Chapter 11

all tissues have

all tissue consist of two components: cells and extracellular matrix (ECM) (see Chapter 4). Some tissues, such as epithelial tissue, are primarily cellular with very little ECM. Others, such as many connective tissues, have few cells and are mostly ECM. Like epithelial tissue, nervous tissue is highly cellular; about 80% of nervous tissue volume consists of cells (Figure 11.4)

integrative functions

analyze and interpret the detected sensory stimuli and determine an appropriate response. Integration is performed entirely by the CNS, mostly by the brain.

astrocytes

are the most numerous and the largest of the neuroglia in the CNS. Note in Figure 11.6 that each astrocyte has a central portion and numerous processes, all of which terminate in structures called end-feet. This anatomical feature equips astrocytes to perform multiple functions, including the following: Anchoring neurons and blood vessels in place. Astrocytes help form the three-dimensional structure of the brain by using their end-feet to anchor neurons and blood vessels in place. In addition, astrocytes may facilitate the transport of nutrients and gases from the blood vessels to neurons. Regulating the extracellular environment of the brain. Astrocytes are connected by gap junctions that allow them to communicate with one another about the local extracellular environment within the brain. Via this communication they can act as a "clean-up crew," removing excess extracellular potassium ions as well as chemicals known as neurotransmitters. Although neurons use neurotransmitters to send signals, their extracellular accumulation can lead to toxicity. Assisting in the formation of the blood brain barrier. Astrocytes facilitate the formation of a protective structure called the blood brain barrier by ensheathing capillaries and inducing their cells to form tight junctions. These tight junctions prevent most proteins and polar compounds from leaving the capillaries and entering the brain extracellular fluid (ECF). The only substances that can cross these capillaries easily are those that are nonpolar and lipid-soluble and/or those for which special transporters exist. The double barrier separates the blood from the brain ECF, which ensures selective transport of substances between the two fluids. The blood brain barrier is discussed fully in the CNS chapter (see Chapter 12). Repairing damaged brain tissue. When brain injury occurs, astrocytes are triggered to divide rapidly. Although this growth stabilizes the damaged tissue, it may also impede complete healing. Recent research has demonstrated that excess astrocyte activity actually inhibits the regrowth of neurons, leading to more permanent defects.

voltage-gated ion channels

open ​or close in response to changes in the cell's membrane potential.

voltage

separation of charges, called a voltage, is a type of gradient referred to as an electrical gradient.

somatic sensory division

(soma- = "body") consists of neurons that carry signals from skeletal muscles, bones, joints, and skin. This division also includes sensory neurons that transmit signals from the organs of vision, hearing, taste, smell, and balance (see ​Chapter 15​). Sometimes these particular neurons are referred to as the special sensory division.

changes in membrane potential: Ion movements

A cell starts with a negative resting membrane potential, meaning it is negative with respect to the extracellular fluid. There is an unequal distribution of ions across the plasma membrane. We have discussed the sodium and potassium ion gradients, but there are also gradients for other ions such as chloride and calcium. These gradients are maintained by gated channels and pumps such as the Na+/K+ pump. When the gated channels for a specific ion are triggered to open, those ions will follow their electrochemical gradient into or out of the cell.

propagation of the action potential

A single action potential in one spot of the membrane can't perform its main function, which is to act as a method of long-distance signaling. To do this, it has to be conducted, or propagated, down the length of the axon. The propagation of the action potential along the axon creates a flow of charged particles, or a current. Action potentials are self-propagating, meaning that each action potential triggers another one in a neighboring section of the axon. You can imagine this process like a string of dominoes—when the first one is tipped over, the next one falls, which triggers the next to fall, and the process continues until the end of the line is reached. Only the first domino needs the "push," and once they start to fall, the process sustains itself until the end. The transmission of action potentials occurs at a constant speed and largely in one direction—from the trigger zone to the axon terminals. Propagation takes place in a single direction because the membrane in the previous section (behind the action potential) is still in the refractory period. Recall that the sodium ion channels in refractory parts of the membrane are in their inactivated state, which means that the wave of depolarization cannot trigger them to open.

A node of Ranvier (myelin sheath gap)

Between each internode is a gap about 1 μm wide, called a node of Ranvier (rahn-vee-AY), or myelin sheath gap, where no myelin is found. Also unmyelinated is a short region from the axon hillock to the first neuroglial cell; this is known as the initial segment.

Sensory input from both divisions is carried from sensory receptors to the spinal cord and/or the brain by cranial and spinal nerves of the PNS.

CNS. The neurons of the CNS put together the many different types of sensory input, or integrate them, to form a more complete picture that can then elicit response if necessary. Interestingly, once the CNS integrates sensory input, it responds by disregarding about 99% of such integrated data, a process that happens subconsciously. For example, you are likely unaware of any jewelry you're wearing or the hum of the air conditioner, because these stimuli are filtered out as unimportant. However, that small percentage of sensory stimuli to which the CNS does respond generally leads to a motor response. PNS motor division. The PNS motor division consists of motor neurons that carry out the motor functions of the nervous system. Motor output traveling from the brain and spinal cord via cranial and spinal nerves of the PNS may be used to control the contraction of muscle cells or secretion from a gland. Organs that carry out the effects of the nervous system are often called ​effectors​. Like the sensory division, the motor division may be further classified based on the organs that the neurons contact:

Nervous system structures

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

Hyperpolarization

Change in the membrane potential of an excitable cell to a value more negative than its resting membrane potential Hyperpolarization may also result from the opening of channels for anions, such as chloride ions, which would allow these negatively charged ions to flow into the cell.

The messenger of synaptic transmission is the neurotransmitter released by the presynaptic neuron.

Diffusion and absorption. Some neurotransmitters simply diffuse away from the synaptic cleft through the extracellular matrix, where they diffuse through the plasma membrane of a neuron or astrocyte and are then returned to the presynaptic neuron. Degradation in the synaptic cleft. Certain neurotransmitters are broken down by reactions catalyzed by enzymes that reside in the synaptic cleft. The components of the destroyed neurotransmitter are often then taken back up by the presynaptic neuron and resynthesized into the original neurotransmitter. Reuptake into the presynaptic neuron. Some neurotransmitters are removed by a process called reuptake, in which they are transported back into the presynaptic neuron by proteins in its axolemma. Depending on their type, these neurotransmitters may be repackaged into synaptic vesicles or degraded in reactions catalyzed by enzymes. Figure 11.23

diverging circuit

Diverging circuit begins with one axon of an input neuron branching to make contacts with multiple postsynaptic neurons. The axons of these postsynaptic neurons then branch to contact more neurons, which in turn make contact with yet more neurons, and so on. So, when a signal is transmitted down the circuit's pathway, an increasing number of neurons are excited. Diverging circuits are critical because they allow a single neuron to communicate with multiple parts of the brain and/or body. Notice in Figure 11.28a, left, that some diverging circuits are amplifying circuits, in which the signal passes through a progressively greater number of neurons. We start with one neuron, which branches to excite two, the two then excite four, and so on. Some diverging circuits (​Figure 11.28a​, right) split into multiple tracts, each of which goes in a different direction. This type of circuit is characteristic of those transmitting incoming sensory stimuli, which is sent from neurons in the spinal cord to different neuronal pools in the brain for processing.

Events of an action potential

During the depolarization phase, the membrane potential rises toward zero and then becomes briefly positive. The membrane potential returns to a negative value during the repolarization phase, and then becomes temporarily more negative than resting during the hyperpolarization phase. Each phase occurs because of the selective opening and closing of specific voltage-gated ion channels. Note that before the action potential, when the membrane is at rest, both the sodium and the potassium ion channels are in the resting state.

regeneration

Human nervous tissue has a fairly limited capacity for the process of regeneration, or replacement of damaged tissue with the original tissue. Damaged axons and dendrites in the CNS almost never regenerate, a phenomenon apparently due to several factors. For example, oligodendrocytes may inhibit the process of neuronal growth, and chemicals called growth factors that trigger mitosis are largely absent in the CNS. In addition, the growth of astrocytes creates space-filling scar tissue that also prohibits regeneration. For these reasons, injuries to the brain or spinal cord have largely permanent effects.

Synaptic transmission is bidirectional.

In an electrical synapse, transmission is usually bidirectional, which means that either neuron may act as the presynaptic or the postsynaptic neuron and that current may flow in either direction between the two cells. These features of electrical synapses allow the activity of a group of cells to be synchronized—when stimulated, the cells will produce action potentials in unison. Electrical synapses are found primarily in areas of the brain that are responsible for programmed, automatic behaviors such as breathing. They are also present in developing nervous tissue in the embryo and fetus and are thought to assist in the development of the brain. In addition, electrical synapses are found outside the nervous system in locations such as cardiac and visceral smooth muscle, where they allow those tissues to engage in coordinated muscle activity.

neurotransmitters receptors

In chemical synapses, the postsynaptic neuron must have receptors to which the neurotransmitters released by the presynaptic neuron can bind, or it cannot respond to the signal being transmitted. Receptors are generally linked either directly or indirectly to ion channels. This is how the chemical signal is converted back into an electrical signal.

nuclei

In the CNS and PNS, cell bodies of neurons are typically found within clusters, most of which are in the CNS

electrochemical gradients

Ions moving against their electrochemical gradients move via ATP-consuming pumps. One of the most important pumps in electrophysiology is the sodium-potassium ion pump, or Na+/K+ ATPase, which brings two potassium ions into the cytosol as it moves three sodium ions into the extracellular fluid. This pump maintains, and to some extent creates, the vital concentration gradients of sodium and potassium ions that exist across the plasma membrane, an example of the Gradients Core Principle (Module 1.5.5). In a neuron, as in a muscle fiber, the concentration of sodium ions is higher in the extracellular fluid than in the cytosol, and the opposite is true for potassium ions—their concentration is higher in the cytosol than in the extracellular fluid.

neuroglia in the cns

Neuroglia are about 10 times more abundant in the CNS than neurons, and they make up about half the mass of the brain. Within the CNS we find four types of neuroglia: astrocytes, oligodendrocytes, microglia, and ependymal cells

termination of synaptic transmission

Now we have reached the final step of synaptic transmission— termination. But why terminate synaptic transmission? The answer is simple: After presynaptic neurons have generated a specific response in the postsynaptic neuron, the response cannot be initiated again until the postsynaptic neuron stops being stimulated. In other words, it has to be "turned off" in order to be "turned on" again. The neurons involved in breathing provide a simple example. When we need to inhale, specific neurons are stimulated to trigger our respiratory muscles to contract. Once we have taken a breath, our nervous system needs to stop stimulating these neurons or we will continue to inhale. This is accomplished by stopping synaptic transmission.

multipolar neurons

Over 99% of neurons in the human body fall into the group known as multipolar neurons. These neurons have a single axon and typically multiple highly branched dendrites. This group of neurons has the widest variability in terms of shape and size.

difference found between myelination in the PNS and CNS,

Presence or absence of a neurolemma. Note in Figure 11.8a that we find the nucleus and the bulk of the Schwann cell's cytoplasm and organelles on the outer surface of a myelinated axon, a structure called the neurolemma (noor-oh-LEM-ah). No outer neurolemma is found in the CNS because the nucleus and cytoplasm of the oligodendrocyte remain in a centralized location (see Figure 11.8b). Number of axons myelinated by a single glial cell. Each oligodendrocyte may send out multiple processes to envelop parts of several axons. However, Schwann cells can encircle only a portion of a single axon. Timing of myelination. The timing of myelination is also different within the CNS and the PNS. In the PNS myelination begins during the early fetal period, whereas myelination in the CNS, particularly in the brain, begins much later. Very little myelin is present in the brain of the newborn (which is why babies and toddlers need adequate fat in their diets).

myelin sheath

Schwann cells and oligodendrocytes wrap themselves around the axons of certain neurons to create a structure known as the myelin sheath (Figure 11.8). Myelin is composed of repeating layers of the plasma membrane of the neuroglial cell, so it has the same substances as any plasma membrane: phospholipids, other lipids such as cholesterol, and proteins. In addition, myelin contains lipids unique to Schwann cells and oligodendrocytes. Myelin plays an important role in the electrophysiology of many neurons. Recall that in the body, the flow of charged particles, or ions, creates an electric current. In unmyelinated axons, the electric current "leaks" out of the axon and has to be continually regenerated. However, as you learned in the muscle tissue chapter, ions do not pass easily through the hydrophobic portion of the phospholipid bilayer (see Chapter 10). For this reason, the high lipid content of myelin makes it an excellent insulator, akin to the rubber tubing around a copper wire. The overall effect of this insulation is to increase the speed of conduction of action potentials: Myelinated axons conduct action potentials about 15-150 times faster than unmyelinated axons. This is a good example of the Structure-Function Core Principle (Module 1.5.5).

PNS sensory division

Sensory stimuli are first detected by structures of the PNS called sensory receptors. The form of these receptors is diverse—they range from small tips of neurons found in the skin that sense temperature to complex receptors within muscles that sense muscle stretch. Depending on the location of the sensory receptors, the PNS sensory division may be further classified as follows:

synaptic vesicles

The axon terminal of the presynaptic neuron of every chemical synapse houses synaptic vesicles. These vesicles contain chemical messengers called neurotransmitters that transmit signals from the presynaptic to the postsynaptic neuron. This is how the electrical signal of the action potential is converted into a chemical signal.

Synaptic transmission is nearly instantaneous.

The delay between depolarization of the presynaptic neuron and change in potential of the postsynaptic neuron is less than 0.1 ms (millisecond), which is extraordinarily fast (we will see that transmission at most chemical synapses requires from one to a few milliseconds). These features of electrical synapses allow the activity of a group of cells to be synchronized—when stimulated, the cells will produce action potentials in unison. Electrical synapses are found primarily in areas of the brain that are responsible for programmed, automatic behaviors such as breathing. They are also present in developing nervous tissue in the embryo and fetus and are thought to assist in the development of the brain. In addition, electrical synapses are found outside the nervous system in locations such as cardiac and visceral smooth muscle, where they allow those tissues to engage in coordinated muscle activity.

deplorization

The influx of positive charges makes the membrane potential less negative cell becomes less polarized as its membrane potential approaches 0 mV. (Find out what happens when depolarization is blocked in A&P in the Real World: Local Anesthetic Drugs.) When a cell returns to its resting membrane potential, Repolarization has occurred.

microglia

The least numerous neuroglial cells are the small and branching -Although many functions of microglia are still under investigation, we do know that they are activated by injury within the brain and become wandering phagocytes—cells that "clean up" the environment in the brain. When activated, microglia ingest disease-causing organisms, dead neurons, and other cellular debris. They also secrete chemicals that stimulate inflammation.

depending on which channels are opened, one of two events may occur:

The membrane potential of the postsynaptic neuron moves closer to threshold. A small, local depolarization called an excitatory postsynaptic potential (EPSP) may occur, which brings the membrane potential at the trigger zone closer to threshold (Figure 11.21a). If the membrane potential reaches threshold, an action potential is generated. The membrane potential of the postsynaptic neuron moves away from threshold. Alternatively, a small, local hyperpolarization known as an inhibitory postsynaptic potential (IPSP) may occur, moving the membrane potential at the trigger zone farther away from threshold (Figure 11.21b). This tends to inhibit an action potential from firing. EPSPs typically result when ligand-gated channels such as sodium or calcium ion channels open, and these positively charged ions enter the postsynaptic neuron (see Figure 11.21a). A single EPSP produces only a very small, local potential across the membrane. However, each successive EPSP makes the membrane more depolarized, and so makes the trigger zone more likely to reach threshold and fire an action potential. Note in Figure 11.21b that an IPSP can be produced in two ways. First, ligand-gated potassium ion channels can open, which causes the cytosol to lose positive charges and so makes the membrane potential become more negative. Second, ligand-gated chloride ion channels can open. Chloride ions are more abundant in the extracellular fluid than in the neuron, so when chloride ion channels open, these anions enter the neuron and make the membrane potential more negative. The opening of either type of channel will yield the same result: The membrane potential at the trigger zone moves farther away from threshold, and an action potential becomes less likely. Summation of Postsynaptic Potentials and Neural Integration A neuron very rarely receives input from a single source; rather, it receives input from multiple presynaptic neurons, each of which causes an EPSP or IPSP. To complicate matters, synaptic transmission in the CNS occurs continuously for most neurons—they are constantly bombarded by synaptic inputs from hundreds to thousands of presynaptic neurons. Additionally, the input from each presynaptic neuron may be different: The input may be excitatory or inhibitory, and the strength and location of each input may vary. All this input from presynaptic neurons combines to have one cumulative effect on the postsynaptic neuron. The process by which this occurs is known as neural integration. Recall that in Module 11.1 you read about the integrative functions of the nervous system. Put simply, these integrative functions refer to this process of putting together all the excitatory and inhibitory stimuli that determine whether a neuron will or won't fire an action potential. As we have discussed, to fire an action potential, the trigger zone of an axon must be depolarized to threshold. But a single EPSP produces only a small, local depolarization that is often quite far away from the trigger zone. The small size of the EPSP and the distance from the trigger zone mean that a single EPSP is insufficient to trigger an action potential. However, the depolarizations of many EPSPs can be added together to produce a much greater overall effect. This phenomenon of adding the input from several postsynaptic potentials to affect the membrane potential at the trigger zone is known as summation. There are two types of summation, temporal and spatial. The first, temporal summation, occurs when neurotransmitters are released repeatedly from the axon terminal of a single presynaptic neuron (Figure 11.22a). The EPSPs must occur rapidly in succession for temporal summation to occur, because each EPSP lasts no more than about 15 msec.

cell body

The most conspicuous part of a neuron is its large cell body, or soma, which ranges from 5 to 100 μm in diameter. The cell body is the most metabolically active part of the neuron, because it is responsible for maintaining the sometimes huge cytoplasmic volume of the neuron and also for manufacturing all the proteins the neuron needs. This high level of biosynthetic activity is reflected in the composition of the organelles within its cytoplasm: Free ribosomes and rough endoplasmic reticulum (RER) are found in abundance, reflecting the commitment of the cell body to protein synthesis. Note that the association of ribosomes and RER forms what appears under a microscope as dark-staining clusters called Nissl bodies; these are represented in Figure 11.5. Other organelles involved in protein synthesis, including the Golgi apparatus and one or more prominent nucleoli, are present. Mitochondria are found in large numbers, indicating the high metabolic demands of the neuron. Additionally, the cytoplasm of the cell body includes lysosomes, smooth ER, and other organelles found in most cells. The characteristic shape of the cell body is maintained by another component of the cytoplasm—the neuronal cytoskeleton, which is composed largely of intermediate filaments. These filaments bundle together to form larger structures called neurofibrils, which extend out into the dendrites and axon of the neuron (see Figure 11.5). The cytoskeleton also contains microtubules that provide structural support and a means for transporting chemicals between the cell body and the axon.

neural circuits

The patterns of synaptic connection between neuronal pools are called neural circuits. Each neuronal pool in a circuit receives input from other pools, and then produces output that travels to additional pools. How the pools are connected determines the function of the circuits.

presynaptic neuron

The presynaptic neuron is the neuron that is sending the message from its axon terminal.

conduction speed

The rate at which propagation occurs is called ​conduction speed​, and it determines how rapidly signaling can occur within the nervous system. Conduction speed is influenced by two main factors: the diameter of the axon and the presence or absence of a myelin sheath. The diameter of the axon affects the conduction of current through the axon because larger axons have lower resistance to conduction, and therefore current flows through them more easily. The second determinant of conduction speed is the presence or absence of a myelin sheath. Two types of conduction can take place in an axon: saltatory conduction, in which the myelin sheath is present, and continuous conduction, in which it is absent (Figure 11.16). Recall from our discussion in Module 11.2 that myelin is an excellent insulator of electrical charge. So, the flow of current is far more efficient in a myelinated axon, which causes saltatory conduction to be significantly faster than continuous conduction the myelinated axon, the nodes of Ranvier are the only segments that must be depolarized to threshold. When the node, rich in voltage-gated sodium ion channels, is depolarized to threshold, an action potential is triggered. This action potential generates a current that flows passively and efficiently with little loss of charge through the next myelinated segment, or internode (see Figure 11.8 for a review of anatomical terminology). When the current reaches the next node of Ranvier, another action potential is generated. This cycle is repeated down the length of the axon, and the current "jumps" from one node to the next (in fact, "saltatory" comes from the Latin word saltare, which means "leaping"). Compare these "leaping" action potentials with the much slower and more gradual continuous conduction seen in the unmyelinated axon. Figure 11.16 shows that in continuous conduction, absence of the myelin sheath means that each section of the axolemma must be depolarized to threshold. This is why conduction of this sort is called continuous—action potentials must be generated in a continuous sequence along the entire axolemma for the current to spread down the length of the axon. The flow of current in a myelinated axon is much faster than the process of triggering action potentials in each part of an unmyelinated axon..

neuronal synapses

These types are called axodendritic, axosomatic, and axoaxonic synapses, respectively

states of voltage-gated channels

Two types of voltage-gated channels are involved in the depolarization and repolarization of the action potential—one for sodium ions and one for potassium ions. Voltage-gated channels are found most abundantly in the axolemma of the neuron, which is why only axons have action potentials. The structures of voltage-gated potassium and sodium ion channels are depicted in Figure 11.12. Notice in Figure 11.12a that the voltage-gated potassium ion channel has two possible states: resting and activated. In the resting state, the channel is closed. In the activated state, the channel is open and allows potassium ions to cross the axolemma. Figure 11.12

post synaptic potential

a positive or negative change in the membrane potential of a neuron as a result of synaptic transmission

synaptic cleft

Whereas the cells of an electrical synapse are electrically connected by gap junctions, the cells of a chemical synapse are separated by a larger but still microscopic space called the synaptic cleft. The synaptic cleft measures 20-50 nm and is filled with extracellular fluid and proteins such as enzymes.

ganglia

Within the PNS, clusters of cell bodies In addition, axons tend to be bundled together in the CNS and the PNS. In the CNS, these bundles are referred to as tracts, and in the PNS, as nerves.

local potential

a small, local change in the membrane potential of the neuron, A local potential may have one of two effects: It may cause a depolarization, in which positive charges enter the cytosol and make the membrane potential less negative (e.g., a change from −70 to −60 mV). Alternatively, it may cause a hyperpolarization, in which either positive charges exit or negative charges enter the cytosol to make the membrane potential more negative (e.g., a change from −70 to −80 mV). Local potentials are sometimes called graded potentials because they vary greatly in size—some produce a larger change in membrane potential than others. The degree of change in the membrane potential during a local potential depends on multiple factors, including the length of stimulation, number of ion channels that open, and type(s) of ion channels that open. Another feature of local potentials is that they are reversible; when the stimulus that caused the ion channels to open stops, the neuron quickly returns to its resting potential. Local potentials are also decremental in nature: The changes in membrane potential they produce are small, and the current generated is lost across the membrane over the distance of a few millimeters. Consequently, local potentials cannot send signals over great distances, and are useful for short-distance signaling only (which is why they're called local potentials). However, even though they occur only over short distances, we will see in the next section that local potentials are vital triggers for action potentials, our long-distance signals.

chemical synapse

a type of synapse in which presynaptic neuron releases neurotransmitters to trigger a change in a postsynaptic neuron These synapses are more common because they are more efficient—the current in electrical synapses eventually becomes weaker as it dissipates into the extracellular fluid. In contrast, a chemical synapse converts an electrical signal into a controlled chemical signal, so no strength is lost. The chemical signal is reconverted into an electrical signal in the postsynaptic neuron. Recall that we saw this same pattern in the interaction between a motor neuron and a muscle fiber: The electrical signal of the neuronal action potential is converted into the chemical signal of acetylcholine, which is then converted back into the electrical signal of the muscle action potential. In the upcoming sections, we explore how this takes place in the neuron. But first let's look a little more closely at the differences between chemical and electrical synapses.

spinal cord

organ of central nervous system that connects the brain with the peripheral nervous system and performs certain integrative functions.

afferent neurons (sensory neurons)

arry signals toward the central nervous system. The sensory receptors of these neurons detect stimuli, and the electrical changes are transmitted to their cell bodies in the PNS, then down their axons to the brain or spinal cord. They are generally pseudounipolar or bipolar in structure because they receive stimuli from only one area. Sensory neurons detect the internal and external environments (such as from the skin and viscera) and facilitate motor coordination (such as in joints and muscles).

visceral motor division

autonomic nervous system (aw-toh-NAHM-i​k; ANS), ​consists of neurons that carry signals primarily to thoracic and abdominal viscera. The ANS regulates secretion from certain glands, the contraction of smooth muscle, and the contraction of cardiac muscle in the heart. These functions are not generally under voluntary control, so the ANS is sometimes called the involuntary motor division. The ANS, which is very important for maintaining homeostasis of the internal environment,

CNS (central nervous system)

brain and spinal cord

motor neurons (efferent)

carry stimuli away from their cell bodies in the CNS to muscles and glands. Most motor neurons are multipolar, as motor tasks are generally complicated and require input from many other neurons.

Events at a chemical synapse

channels in the axon terminal to open. An action potential reaches the axon terminal of the presynaptic neuron, which triggers the opening of voltage-gated calcium ion channels in its axolemma. ❷❷ Influx of calcium ions causes synaptic vesicles to release neurotransmitters into the synaptic cleft. Calcium ions enter the axon terminal, causing synaptic vesicles in the area to fuse with the presynaptic membrane. This releases neurotransmitters into the synaptic cleft via exocytosis. ❸❸ Neurotransmitters bind to receptors on the postsynaptic neuron. The neurotransmitters diffuse across the synaptic cleft, where they bind to neurotransmitter receptors on the membrane of the postsynaptic neuron. ❹❹ Ion channels open, leading to a local potential and possibly an action potential. The binding of neurotransmitters to receptors generally either opens or closes ligand-gated ion channels in the postsynaptic membrane, resulting in a local potential. Such local potentials may or may not lead to an action potential in the postsynaptic neuron.

ependymal cells

ciliated cells with a variety of functions. One of their main functions is circulating cerebrospinal fluid, which is the fluid in the cavities of the brain and spinal cord. Certain ependymal cells also play a role in the formation of this fluid, and others are thought to monitor its composition. Neuroglia in the PNS In the PNS the two types of neuroglia are Schwann cells and satellite cells (Figure 11.7). Like those in the CNS, the neuroglia of the PNS serve supportive and protective functions, with, once again, their form specialized for their function.

white matter

collection of myelinated axons that appear white

cyclic adenosine monophosphate (cAMP)

common second messenger is cyclic adenosine monophosphate ​(or cAMP),​ which is derived from ATP. In the neuron, cAMP has multiple functions, including binding a group of enzymes that catalyze reactions adding phosphate groups to ligand-gated ion channels, triggering them to open or close. Second messengers are covered more fully in the endocrine chapter

visceral sensory division

consists of neurons that transmit signals from viscera (organs) such as the heart, lungs, stomach, intestines, kidneys, and urinary bladder.

somatic motor division

consists of neurons that transmit signals to skeletal muscles. Skeletal muscle tissue is under conscious control, so this division is sometimes referred to as the voluntary motor division.

Peripheral Nervous System (PNS)

cranial nerves and spinal nerves The peripheral nervous system is made up of the most numerous organs of the nervous system, the Nerves, which carry signals to and from the central nervous system. A nerve consists of a bundle of long neuron "arms" known as axons that are packaged together with blood vessels and surrounded by connective tissue sheaths. Nerves are classified according to their origin or destination: Those originating from or traveling to the brain are called cranial nerves, and those originating from or traveling to the spinal cord are called spinal nerves (see Figure 11.1). There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. The PNS has separate functional divisions, which we discuss next.

internodes

segments of an axon that are covered by neuroglia

membrane potential

e electrical gradient across the plasma membrane is known as a membrane potential, named for the fact that, like any gradient, an electrical gradient is a source of potential energy for the cell.

Acetylcholine (ACh)

e small-molecule neurotransmitter acetylcholine (ACh) (ah-seet'l-KOH-leen). Synapses that use ACh, called cholinergic synapses, are located at the neuromuscular junction, within the brain and spinal cord, and within the autonomic nervous system (ANS). Its effects are largely excitatory; however, it does exhibit inhibitory effects at some PNS synapses. ACh is synthesized from the precursors choline and acetyl-CoA (an acetic acid molecule bound to coenzyme A) and then packaged into synaptic vesicles. Once ACh is released from the synaptic vesicles, its activity is rapidly terminated by an enzyme in the synaptic cleft known as acetylcholinesterase (AChE; ah-seetʼ1′-koh-leh-NESS-ter-ayz). AChE degrades ACh back into acetic acid and choline. The presynaptic neuron then takes the choline back up, to be used in the synthesis of new ACh molecules.

psuedounipolar neurons

egin developmentally as bipolar neurons, but their two processes fuse to give rise to a single axon. As the axon extends from the cell body, it splits into two processes: one that brings stimuli from sensory receptors to the cell body, called the peripheral process or axon, and one that travels to the spinal cord away from the cell body, called the central process or axon. The pseudounipolar neurons are sensory neurons that detect stimuli such as touch, pressure, and pain.

neuron

excitable cell type responsible for sending and receiving signals in the form of action potentials. call that most neurons are generally amitotic, meaning that at a certain point in development, they lose their centrioles and after that lack the ability to undergo mitosis (see Chapter 4). Luckily, neurons are very long-lived cells, and some can easily survive the entire lifespan of an organism if given adequate nutrition and oxygen in a supportive environment. Neurons vary greatly in size. Some tiny neurons in the CNS are only 1 mm long, whereas some PNS neurons may be up to 1 m or longer. As Figure 11.5 shows, most neurons consist of three parts: the central cell body, where the majority of the biosynthetic processes of the cell occur; one or more dendrites, which carry electrical signals to the cell body; and one axon, the long "arm" that generally carries electrical signals away from the cell body. Let's examine each of these parts in greater detail. Note that we discuss the electrophysiology of neurons in Module 11.3.

satellite cells

flat cells that surround the cell bodies of neurons in the PNS (see Figure 11.7, right). The most poorly understood of the neuroglia, they appear to enclose and support the cell bodies, and have intertwined processes that link them with other parts of the neuron, other satellite cells, and also neighboring Schwann cells. They also appear to regulate the extracellular environment around the neuronal cell body, a function analogous to that of astrocytes in the CNS.

brain merges with spinal cord where?

foramen magnum At the foramen magnum, the brain merges with the other organ of the central nervous system: the spinal cord. The spinal cord passes through the vertebral foramen of the first cervical vertebra and continues inferiorly to the first or second lumbar vertebra (see Chapter 7). It contains fewer cells than the brain, with only about 100 million neurons. The spinal cord enables the brain to communicate with most parts of the body below the head and neck; it is also able to carry out certain functions on its own (which are discussed in later chapters).

neuroglial cells or neuroglia

generally does not transmit signals but rather serves a variety of supportive functions.

neuronal pools

groups of interneurons within the CNS. These pools typically are a tangled mat of neuroglial cells, dendrites, and axons in the brain, whereas their cell bodies may lie in other parts of the CNS. The type of information that can be processed by a pool is defined by the synaptic connections of that pool. The connections between pools allow for complex mental activity such as planned movement, cognition, and personality. Each neuronal pool begins with one or more neurons called input neurons that initiate the series of signals. The input neuron branches repeatedly to serve multiple neurons in the pool; however, it may have different effects on different neurons. For some neurons, it may generate EPSPs that trigger an action potential, and for others, it may simply bring the trigger zone closer to threshold. This difference is determined by the number of contacts the input neuron makes with the postsynaptic neuron. A small neuronal pool with one input neuron and its postsynaptic neurons is illustrated in Figure 11.27. You can see that the postsynaptic neurons in the center (surrounded by green) have the highest number of synaptic contacts with the input neuron. Because of these connections, spatial summation is possible and the firing of the input neuron is likely to generate adequate EPSPs to trigger an action potential. Notice, however, that the neurons in the light orange area on either side have fewer synaptic contacts with the input neuron. As a result, the input neuron acting alone will not be able to bring the trigger zones of these neurons to threshold and elicit action potentials. However, it can help another input neuron trigger action potentials. Until now we've been discussing only excitatory input; however, remember that inhibitory synapses occur as well. The degrees of inhibition also correlate strongly with the number of synaptic contacts. Action potentials are effectively prevented in the postsynaptic neurons that receive the greatest number of IPSPs from the input neuron.

bipolar neurons

has only two processes: one axon and one dendrite. In humans the majority of bipolar neurons are sensory neurons, located in places such as the retina of the eye and the olfactory epithelium of the nasal cavity.

neurotransmitters induce IPSPS

have inhibitory effects. Most neurotransmitters can have both excitatory and inhibitory effects, depending on which postsynaptic neuron receptors they bind. In fact, a single neurotransmitter can have several receptor types. This makes a purely functional classification of neurotransmitters difficult. For this reason, the major neurotransmitters operating within the nervous system are usually classified into four groups by their chemical structures, which we will now explore.

Oligodendrocytes

have radiating processes, but they are fewer in number and smaller than those of astrocytes. The flattened ends of some of these processes wrap around part of the axons of certain neurons. These wrapped processes form concentric layers of plasma membrane that are collectively called myelin (MY-eh-lin). Repeating segments of myelin along the length of an axon form the myelin sheath. As you can see in Figure 11.6, each oligodendrocyte has several of these processes that wrap around multiple axons. We consider the formation of the myelin sheath and its functional significance later in this module.

Resting Membrane Potential (RMP)

here is a separation of charges across the plasma membrane—there is a thin layer of negative charges in the cytosol lining the inside of the membrane and a thin layer of positive charges in the extracellular fluid lining the outside of the membrane, as you can see here: When you measure the membrane potential of a neuron at rest, or the Resting membrane potential, it measures about −70 mV. Recall that the resting membrane potential is negative because the cell constantly loses small numbers of positively charged potassium ions. The potassium ions are able to exit the cell due to the presence of proteins in the plasma membrane known as leak channels, which are always open. This continual loss of positive charges makes the cytosol on the inside of the membrane negative with respect to the extracellular fluid. You may have noticed that the resting membrane potential for a neuron is less negative at −70 mV than that of a skeletal muscle fiber at −90 mV. This is largely due to the number of potassium ion leak channels in the skeletal muscle fiber membrane—the skeletal muscle fiber loses more positive charges and so has a more negative resting membrane potential. A cell at its resting membrane potential is​ polarized​. The word polarized simply means that the membrane potential is at the negative or positive side (think of the negative and positive "poles" of a magnet). The closer the membrane potential comes to 0 mV, the less polarized the membrane potential becomes. As we discuss shortly, changes of this sort are responsible for the electrical events of a neuron.

sensory functions

involve gathering information about the internal and external environments of the body. gathered by the sensory, or afferent, division (AF-er-ent; "carrying toward") of the PNS

spatial summation

involves the simultaneous release of neurotransmitters from the axon terminals of multiple presynaptic neurons (Figure 11.22b). Notice the difference between the graphs of spatial and temporal summation in Figure 11.22. The graph of temporal summation shows a staircase-like rise in membrane potential as the postsynaptic neuron is hit with successive bursts of neurotransmitters. In contrast, the membrane potential in the graph of spatial summation shows a smooth rise as large quantities of neurotransmitters are released at once. Spatial summation can combine with temporal summation. When several presynaptic neurons fire together and trigger EPSPs in the postsynaptic neuron, spatial summation occurs and the membrane potential at the trigger zone approaches threshold. The closer the membrane potential gets to threshold, the more likely it becomes that the next EPSP will trigger an action potential due to temporal summation, even if the stimulus is smaller. Although we have discussed summation of EPSPs, IPSPs can summate both temporally and spatially as well. With summation of IPSPs, the postsynaptic neuron becomes less and less likely to fire an action potential. Additionally, IPSPs and EPSPs can summate. The overall result of this will depend on the individual strength of the IPSP and EPSP—if the IPSP is stronger, the membrane potential will hyperpolarize slightly, and if the EPSP is stronger, the membrane potential will depolarize slightly.

biogenic amines

lass of neurotransmitters synthesized from amino acids. Most biogenic amines are widely used by the CNS and the PNS, and have diverse functions including maintenance of homeostasis and cognition (thinking). The biogenic amines are implicated in a wide variety of psychiatric disorders and are often the targets of drug therapy for these disorders. Three of the biogenic amines form a subgroup called the catecholamines (kat′-eh-KOHL-ah-meenz), all of which are synthesized from the amino acid tyrosine and share a similar chemical structure. Though many of their synapses are excitatory, like most neurotransmitters, catecholamines can cause inhibition as well. The three catecholamines are as follows: Norepinephrine. Norepinephrine (nor′-ep-ih-NEF-rin; also called noradrenalin) is widely used by the ANS, where it influences functions such as heart rate, blood pressure, and digestion. Neurons that secrete norepinephrine in the CNS are largely confined to the brainstem, where they work to regulate the sleep/wake cycle, attention, and feeding behaviors. Epinephrine. Epinephrine (also called adrenalin) is also used by the ANS, where it has the same effects as norepinephrine. However, it is more widely used as a hormone by the endocrine system (see Chapter 16 for details). Dopamine. Dopamine, used extensively in the CNS, has a variety of functions. It helps to coordinate movement, and is also involved in emotion and motivation. The receptor for dopamine in the brain is a target for certain illegal drugs, such as cocaine and amphetamine, and is likely responsible for the behavioral changes seen with addiction to these drugs. Another biogenic amine is serotonin (sehr-oh-TOH-nin), which is synthesized from the amino acid tryptophan. Most neurons that use serotonin are found in the brainstem, and their axons project to multiple places in the brain. Serotonin is thought to be one of the major neurotransmitters involved in mood regulation (likely along with norepinephrine), and it is a common target in the treatment of depression. Additionally, serotonin acts to affect emotions, attention and other cognitive functions, motor behaviors, feeding behaviors, and daily rhythms. The final biogenic amine we'll discuss is histamine (HISS-tah-meen), which is synthesized from the amino acid histidine. Histamine is involved in a large number of processes in the CNS, including regulation of arousal and attention. In addition, outside the nervous system, histamine is an important mediator of allergic responses. Drugs called antihistamines block histamine receptors outside the nervous system to alleviate allergy symptoms, but most also block histamine receptors in the CNS. As histamine plays a part in arousal, blocking its actions often leads to the common side effect of drowsiness seen with these drugs.

gray matter

made up primarily of cell bodies and dendrites, which are never myelinated, as well as small unmyelinated axons.

relative refractory period

mmediately following the absolute refractory period is the relative refractory period, during which only a strong stimulus will produce an action potential. The relative refractory period is marked by a return of voltage-gated sodium ion channels to their resting state while some potassium ion channels remain activated. It's difficult to depolarize the membrane to threshold during this period because the potassium ion channels are activated and the membrane is repolarizing or even hyperpolarizing. However, if a greater than normal stimulus is applied, the membrane may depolarize to threshold, and the axon may fire off another action potential. The absolute and relative refractory periods limit the frequency of action potential production. In addition, the relative refractory period ensures that stronger stimuli trigger more frequent action potentials.

voltage-gated sodium ion channels

more complicated. It has two gates: an activation gate and an inactivation gate. This means a sodium ion channel has three potential "states": Resting state: Inactivation gate opened, activation gate closed. During the resting state the neuron is not being stimulated, and the activation gate is closed and the inactivation gate is open. No sodium ions cross the membrane when the channel is in the resting state. Activated state: Both activation and inactivation gates opened. When an action potential is initiated, the voltage change opens the activation gates and the channel is in its activated state. The channel in the activated state allows sodium ions to cross the axolemma. Inactivated state: Inactivation gate closed, activation gate opened. When the inactivation gate closes, the channel is in its inactivated state. The channel in this state no longer allows sodium ions to pass through. Notice that during this state, the activation gate remains open. When the action potential is finished, the channel returns to the resting state.

Neuotransmitter receptors

nduce postsynaptic potentials by binding to their receptors in the postsynaptic membrane. The type of receptor to which a neurotransmitter binds determines the postsynaptic response. Two types of neurotransmitter receptors have been identified: ionotropic and metabotropic (Figure 11.26): Ionotropic receptors (aye-AHN-oh-troh′-pik) are simply receptors that are part of ligand-gated ion channels. They are called ionotropic because they directly control the movement of ions into or out of the neuron when bound by a neurotransmitter. Neurotransmitters that bind ionotropic receptors have very rapid but short-lived effects on the membrane potential of the postsynaptic neuron. Metabotropic receptors (meh-TAB-oh-troh′-pik) are receptors within the plasma membrane that are connected to a separate ion channel in some fashion. They are called metabotropic because they are directly connected to metabolic processes that begin when they are bound by neurotransmitters. Most are connected through a group of intracellular enzymes called G-proteins. When the neurotransmitter molecule binds to the receptor, it activates one or more G-proteins and begins a cascade of enzyme-catalyzed reactions. The end result of the cascade is the formation of a compound inside the postsynaptic neuron called a second messenger (in this system the neurotransmitter molecule is considered the "first messenger"). The second messenger then opens or closes a ligand-gated ion channel in the plasma membrane of the postsynaptic neuron.

Schwann cells

neuroglial cell of the PNS that myelinates the axons of certain neurons

neuroglia

neuroglial cells, were named for the early scientific idea that these cells "glued together" the neurons, as the word root glia means "glue." However, we now recognize that neuroglia also serve many more functions. Examples include maintaining the environment around neurons, protecting them, and assisting in their proper functioning. Unlike the mostly amitotic neurons, neuroglia retain their ability to divide, and they fill in gaps left when neurons die. Six different types of neuroglia can be found in the nervous system, four in the CNS and two in the PNS. Like all cells we've covered, the form of each type of neuroglial cell is specialized for its function, another example of the Structure-Function Core Principle (Module 1.5.5). Keep this in mind as we examine the six types of cells.

receptive region

neuron consists of the dendrites and cell body. The dendrites may receive signals from other neurons, or may monitor the external and internal environments via sensory receptors. The received signals are collected in the cell body, which then may transmit a signal to the axon, the conducting region of the neuron. When the signal reaches the axon terminals of the secretory region, they secrete chemicals that trigger changes in their target cells.

postsynaptic neuron

neuron that is receiving the message from its dendrite, cell body, or axon

absolute refractory period

no additional stimulus, no matter how strong, is able to produce an additional action potential. Notice in Figure 11.14 that this period coincides with the voltage-gated sodium ion channels being in their activated and inactivated states; sodium ion channels may not be activated until they return to their resting states with their activation gates closed and their inactivation gates open.

electrical synapse

occurs between cells that are electrically coupled via gap junctions. In these synapses, the axolemmas of the two neurons are nearly touching (they are separated by only about 3.5 nm) and the gap junctions contain precisely aligned channels that form pores through which ions and other small substances may travel. This allows the electric current to flow directly from the axoplasm of one neuron to that of the next.

ligand-gated channels

open in response to a certain chemical, called a ligand, binding to the channel or to a receptor associated with the channel.

mechanically gated channels

open or close in response to mechanical stimulation such as stretch, pressure, and vibration.

brain

organ of central nervous system that performs multiple integrative functions The organ of the central nervous system that is likely most familiar to you, yet still holds the greatest mysteries for physiologists, is the brain. Enclosed completely by the skull, the brain is composed primarily of nervous tissue. This remarkable organ consists of about 100 billion cells called neurons (NOOR-onz), or nerve cells, that enable everything from the regulation of breathing and the processing of algebra to performing in the creative arts. The cells that make up nervous tissue are discussed in Module 11.2.

Nerves

organs of peripheral nervous system that consist of bundles of axons, connective tissue, sheates, and blood vessels.

refractory period

period during which an excitable cell either can't respond to another stimulus (the absolute refractory period) or requires a stronger stimulus to respond ( the relative refractory period)

myelination

process of myelin sheath formation. During this process in the PNS, a Schwann cell wraps itself outward away from the axon in successively tighter bands, forming a myelin sheath up to 100 layers thick (see Figure 11.8a). The basic process is similar for an oligodendrocyte in the CNS. However, in the CNS the arms of an oligodendrocyte wrap inward toward the axon—the opposite direction from the Schwann cells

Interneurons (association neurons)

relay messages within the CNS, primarily between sensory and motor neurons, and are the location of most information processing. The vast majority of neurons are interneurons. Multipolar in structure, interneurons generally communicate with many other neurons (for example, one Purkinje cell [per-KIN-jee] of the cerebellum can receive as many as 150,000 contacts from other neurons).

axon

sometimes called a nerve fiber. Traditionally, an axon was defined as a process that carried a signal away from the cell body. However, the axons of certain neurons can carry a signal both toward and away from the cell body. For this reason, new criteria have been developed to define an axon: They are considered processes that can generate and conduct action potentials. Notice in Figure 11.5 that each axon arises from an area of the cell body called the Axon hillock, and then tapers to form the slender axon, which is often wrapped in the insulating myelin sheath. Depending on the type of neuron, the axon may range in length from short to very long; in some neurons the axon accounts for most of the length of the neuron. For example, the axons of motor neurons going to the foot must extend from the lumbar portion of the spinal cord all the way down to the foot. Extending from some axons are branches that typically arise at right angles to the axon, called axon collaterals. Both the axon and its collaterals split near their ends to produce multiple fine branches known as telodendria (tee′-loh-DEN-dree-ah). The telodendria terminate in axon terminals, or synaptic knobs, that communicate with a target cell. Each axon generally splits into 1000 or more axon terminals. The plasma membrane that envelops the axon is called the axolemma (aks-oh-LEM-ah), and its cytoplasm is known as axoplasm. Although dendrites have most of the same organelles as the cell body, axons do not. Axons contain mitochondria, abundant intermediate filaments, vesicles, and lysosomes; however, they do not contain protein-making organelles such as ribosomes or Golgi apparatus. The composition of the axoplasm is dynamic, as substances move both toward and away from the cell body along the axon's length. Substances may travel through the axoplasm using one of two types of transport, which are together termed axonal transport or flow: Slow axonal transport. Substances within the axoplasm, such as cytoskeletal proteins and other types of proteins, move by slow axonal transport. These substances move only away from the cell body and do so at a rate of about 1-3 mm/day. Fast axonal transport. Vesicles and membrane-enclosed organelles use fast axonal transport to travel much more rapidly through the axon. This type of transport relies on motor proteins in the axoplasm that consume ATP to move components along microtubules. Components may move either toward the cell body at a maximum rate of about 200 mm/day, a process called retrograde axonal transport, or away from the cell body at a maximum rate of about 400 mm/day, a process called anterograde axonal transport.

action potential

uniform, rapid depolarization and repolarization of the membrane potential of a cell (see Chapter 10). This change in the membrane potential causes a response—or action—of some sort. For a muscle fiber, the change initiates events that lead to muscle fiber contraction. Within the nervous system, signals are sent through an axon to another neuron, a muscle fiber, or a gland. Recall that only axons generate action potentials; dendrites and cell bodies generate local potentials only. Action potentials are generated in the initial segment of the axon; for this reason, we refer to this region as the trigger zone. In this section we look at what happens during an action potential. First, however, we need to delve deeper into the function of the voltage-gated channels that allow ions to move and change the membrane potential of the neuron.

regeneration PNS

the PNS is capable of regeneration to some extent, but only if the cell body remains intact. When a peripheral axon is damaged, the following sequence of events repairs the damaged neuron (Figure 11.10): ❶❶ The axon and myelin sheath distal to the injury degenerate. The damaged axon distal to the injury is cut off from the cell body where all the protein-synthesis machinery is housed. It therefore has no way to repair itself, and so this part of the axon, along with its myelin sheath, begins to degenerate. This occurs via a process called Wallerian degeneration (vah-LEHR-ee-an), in which phagocytes digest the cellular debris. ❷❷ Growth processes form from the proximal end of the axon. As Wallerian degeneration occurs, protein synthesis within the cell body increases, and several small growth processes sprout from the proximal end of the axon. ❸❸ Schwann cells and the basal lamina form a regeneration tube. Schwann cells near the site of the injury begin to proliferate along the length of a collagen-rich surrounding structure known as the basal lamina (also called the external lamina), which is made by connective tissue cells around the neuron. This forms a cylinder called the regeneration tube. ❹❹ A single growth process grows into the regeneration tube. Note in step 2 of Figure 11.10 that several growth processes form; however, only one will make it into the regeneration tube. In the tube, Schwann cells secrete growth factors that stimulate regrowth of the axon. The regeneration tube then guides the axon to grow toward its target cell at an average rate of about 1.5-3 mm/day. ❺❺ The axon is reconnected with the target cell. If the axon continues to grow, it most likely will meet up with its target cell and re-establish its synaptic contacts. Over time, the Schwann cells re-form the myelin sheath.

motor functions

the actions performed in response to integration Motor output is performed by the motor, or efferent, division (EE-fer-ent; "carrying away") of the PNS (remember this with the mnemonic "The Motor Efferent division moves ME"

synaptic delay

the short delay between the arrival of the action potential at the axon terminal of a presynaptic neuron and the postsynaptic potential of a postsynaptic neuron there is about a 0.5-ms gap between the arrival of the action potential at the axon terminal and the effects on the postsynaptic neuron's membrane Also, chemical synapses are unidirectional—the message can be sent only by the presynaptic neuron. However, these three structural differences also allow something not permitted by the structure of the electrical synapse: The signal can vary in size. If more neurotransmitters are released, then the presynaptic neuron has a greater effect on the postsynaptic neuron. The signal in an electrical synapse, by contrast, will always be the same size. In addition, the effect that the presynaptic neuron triggers can vary with different neurotransmitters and receptors.

all-or-none principle

this principle refers to an event, in this case an action potential, that either happens completely or doesn't happen at all. If a neuron does not depolarize to threshold, an action potential does not occur. If the neuron does depolarize to threshold, the result is an action potential of a characteristic strength. The size of the action potential is not determined by the strength, frequency, or length of the stimulus, and therefore is not graded like a local potential. The all-or-none principle leads us to a second difference between local potentials and action potentials: their reversibility. Recall that a local potential is reversible; once the stimulus stops, the ion channels close and the resting membrane potential is restored. However, a key feature of an action potential is that when one occurs, it is irreversible—once threshold is reached, it cannot be stopped and will proceed to completion. Finally, a third important difference between local potentials and action potentials is the distance the signal must travel. Whereas local potentials are decremental and decrease over short distances, action potentials are nondecremental; that is, their strength does not diminish. This property of action potentials is key, as otherwise, signals could not be sent over long distances in the nervous system.

amino acid neurotransmitters

three major amino acid neurotransmitters: glutamate; glycine; and γ-aminobutyric acid, or GABA. Glutamate is the most important excitatory neurotransmitter in the brain—most neurons in the brain are thought to have at least one type of receptor for it. There are both ionotropic and metabotropic receptors for glutamate; both generally lead to the opening of sodium or calcium ion channels and the production of EPSPs. Glycine and GABA are the two major inhibitory neurotransmitters of the nervous system. Both induce IPSPs in the postsynaptic neurons, primarily by opening ligand-gated chloride ion channels and hyperpolarizing the axolemma. GABA is the most important inhibitory neurotransmitter in the brain, whereas glycine is the most widely used inhibitory neurotransmitter in the spinal cord.

neuropeptides

three major amino acid neurotransmitters: glutamate; glycine; and γ-aminobutyric acid, or GABA. Glutamate is the most important excitatory neurotransmitter in the brain—most neurons in the brain are thought to have at least one type of receptor for it. There are both ionotropic and metabotropic receptors for glutamate; both generally lead to the opening of sodium or calcium ion channels and the production of EPSPs. Glycine and GABA are the two major inhibitory neurotransmitters of the nervous system. Both induce IPSPs in the postsynaptic neurons, primarily by opening ligand-gated chloride ion channels and hyperpolarizing the axolemma. GABA is the most important inhibitory neurotransmitter in the brain, whereas glycine is the most widely used inhibitory neurotransmitter in the spinal cord.

action potential proceeds as follows

threshold. The action potential begins when the voltage-gated sodium ion channels in the axolemma of the trigger zone enter the activated (open) state (see Figure 11.12b). However, these voltage-gated channels will become activated only if the membrane is already depolarized to a level known as threshold, usually −55 mV. The source of this depolarization is generally local potentials that arrive from the cell body. ❷❷ Voltage-gated sodium ion channels activate, sodium ions enter, and the axon section depolarizes. When threshold is reached, the sodium ion channels in the trigger zone are activated (open) and sodium ions rush into the neuron with their electrochemical gradient. As the membrane potential becomes more positive, more voltage-gated sodium ion channels are activated. This cycle continues, and the more the axon depolarizes, the more voltage-gated sodium ion channels are activated. This influx of positive charges causes rapid depolarization to about +30 mV. You may recognize this as an example of a positive feedback loop—the initial input (activation of sodium ion channels and depolarization) amplifies the output (more sodium ion channels are activated and the axolemma depolarizes further), an example of the Feedback Loops Core Principle (Module 1.5.5). ❸❸ Sodium ion channels inactivate and voltage-gated potassium ion channels activate, and repolarization begins. When the axolemma is fully depolarized (about +30 mV), the inactivation gates of the voltage-gated sodium ion channels close, and sodium ions stop entering the axon. As this occurs, voltage-gated potassium ion channels slowly open and potassium ions flow out of the axon along their electrochemical gradient, causing the axolemma of the trigger zone to lose positive charges and so to begin repolarization. ❹❹ Sodium ion channels return to the resting state and repolarization continues. As potassium ions exit the axon and repolarization continues, the activation gates of the sodium ion channels close and the inactivation gates open, returning the sodium ion channels to their resting state. ❺❺ The axolemma may hyperpolarize before potassium ion channels return to the resting state; after this, the axolemma returns to the resting membrane potential. In many axons, the outflow of potassium ions continues until the membrane potential of the axolemma hyperpolarizes, possibly becoming as negative as −90 mV. The axolemma hyperpolarizes because the gates of the potassium ion channels are slow to close, allowing additional potassium ions to leak out of the cell. Hyperpolarization finishes as the voltage-gated potassium ion channels return to their resting state. After the action potential, the potassium leak channels and Na+/K+ pumps re-establish the resting membrane potential.

synaptic transmission

transfer of chemical or electrical signals between neurons at a synapse it is the fundamental process for most functions of the nervous system. Synaptic transmission allows voluntary movement, cognition, sensation, and emotion, as well as countless other processes. Each neuron has an enormous number of synapses. Recall from Module 11.2 that each axon generally splits into 1000 or more axon terminals, and each terminal meets up with another axon, dendrite, or cell body. An average presynaptic neuron, then, generally forms synapses with about 1000 postsynaptic neurons. A postsynaptic neuron can receive input from even more synapses—an average neuron can have as many as 10,000 synaptic connections from different presynaptic neurons.

dendrites

typically short, highly forked processes that resemble the branches of a tree limb. They receive input from other neurons, which they transmit in the form of electrical impulses toward the cell body. Note, however, that dendrites by definition do not generate or conduct action potentials. Their cytoplasm contains most of the same organelles as the cell body, including mitochondria, ribosomes, and smooth endoplasmic reticulum. The extensively forked "dendritic trees" of most neurons give them a huge receptive surface area. Interestingly, the branches of the dendritic tree change throughout an individual's lifetime: They grow and are "pruned" as a person develops and matures and as functional demands on the nervous system change.

synapse

where a neuron meets its target cell

Event Propagation

❶❶ The axolemma depolarizes to threshold due to local potentials. The axolemma of the trigger zone is depolarized to threshold by local potentials from the dendrites, cell body, or axon. ❷❷ As sodium ion channels activate, an action potential is triggered and spreads positive charges down the axon. Voltage-gated sodium ion channels are activated (open) and an action potential occurs. When this happens, positive charges flow down the axon through the axoplasm. ❸❸ The next section of the axolemma depolarizes to threshold and fires an action potential as the previous section of the axolemma repolarizes. As the depolarizing current reaches the next section of the axolemma, it depolarizes that section to threshold. The voltage-gated sodium ion channels in that part of the axolemma are activated, and that section then generates an action potential. The current then flows down to the next section of the axon. Note that the section of the axolemma that had an action potential in step 2 is repolarizing (its potassium ion channels are activated and potassium ions are exiting the axoplasm) and is in its refractory period, so any current that flows backward can't trigger an action potential. ❹❹ The current continues to move down the axon, and the process repeats. The current flowing into the next section of the axolemma causes it to depolarize to threshold, activating voltage-gated sodium ion channels and producing an action potential there.