usually near the midpoint of the muscle fiber, once the muscle action potential arises, it propagates throughout the muscle fiber membrane in both directions away from the NMJ toward the ends of the fiber. The action potential triggers a chain of events that ultimately leads to contraction of the muscle fiber. 9 The effect of ACh binding to its receptor lasts only briefly because ACh is rapidly broken down by an enzyme called acetylcholinesterase (AChE), which is located on the end plate membrane. AChE breaks down ACh into acetate and choline, products that cannot individually activate the nicotinic ACh receptor.

synaptic end bulb of a somatic motor neuron.

In most neurons, the tips of the axon terminals swell into A synapse (SIN-aps) is a site of Within the synaptic end bulbs are many tiny membrane-enclosed sacs called The arrival of an action potential at the synaptic end bulb ultimately causes the The released neurotransmitter molecules, in turn, For an axon to function, materials must move between the cell body and Axonal transport uses proteins called kinesins and dyneins as "motors" to transport materials along the surfaces of microtubules of the neuron's cytoskeleton (Figure 7.2). Each of these motor proteins has a region that The bound particle is carried by the Axonal transport moves materials in both directions—away from and toward the cell body. Axonal transport that occurs in an anterograde (____) involves Anterograde transport moves organelles and synaptic vesicles from the Axonal transport that occurs in a retrograde (___) direction involves Retrograde transport moves Substances that enter the neuron at the axon terminals are also moved to the cell body by These substances include (1)

synaptic end bulbs, which are so-named because these bulb-shaped structures can form synapses with other cells. communication between a neuron and a target cell, which can be another neuron, a muscle fiber, or a gland cell. synaptic vesicles that store chemical neurotransmitters (Figure 7.2). release of neurotransmitters from the synaptic vesicles. excite or inhibit the target cell. axon terminals, a process known as axonal transport. binds to the particle to be transported and a region that binds to a microtubule. motor protein as the motor protein uses energy from ATP hydrolysis to "walk" along the surface of the microtubule. forward) direction involves kinesins. cell body to the axon terminals. backward) direction involves dyneins. membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled. retrograde transport. trophic chemicals such as nerve growth factor and (2) harmful agents such as tetanus toxin and the viruses that cause rabies, herpes simplex, and polio.

The parasympathetic nervous system originates in Once ACh is released from a cholinergic neuron, it Recall that there are two types of cholinergic receptors: Nicotinic acetylcholine receptors are present in the plasma membranes of the They are so-named because the . Muscarinic acetylcholine receptors are present in the In addition, most sweat glands receive their innervation from These receptors are so-named because a mushroom poison called muscarine mimics the . Nicotine does not

the brain stem and sacral region of the spinal cord; the sympathetic nervous system originates in the thoracic and lumbar regions of the spinal cord. diffuses across the synaptic cleft and binds with specific cholinergic receptors, integral membrane proteins in the postsynaptic plasma membrane. nicotinic acetylcholine receptors and muscarinic acetylcholine receptors (see Section 7.5). dendrites and cell bodies of both parasympathetic and sympathetic postganglionic neurons (Figure 10.6a-c), in the plasma membranes of chromaffin cells of the adrenal medulla, and in skeletal muscle at the neuromuscular junction (NMJ). drug nicotine mimics the action of ACh by binding to these receptors plasma membranes of the effectors (smooth muscle, cardiac muscle, and glands) innervated by parasympathetic postganglionic axons (Figure 10.6a). cholinergic sympathetic postganglionic neurons and possess muscarinic ACh receptors (Figure 10.6c). actions of ACh by binding to them activate muscarinic receptors, and muscarine does not activate nicotinic receptors, but ACh does activate both types of cholinergic receptors.

The production of electrical signals depends on two basic features of the plasma membrane of excitable cells: When ion channels are open, they allow Recall that ions move from areas of Also, positively charged cations move toward a As ions move, they create a Ion channels open and close due to the presence The gate is a part of the channel protein that can Four types of ion channels are important to neuron function:

the presence of specific types of ion channels and the existence of a resting membrane potential. specific ions to move across the plasma membrane, down their electrochemical gradient—a concentration (chemical) difference plus an electrical difference. higher concentration to areas of lower concentration (the chemical part of the gradient). negatively charged area, and negatively charged anions move toward a positively charged area (the electrical aspect of the gradient). flow of electrical current that can change the membrane potential. of "gates." seal the channel pore shut or move aside to open the pore (see Figure 5.7). leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels (Figure 7.8).

In the sympathetic nervous system, preganglionic neurons have their cell bodies in the For this reason, the sympathetic nervous system is also called the Sympathetic preganglionic axons exit the CNS through After leaving the CNS, most sympathetic preganglionic axons extend to sympathetic Other sympathetic preganglionic axons extend to sympathetic postganglionic neurons in From the sympathetic trunk or collateral ganglia, sympathetic postganglionic axons extend to the Because sympathetic trunk ganglia are located near the spinal cord, most sympathetic preganglionic axons are

thoracic and upper lumbar regions of the spinal cord (Figure 10.2). thoracolumbar division of the ANS. thoracic and lumbar spinal nerves. postganglionic neurons in the sympathetic trunk, a chain of ganglia located on either side of the spinal cord (Figure 10.2). collateral ganglia, individual ganglia that are not associated with the sympathetic trunk (Figure 10.2). visceral effector. short and most sympathetic postganglionic axons are long.

Action of the Na+/K+ ATPases. The Na+/K+ ATPases (sodium-potassium pumps) also contribute to the generation of the resting membrane potential (Figure 7.10). As you learned in Chapter 5, the Na+/K+ ATPases are transporters that expel This function of the Na+/K+ ATPase is relevant to resting membrane potential because it Na+ and K+ ions in turn use these concentration gradients to help generate the Without the Na+/K+ ATPases, the Na+ and K+ concentration gradients would Another aspect of Na+/K+ ATPase function that is relevant to resting membrane potential is that the Na+/K+ ATPases expel This action of the Na+/K+ ATPases stabilizes the resting membrane potential, keeping it at This means that at −70 mV, there is no Here is one final point about the role of the Na+/K+ ATPases in helping to generate the resting membrane potential: Because the Na+/K+ ATPases remove more positive charges from the cell than they bring into the cell (three Na+ ions exported for every two 2 K+ ions imported), these transporters are However, their total contribution is very small, only a few millivolts of the

three Na+ ions from the cytosol into the ECF and import two K+ ions from the ECF into the cytosol using the energy derived from the hydrolysis of ATP (see Section 5.5). maintains the Na+ and K+ concentration gradients. resting membrane potential. eventually dissipate, K+ would no longer be able to leave the cell and Na+ would no longer be able to enter the cell, and a normal resting membrane potential would no longer be established. Na+ ions out of the neuron as fast as they leak in and import K+ ions into the neuron as fast as they leak out. −70 mV. net movement of Na+ or K+ ions across the membrane of a resting neuron. electrogenic, which means they contribute to the negativity of the resting membrane potential. total −70 mV resting membrane potential in a typical neuron.

So an action potential is generated in response to a ____, but In other words, an action potential either completely occurs or it does not occur at all. This characteristic of an action potential is known as the . When the push on the first domino is strong enough (when depolarization reaches threshold), that domino falls against the second domino, and the entire row topples (an action potential occurs). Stronger pushes on the first domino produce the identical effect—toppling of the entire row. Thus, pushing on the first domino produces an all-or-none event: The dominoes all fall or none fall. Depolarizing Phase During the depolarizing phase of the action potential, membrane permeability to The depolarizing phase begins when a depolarizing graded potential or some other stimulus causes the Once threshold is reached, voltage- Because both the Na+ concentration and electrical gradients favor inward movement of Na+, there is a The inflow of Na+ causes the membrane potential However, the membrane potential never reaches ENa because, during the repolarizing phase (described next), the voltage-gated Na+ channels . This causes the membrane potential to peak at +30 mV at the end of the The total change in membrane potential from resting conditions to the end of the depolarizing phase is about 100 mV (from −70 mV to +30 mV).

threshold stimulus, but it does not form when there is a subthreshold stimulus. all‐or‐none principle Na+ ions increases (see Figure 7.20b). membrane of the axon to depolarize to threshold. gated Na+ channels open rapidly. rush of Na+ ions into the neuron. to move above −55 mV toward the sodium equilibrium potential (ENa) of +60 mV (see Figure 7.20a). close and Na+ membrane permeability decreases depolarizing phase.

To communicate information from one part of the body to another, action potentials in a neuron must travel from the In contrast to a graded potential, an action potential is not Instead, an action potential maintains its strength as its spreads along the membrane. This mode of conduction, called . When sodium ions flow in, they cause voltage-gated Na+ channels in Thus, the action potential travels along the membrane rather like the The propagation of an action potential is accomplished by local current flow (see Figure 7.17). Recall that local current flow refers to the In a neuron, an action potential can propagate along the axon . You should realize that it is not the same action potential that propagates along the entire axon. Rather, the action potential Regeneration of the action potential along an unmyelinated axon of a neuron is similar to the " . Like the action potential, the wave is regenerated throughout the stadium as each fan Therefore, it is the wave that travels around the stadium, not the actual fans. Because they can travel along a membrane without dying out, action potentials function in

trigger zone of the axon (their point of origin) to the axon terminals. decremental (it does not die out). propagation, depends on positive feedback adjacent segments of the membrane to open. activity of that long row of dominoes. passive movement of charges from one membrane region to adjacent membrane regions due to differences in membrane potential in these areas. away from the cell body only—it cannot propagate back toward the cell body because any region of membrane that has just undergone an action potential is temporarily in the absolute refractory period and cannot generate another action potential regenerates over and over at adjacent regions of membrane from the trigger zone to the axon terminals (Figure 7.23). wave" that is performed by fans at a football game sequentially stands up (the depolarizing phase of the action potential) and then sits down (the repolarizing phase of the action potential). communication over long distances.

Autonomic motor pathways (both parasympathetic and sympathetic) consist of The first neuron, called the Its axon exits the CNS via a The second neuron, called the Its cell body is located in the Thus, preganglionic neurons convey action potentials from the

two autonomic motor neurons in series and a visceral effector (smooth muscle, cardiac muscle, or a gland) (Figure 10.1). preganglionic neuron, has its cell body in the brain or spinal cord. cranial or spinal nerve and then extends to an autonomic ganglion, where it synapses with the second neuron. (Recall that a ganglion is a cluster of neuronal cell bodies in the PNS). postganglionic neuron, lies entirely in the PNS. autonomic ganglion, and its axon extends from the ganglion to the visceral effector CNS to autonomic ganglia, and postganglionic neurons relay the action potentials from autonomic ganglia to visceral effectors.

input zone integration zone conduction zone output zone

where neurons collect and process information — either from environment or other cells (dendritic a and cell body) where decision to produce neural sign is made where information can electrically be transmitted over great distances where neuron transfers info to other cells

Ed cuts his finger while slicing onions for his world-famous chili. Based on the gate-control theory of pain modulation, if the C fiber produces an EPSP of +16 mV (assuming that a resting potential of -70mV and a threshold of -55mV), will the ascending pain pathway carry the message to the brain?

yes (more positive -54!!!)