Chapter 12 Nervous Tissue

continuous conduction

-involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane. In this conduction, ions flow through their voltage-gated channels in each adjacent segment of the membrane. -the action potential propagates only a relatively short distance in a few milliseconds. -occurs in unmyelinated axons and in muscle fibers. -slower than saltatory conduction

sensory receptor

-structure of the nervous system that monitors changes in the external or internal environment -an example is touch receptors in skin

what are step 1 and 2 of a chemical synapse signal transmission?

1. A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon. 2. The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels.

1. synaptic vesicles 2. neurotransmitter

1a. inside Both synaptic end bulbs and varicosities 1b. tiny membrane-enclosed sacs that store neurotransmitters. 2a. a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell. 2b. Many neurons contain two or even three types of these molecules, each with different effects on the postsynaptic cell.

absolute refractory period

During this period, even a very strong stimulus cannot initiate a second action potential. This period coincides with the period of Na+ channel activation and inactivation (steps 2-4 in Figure 12.20). Inactivated Na+ channels cannot reopen; they first must return to the resting state.

examples of amino acid neurotransmitter

Several amino acids are neurotransmitters in the CNS. Glutamate (glutamic acid) and aspartate (aspartic acid) have powerful excitatory effects.

Local anesthetics and how do they work?

drugs that block pain and other somatic sensations. Examples include procaine (Novocaine®) and lidocaine, which may be used to produce anesthesia in the skin during suturing of a gash, in the mouth during dental work, or in the lower body during childbirth. Like TTX, these drugs act by blocking the opening of voltage-gated Na+ channels. Action potentials cannot propagate past the obstructed region, so pain signals do not reach the CNS.

Attack of MS

-An attack is followed by a period of remission during which the symptoms temporarily disappear. One attack follows another over the years, usually every year or two. The result is a progressive loss of function interspersed with remission periods, during which symptoms abate.

Soma

-cell body -contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, a Golgi complex, and Nissl bodies

1. Neurotoxins 2. tetrodotoxin

1. substances that produce their poisonous effects by acting on the nervous system. -Certain shellfish and other organisms contain these substances 2. present in the viscera of Japanese puffer fish. TTX effectively blocks action potentials by inserting itself into voltage-gated Na+ channels so they cannot open.

what are step 6 of a chemical synapse signal transmission?

As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential. Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition). For example, opening of Na+ channels allows inflow of Na+, which causes depolarization. However, opening of Cl− or K+ channels causes hyperpolarization. Opening Cl− channels permits 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

Voltage-gated Na+ channel's gates

Each voltage-gated Na+ channel has two separate gates: -In the resting state of a voltage-gated Na+ channel, the inactivation gate is open, but the activation gate is closed. -so Na+ can't move into cell through these channels. -At threshold, voltage-gated Na+ channels are activated. -both the activation and inactivation gates in the channel are open and Na+ inflow begins.

Is NO toxic?

In larger quantities, NO is highly toxic. Phagocytic cells, such as macrophages and certain white blood cells, produce NO to kill microbes and tumor cells.

oligodendrocyte myelin sheath

In the CNS, an oligodendrocyte myelinates parts of several axons. Each oligodendrocyte puts forth about 15 broad, flat processes that spiral around CNS axons, forming a myelin sheath. A neurolemma is not present because the oligodendrocyte cell body and nucleus do not envelop the axon. Nodes of Ranvier are present, but are fewer in number. Axons in the CNS display little regrowth after injury. This is thought to be due, in part, to the absence of a neurolemma, and in part to an inhibitory influence exerted by the oligodendrocytes on axon regrowth.

describe synapses

Most synapses between neurons are axodendritic (from axon to dendrite), while others are axosomatic (from axon to cell body) or axoaxonic (from axon to axon). In addition, synapses may be electrical or chemical and they differ both structurally and functionally. -essential for homeostasis because they allow information to be filtered and integrated. -During learning, the structure and function of particular synapses change. The changes may allow some signals to be transmitted while others are blocked

Basic functions of Nervous system

Sensory function. Integrative function. Motor function.

neurotransmitter transporters

The membrane proteins that accomplish uptake of neurotransmitters

the common form of MS

The most common form of the condition is relapsing-remitting MS, which usually appears in early adulthood. The first symptoms may include a feeling of heaviness or weakness in the muscles, abnormal sensations, or double vision.

Integrative function

The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration.

causes of epilepsy

there are many causes, including brain damage at birth (the most common cause); metabolic disturbances (hypoglycemia, hypocalcemia, uremia, hypoxia); infections (encephalitis or meningitis); toxins (alcohol, tranquilizers, hallucinogens); vascular disturbances (hemorrhage, hypotension); head injuries; and tumors and abscesses of the brain. Seizures associated with fever are most common in children under the age of two. However, most epileptic seizures have no demonstrable cause.

Neuroblastoma

(noor-ō-blas-TŌ-ma) A malignant tumor that consists of immature nerve cells (neuroblasts); occurs most commonly in the abdomen and most frequently in the adrenal glands. Although rare, it is the most common tumor in infants.

excitatory postsynaptic potential (EPSP)

-A depolarizing postsynaptic potential -Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. -Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs. -caused by more positive ions inside the cell

Motor Neurons

-convey action potentials away from the CNS to effectors (muscles and glands) in the PNS through cranial or spinal nerves. -are multipolar in structure. -Also called efferent neurons.

dopamine

Brain neurons containing the neurotransmitter dopamine (DA) are active during emotional responses, addictive behaviors, and pleasurable experiences. In addition, dopamine-releasing neurons help regulate skeletal muscle tone and some aspects of movement due to contraction of skeletal muscles. The muscular stiffness that occurs in Parkinson's disease is due to degeneration of neurons that release dopamine. One form of schizophrenia is due to accumulation of excess dopamine.

1. dysthymia 2. bipolar disorder, or manic-depressive illness

1. A person experiences episodes of depression that alternate with periods of feeling normal. 2. A person experiences recurrent episodes of depression and extreme elation (mania)

1. suprathreshold stimulus

1. a stimulus that is strong enough to depolarize the membrane above threshold so several action potentials will form in response to this stimulus. -Each of the action potentials caused by this stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus.

1. subthreshold stimulus 2. threshold stimulus

1. a weak depolarization that cannot bring the membrane potential to threshold so An action potential will not occur in response to this stimulus 2. a stimulus that is just strong enough to depolarize the membrane to threshold so an action potential will occur in response to this stimulus.

1. membrane potential 2. resting membrane potential 3. electrochemical gradient

1. an electrical potential difference (voltage) across the membrane. 2. this voltage is in excitable cells 3. a concentration (chemical) difference plus an electrical difference

1. stimulus 2. action potential

1. any change in the environment that is strong enough to initiate an action potential. 2. an electrical signal that propagates along the surface of the membrane of a neuron. It begins and travels due to the movement of ions between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength. It travels at 1-290 mph

Gamma-aminobutyric acid and glycine

-Gamma-aminobutyric acid (GABA) and glycine are important inhibitory neurotransmitters. -At many synapses, the binding of GABA to ionotropic receptors opens Cl− channels. -GABA is found only in the CNS, where it is the most common inhibitory neurotransmitter. As many as one-third of all brain synapses use GABA. -Antianxiety drugs such as diazepam (Valium®) enhance the action of GABA. Like GABA, the binding of glycine to ionotropic receptors opens Cl− channels. About half of the inhibitory synapses in the spinal cord use the amino acid glycine; the rest use GABA.

All-or-None principle

-If a stimulus depolarizes a neuron to threshold, the neuron fires at its maximum voltage (all); if threshold is not reached, the neuron does not fire at all (none). -In other words, an action potential either occurs completely or it does not occur at all.

Who does multiple sclerosis affect more?

-It afflicts about 350,000 people in the United States and 2 million people worldwide. -appears between the ages of 20 and 40, affecting females twice as often as males. MS is most common in whites, less common in blacks, and rare in Asians

Enteric Nervous system (ENS)

-involuntary -consists of over 100 million neurons in enteric plexuses that extend most of the length of the gastrointestinal (GI) tract -independent in function but it does communicate with CNS via sympathetic and parasympathetic neurons

B fibers

-myelinated axons with diameters of 2-3 -exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/hr). -have a somewhat longer absolute refractory period than A fibers. -conduct sensory nerve impulses from the viscera to the brain and spinal cord. -They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.

relationship of neurotransmitter and neurotransmitter receptor

-neurotransmitters released from a presynaptic neuron bind to neurotransmitter receptors in the plasma membrane of a postsynaptic cell. -Each type of neurotransmitter receptor has one or more neurotransmitter binding sites where its specific neurotransmitter binds. -When a neurotransmitter binds to the correct neurotransmitter receptor, an ion channel opens and a postsynaptic potential (either an EPSP or IPSP) forms in the membrane of the postsynaptic cell.

What are the first three steps after you picked up a pen?

1. As you touch the pen, a graded potential develops in a sensory receptor in the skin of the fingers. 2. The graded potential triggers the 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. 3. The neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell body.

1. axon hillock 2. inital segment 3. trigger zone

1. cone-shaped elavation that joins axon to cell body 2. part of axon closest to axon hillock 3. area at junction of initial segment and axon hillock -This is where nerve impulses arises

1. Depression 2. major depression

1. disorder that affects over 18 million people each year in the United States. People who are depressed feel sad and helpless, have a lack of interest in activities that they once enjoyed, and experience suicidal thoughts 2. A person experiences symptoms of depression that last for more than two weeks

What are the 3 major factors from which the resting membrane potential arises?

1. unequal distribution of ion in ECF and cytosol 2. inability of most anions to leave the cell 3. electrogenic nature of Na+ - K+ ATPases

1. multipolar neurons 2. bipolar neurons

1. usually have several dendrites and one axon. Most neurons in the brain and spinal cord are of this type, as well as all motor neurons. 2. Bipolar neurons have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area of the brain.

Neurons structure

Neurons display great diversity in size and shape The pattern of dendritic branching is varied and distinctive for neurons in different parts of the nervous system. A few small neurons lack an axon, and many others have very long or short axons

what are neurotransmitter receptors are classified into?

Neurotransmitter receptors are classified as either ionotropic receptors or metabotropic receptors based on whether the neurotransmitter binding site and the ion channel are components of the same protein or are components of different proteins.

neuropeptides

Neurotransmitters consisting of 3 to 40 amino acids linked by peptide bonds called neuropeptides are numerous and widespread in both the CNS and PNS. Neuropeptides bind to metabotropic receptors and have excitatory or inhibitory actions, depending on the type of metabotropic receptor at the synapse. Neuropeptides are formed in the neuron cell body, packaged into vesicles, and transported to axon terminals. many neuropeptides also serve as hormones that regulate physiological responses elsewhere in the body.

catecholamines

Norepinephrine, dopamine, and epinephrine are classified chemically as catecholamines. They all have an amino group (—NH2) and a catechol ring composed of six carbons and two adjacent hydroxyl groups. Catecholamines are synthesized from the amino acid tyrosine. Inactivation of catecholamines occurs via reuptake into synaptic end bulbs. Then they are either recycled back into the synaptic vesicles or destroyed by enzymes. The two enzymes that break down catecholamines are catechol-O-methyltransferase (COMT), and monoamine oxidase (MAO).

epileptic seizures

The attacks afflict about 1% of the world's population. They are initiated by abnormal, synchronous electrical discharges from millions of neurons in the brain, perhaps resulting from abnormal reverberating circuits. The discharges stimulate many of the neurons to send nerve impulses over their conduction pathways. this causes lights, noise, or smells to be sensed when the eyes, ears, and nose have not been stimulated. also causes involuntary muscle contraction.

First recognition of NO? Effects of NO?

The first recognition of NO as a regulatory molecule was the discovery in 1987 that a chemical called EDRF (endothelium-derived relaxing factor) was actually NO. Endothelial cells in blood vessel walls release NO, which diffuses into neighboring smooth muscle cells and causes relaxation. The result is vasodilation, an increase in blood vessel diameter. The effects of such vasodilation range from a lowering of blood pressure to erection of the penis in males. Sildenafil (Viagra®) alleviates erectile dysfunction (impotence) by enhancing the effect of NO.

What are the Two types of voltage-gated channels open and then close during an action potential?

These channels are present mainly in the axon plasma membrane and axon terminals. -The first channels that open, the voltage-gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. -Then voltage-gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase. -The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends.

Satellite Cells

These flat cells surround the cell bodies of neurons of PNS ganglia. -Besides providing structural support, they regulate the exchanges of materials between neuronal cell bodies and interstitial fluid. -monitors chemical environment of neural cell bodies

convergence

In another arrangement, called convergence, several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the postsynaptic neuron. In a converging circuit, the postsynaptic neuron receives nerve impulses from several different sources.

Rabies

A fatal disease caused by a virus that reaches the CNS via fast axonal transport. It is usually transmitted by the bite of an infected dog or other meat-eating animal. The symptoms are excitement, aggressiveness, and madness, followed by paralysis and death.

a neurotransmitter causes what kind of graded potential?

A neurotransmitter causes either an excitatory or an inhibitory graded potential. -A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold -A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory.

seasonal affective disorder (SAD)

A person experiences depression during the winter months, when day length is short. Although the exact cause is unknown, research suggests that depression is linked to an imbalance of the neurotransmitters serotonin, norepinephrine, and dopamine in the brain. Factors that may contribute to depression include heredity, stress, chronic illnesses, certain personality traits (such as low self-esteem), and hormonal changes.

Divergence

A single presynaptic neuron may synapse with several postsynaptic neurons. this arrangement, called divergence, permits one presynaptic neuron to influence several postsynaptic neurons (or several muscle fibers or gland cells) at the same time. In a diverging circuit, the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit. Sensory signals are also arranged in diverging circuits, allowing a sensory impulse to be relayed to several regions of the brain. This arrangement amplifies the signal.

How does the membrane potential change?

AS IONS MOVE, THEY CREATE A FLOW OF ELECTRICAL CURRENT THAT CAN CHANGE THE MEMBRANE POTENTIAL!!!!! ions move from areas of higher concentration to areas of lower concentration (the chemical part of the gradient). Also, positively charged cations move toward a negatively charged area, and negatively charged anions move toward a positively charged area (the electrical aspect of the gradient)

causes and treatment of MS

Although the cause of MS is unclear, both genetic susceptibility and exposure to some environmental factor (perhaps a herpes virus) appear to contribute. Since 1993, many patients with relapsing-remitting MS have been treated with injections of beta-interferon. This treatment lengthens the time between relapses, decreases the severity of relapses, and slows formation of new lesions in some cases. Unfortunately, not all MS patients can tolerate beta-interferon, and therapy becomes less effective as the disease progresses.

Guillain-Barré syndrome (GBS)

An acute demyelinating disorder in which macrophages strip myelin from axons in the PNS. It is the most common cause of acute paralysis in North America and Europe and may result from the immune system's response to a bacterial infection. Most patients recover completely or partially, but about 15% remain paralyzed.

what are step 3 of a chemical synapse signal transmission?

An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. Each synaptic vesicle contains several thousand molecules of neurotransmitter.

Neuropathy

Any disorder that affects the nervous system but particularly a disorder of a cranial or spinal nerve. An example is facial neuropathy (Bell's palsy), a disorder of the facial (VII) nerve.

what are step 5 of a chemical synapse signal transmission?

Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane.

biogenic amines

Certain amino acids are modified and decarboxylated (carboxyl group removed) to produce biogenic amines. Those that are prevalent in the nervous system include norepinephrine, epinephrine, dopamine, and serotonin. Most biogenic amines bind to metabotropic receptors; there are many different types of metabotropic receptors for each biogenic amine. Biogenic amines may cause either excitation or inhibition, depending on the type of metabotropic receptor at the synapse.

localized cooling

Localized cooling of a nerve can also produce an anesthetic effect because axons propagate action potentials at lower speeds when cooled. The application of ice to injured tissue can reduce pain because propagation of the pain sensations along axons is partially blocked.

relationship between neurotransmitter and hormones?

Many neurotransmitters are also hormones released into the bloodstream by endocrine cells in organs throughout the body. Within the brain, certain neurons, called neurosecretory cells, also secrete hormones.

Glutamate

Most excitatory neurons in the CNS and perhaps half of the synapses in the brain communicate via glutamate. At some glutamate synapses, binding of the neurotransmitter to ionotropic receptors opens cation channels. The consequent inflow of cations (mainly Na+ ions) produces an EPSP. Inactivation of glutamate occurs via reuptake. Glutamate transporters actively transport glutamate back into the synaptic end bulbs and neighboring neuroglia

Interneurons

mainly located within the CNS between sensory and motor neurons. -largest group of neurons -integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. -are multipolar in structure. -Also called association neurons.

How is NO packaged?

Unlike all previously known neurotransmitters, NO is not synthesized in advance and packaged into synaptic vesicles. it is formed on demand and acts immediately. Its action is brief because NO is a highly reactive free radical. It exists for less than 10 seconds before it combines with oxygen and water to form inactive nitrates and nitrites. Because NO is lipid-soluble, it diffuses from cells that produce it into neighboring cells, where it activates an enzyme for production of a second messenger called cyclic GMP. Some research suggests that NO plays a role in memory and learning.

Norepinephrine (NE)

plays roles in arousal (awakening from deep sleep), dreaming, and regulating mood. A smaller number of neurons in the brain use epinephrine as a neurotransmitter. Both epinephrine and norepinephrine also serve as hormones. Cells of the adrenal medulla, the inner portion of the adrenal gland, release them into the blood.

what are two types of summation?

spatial summation and temporal summation. Summation is rather like a vote 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.

frequency of action potentials what happens with a light touch? how about with a firm touch? (stronger action potential maybe?)

—how often they are generated at the trigger zone. -A light touch generates a low frequency of action potentials. -A firmer pressure elicits action potentials that pass down the axon at a higher frequency. -a second factor is the number of sensory neurons recruited (activated) by the stimulus. A firm pressure stimulates a larger number of pressure-sensitive neurons than does a light touch.

1. Graded potentials 2. action potentials

(1) electrical signals that are used for short-distance communication only. (2) electrical signal that allows communication over long distances within the body.

(a) corpuscle of touch (b) type I cutaneous mechanoreceptor

(a) a touch receptor that consists of a mass of dendrites enclosed by a capsule of connective tissue. (b) a touch receptor that consists of free nerve endings (bare dendrites) that make contact with tactile epithelial cells of the stratum basale of the skin.

1. axoplasm 2. axolemma

1. cytoplasm of axon 2. plasma membrane of axon

nodes of Ranvier

Gaps in the myelin sheath that appear at intervals along the axon

How are neurons organized?

The CNS contains billions of neurons organized into complicated networks called neural circuits, functional groups of neurons that process specific types of information. In a simple series circuit, a presynaptic neuron stimulates a single postsynaptic neuron. The second neuron then stimulates another, and so on. However, most neural circuits are more complex.

Nitric oxide synthase

The enzyme nitric oxide synthase (NOS) catalyzes formation of NO from the amino acid arginine. Based on the presence of NOS, it is estimated that more than 2% of the neurons in the brain produce NO.

The small-molecule neurotransmitters

include acetylcholine, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide.

1. epilepsy 2. partial seizures

1. short, recurrent attacks of motor, sensory, or psychological malfunction, although it almost never affects intelligence. 2. begin in a small area on one side of the brain and produce milder symptoms; generalized seizures involve larger areas on both sides of the brain and loss of consciousness.

what are step 4 of a chemical synapse signal transmission?

The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron's plasma membrane. you will soon learn that this type of neurotransmitter receptor is called an ionotropic receptor. Not all neurotransmitters bind to ionotropic receptors; some bind to metabotropic receptors

what are step 7 of a chemical synapse signal transmission7

When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron.

neurotransmitter

About 100 substances are either known or suspected to be neurotransmitters. Some neurotransmitters bind to their receptors and act quickly to open/close ion channels in the membrane. Others act more slowly via second-messenger systems to influence chemical reactions inside cells. The result of either process can be excitation or inhibition of postsynaptic neurons. Neurotransmitters can be divided into two classes based on size: small-molecule neurotransmitters and neuropeptides

substance P

Another neuropeptide, substance P, is released by neurons that transmit pain-related input from peripheral pain receptors into the central nervous system, enhancing the perception of pain. Enkephalin and endorphin suppress the release of substance P, thus decreasing the number of nerve impulses being relayed to the brain for pain sensations. Substance P has also been shown to counter the effects of certain nerve-damaging chemicals, prompting speculation that it might prove useful as a treatment for nerve degeneration.

why do action potentials only go one direction?

At most chemical synapses, only one-way information transfer can occur—from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell. Only synaptic end bulbs of presynaptic neurons can release neurotransmitter, and only the postsynaptic neuron's membrane has the receptor proteins that can recognize and bind that neurotransmitter. As a result, action potentials move in one direction.

inhibitory postsynaptic potential (IPSP)

During hyperpolarization, generation of an action potential 36l,2...00.1.kj 6,kis more difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state. -A hyperpolarizing postsynaptic potential that is caused by more negative ions or less positive ions inside of cells

treatment for epileptic seizures

Epileptic seizures often can be eliminated or alleviated by antiepileptic drugs, such as phenytoin, carbamazepine, and valproate sodium. An implantable device that stimulates the vagus (X) nerve has produced dramatic results in reducing seizures in some patients whose epilepsy was not well controlled by drugs. In very severe cases, surgical intervention may be an option.

Neuropeptides SUBSTANCE DESCRIPTION

Hypothalamic releasing and inhibiting hormones: Produced by hypothalamus; regulate release of hormones by anterior pituitary. Angiotensin II: Stimulates thirst; may regulate blood pressure in brain. As a hormone, causes vasoconstriction and promotes release of aldosterone, which increases rate of salt and water reabsorption by kidneys.

parallel after-discharge circuit

In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. Parallel after-discharge circuits may be involved in precise activities such as mathematical calculations.

describe the different axons and their absolute refractory period

Large-diameter axons have a larger surface area and have a brief absolute refractory period of about 0.4 msec. Because a second nerve impulse can arise very quickly, up to 1000 impulses per second are possible. Small-diameter axons have absolute refractory periods as long as 4 msec, enabling them to transmit a maximum of 250 impulses per second. Under normal body conditions, the maximum frequency of nerve impulses in different axons ranges between 10 and 1000 per second.

what inhibitory ion channels do ionotropic receptors exist on?

Many inhibitory neurotransmitters bind to ionotropic receptors that contain chloride channels. IPSPs result from opening these Cl− channels. When Cl− channels open, a larger number of chloride ions diffuse inward. The inward flow of Cl− ions causes the inside of the postsynaptic cell to become more negative (hyperpolarized).

Treatment of depression

Medication is the most common treatment for depression. a type of meds would be selective serotonin reuptake inhibitors (SSRIs) are drugs that provide relief from some forms of depression. By inhibiting reuptake of serotonin by serotonin transporters, SSRIs prolong the activity of this neurotransmitter at synapses in the brain. SSRIs include fluoxetine (Prozac®), paroxetine (Paxil®), and sertraline (Zoloft®).

1. Summation 2. describe the summation of 2 hyperpolarizing or 2 depolarizing graded potential and the summation of opposite graded potential

1. the process by which graded potentials add together, making the graded potentials stronger and last longer 2. If two depolarizing or two hyperpolarizing graded potentials summate, the net result is a larger depolarizing or hyperpolarizing graded potential If two equal but one depolarizing and the other hyperpolarizing, then they cancel each other out and the overall graded potential disappears.

1. propagation 2. why does an action potential propagate in one direction only?

1.mode of conduction that keeps its strength as it spreads along the membrane. -an action potential is not decremental (it does not die out). -two types of propagation: continuous conduction and saltatory conduction. 2. an action potential can propagate in this direction 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.

what affects a postsynaptic neuron and its potential?

A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters or inhibitory neurotransmitters. The sum of all the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron, which may respond in the following ways: 1. 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 EPSP that does not reach threshold. after an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized. 2. Nerve impulse(s). If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long as the EPSP is at or above the threshold level. 3. IPSP. If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse.

Summation of postsynaptic potentials

A typical neuron in the CNS receives input from 1000 to 10,000 synapses. Integration of these inputs involves summation of the postsynaptic potentials that form in the postsynaptic neuron. Recall that summation is the process by which graded potentials add together. The greater the summation of EPSPs, the greater the chance that threshold will be reached. At threshold, one or more nerve impulses (action potentials) arise.

Strychnine Poisoning

Normally, inhibitory neurons in the spinal cord called Renshaw cells release the neurotransmitter glycine at inhibitory synapses with somatic motor neurons. This inhibitory input to their motor neurons prevents excessive contraction of skeletal muscles. Strychnine (STRIK-nīn) is a lethal poison that is mainly used as a pesticide (rats, moles, gophers, and coyotes). When ingested, it binds to and blocks glycine receptors. The normal, delicate balance between excitation and inhibition in the CNS is disturbed, and motor neurons generate nerve impulses without restraint. All skeletal muscles, including the diaphragm, contract fully and remain contracted. Because the diaphragm cannot relax, the victim cannot inhale, and suffocation results.

opiate drugs

Scientists discovered that certain brain neurons have plasma membrane receptors for opiate drugs such as morphine and heroin. The quest to find the naturally occurring substances that use these receptors brought to light the first neuropeptides: two molecules, each a chain of five amino acids, named enkephalins. Other so-called opioid peptides include the endorphins and dynorphins. It is thought that opioid peptides are the body's natural painkillers. Acupuncture may produce analgesia (loss of pain sensation) by increasing the release of opioids. These neuropeptides have also been linked to improved memory and learning; feelings of pleasure or euphoria; control of body temperature; regulation of hormones that affect the onset of puberty, sexual drive, and reproduction; and mental illnesses such as depression and schizophrenia.

reverberating circuit

Some circuits are organized so that stimulation of the presynaptic cell causes the postsynaptic cell to transmit a series of nerve impulses. This is called a reverberating circuit. In this pattern, the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from later neurons synapse with earlier ones. This arrangement sends impulses back through the circuit again and again. The output signal may last from a few seconds to many hours, depending on the number of synapses and the arrangement of neurons in the circuit. Inhibitory neurons may turn off a reverberating circuit after a period of time. Among the body responses thought to be the result of output signals from reverberating circuits are breathing, coordinated muscular activities, waking up, and short-term memory.

whta do some inhibitory neurotransmitters bind to?

Some inhibitory neurotransmitters bind to metabotropic receptors that are linked to K+ channels. IPSPs result from the opening of these K+ channels. -When K+ channels open, a larger number of potassium ions diffuses outward. The outward flow of K+ ions causes the inside of the postsynaptic cell to become more negative (hyperpolarized).

Neurons

-possess electrical excitability -forms networks within brain and spinal cord -connects all regions of body to brain and spinal cord -lost ability to undergo mitotic division -vary in length -consists of cell body, dendrites and axon

Axon

-propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. -a long, thin, cylindrical projection that often joins to the cell body at an axon hillock -contains mitochondria, microtubules, and neurofibrils (No rough ER = no protein synthesis)

Substances naturally present in the body as well as drugs and toxins can modify the effects of neurotransmitters in what ways?

3. The neurotransmitter receptors can be activated or blocked. An agent that binds to receptors and enhances or mimics the effect of a natural neurotransmitter is an agonist. Isoproterenol (Isuprel®) is a powerful agonist of epinephrine and norepinephrine. It can be used to dilate the airways during an asthma attack. An agent that binds to and blocks neurotransmitter receptors is an antagonist. Zyprexa®, a drug prescribed for schizophrenia, is an antagonist of serotonin and dopamine. 4. Neurotransmitter removal can be stimulated or inhibited. For example, cocaine produces euphoria—intensely pleasurable feelings—by blocking transporters for dopamine reuptake. This action allows dopamine to linger longer in synaptic clefts, producing excessive stimulation of certain brain regions.

Neuropeptides SUBSTANCE DESCRIPTION

Enkephalins: Inhibit pain impulses by suppressing release of substance P; may have role in memory and learning, control of body temperature, sexual activity, and mental illness. Their potent analgesic effect is 200 times stronger than morphine. Cholecystokinin (CCK): Found in brain and small intestine; may regulate feeding as a "stop eating" signal. As a hormone, regulates pancreatic enzyme secretion during digestion, and contraction of smooth muscle in gastrointestinal tract.

Neuropeptides SUBSTANCE DESCRIPTION

Substance P: Found in sensory neurons, spinal cord pathways, and parts of brain associated with pain; enhances perception of pain. Endorphins: Inhibit pain by blocking release of substance P; may have role in memory and learning, sexual activity, control of body temperature, and mental illness. Dynorphins: May be related to controlling pain and registering emotions.

Ligand-gated channels

- opens and closes in response to the binding of a ligand (chemical) stimulus. -A wide variety of chemical ligands—including neurotransmitters, hormones, and particular ions—can open or close these channels. - are located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons.

Central Nervous System (CNS)

-Consists of brain and spinal cord -Processes many different kinds of sensory info -Source of thoughts, emotions, and memories -Most signals that stimulate muscles and glands originate here

Excitotoxicity causes and treatment

-A high level of glutamate in the interstitial fluid of the CNS causes it —destruction of neurons through prolonged activation of excitatory synaptic transmission. -The most common cause is oxygen deprivation of the brain due to ischemia (inadequate blood flow), as happens during a stroke. Lack of oxygen causes the glutamate transporters to fail, and glutamate accumulates in the interstitial spaces between neurons and neuroglia, literally stimulating the neurons to death. Clinical trials are underway to see whether antiglutamate drugs administered after a stroke can offer some protection from excitotoxicity.

Saltatory conduction (SAL-ta-tō-rē; saltat- = leaping)

-Action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. (also faster in larger diamater axons) -the special mode of action potential propagation that occurs because of the uneven distribution of voltage-gated channels. -Few voltage-gated channels are present in regions where a myelin sheath covers the axolemma. -at the nodes of Ranvier (where there is no myelin sheath), the axolemma has many voltage-gated channels. Hence, current carried by Na+ and K+ flows across the membrane mainly at the nodes. -occurs along myelinated axons,

Na+ inflow to cell

-As more channels open, Na+ inflow increases, the membrane depolarizes further, and more Na+ channels open. -During the few ten-thousandths of a second that the voltage-gated Na+ channel is open, about 20,000 Na+ flow across the membrane and change the membrane potential considerably.

what excitatory ion channels do ionotropic receptors exist on?

-Many excitatory neurotransmitters bind to ionotropic receptors that contain cation channels. -EPSPs result from opening these cation channels. -When cation channels open, they allow passage of the three most plentiful cations (Na+, K+, and Ca2+) through the postsynaptic cell membrane, but Na+ inflow is greater than either Ca2+ inflow or K+ outflow and the inside of the postsynaptic cell becomes less negative (depolarized).

Adenosine

-The characteristic ring structure of the adenosine portion of ATP is called a purine ring. Adenosine itself, as well as its ATP, ADP, and AMP derivatives , is an excitatory neurotransmitter in both the CNS and the PNS. -Most of the synaptic vesicles that contain ATP also contain another neurotransmitter. In the PNS, ATP and norepinephrine are released together from some sympathetic neurons; some parasympathetic neurons release ATP and acetylcholine in the same vesicles.

Differences between muscle and nerve cells

-The typical resting membrane potential of a neuron is , but it is closer to in skeletal and cardiac muscle fibers. -The duration of a nerve impulse is 0.5-2 msec, but a muscle action potential is considerably longer—about 1.0-5.0 msec for skeletal muscle fibers and 10-300 msec for cardiac and smooth muscle fibers. -the propagation speed of action potentials along the largest diameter, myelinated axons is about 18 times faster than the propagation speed along the sarcolemma of a skeletal muscle fiber.

How do the nodes work in action potentials?

-When an action potential propagates along a myelinated axon, an electric current (carried by ions) flows through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node to the next. 1. The action potential at the first node generates ionic currents in the cytosol and extracellular fluid that depolarize the membrane to threshold, opening voltage-gated Na+ channels at the second node. 2.The resulting ionic flow through the opened channels constitutes an action potential at the second node. Then, the action potential at the second node generates an ionic current that opens voltage-gated Na+ channels at the third node, and so on. -Each node repolarizes after it depolarizes.

An ionotropic receptor

-a type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel. -the neurotransmitter binding site and the ion channel are components of the same protein. -a type of ligand-gated channel. -In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor is closed. -When the correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell.

A metabotropic receptor (me-tab′-ō-TRO-pik)

-a type of neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel as part of its structure. -However, a metabotropic receptor is coupled to a separate ion channel by a type of membrane protein called a G protein. -When a neurotransmitter binds to a metabotropic receptor, the G protein either directly opens/closes the ion channel or it may act indirectly by activating another molecule, a "second messenger," in the cytosol, which in turn opens/closes the ion channel. -Thus, a metabotropic receptor differs from an ionotropic receptor in that the neurotransmitter binding site and the ion channel are components of different proteins.

The best-studied neurotransmitter

-acetylcholine (ACh), which is released by many PNS neurons and by some CNS neurons. - an excitatory neurotransmitter at some synapses, such as the neuromuscular junction, where the binding of ACh to ionotropic receptors opens cation channels. -also an inhibitory neurotransmitter at other synapses, where it binds to metabotropic receptors coupled to G proteins that open K+ channels. For example, ACh slows heart rate at inhibitory synapses made by parasympathetic neurons of the vagus (X) nerve. -The enzyme acetylcholinesterase (AChE) inactivates ACh by splitting it into acetate and choline fragments.

electrical synapse

-action potentials conduct directly between the plasma membranes of adjacent neurons through gap junctions. Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. -As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell. -Gap junctions are common in visceral smooth muscle, cardiac muscle, and the developing embryo. They also occur in the brain.

Action potential

-action potentials must travel from the trigger zone of the axon to the axon terminals. it depends on positive feedback to keep its strength. -it is not the same action potential that propagates along the entire axon. Instead, the action potential regenerates over and over at adjacent regions of membrane -Because they can travel along a membrane without dying out, action potentials function in communication over long distances.

Serotonin

-also known as 5-hydroxytryptamine (5-HT), - is concentrated in the neurons in a part of the brain called the raphe nucleus. It is thought to be involved in sensory perception, temperature regulation, control of mood, appetite, and the induction of sleep.

nitric oxide (NO)

-an important excitatory neurotransmitter secreted in the brain, spinal cord, adrenal glands, and nerves to the penis and has widespread effects throughout the body. NO contains a single nitrogen atom, in contrast to nitrous oxide (N2O), or laughing gas, which has two nitrogen atoms. N2O is sometimes used as an anesthetic during dental procedures.

A fibers

-are the largest diameter axons (5-20 ) and are myelinated. -have a brief absolute refractory period -conduct nerve impulses (action potentials) at speeds of 12 to 130 m/sec (27-290 mi/hr). -The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are these fibers, as are the axons of motor neurons that conduct impulses to skeletal muscles.

C fibers

-are the smallest diameter axons (0.5-1.5 ) -all are unmyelinated. Nerve impulse propagation along this fiber ranges from 0.5 to 2 m/sec (1-4 mi/hr). -exhibit the longest absolute refractory periods. -These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin, and pain impulses from the viscera. Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are these fibers. Examples of motor functions of B and these fibers are constricting and dilating the pupils, increasing and decreasing the heart rate, and contracting and relaxing the urinary bladder.

multiple sclerosis

-autoimmune disease that causes a progressive destruction of myelin sheaths surrounding neurons in the CNS. -In multiple regions the myelin sheaths deteriorate to scleroses, which are hardened scars or plaques shown by Magnetic resonance imaging (MRI) in the white matter of the brain and spinal cord. -The destruction of myelin sheaths slows and then short-circuits propagation of nerve impulses.

Carbon monoxide (CO)

-like NO, is not produced in advance and packaged into synaptic vesicles. -is formed as needed and diffuses out of cells that produce it into adjacent cells. -an excitatory neurotransmitter produced in the brain and in response to some neuromuscular and neuroglandular functions. -might protect against excess neuronal activity and might be related to dilation of blood vessels, memory, olfaction, vision, thermoregulation, insulin release, and anti-inflammatory activity.

a chemical synapse

-plasma membranes of presynaptic and postsynaptic neurons do not touch. They are separated by the synaptic cleft, a space of 20-50 nm* that is filled with interstitial fluid. Nerve impulses cannot conduct across the synaptic cleft, so indirect form of communication occurs. In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron. The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential. Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential). The time required for these processes at a chemical synapse, a synaptic delay of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.

Spatial summation

-summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. For example, spatial summation results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs.

Temporal summation

-summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times. For example, temporal summation results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession. Because a typical EPSP lasts about 15 msec, the second (and subsequent) release of neurotransmitter must occur soon after the first one if temporal summation is to occur.

The speed of propagation of an action potential is affected by what three major factors?

1. Amount of myelination. As you have just learned, action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. 2. Axon diameter. Larger diameter axons propagate action potentials faster than smaller ones due to their larger surface areas. 3. Temperature. Axons propagate action potentials at lower speeds when cooled. -Axons can be classified into three major groups based on the amount of myelination, their diameters, and their propagation speeds

Electrical synapses have what two main advantages?

1. Faster communication. Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. The events that occur at a chemical synapse take some time and delay communication slightly. 2. Synchronization. Electrical synapses can synchronize activity of a group of neurons or muscle fibers. a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions. -in the heart or in visceral smooth muscle is coordinated contraction of these fibers

Substances naturally present in the body as well as drugs and toxins can modify the effects of neurotransmitters in what ways?

1. Neurotransmitter synthesis can be stimulated or inhibited. For instance, many patients with Parkinson's disease receive benefit from the drug l-dopa because it is a precursor of dopamine. For a limited period of time, taking l-dopa boosts dopamine production in affected brain areas. 2. Neurotransmitter release can be enhanced or blocked. Amphetamines promote release of dopamine and norepinephrine. Botulinum toxin causes paralysis by blocking release of acetylcholine from somatic motor neurons.

what are the two consequences of the flow of current across the membrane occuring only at the nodes of Ranvier?

1. The action potential appears to "leap" from node to node as each nodal area depolarizes to threshold. -Because an action potential leaps across long segments of the myelinated axolemma as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter. 2. Opening a smaller number of channels only at the nodes represents a more energy-efficient mode of conduction. -Because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs. less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

1. synapse 2. presynaptic neuron 3. A postsynaptic cell

1. a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell). -one neuron synapses with 5000 other neurons 2. a nerve cell that carries a nerve impulse toward a synapse. -It is the cell that sends a signal. 3. the cell that receives a signal. It may be a nerve cell called a postsynaptic neuron that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse.

(c) lamellated corpuscle (d) nociceptor

(c) a pressure receptor composed of a multilayered connective tissue capsule that encloses a dendrite. (d) a pain receptor that consists of free nerve endings (bare dendrites). Thermoreceptors (which detect thermal sensations), itch receptors, and tickle receptors resemble these receptors in that they are unipolar neurons with free nerve endings that serve as sensory receptors.

Ependymal Cells

- are cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. -These cells line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord). -produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. -They also form the blood-cerebrospinal fluid barrier

sympathetic division and parasympathetic division

- effectors receive nerves from both divisions, and usually the two divisions have opposing actions -the sympathetic division helps support exercise or emergency actions, the "fight-or-flight" responses, and the parasympathetic division takes care of "rest-and-digest" activities.

Threshold

-An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level. -about -55 mV in many neurons -Different neurons may have different thresholds for generation of an action potential, but the threshold in a particular neuron usually is constant. -The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold.

Peripheral Nervous System (PNS)

-Consists of all nervous tissue outside of CNS -Components are nerves ganglia, enteric plexuses and sensory receptors -Divided into somatic nervous system, autonomic nervous system and enteric nervous system

Unequal distribution of ion in ECF and cytosol

-Extracellular fluid is rich in Na+ and Cl−. -In cytosol, the main cation is K+, and the two dominant anions are phosphates attached to molecules (Ex ATP). -the plasma membrane typically has more K+ leak channels than Na+ leak channels, the number of K+ "leak" out of the cell into the ECF is greater than the number of Na+ that "leak" into the cell. -As more and more positive K+ exit, the inside of the membrane becomes increasingly negative, and the outside of the membrane becomes increasingly positive.

Electrogenic nature of the Na+-K+ ATPases

-Left unchecked, inward leakage of Na+ would eventually destroy the resting membrane potential. -The small inward Na+ leak and outward K+ leak are offset by the Na+-K+ ATPases (sodium-potassium pumps). -These pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in. At the same time, the Na+-K+ ATPases bring in K+. -the potassium ions eventually leak back out of the cell -the Na+-K+ ATPases expel three Na+ for each two K+ imported. -Since these pumps remove more positive charges from the cell than they bring into the cell, they are electrogenic, which means they contribute to the negativity of the resting membrane potential. Their total contribution, however, is very small

After-hyperpolarizing Phase

-May occur if K+ outflow is large enough -During this phase, the voltage-gated K+ channels remain open and the membrane potential becomes even more negative (about -75 mV). -As the voltage-gated K+ channels close, the membrane potential returns to the resting level of -70mV. -most voltage-gated K+ channels do not exhibit an inactivated state, just closed and open states.

Schwann cells myelin sheaths

-Schwann cells begin to form myelin sheaths around axons during fetal development. -Each Schwann cell wraps about 1 millimeter of a single axon's length by spiraling many times around the axon. -Eventually, multiple layers of glial plasma membrane surround the axon, with the Schwann cell's cytoplasm and nucleus forming the outermost layer. -Each Schwann cell wraps one axon segment between two nodes of Ranvier

ENS Functions

-Sensory neurons of the ENS monitor chemical changes within the GI tract as well as the stretching of its walls. -Enteric motor neurons govern contractions of GI tract smooth muscle to propel food through the GI tract, secretions of GI tract organs and activities of GI tract endocrine cells, which secrete hormones.

Sensory function

-Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). -This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves.

Leak channels

-The gates randomly alternate between open and closed positions. Typically, plasma membranes have many more K+ leak channels than Na+ leak channels, and the K+ leak channels are leakier than the sodium ion leak channels. Thus, the membrane's permeability to K+ is much higher than its permeability to Na+. -found in nearly all cells, including the dendrites, cell bodies, and axons of all types of neurons.

slow axonal transport

-The slower system, which moves materials about 1-5 mm per day -It conveys axoplasm in one direction only—from the cell body toward the axon terminals. -supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons.

Schwann Cells

-These cells encircle PNS axons. -Like oligodendrocytes, they form the myelin sheath around axons. -myelinates a single axon -A single Schwann cell can also enclose as many as 20 or more unmyelinated axons (axons that lack a myelin sheath). -participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.

microglial cells AKA microglia

-These neuroglia are small cells with slender processes that give off numerous spinelike projections. -function as phagocytes they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

Astrocytes

-These star-shaped cells have many processes and are the largest and most numerous of the neuroglia. -There are two types Protoplasmic astrocytes have many short branching processes and are found in gray matter. Fibrous astrocytes have many long unbranched processes and are located mainly in white matter. -The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord).

Ion channels The gate

-When ion channels are open, they allow specific ions to move across the plasma membrane, down their electrochemical gradient -Ion channels open and close due to "gates." -The gate is a part of the channel protein that can seal the channel pore shut or move aside to open the pore. -The electrical signals produced by neurons and muscle fibers rely on four types of ion channels: leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels -cell places these channels wherever it wants to change its properties

Describe depolarizing phase

-a depolarizing graded potential or some other stimulus causes the membrane of the axon to depolarize to threshold -This cause voltage-gated Na+ channels to open rapidly. Both the electrical and the chemical gradients favor inward movement of Na+, so the resulting inrush of Na+ changes the membrane potential from -55 to peak of 30mV (more positive inside than outside)

Lipofuscin

-a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. -a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron.

action potential (AP) or impulse

-a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. -Occurs only in axons) -has two main phases (takes about .002 sec) : a depolarizing phase and a repolarizing phase (RP may have a hyperpolarizing phase)

Autonomic nervous system (ANS)

-consists of (1) sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs, and (2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands -Part of PNS that is involuntary - Consists of two branches : sympathetic division and parasympathetic division

Somatic Nervous System (SNS)

-consists of (1) sensory neurons that convey information to the CNS from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell, and (2) motor neurons that conduct impulses from the CNS to skeletal muscles only. -Part of PNS that is voluntary

Inability of most anions to leave the cell

-contributes to the negative resting membrane potential -Most anions inside the cell are not free to leave. -They cannot follow the K+ out of the cell because they are attached to nondiffusible molecules such as ATP and large proteins.

The production of graded potentials and action potentials

-depends on two basic features of the plasma membrane of excitable cells: the existence of a resting membrane potential and the presence of specific types of ion channels. -the plasma membrane of excitable cells exhibits a resting membrane potential. -occur because the membranes of neurons contain many different kinds of ion channels that open or close in response to specific stimuli. -Because the lipid bilayer of the plasma membrane is a good electrical insulator, the main paths for ions to flow across the membrane are through the ion channels.

Sensory neurons

-either contain sensory receptors at their dendrites or are located just after sensory receptors that are separate cells. -Once an appropriate stimulus activates a sensory receptor, the neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. -are unipolar in structure. -Also called afferent neurons.

enteric plexuses

-extensive networks of neurons located in the walls of organs of the gastrointestinal tract. -The neurons of these plexuses help regulate the digestive system

Nissl bodies

-free ribosomes and prominent clusters of rough endoplasmic reticulum, -The ribosomes are the sites of protein synthesis. -Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS.

Nerve Fiber

-general term for any neuronal process that emerges from the cell body of a neuron. -Most neurons have two kinds of processes: multiple dendrites and a single axon

Unipolar neurons

-have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body - The dendrites function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli -The trigger zone for nerve impulses is at the junction of the dendrites and axon. The impulses then propagate toward the synaptic end bulbs. The cell bodies are located in the ganglia of spinal and cranial nerves.

cytoskeleton

-includes both neurofibrils, composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon.

Neuroglia

-make up about half the volume of the CNS. -actively participate in the activities of nervous tissue (Neurons can NOT function with them). -are smaller than neurons and are 5 to 25 times more numerous. -do not generate or propagate action potentials, -they can multiply and divide in the mature nervous system. -four types of glia—astrocytes, oligodendrocytes, microglia, and ependymal cells—are found only in the CNS. -The remaining two types—Schwann cells and satellite cells—are present in the PNS.

Fast axonal transport

-moves materials a distance of 200-400 mm per day -uses proteins that function as "motors" to move materials along the surfaces of microtubules of the neuron's cytoskeleton -moves materials in both directions (anterograde and retrograde)

1. resting membrane potential 2. polarized

1. exists because of an unequal distribution of ions -measured in mV or V -buildup of charge happens very close to membrane -The cytosol or extracellular fluid elsewhere in the cell contains equal numbers of positive and negative charges (neutral = no charge). 2. describes a cell that exhibits a membrane potential

graded potential occurs when and where? what are the ion channels in sensory, interneurons and motor neurons?

-occurs when a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell's plasma membrane. -Typically, mechanically-gated channels and ligand-gated channels can be present in the dendrites of sensory neurons -ligand-gated channels are numerous in the dendrites and cell bodies of interneurons and motor neurons. -occur mainly in the dendrites and cell body of a neuron.

voltage-gated channel

-opens in response to a change in membrane potential (voltage). -participate in the generation and conduction of action potentials in the axons of all types of neurons.

mechanically-gated channel

-opens or closes in response to mechanical stimulation in the form of vibration (such as sound waves), touch, pressure, or tissue stretching. -The force distorts the channel from its resting position, opening the gate. -Examples are those found in auditory receptors in the ears, in receptors that monitor stretching of internal organs, and in touch receptors and pressure receptors in the skin.

Ganglia

-small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord. -closely associated with cranial and spinal nerves.

Dendrites

-the receiving or input portions of a neuron. -The plasma membranes of dendrites (and cell bodies) contain numerous receptor sites for binding chemical messengers from other cells. -short, tapering, and highly branched. -form a tree-shaped array of processes extending from the cell body. -their cytoplasm contains Nissl bodies, mitochondria, and other organelles.

pseudounipolar neurons

-they begin in the embryo as bipolar neurons. -During development, the dendrites and axon fuse together and become a single process -AKA unipolar neurons

How is the resting potential measured?

-tip of recording microelectrode inside cell and reference electrode outside of cell are connected to voltmeter -typical voltage is -70mV

Functions of astrocytes

1. Astrocytes contain microfilaments that give them considerable strength, which enables them to support neurons. 2. Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from various potentially harmful substances in blood 3. In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnection among neurons in the brain. 4. Astrocytes help to maintain the appropriate chemical environment for the generation of nerve impulses. For example they serve as a conduit for the passage of nutrients and other substances between blood capillaries and neurons. 5. Astrocytes may also play a role in learning and memory by influencing the formation of neural synapses.

1. fast anterograde transport 2. Fast retrograde transport

1. Fast axonal transport that occurs in a forward direction moves organelles and synaptic vesicles from the cell body to the axon terminals 2. Fast axonal transport that occurs in a backward direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled. Substances that enter the neuron at the axon terminals are also moved to the cell body such as toxins or viruses

1. myelin sheath 2. neurolemma

1. The inner portion that consists of up to 100 layers of Schwann cell membrane 2. The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath -found only around axons in the PNS. When an axon is injured, this layer aids in regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon.

1. refractory period 2. relative refractory period

1. The period of time after an action potential that begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus -In contrast to action potentials, graded potentials do not exhibit a refractory period. 2. the period of time during which a second action potential can be initiated, but only by a larger than normal stimulus. -It coincides with the period when the voltage-gated K+ channels are still open after inactivated Na+ channels have returned to their resting state

1. synapse 2. synaptic end bulb 3. varicosities

1. The site of communication between two neurons or between a neuron and an effector cell 2. The tips of some axon terminals that swell into bulb-shaped structures 3. others exhibit a string of swollen bumps

1. ganglion 2. nucleus 3. tract

1. Usually, a group of neuronal cell bodies lying outside the central nervous system (CNS). Plural is ganglia 2. a cluster of neuronal cell bodies located in the CNS. 3. a bundle of axons that is located in the CNS. they interconnect neurons in the spinal cord and brain.

1. myelin sheath 2. unmyelinated

1. a multilayered lipid and protein covering, that surrounds an axon. -it electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction. -amount of myelin increase from birth to maturity. 2. Axons without such a covering

1. graded potential 2. hyperpolarizing graded potential 3. depolarizing graded potential

1. a small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative). - occurs in cell body and dendrites 2. a graded potential When the response makes the membrane more polarized (inside more negative), -mV moves away from 0 3. a graded potential When the response makes the membrane less polarized (inside less negative = more positive). -mV moves toward 0

1. Nerve 2. cranial nerve 3. spinal nerve

1. bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lie outside of brain and spinal cord -follows defined path and serves specific region in body 2. 12 pairs of nerves that emerge from brain 3. 21 pairs of nerves that emerge from spinal cord

1. white matter 2. gray matter

1. composed primarily of myelinated axons. Myelin is a whitish color -In the spinal cord, the white matter surrounds an inner core of gray matter that looks like a butterfly or the letter H in transverse section 2. contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish because the Nissl bodies impart a gray color and there is little or no myelin in these areas. - in the brain, a thin shell of gray matter covers the surface of the largest portions of the brain, the cerebrum and cerebellum. Blood vessels are present in both white and gray matter.

1. What does graded refer to in graded potential? 2. what are graded potentials useful for?

1. graded means that they vary in amplitude, depending on the strength of the stimulus. They are larger or smaller depending on how many ligand-gated or mechanically-gated channels have opened/closed and how long each remains open. 2.Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only.

what happen in cases of injury or disease? gliomas

1. neuroglia multiply to fill in the spaces formerly occupied by neurons. Brain tumors derived from glia, tend to be highly malignant and to grow rapidly.

1. oligodendrocyte 2. myelin sheath

1. resemble astrocytes but are smaller and contain fewer processes. -the Processes are responsible for forming and maintaining the myelin sheath around CNS axons. -wraps 5-20 sections 2. a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Such axons are said to be myelinated

1. axon terminals 2. axon collaterals

1.Terminal branches of an axon where synaptic vesicles undergo exocytosis to release neurotransmitter molecules. Also called telodendria 2. side branches at usually 90 degrees along the length of axon

1. decremental conduction. 2. what kind of current does a graded potential make?

1.mode of travel by which graded potentials die out as they spread along the membrane 2.produced a flow of current that is localized, which means that it spreads to adjacent regions along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels.

2. what depends on stimulus intensity?

2. once an action potential is generated, the amplitude of an action potential is always the same and does not depend on stimulus intensity. Instead, the greater the stimulus strength above threshold, the greater the frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period.

what are steps 4 and 5 after you have picked up a pen?

4. In response to the graded potential, the axon of the interneuron forms a nerve action potential. The nerve action potential travels along the axon, which results in neurotransmitter release at the next synapse with another interneuron. 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 interneurons in higher parts of the brain (such as the thalamus and cerebral cortex) are activated. Once interneurons in the cerebral cortex, the outer part of the brain, are activated, perception occurs and you are able to feel the smooth surface of the pen touch your fingers.

Nervous system

4.5 lbs -Smallest and most complex body system -Consists of neurons and neuroglia -Organized into central nervous system and peripheral nervous system

what is step 6 after you have picked up a pen? 6a. Upper motor neuron

6. A stimulus in the brain causes a graded potential to form in the dendrites and cell body of an upper motor neuron. The graded potential subsequently causes a nerve action potential to occur in the axon of the upper motor neuron, followed by neurotransmitter release. 6a. a type of motor neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle.

what is step 7 after you have picked up a pen? 7a. lower motor neuron

7. The neurotransmitter generates a graded potential in a lower motor neuron. The graded potential triggers the formation of a nerve action potential and then release of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers. 7a. a type of motor neuron that directly supplies skeletal muscle fibers.

what is step 8 after you have picked up a pen?

8. The neurotransmitter stimulates the muscle fibers that control finger movements to form muscle action potentials. The muscle action potentials cause these muscle fibers to contract, which allows you to write with the pen.

Describe the three phases of an action potential

During the depolarizing phase (up to 30 mV), the negative membrane potential becomes less negative, reaches zero, and then becomes positive. During the repolarizing phase (down to -70 mV), the membrane potential is restored to the resting state of . Following the repolarizing phase there may be an after-hyperpolarizing phase (down to -75 mV), during which the membrane potential temporarily becomes more negative than the resting level.

Give examples of named graded potentials

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 postsynaptic potential. On the other hand, the graded potentials that occur in sensory receptors and sensory neurons are termed receptor potentials and generator potentials.

Motor Function

Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.

describe Repolarizing phase

Shortly after the activation gates of the voltage-gated Na+ channels open, the inactivation gates close (inactivated state). In addition to opening voltage-gated Na+ channels, a threshold-level depolarization also opens voltage-gated K+ channels. Because the voltage-gated K+ channels open more slowly, their opening occurs at about the same time the voltage-gated Na+ channels are closing. this produces the repolarizing phase of the action potential. This also causes Na+ inflow to slow and the K+ channels to open. This fast and slow flows cause the membrane potential to change from 30mV to -70mV . Repolarization also allows inactivated Na+ channels to revert to the resting state.

blood barrier

astrocytes secrete chemicals that maintain the unique selective permeability characteristics of the endothelial cells of the capillaries. In effect, the endothelial cells (Who are primarily responsible) create a blood-brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS.

How are neurons classified?

classified and named for the histologist who first described them or for an aspect of their shape or appearance examples are Purkinje cells in cerebellum and pyramidal cells whose cell body are pyramidal shaped also classified by structural and functional features examples are the -polar classifications and sensory, motor and interneuron classification