CH 12 - Nervous Tissue

(X) is the ability to hold something in mind for just a few seconds. By remembering what just happened, we get a feeling for the flow of events and a sense of the present. Immediate memory is indispensable to the ability to read;

Immediate memory

(X) is the memory of things that come reflexively or unconsciously, including emotional memories (such as the fear of being stung if a wasp lands on you) and procedural memory, the retention of motor skills—how to tie your shoes, play a musical instrument, or type on a keyboard.

Implicit memory

(X) lasts up to a lifetime and is less limited than STM in the amount of information it can store.

Long-term memory (LTM)

Neuroscientists still argue about how to classify the various forms of memory, but three kinds often recognized are

immediate memory, short-term memory, and long-term memory.

Immediate and short-term memories vanish simply as neural circuits cease to fire. Long-term memories can be erased by a process of (X)

long-term depression (LTD).

The control center of the neuron is the

neurosoma, also called the soma or cell body.

During an action potential and for a few milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again. This period of resistance to restimulation is called the (X)

refractory period.

Schwann cells, the basal lamina, and the neurilemma form a (X)

regeneration tube.

The peripheral nervous system is functionally divided into

sensory and motor divisions, and each of these is further divided into somatic and visceral subdivisions.

At the distal end, an axon usually has a (X)—an extensive complex of fine branches.

terminal arborization

A 20 to 40 nm gap between neurons

the synaptic cleft

Nerve fibers of the CNS have no neurilemma or endoneurium. The contrasting modes of myelination are called (X) ("away from the center") in the PNS and ("toward the center") in the CNS.

X = centrifugal myelination Y = centripetal myelination

The (X) division carries signals to the skeletal muscles. This produces voluntary muscle contractions as well as involuntary somatic reflexes.

somatic motor

The sensory (afferent) division carries signals from (X) to the CNS. This pathway informs the CNS of stimuli within and around the body.

various receptors (sense organs and simple sensory nerve endings)

The resting membrane potential (RMP) results from the combined effect of three factors:

(1) the diffusion of ions down their concentration gradients through the membrane; (2) selective permeability of the membrane, allowing some ions to pass more easily than others; and (3) the electrical attraction of cations and anions to each other.

Any voltage change in that direction makes a neuron more likely to fire and is therefore called an (X)

excitatory postsynaptic potential (EPSP). EPSPs usually result from Na+ flowing into the cell and neutralizing some of the negative charge on the inside of the membrane.

There are two forms of long-term memory:

explicit and implicit.

Saltatory conduction is based on a process that is very fast in the internodes (transfer of energy from ion to ion), but decremental. In the nodes, conduction is slower but nondecremental. Since most of the axon is covered with myelin, conduction occurs mainly by the (X). This is why myelinated fibers conduct signals much faster (up to 120 m/s) than unmyelinated ones (up to 2 m/s).

fast internodal process

The physical basis of memory is a pathway through the brain called a (X), in which new synapses have formed or existing synapses have been modified to make transmission easier. In other words, synapses are not fixed for life; in response to experience, they can be added, taken away, or modified to make transmission easier or harder

memory trace (engram)

Long term memory LTM can also be grounded in molecular changes called (X)

long-term potentiation (LTP) - This involves NMDA receptors, which are glutamate-binding receptors found on the dendritic spines of pyramidal cells.

The (X) is a spiral layer of insulation around a nerve fiber, formed by oligodendrocytes in the CNS and Schwann cells in the PNS.

myelin sheath

Types of Glial cells - Neuroglia of PNS - Surround somas of neurons in the ganglia; provide electrical insulation and regulate chemical environment of neurons

Satellite cells - surround the somas in ganglia of the PNS. They provide insulation around the soma and regulate the chemical environment of the neurons.

(X) lasts from a few seconds to a few hours. Information stored in STM may be quickly forgotten if you stop mentally reciting it, you are distracted, or you have to remember something new.

Short-term memory (STM)

(X) is the process of adding up postsynaptic potentials and responding to their net effect. It occurs in the trigger zone.

Summation

The (X) division carries signals from receptors in the skin, muscles, bones, and joints.

somatic sensory

The (X) carries signals to glands, cardiac muscle, and smooth muscle.

visceral motor division (autonomic nervous system, ANS)

(X) neurons have only a single process leading away from the soma. They are represented by the neurons that carry signals to the spinal cord for such senses as touch and pain. They are also called pseudounipolar because they start out as bipolar neurons in the embryo, but their two processes fuse into one as the neuron matures.

Unipolar

An (X) employs the neurotransmitter norepinephrine (NE), also called noradrenaline. NE, other monoamines, and neuropeptides act through second-messenger systems such as cyclic AMP (cAMP).

adrenergic synapse

The presynaptic neuron may synapse with a dendrite, the soma, or the axon of a postsynaptic neuron, forming an (X)

axodendritic, axosomatic, or axoaxonic synapse, respectively

Each branch ends in a bulbous (X), which forms a junction (synapse) with the next cell.

axon terminal (terminal button)

Neurons routinely work in groups to modify each other's actions. (X) is a process in which one neuron enhances the effect of another.

Presynaptic facilitation

We have two organ systems dedicated to maintaining internal coordination— the (X) system, which communicates by means of chemical messengers (hormones) secreted into the blood, and the (Y) system, which employs electrical and chemical means to send messages very quickly from cell to cell.

X =endocrine Y = nervous

An (X) is a more dramatic change produced by voltage-gated ion channels in the plasma membrane.

action potential

An electrical (X) is a flow of charged particles from one point to another.

current

Local potentials can be either (X)

excitatory or inhibitory.

There are two types of axonal transport:

fast and slow

The major inclusions in a neuron are

glycogen granules, lipid droplets, melanin, and a golden brown pigment called lipofuscin, produced when lysosomes degrade worn-out organelles and other products.

Local potentials are (X), meaning they vary in magnitude (voltage) according to the strength of the stimulus.

graded - An intense or prolonged stimulus opens more gated ion channels than a weaker stimulus, and they stay open longer. Thus, more Na+ enters the cell and the voltage changes more than it does with a weaker stimulus.

Action potentials are (X). If a neuron reaches threshold, the action potential goes to completion; it cannot be stopped once it begins.

irreversible

The incoming Na+ diffuses for short distances along the inside of the plasma membrane, creating a wave of excitation that spreads out from the point of stimulation, like ripples spreading across a pond when you drop a stone into it. This short-range change in voltage is called a (X)

local potential.

The responses of the ANS and its effectors are visceral reflexes. The ANS has two further divisions:

• The sympathetic division tends to arouse the body for action—for example, by accelerating the heartbeat and increasing respiratory airflow—but it inhibits digestion. • The parasympathetic division tends to have a calming effect—slowing the heartbeat, for example—but it stimulates digestion.

Most neurotransmitters fall into the following categories:

1. Acetylcholine is in a class by itself. It is formed from acetic acid (acetate) and choline. 2. Amino acid neurotransmitters include glycine, glutamate, aspartate, and γ-aminobutyric acid (GABA). 3. Monoamines (biogenic amines) are synthesized from amino acids by removal of the —COOH group. They retain the —NH2 (amino group), hence their name. Some monoamine neurotransmitters are epinephrine, norepinephrine, dopamine, histamine, ATP, and serotonin (5-hydroxytryptamine, or 5-HT). The first three of these are in a subclass called catecholamines 4. Purines serving as neurotransmitters include adenosine and ATP (adenosine triphosphate). 5. Gases, specifically nitric oxide (NO) and carbon monoxide (CO), are inorganic exceptions to the usual definition of neurotransmitters. They are synthesized as needed rather than stored in synaptic vesicles; they simply diffuse out of the axon terminal rather than being released by exocytosis; and they diffuse into the postsynaptic neuron rather than bind to a surface receptor. 6. Neuropeptides are chains of 2 to 40 amino acids. Some examples are cholecystokinin (CCK) and the endorphins. Neuropeptides are stored in secretory granules (dense-core vesicles) that are about 100 nm in diameter, twice as large as typical synaptic vesicles. Some neuropeptides also function as hormones or as neuromodulators, whose action is discussed later in this chapter. Some are produced not only by neurons but also by the digestive tract; thus, they are known as gut-brain peptides. Some of these cause cravings for specific nutrients such as fat, protein, or carbohydrates and may be associated with certain eating disorders.

The communicative role of the nervous system is carried out by nerve cells, or neurons. These cells have three fundamental physiological properties that enable them to communicate with other cells:

1. Excitability. All cells are excitable—that is, they respond to environmental changes (stimuli). Neurons exhibit this property to the highest degree. 2. Conductivity. Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations. 3. Secretion. When the signal reaches the end of a nerve fiber, the neuron secretes a neurotransmitter that crosses the gap and stimulates the next cell.

The functions of a neural pool are partly determined by its neural circuit—the pathways among its neurons. Just as a wide variety of electronic devices are constructed from a relatively limited number of circuit types, a wide variety of neural functions result from the operation of four principal kinds of neural circuits:

1. In a diverging circuit, an individual neuron sends signals to multiple downstream neurons, or one neural pool may send output to multiple downstream neural pools. Each of those neurons or neural pools may communicate with several more, so input from just one pathway may produce output through hundreds of others. Such a circuit allows signals from one motor neuron of the brain, for example, to ultimately stimulate thousands of muscle fibers. 2. A converging circuit is the opposite of a diverging circuit— input from many nerve fibers or neural pools is funneled to fewer and fewer intermediate or output pathways. For example, you have a brainstem respiratory center that receives converging information from other parts of your brain, blood chemistry sensors in your arteries, and stretch receptors in your lungs. The respiratory center can then produce an output that takes all of these factors into account and sets an appropriate pattern of breathing. 3. In a reverberating circuit, neurons stimulate each other in a linear sequence from input to output neurons, but some of the neurons late in the path send axon collaterals back to neurons earlier in the path and restimulate them. As an exceedingly simplified model of such a circuit, consider a path such as A → B → C → D, in which neuron C sends an axon collateral back to A. As a result, every time C fires it not only stimulates output neuron D, but also restimulates A and starts the process over. Such a circuit produces a prolonged or repetitive effect that lasts until one or more neurons in the circuit fail to fire, or an inhibitory signal from another source stops one of them from firing. A reverberating circuit sends repetitious signals to your diaphragm and intercostal muscles, for example, to make you inhale. Sustained output from the circuit ensures that the respiratory muscles contract for the 2 seconds or so that it normally takes to fill the lungs. When the circuit stops firing, you exhale; the next time it fires, you inhale again. Reverberating circuits may also be involved in short-term memory, as discussed in the next section, and they may play a role in the uncontrolled "storms" of neural activity that occur in epilepsy. 4. In a parallel after-discharge circuit, an input neuron diverges to stimulate several chains of neurons. Each chain has a different number of synapses, but eventually they all reconverge on one or a few output neurons. Since the chains differ in total synaptic delay, their signals arrive at the output neurons at different times, and the output neurons may go on firing for some time after input has ceased. Unlike a reverberating circuit, this type has no feedback loop. Once all the neurons in the circuit have fired, the output ceases. Continued firing after the stimulus stops is called after-discharge. It explains why you can stare at a lamp, then close your eyes and continue to see an image of it for a while. Such a circuit is also important in withdrawal reflexes, in which a brief pain produces a longer-lasting output to the limb muscles and causes you to draw back your hand or foot from danger.

There are six kinds of neuroglia, each with a unique function. The first four types occur only in the central nervous system:

1. Oligodendrocytes somewhat resemble an octopus; they have a bulbous body with as many as 15 arms. Each arm reaches out to a nerve fiber and spirals around it like electrical tape wrapped repeatedly around a wire. This wrapping, called the myelin sheath, insulates the nerve fiber from the extracellular fluid. For reasons explained later, it speeds up signal conduction in the nerve fiber. 2. Ependymal cells resemble a cuboidal epithelium lining the internal cavities of the brain and spinal cord. Unlike true epithelial cells, however, they have no basement membrane and they exhibit rootlike processes that penetrate into the underlying tissue. Ependymal cells produce cerebrospinal fluid (CSF), a liquid that bathes the CNS and fills its internal cavities. They have patches of cilia on their apical surfaces that help to circulate the CSF. 3. Microglia are small macrophages that develop from white blood cells called monocytes. They wander through the CNS, putting out fingerlike extensions to constantly probe the tissue for cellular debris or other problems. They are thought to perform a complete checkup on the brain tissue several times a day, phagocytizing dead tissue, microorganisms, and other foreign matter. They become concentrated in areas damaged by infection, trauma, or stroke. Pathologists look for clusters of microglia in brain tissue as a clue to sites of injury. Microglia also aid in synaptic remodeling, changing the connections between neurons. 4. Astrocytes are the most abundant glial cells in the CNS and constitute over 90% of the tissue in some areas of the brain. They cover the entire brain surface and most nonsynaptic regions of the neurons in the gray matter. They are named for their many-branched, somewhat starlike shape. They have the most diverse functions of any glia:

Weak stimulation of a postsynaptic neuron may generate an EPSP, but it fades before reaching threshold. A typical EPSP is a voltage change of only 0.5 mV and lasts only 15 to 20 ms. If a neuron has an RMP of -70 mV and a threshold of -55 mV, it needs at least 30 EPSPs to reach threshold and fire. There are two ways in which EPSPs can add up to do this, and both may occur simultaneously:

1. Temporal summation. This occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades. This allows the EPSPs to add up over time to a threshold voltage that triggers an action potential (fig. 12.26). Temporal summation can occur if even one presynaptic neuron stimulates the postsynaptic neuron at a fast enough rate. 2. Spatial summation. This occurs when EPSPs from several synapses add up to threshold at the axon hillock. Any one synapse may generate only a weak signal, but several synapses acting together can bring the hillock to threshold. The presynaptic neurons collaborate to induce the postsynaptic neuron to fire.

Types of Glial cells - Neuroglia of CNS - Cover brain surface and nonsynaptic regions of neurons; form supportive framework in CNS; induce formation of blood-brain barrier; nourish neurons; produce growth factors that stimulate neurons; communicate electrically with neurons and may influence synaptic signaling; remove K+ and some neurotransmitters from ECF of brain and spinal cord; help to regulate composition of ECF; form scar tissue to replace damaged nervous tissue

Astrocytes - are the most abundant glial cells in the CNS and constitute over 90% of the tissue in some areas of the brain. They cover the entire brain surface and most nonsynaptic regions of the neurons in the gray matter. They are named for their many-branched, somewhat starlike shape. They have the most diverse functions of any glia

Types of Glial cells - Neuroglia of CNS - Line cavities of brain and spinal cord; secrete and circulate cerebrospinal fluid

Ependymal cells - resemble a cuboidal epithelium lining the internal cavities of the brain and spinal cord. Unlike true epithelial cells, however, they have no basement membrane and they exhibit rootlike processes that penetrate into the underlying tissue. Ependymal cells produce cerebrospinal fluid (CSF), a liquid that bathes the CNS and fills its internal cavities. They have patches of cilia on their apical surfaces that help to circulate the CSF.

(X) is the retention of events and facts that you can put into words—numbers, names, dates, and so forth. You must think to remember these things.

Explicit or declarative memory

(X) neurons are those, like the preceding, that have one axon and multiple dendrites. This is the most common type and includes most neurons of the brain and spinal cord.

Multipolar

Not all neurons fit the preceding description. Neurons are classified structurally according to the number of processes extending from the soma:

Multipolar neurons, bipolar neurons, unipolar neurons, and anaxonic neurons

Types of Glial cells - Neuroglia of PNS - Form neurilemma around all PNS nerve fibers and myelin around most of them; aid in regeneration of damaged nerve fibers

Schwann Cells - or neurilemmocytes, envelop nerve fibers of the PNS. In most cases, a Schwann cell winds repeatedly around a nerve fiber and produces a myelin sheath similar to the one produced by oligodendrocytes in the CNS. There are some important differences in myelin production between the CNS and PNS, which we consider shortly. Schwann cells also assist in the regeneration of damaged fibers

(X) is a form of STM that allows you to hold an idea in mind long enough to carry out an action such as calling a telephone number you just looked up, working out the steps of a mathematics problem, or searching for a lost set of keys while remembering where you have already looked.

Working memory

It is divided into two phases: an (X) in which no stimulus of any strength will trigger a new action potential, followed by a (Y) in which it is possible to trigger a new action potential, but only with an unusually strong stimulus

X = absolute refractory period Y = relative refractory period

Movement away from the soma down the axon is called (X) and movement up the axon toward the soma is called (Y)

X = anterograde transport Y = retrograde transport.

On one side of the neurosoma is a mound called the (X), from which the axon (nerve fiber) originates.

axon hillock - The axon is cylindrical and relatively unbranched for most of its length, although it may give rise to a few branches called axon collaterals near the soma, and most axons branch extensively at their distal end.

Other substances are transported from the axon terminals back to the soma for disposal or recycling. The two-way passage of proteins, organelles, and other materials along an axon is called

axonal transport.

An axon is specialized for rapid conduction of nerve signals to points remote from the soma. Its cytoplasm is called the (X) and its membrane the axolemma. A neuron never has more than one axon, and some neurons have none.

axoplasm

Sodium and potassium channels open and close just as they did at the trigger zone, and a new action potential is produced. By repetition, this excites the membrane immediately distal to that. This chain reaction continues until the traveling signal reaches the end of the axon. Because this produces an uninterrupted wave of electrical excitation all along the fiber, this mechanism is called (X)

continuous conduction.

Local potentials are (X), meaning they get weaker as they spread from the point of origin.

decremental - The decline in strength occurs partly because the Na+ leaks back out of the cell through channels along its path, and partly because as Na+ spreads out under the plasma membrane and depolarizes it, K+ flows out and reverses the effect of the Na+ inflow. Therefore, the voltage shift caused by Na+ diminishes rapidly with distance. This prevents local potentials from having long-distance effects.

The somas of most neurons give rise to a few thick processes that branch into a vast number of (X)—named for their striking resemblance to the bare branches of a tree in winter.

dendrites

The motor (efferent) division carries signals from the CNS mainly to gland and muscle cells that carry out the body's responses. Cells and organs that respond to these signals are called (X).

effectors

An (X) is a difference in the concentration of charged particles between one point and another. It is a form of potential energy that, under the right circumstances, can produce a current.

electrical potential

Some neurons, neuroglia, and cardiac and single-unit smooth muscle do indeed have (X), where adjacent cells are joined by gap junctions and ions diffuse directly from one cell into the next. These junctions have the advantage of quick transmission because there is no delay for the release and binding of neurotransmitter.

electrical synapses

These layers constitute the myelin sheath. The Schwann cell spirals outward as it wraps the nerve fiber, finally ending with a thick outermost coil called the neurilemma. Here, the bulging body of the Schwann cell contains its nucleus and most of its cytoplasm. External to the neurilemma is a basal lamina and then a thin sleeve of fibrous connective tissue called the (X)

endoneurium.

The neuropeptides are neuromodulators; among these are the (X)

enkephalins and endorphins - which inhibit spinal neurons from transmitting pain signals to the brain. Other neuromodulators include hormones and some neurotransmitters such as dopamine, serotonin, and histamine. The last point may seem confusing, but the terms neurotransmitter, hormone, and neuromodulator define not so much the chemical itself, but the role it plays in a given context.

A (X) is a knot like swelling in a nerve where the cell bodies of peripheral neurons are concentrated.

ganglion (plural, ganglia)

Local potentials can be either excitatory or inhibitory. So far, we have considered only excitatory local potentials, which depolarize a cell and make a neuron more likely to produce an action potential. Acetylcholine usually has this effect. Other neurotransmitters, such as glycine, cause an opposite effect—they (X) a cell, or make the membrane more negative. This inhibits a neuron, making it less sensitive and less likely to produce an action potential.

hyperpolarize

In other cases, a neurotransmitter hyperpolarizes the postsynaptic cell and makes it more negative than the RMP. Since this makes the postsynaptic cell less likely to fire, it is called an (X)

inhibitory postsynaptic potential (IPSP)

The most important mechanism for transmitting qualitative information is the (X)

labeled line code - This code is based on the fact that each nerve fiber to the brain leads from a receptor that specifically recognizes a particular stimulus type. Nerve fibers in the optic nerve, for example, carry signals only from light receptors in the eye; these fibers never carry information about taste or sound.

This wrapping, called the (X), insulates the nerve fiber from the extracellular fluid. For reasons explained later, it speeds up signal conduction in the nerve fiber.

myelin sheath

Production of the myelin sheath is called (X)

myelination - It begins in the fourteenth week of fetal development, yet hardly any myelin exists in the brain at the time of birth. Myelination proceeds rapidly in infancy and isn't completed until late adolescence. Since myelin has such a high lipid content, dietary fat is important to early nervous system development. It is best not to give children under 2 years old the sort of low-fat diets (skimmed milk, etc.) that may be beneficial to an adult.

A (X) is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue.

nerve

The functions of a neural pool are partly determined by its (X)

neural circuit —the pathways among its neurons.

The way in which the nervous system converts information to a meaningful pattern of action potentials is called (X)

neural coding (or sensory coding when it occurs in the sense organs).

The ability of your neurons to process information, store and recall it, and make decisions is called (X)

neural integration.

Neurons function in larger ensembles called

neural pools

In the PNS, a Schwann cell spirals repeatedly around a single nerve fiber, laying down up to 100 compact layers of its own membrane with almost no cytoplasm between the membranes. These layers constitute the myelin sheath. The Schwann cell spirals outward as it wraps the nerve fiber, finally ending with a thick outermost coil called the (X)

neurilemma

The study of the nervous system is called

neurobiology

The neuron cytoskeleton consists of a dense mesh of microtubules and (X), which compartmentalize the rough ER into dark staining regions called chromatophilic substance

neurofibrils (bundles of actin filaments)

In both the PNS and CNS, a nerve fiber is much longer than the reach of a single glial cell, so it requires many Schwann cells or oligodendrocytes to cover one nerve fiber. Consequently, the myelin sheath is segmented. Each gap between segments is called a (X)

node of Ranvier or myelin sheath gap

(V). If a lightbulb and the two poles of the battery are connected by a wire, electrons flow through the wire from one pole to the other, creating a current that lights the bulb. As long as the battery has a potential (voltage), we say it is

polarized.

that cell is more likely to fire. Memories lasting for a few hours, such as remembering what someone said to you earlier in the day or remembering an upcoming appointment, may involve (X)

post-tetanic potentiation.

A weak stimulus excites sensitive neurons with the lowest thresholds, while a strong stimulus excites less sensitive high-threshold neurons. Bringing additional neurons into play as the stimulus becomes stronger is called .

recruitment - It enables the nervous system to judge stimulus strength by which neurons, and how many of them, are firing.

Living cells are also polarized. The charge difference across the plasma membrane is called the (X)

resting membrane potential (RMP) - typically about -70 millivolts (mV) in an unstimulated, "resting" neuron. The negative value means there are more negatively charged particles on the inside of the membrane than on the outside.

Local potentials are (X), meaning that if stimulation ceases, cation diffusion out of the cell quickly returns the membrane voltage to its resting potential.

reversible

Myelinated fibers conduct signals in a very different manner called (X)—meaning "leaping" or "jumping." These fibers cannot conduct a signal in continuous mode, like a burning fuse, because voltage-gated ion channels are too scarce in the myelin-covered internodes—fewer than 25/µm2 in these regions compared with 2,000 to 12,000/µm2 at the nodes of Ranvier.

saltatory conduction

relation to these circuit types, the nervous system handles information in two modes called (X)

serial and parallel processing.

In (X), neurons and neural pools relay information along a pathway in a relatively simple linear fashion and can process only one flow of information at a time. For example, you can read a book or watch a television movie and the language recognition centers of your brain can process one linguistic input or the other, but you cannot do both simultaneously.

serial processing -

Although slower to respond than cholinergic and GABA-ergic synapses, adrenergic synapses do have an advantage—(X) A single NE molecule binding to a receptor can induce the formation of many cAMPs, each of those can activate many enzyme molecules or induce the transcription of a gene to generate numerous mRNA molecules, and each of those can result in the production of a vast number of enzyme molecules and metabolic products such as glucose molecules.

signal amplification.

As complex as synaptic events may seem, they typically require only 0.5 ms or so—an interval called (X). This is the time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell.

synaptic delay

The ability of synapses to change is called (X)

synaptic plasticity.

Think about when you learned as a child to tie your shoes. The procedure was very slow, confusing, and laborious at first, but eventually it became so easy you could do it with little thought— like a motor program playing out in your brain without requiring your conscious attention. It became easier to do because the synapses in a certain pathway were modified to allow signals to travel more easily across them than across "untrained" synapses. The process of making transmission easier is called (X)

synaptic potentiation (one form of synaptic plasticity).

Each branch ends in a bulbous axon terminal (terminal button), which forms a junction (synapse) with the next cell. It contains (X) full of neurotransmitter.

synaptic vesicles - In autonomic neurons, however, the axon has numerous beads called varicosities along its length. Each varicosity contains synaptic vesicles and secretes neurotransmitter.

Such synaptic facilitation, as it is called (different from the facilitation of one neuron by another that we studied earlier in the chapter), can be induced by (X), the rapid arrival of repetitive signals at a synapse.

tetanic stimulation

Since the axon hillock and initial segment play an important role in initiating a nerve signal, they are collectively called the (X)

trigger zone.

The (X) division carries signals mainly from the viscera of the thoracic and abdominal cavities, such as the heart, lungs, stomach, and urinary bladder.

visceral sensory

Fast axonal transport occurs at a rate of 200 to 400 mm/day and may be either anterograde or retrograde:

• Fast anterograde transport moves mitochondria; synaptic vesicles; other organelles; components of the axolemma; calcium ions; enzymes such as acetylcholinesterase; and small molecules such as glucose, amino acids, and nucleotides toward the distal end of the axon. • Fast retrograde transport returns used synaptic vesicles and other materials to the soma and informs the soma of conditions at the axon terminals. Some pathogens exploit this process to invade the nervous system. They enter the distal tips of an axon and travel to the soma by retrograde transport. Examples include tetanus toxin and the herpes simplex, rabies, and polio viruses. In such infections, the delay between infection and the onset of symptoms corresponds to the time needed for the pathogens to reach the somas.

The other two types of glial cells occur only in the peripheral nervous system:

5. Schwann21 cells, or neurilemmocytes, envelop nerve fibers of the PNS. In most cases, a Schwann cell winds repeatedly around a nerve fiber and produces a myelin sheath similar to the one produced by oligodendrocytes in the CNS. There are some important differences in myelin production between the CNS and PNS, which we consider shortly. Schwann cells also assist in the regeneration of damaged fibers, as described later. 6. Satellite cells surround the somas in ganglia of the PNS. They provide insulation around the soma and regulate the chemical environment of the neurons.

There are three general classes of neurons corresponding to the three major aspects of nervous system function listed earlier:

1 Sensory (afferent) neurons are specialized to detect stimuli such as light, heat, pressure, and chemicals, and transmit information about them to the CNS. Such neurons begin in almost every organ of the body and end in the CNS; the word afferent refers to signal conduction toward the CNS. Some receptors, such as those for pain and smell, are themselves neurons. In other cases, such as taste and hearing, the receptor is a separate cell that communicates directly with a sensory neuron. 2 Interneurons lie entirely within the CNS. They receive signals from many other neurons and carry out the integrative function of the nervous system—that is, they process, store, and retrieve information and "make decisions" that determine how the body responds to stimuli. About 90% of our neurons are interneurons. The word interneuron refers to the fact that they lie between, and interconnect, the incoming sensory pathways and the outgoing motor pathways of the CNS. 3 Motor (efferent) neurons send signals predominantly to muscle and gland cells, the effectors. They are called motor neurons because most of them lead to muscle cells, and efferent neurons to signify signal conduction away from the CNS.

(X) neurons have multiple dendrites but no axon. They communicate locally through their dendrites and do not produce action potentials.

Anaxonic - Some anaxonic neurons are found in the brain, retina, and adrenal medulla. In the retina, they help in visual processes such as the perception of contrast.

(X) neurons have one axon and one dendrite. Examples include olfactory cells of the nose, certain neurons of the retina, and sensory neurons of the ear.

Bipolar

Synapses that employ γ-aminobutyric acid (GABA) as their neurotransmitter are called (X). Amino acid neurotransmitters such as GABA work by the same mechanism as ACh; they bind to ion channels and cause immediate changes in membrane potential.

GABA-ergic synapses

The short section of nerve fiber between the axon hillock and the first glial cell is called the (X)

initial segment.

Types of Glial cells - Neuroglia of CNS - Phagocytize and destroy microorganisms, foreign matter, and dead nervous tissue

Microglia - are small macrophages that develop from white blood cells called monocytes. They wander through the CNS, putting out fingerlike extensions to constantly probe the tissue for cellular debris or other problems. They are thought to perform a complete checkup on the brain tissue several times a day, phagocytizing dead tissue, microorganisms, and other foreign matter. They become concentrated in areas damaged by infection, trauma, or stroke. Pathologists look for clusters of microglia in brain tissue as a clue to sites of injury. Microglia also aid in synaptic remodeling, changing the connections between neurons.

Types of Glial cells - Neuroglia of CNS - Form myelin in brain and spinal cord

Oligodendrocytes - somewhat resemble an octopus; they have a bulbous body with as many as 15 arms. Each arm reaches out to a nerve fiber and spirals around it like electrical tape wrapped repeatedly around a wire. This wrapping, called the myelin sheath, insulates the nerve fiber from the extracellular fluid. For reasons explained

(X) is the opposite of facilitation, a mechanism in which one presynaptic neuron suppresses another one. This mechanism is used to reduce or halt unwanted synaptic transmission.

Presynaptic inhibition

(X) transport is an anterograde process that works in a stop-and-go fashion. If we compare fast axonal transport to an express train traveling nonstop to its destination, slow axonal transport is like a local train that stops at every station.

Slow axonal - When moving, it goes just as fast as the express train, but the frequent stops result in an overall progress of only 0.2 to 0.5 mm/day. It moves enzymes and cytoskeletal components down the axon, renews worn-out axoplasmic components in mature neurons, and supplies new axoplasm for developing or regenerating neurons. Damaged nerves regenerate at a speed governed by slow axonal transport.

Information arrives at a neural pool through one or more input neurons, which branch repeatedly and synapse with numerous interneurons in the pool. Some input neurons form multiple synapses with a single postsynaptic cell. They can produce EPSPs at all points of contact with that cell and, through spatial summation, make it fire more easily than if they synapsed with it at only one point. Within the (X) of an input neuron, that neuron acting alone can make the postsynaptic cells fire. But in a broader (Y), it synapses with still other neurons in the pool, with fewer synapses on each of them. It can stimulate those neurons to fire only with the assistance of other input neurons; that is, it facilitates the others.

X = discharge zone Y = facilitated zone

Anterograde transport employs a motor protein called (X) and retrograde transport uses one called (Y)

X = kinesin Y = dynein (the same protein responsible for the motility of cilia and flagella).

Signals arrive at the synapse by way of the (X) , which releases a neurotransmitter. The next neuron, which responds to it, is called the (Y)

X = presynaptic neuron Y = postsynaptic neuron

Ependymal cells produce (X), a liquid that bathes the CNS and fills its internal cavities.

cerebrospinal fluid (CSF)

Their disadvantage of electrical synapses is that they cannot integrate information and make decisions. The ability to do that is a property of (X), in which neurons communicate by neurotransmitters. Chemical synapses are also the site of learning and memory, the target of many prescription drugs, and the site of action of drugs of addiction, among other things.

chemical synapses

A (X) employs acetylcholine (ACh) as its neurotransmitter.

cholinergic synapse - ACh excites some postsynaptic cells (such as skeletal muscle) and inhibits others (such as cardiac muscle),

Neurons sometimes secrete chemical signals that have long-term effects on entire groups of neurons instead of brief, quick effects at an individual synapse. Some call these (X) to distinguish them from neurotransmitters; others use the term neurotransmitter broadly to include these.

neuromodulators

The simplest neuromodulator is the gas (X)

nitric oxide (NO) - NO diffuses readily into a postsynaptic cell and activates second messenger pathways with such effects as relaxing smooth muscle. This has the effect of dilating small arteries and increasing blood flow to a tissue;

Action potentials are (X). For reasons examined shortly, they do not get weaker with distance. The last action potential at the end of a nerve fiber is just as strong as the first one in the trigger zone, no matter how far away— even in a pain fiber that extends from your toes to your brainstem.

nondecremental -

In (X), information is transmitted along diverging circuits through different pathways that act on it simultaneously, to different purposes. For example, when you're driving your car, your visual system (eye and brain) must simultaneously process information about color, shape, depth of field, and motion in the scene before your eyes. This requires complex parallel processing circuits from the retinas of your eyes through the visual centers at the rear of your brain.

parallel processing

The nervous system has two major anatomical subdivisions:

• The central nervous system (CNS) consists of the brain and spinal cord, which are enclosed and protected by the cranium and vertebral column. • The peripheral nervous system (PNS) consists of all the rest; it is composed of nerves and ganglia. A nerve is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue. Nerves emerge from the CNS through foramina of the skull and vertebral column and carry signals to and from other organs of the body. A ganglion1 (plural, ganglia) is a knot like swelling in a nerve where the cell bodies of peripheral neurons are concentrated.

Functions of astrocytes:

• They form a supportive framework for the nervous tissue. • They have extensions called perivascular feet, which contact the blood capillaries and stimulate them to form a tight, protective seal called the blood-brain barrier. • They monitor neuron activity, stimulate dilation and constriction of blood vessels, and thus regulate blood flow in the brain tissue to meet changing needs for oxygen and nutrients. • They convert blood glucose to lactate and supply this to the neurons for nourishment. • They secrete nerve growth factors that regulate nerve development. • They communicate electrically with neurons and influence synaptic signaling between them. • They regulate the composition of the tissue fluid. When neurons transmit signals, they release neurotransmitters and potassium ions. Astrocytes absorb these and prevent them from reaching excessive levels in the tissue fluid. • When neurons are damaged, astrocytes form hardened scar tissue and fill space formerly occupied by the neurons. This process is called astrocytosis or sclerosis.