Nervous Tissue An Intricate Network of Neurons and Glia Orchestrating Complex Signaling and Information Processing in the Central and Peripheral Nervous System.

Nervous Tissue and Homeostasis

Both the nervous and endocrine systems have the same objective:
to keep controlled conditions within limits that maintain life. The
nervous system regulates body activities by responding rapidly
using nerve impulses; the endocrine system responds by releasing
hormones. Chapter 18 compares the roles of both systems in
maintaining homeostasis.
The nervous system is also responsible for our perceptions,
behaviors, and memories, and it initiates all voluntary movements.
Because this system is quite complex, we discuss its structure and
function in several chapters. This chapter focuses on the organization
of the nervous system and the properties of neurons (nerve cells)
and neuroglia (cells that support the activities of neurons). We then
examine the structure and functions of the spinal cord and spinal
nerves (Chapter 13), and of the brain and cranial nerves (Chapter 14).
The autonomic nervous system, which operates without voluntary
control, will be covered in Chapter 15. Chapter 16 will discuss the
somatic senses—touch, pressure, warmth, cold, pain, and others—
and their sensory and motor pathways to show how nerve impulses
pass into the spinal cord and brain or from the spinal cord and brain
to muscles and glands. Exploration of the nervous system concludes
with a discussion of the special senses: smell, taste, vision, hearing,
and equilibrium (Chapter 17).

Q Did you ever wonder how the human nervous system
coordinates and integrates all body systems so rapidly and
efficiently?

Overview of the Nervous
System

OBJECTIVES

• Describe the organization of the nervous system.
• Describe the three basic functions of the nervous system.
Organization of the Nervous System
With a mass of only 2 kg (4.5 lb), about 3% of total body weight, the
nervous system is one of the smallest and yet the most complex of
the 11 body systems. This intricate network of billions of neurons and
even more neuroglia is organized into two main subdivisions: the cen-
tral nervous system and the peripheral nervous system. Neurology
deals with normal functioning and disorders of the nervous system. A
neurologist (noo-ROL-oˉ-jist) is a physician who diagnoses and treats
disorders of the nervous system.
Central Nervous System The central nervous system
(CNS) consists of the brain and spinal cord (Figure 12.1a). The
brain is the part of the CNS that is located in the skull and contains
about 85 billion neurons. The spinal cord is connected to the brain
through the foramen magnum of the occipital bone and is encircled
by the bones of the vertebral column. The spinal cord contains about
100 million neurons. The CNS processes many diff erent kinds of
incoming sensory information. It is also the source of thoughts,
emotions, and memories. Most signals that stimulate muscles to
contract and glands to secrete originate in the CNS.
Peripheral Nervous System The peripheral nervous
system (PNS) (pe-RIF-e-ral) consists of all nervous tissue outside
the CNS (Figure 12.1a). Components of the PNS include nerves and
sensory receptors. A nerve is a bundle of hundreds to thousands of
axons plus associated connective tissue and blood vessels that lies
outside the brain and spinal cord. Twelve pairs of cranial nerves
emerge from the brain and thirty-one pairs of spinal nerves emerge
from the spinal cord. Each nerve follows a defined path and serves
a specific region of the body. The term sensory receptor refers to a
structure of the nervous system that monitors changes in the external
or internal environment. Examples of sensory receptors include touch
receptors in the skin, photoreceptors in the eye, and olfactory (smell)
receptors in the nose.
The PNS is divided into sensory and motor divisions (Figure
12.1b). The sensory or afferent division of the PNS conveys input into
the CNS from sensory receptors in the body. This division provides the
CNS with sensory information about the somatic senses (tactile, ther-
mal, pain, and proprioceptive sensations) and special senses (smell,
taste, vision, hearing, and equilibrium).

The motor or efferent division of the PNS conveys output from
the CNS to eff ectors (muscles and glands). This division is further sub-
divided into a somatic nervous system and an autonomic nervous
system (Figure 12.1b). The somatic nervous system (SNS) (soˉ -MAT-
ik; soma = body) conveys output from the CNS to skeletal muscles
only. Because its motor responses can be consciously controlled, the
action of this part of the PNS is voluntary. The autonomic nervous
system (ANS) (aw′-toˉ -NOM-ik; auto- = self; -nomic = law) conveys
output from the CNS to smooth muscle, cardiac muscle, and glands.
Because its motor responses are not normally under conscious con-
trol, the action of the ANS is involuntary. The ANS is comprised of two
main branches, the sympathetic nervous system and the parasym-
pathetic nervous system. With a few exceptions, eff ectors receive
innervation from both of these branches, and usually the two branch-
es have opposing actions. For example, neurons of the sympathetic
nervous system increase heart rate, and neurons of the parasympa-
thetic nervous system slow it down. In general, the parasympathetic
nervous system takes care of “rest-and-digest” activities, and the
sympathetic nervous system helps support exercise or emergency
actions—the so-called “fight-or-flight” responses. A third branch of
the autonomic nervous system is the enteric nervous system (ENS)
(en-TER-ik; enteron= intestines), an extensive network of over 100 million
neurons confined to the wall of the gastrointestinal (GI) tract. The ENS
helps regulate the activity of the smooth muscle and glands of the GI
tract. Although the ENS can function independently, it communicates
with and is regulated by the other branches of the ANS

Functions of the Nervous System

The nervous system carries out a complex array of tasks. It allows us
to sense various smells, produce speech, and remember past events;
in addition, it provides signals that control body movements and reg-
ulates the operation of internal organs. These diverse activities can be
grouped into three basic functions: sensory (input), integrative (pro-
cess), and motor (output).
• Sensory function. Sensory receptors detect internal stimuli, such
as an increase in blood pressure, or external stimuli (for example, a
raindrop landing on your arm). This sensory information is then car-
ried into the brain and spinal cord through cranial and spinal nerves.
• Integrative function. The nervous system processes sensory in-
formation by analyzing it and making decisions for appropriate
responses—an activity known as integration.
• Motor function. Once sensory information is integrated, the ner-
vous system may elicit an appropriate motor response by activating
eff ectors (muscles and glands) through cranial and spinal nerves.
Stimulation of the eff ectors causes muscles to contract and glands
to secrete.
The three basic functions of the nervous system occur, for exam-
ple, when you answer your cell phone aft er hearing it ring. The sound
of the ringing cell phone stimulates sensory receptors in your ears
(sensory function). This auditory information is subsequently relayed
into your brain where it is processed and the decision to answer the
phone is made (integrative function). The brain then stimulates the

Histology of Nervous Tissue

OBJECTIVES

• Contrast the histological characteristics and the functions of neu-
rons and neuroglia.
• Distinguish between gray matter and white matter.
Nervous tissue comprises two types of cells—neurons and neuroglia.
These cells combine in a variety of ways in diff erent regions of the
nervous system. In addition to forming the complex processing net-
works within the brain and spinal cord, neurons also connect all
regions of the body to the brain and spinal cord. As highly specialized
cells capable of reaching great lengths and making extremely intri-
cate connections with other cells, neurons provide most of the
unique functions of the nervous system, such as sensing, thinking,
remembering, controlling muscle activity, and regulating glandular
secretions. As a result of their specialization, most neurons have lost
the ability to undergo mitotic divisions. Neuroglia are smaller cells,
but they greatly outnumber neurons—perhaps by as much as 25
times. Neuroglia support, nourish, and protect neurons, and main-
tain the interstitial fluid that bathes them. Unlike neurons, neuroglia
continue to divide throughout an individual’s lifetime. Both neurons
and neuroglia diff er structurally depending on whether they are
located in the central nervous system or the peripheral nervous
system. These diff erences in structure correlate with the diff erences
in function of the central nervous system and the peripheral nervous
system.

Neurons

Like muscle cells, neurons (nerve cells) (NOO-rons) possess electri-
cal excitability (ek-sīt′-a-BIL-i-tē), the ability to respond to a stimu-
lus and convert it into an action potential. A stimulus is any change
in the environment that is strong enough to initiate an action
potential. An action potential (nerve impulse) is an electrical signal
that propagates (travels) along the surface of the membrane of a
neuron. It begins and travels due to the movement of ions (such as
sodium and potassium) between interstitial fluid and the inside of
neuron through specific ion channels in its plasma membrane.
Once begun, a nerve impulse travels rapidly and at a constant
strength.
Some neurons are tiny and propagate impulses over a short dis-
tance (less than 1 mm) within the CNS. Others are the longest cells in
the body. The neurons that enable you to wiggle your toes, for exam-
ple, extend from the lumbar region of your spinal cord (just above
waist level) to the muscles in your foot. Some neurons are even longer.
Those that allow you to feel a feather tickling your toes stretch all the
way from your foot to the lower portion of your brain. Nerve impulses
travel these great distances at speeds ranging from 0.5 to 130 meters
per second (1 to 290 mi/hr).
Parts of a Neuron Most neurons have three parts: (1) a cell
body, (2) dendrites, and (3) an axon (Figure 12.2). The cell body, also
known as the perikaryon (per′-i-KAR-ē-on) or soma, contains a nucleus
surrounded by cytoplasm that includes typical cellular organelles
such as lysosomes, mitochondria, and a Golgi complex. Neuronal cell
bodies also contain free ribosomes and prominent clusters of rough
endoplasmic reticulum, termed Nissl bodies (NIS-el). The ribosomes
are the sites of protein synthesis. Newly synthesized proteins
produced by Nissl bodies are used to replace cellular components,
as material for growth of neurons, and to regenerate damaged axons
in the PNS. The cytoskeleton includes both neurofibrils (noo-rō-FĪ-
brils), composed of bundles of intermediate filaments that provide
the cell shape and support, and microtubules (mī-krō-TOO-būls′),
which assist in moving materials between the cell body and axon.
Aging neurons also contain lipofuscin (līp′-o-FYŪS-īn), a pigment
that occurs as clumps of yellowish brown granules in the cytoplasm.
Lipofuscin is a product of neuronal lysosomes that accumulates as
the neuron ages, but does not seem to harm the neuron. A collection
of neuron cell bodies outside the CNS is called a ganglion (GANG-lē-
on = sculling or knot; ganglia is plural).
A nerve fiber is a general term for any neuronal process (exten-
sion) that emerges from the cell body of a neuron. Most neurons have
two kinds of processes: multiple dendrites and a single axon. Den-
drites (DEN-drīts = little trees) are the receiving or input portions of a
neuron. The plasma membranes of dendrites (and cell bodies) con-
tain numerous receptor sites for binding chemical messengers from
other cells. Dendrites usually are short, tapering, and highly branched.
In many neurons the dendrites form a tree-shaped array of processes
extending from the cell body. Their cytoplasm contains Nissl bodies,
mitochondria, and other organelles.
The single axon (= axis) of a neuron propagates nerve impulses
toward another neuron, a muscle fiber, or a gland cell. An axon is a
long, thin, cylindrical projection that oft en joins to the cell body at a
cone-shaped elevation called the axon hillock (HIL-lok = small hill).
The part of the axon closest to the axon hillock is the initial seg-
ment. In most neurons, nerve impulses arise at the junction of the
axon hillock and the initial segment, an area called the trigger zone,
from which they travel along the axon to their destination. An axon
contains mitochondria, microtubules, and neurofibrils. Because
rough endoplasmic reticulum is not present, protein synthesis does
not occur in the axon. The cytoplasm of an axon, called axoplasm, is
surrounded by a plasma membrane known as the axolemma
(lemma = sheath or husk). Along the length of an axon, side branches
called axon collaterals may branch off , typically at a right angle to the
axon. The axon and its collaterals end by dividing into many fine proc-
esses called axon terminals or axon telodendria (tēl′-ō-DEN-drē-a).
The site of communication between two neurons or between a
neuron and an eff ector cell is called a synapse (SIN-aps). The tips of
some axon terminals swell into bulb-shaped structures called synap-
tic end bulbs; others exhibit a string of swollen bumps called vari-
cosities (var′-i-KOS-i-tēz). Both synaptic end bulbs and varicosities
contain many tiny membrane-enclosed sacs called synaptic vesicles
that store a chemical called a neurotransmitter (noo′-rō-trans′-MIT-
ter). A neurotransmitter is a molecule released from a synaptic vesicle
that excites or inhibits another neuron, muscle fiber, or gland cell.
Many neurons contain two or even three types of neurotransmitters,
each with diff erent eff ects on the postsynaptic cell.
Because some substances synthesized or recycled in the neuron
cell body are needed in the axon or at the axon terminals, two types of
transport systems carry materials from the cell body to the axon termi-
nals and back. The slower system, which moves materials about
1–5 mm per day, is called slow axonal transport. It conveys axoplasm
in one direction only—from the cell body toward the axon terminals.
Slow axonal transport supplies new axoplasm to developing or regen-
erating axons and replenishes axoplasm in growing and mature axons.
Fast axonal transport, which is capable of moving materials a
distance of 200–400 mm per day, uses proteins that function as “mo-
tors” to move materials along the surfaces of microtubules of the
neuron’s cytoskeleton. Fast axonal transport moves materials in
both directions—away from and toward the cell body. Fast axonal
transport that occurs in an anterograde (forward) direction moves
organelles and synaptic vesicles from the cell body to the axon termi-
nals. Fast axonal transport that occurs in a retrograde (backward)
direction moves membrane vesicles and other cellular materials
from the axon terminals to the cell body to be degraded or recycled.
Substances that enter the neuron at the axon terminals are also
moved to the cell body by fast retrograde transport. These sub-
stances include trophic chemicals such as nerve growth factor and
harmful agents such as tetanus toxin and the viruses that cause rabies,
herpes simplex, and polio.
Structural Diversity in Neurons Neurons display great
diversity in size and shape. For example, their cell bodies range in
diameter from 5 micrometers (μm) (slightly smaller than a red blood
cell) up to 135 μm (barely large enough to see with the unaided
eye). The pattern of dendritic branching is varied and distinctive for
neurons in diff erent parts of the nervous system. A few small neurons
lack an axon, and many others have very short axons. As we have
already discussed, the longest axons are almost as long as a person is
tall, extending from the toes to the lowest part of the brain.
Classification of Neurons Both structural and functional
features are used to classify the various neurons in the body.
STRUCTURAL CLASSIFICATION Structurally, neurons are classified
according to the number of processes extending from the cell body
(Figure 12.3):

Q Which type of neuron shown in this figure is the most abundant type of neuron in
the CNS?

1. Multipolar neurons usually have several dendrites and one axon
(Figure 12.3a). Most neurons in the brain and spinal cord are of this
type, as well as all motor neurons (described shortly).
2. Bipolar neurons have one main dendrite and one axon (Fig-
ure 12.3b). They are found in the retina of the eye, the inner ear,
and the olfactory area (olfact = to smell) of the brain.
3. Unipolar neurons have dendrites and one axon that are fused
together to form a continuous process that emerges from the cell
body (Figure 12.3c). These neurons are more appropriately called
pseudounipolar neurons (soo′-dō-ū′-ni-PŌ-lar) because they
begin in the embryo as bipolar neurons. During development, the
dendrites and axon fuse together and become a single process. The
dendrites of most unipolar neurons function as sensory receptors
that detect a sensory stimulus such as touch, pressure, pain, or
thermal stimuli. The trigger zone for nerve impulses in a unipolar
neuron is at the junction of the dendrites and axon (Figure 12.3c).
The impulses then propagate toward the synaptic end bulbs. The
cell bodies of most unipolar neurons are located in the ganglia of
spinal and cranial nerves.
In addition to the structural classification scheme just described,
some neurons are named for the histologist who first described them
or for an aspect of their shape or appearance; examples include
Purkinje cells (pur-KIN-jē) in the cerebellum and pyramidal cells
(pi-RAM-i-dal), found in the cerebral cortex of the brain, which have
pyramid-shaped cell bodies (Figure 12.4).
FUNCTIONAL CLASSIFICATION Functionally, neurons are classified
according to the direction in which the nerve impulse (action poten-
tial) is conveyed with respect to the CNS (Figure 12.5).
1. Sensory neurons or afferent neurons (AF-er-ent NOO-ronz; af- =
toward; -ferrent = carried) either contain sensory receptors at their
distal ends (dendrites) (see also Figure 12.10) or are located just
aft er sensory receptors that are separate cells. Once an appropriate
stimulus activates a sensory receptor, the sensory neuron forms an
action potential in its axon and the action potential is conveyed
into the CNS through cranial or spinal nerves. Most sensory neu-
rons are unipolar in structure.
2. Motor neurons or efferent neurons (EF-e-rent; ef- = away from)
convey action potentials away from the CNS to eff ectors (muscles
and glands) in the periphery (PNS) through cranial or spinal nerves
(see also Figure 12.10). Motor neurons are multipolar in structure.
3. Interneurons or association neurons are mainly located within the
CNS between sensory and motor neurons (see also Figure 12.10).
Interneurons integrate (process) incoming sensory information
from sensory neurons and then elicit a motor response by activat-
ing the appropriate motor neurons. Most interneurons are multipolar
in structure.

Neuroglia

Neuroglia (noo-RŌG-lē-a; -glia = glue) or glia (GLĒ-a) make up about
half the volume of the CNS. Their name derives from the idea of early
histologists that they were the “glue” that held nervous tissue
together. We now know that neuroglia are not merely passive bystand-
ers but rather actively participate in the activities of nervous tissue.
Generally, neuroglia are smaller than neurons, and they are 5 to
25 times more numerous. In contrast to neurons, glia do not generate
or propagate action potentials, and they can multiply and divide in
the mature nervous system. In cases of injury or disease, neuroglia
multiply to fill in the spaces formerly occupied by neurons. Brain
tumors derived from glia, called gliomas (glē-Ō-mas), tend to be
highly malignant and to grow rapidly. Of the six types of neuroglia,
four—astrocytes, oligodendrocytes, microglia, and ependymal cells—
are found only in the CNS. The remaining two types—Schwann cells
and satellite cells—are present in the PNS.
Neuroglia of the CNS Neuroglia of the CNS can be classified
on the basis of size, cytoplasmic processes, and intracellular
organization into four types: astrocytes, oligodendrocytes, microglial
cells, and ependymal cells (Figure 12.6).
ASTROCYTES These star-shaped cells have many processes and are
the largest and most numerous of the neuroglia. There are two types
of astrocytes (AS-trō-sīts; astro- = star; -cyte = cell). Protoplasmic
astrocytes have many short branching processes and are found in
gray matter (described shortly). Fibrous astrocytes have many long
unbranched processes and are located mainly in white matter (also
described shortly). The processes of astrocytes make contact with
blood capillaries, neurons, and the pia mater (a thin membrane
around the brain and spinal cord).

Q Which functional class of neurons is responsible for integration?

The functions of astrocytes include the following:
1. Astrocytes contain microfilaments that give them considerable
strength, which enables them to support neurons.
2. Processes of astrocytes wrapped around blood capillaries isolate
neurons of the CNS from various potentially harmful substances
in blood by secreting chemicals that maintain the unique selective
permeability characteristics of the endothelial cells of the capil-
laries. In eff ect, the endothelial cells create a blood–brain barrier,
which restricts the movement of substances between the blood
and interstitial fluid of the CNS. Details of the blood–brain barrier
are discussed in Chapter 14.
3. In the embryo, astrocytes secrete chemicals that appear to regulate
the growth, migration, and interconnection among neurons in the
brain.
4. Astrocytes help to maintain the appropriate chemical environ-
ment for the generation of nerve impulses. For example, they
regulate the concentration of important ions such as K+; take up
excess neurotransmitters; and serve as a conduit for the passage
of nutrients and other substances between blood capillaries and
neurons.
5. Astrocytes may also play a role in learning and memory by influenc-
ing the formation of neural synapses (see Section 16.5).

Q Which CNS neuroglia function as phagocytes?

OLIGODENDROCYTES These resemble astrocytes but are smaller
and contain fewer processes. Processes of oligodendrocytes (OL-i-
gō-den′-drō-sīts; oligo- = few; -dendro- = tree) are responsible for
forming and maintaining the myelin sheath around CNS axons. As you
will see shortly, the myelin sheath is a multilayered lipid and protein
covering around some axons that insulates them and increases the
speed of nerve impulse conduction. Such axons are said to be myeli-
nated (MĪ-e-li-nā-ted).
MICROGLIAL CELLS OR MICROGLIA These neuroglia are small cells
with slender processes that give off numerous spinelike projections.
Microglial cells or microglia (mī-KROG-lē-a; micro- = small) function
as phagocytes. Like tissue macrophages, they remove cellular debris
formed during normal development of the nervous system and
phagocytize microbes and damaged nervous tissue.
EPENDYMAL CELLS Ependymal cells (ep-EN-de-mal; epen- = above;
-dym- = garment) are cuboidal to columnar cells arranged in a single
layer that possess microvilli and cilia. These cells line the ventricles of
the brain and central canal of the spinal cord (spaces filled with cere-
brospinal fluid, which protects and nourishes the brain and spinal
cord). Functionally, ependymal cells produce, possibly monitor, and
assist in the circulation of cerebrospinal fluid. They also form the
blood–cerebrospinal fluid barrier, which is discussed in Chapter 14.
Neuroglia of the PNS Neuroglia of the PNS completely
surround axons and cell bodies. The two types of glial cells in the PNS
are Schwann cells and satellite cells (Figure 12.7).
SCHWANN CELLS These cells encircle PNS axons. Like oligodendro-
cytes, they form the myelin sheath around axons. A single oligoden-
drocyte myelinates several axons, but each Schwann cell (SCHVON or
SCHWON) myelinates a single axon (Figure 12.7a; see also Fig-
ure 12.8a, c). A single Schwann cell can also enclose as many as 20 or
more unmyelinated axons (axons that lack a myelin sheath) (Fig-
ure 12.7b). Schwann cells participate in axon regeneration, which is
more easily accomplished in the PNS than in the CNS.
SATELLITE CELLS These flat cells surround the cell bodies of neurons
of PNS ganglia (Figure 12.7c). Besides providing structural support,
satellite cells (SAT-i-līt) regulate the exchanges of materials between
neuronal cell bodies and interstitial fluid.

Q How do Schwann cells and oligodendrocytes differ with respect to the number of
axons they myelinate?

Myelination

As you have already learned, axons surrounded by a multilayered lipid
and protein covering, called the myelin sheath, are said to be myeli-
nated (Figure 12.8a). The sheath electrically insulates the axon of a
neuron and increases the speed of nerve impulse conduction. Axons
without such a covering are said to be unmyelinated (Figure 12.8b).
Two types of neuroglia produce myelin sheaths: Schwann cells
(in the PNS) and oligodendrocytes (in the CNS). Schwann cells begin
to form myelin sheaths around axons during fetal development. Each
Schwann cell wraps about 1 millimeter (1 mm = 0.04 in.) of a single
axon’s length by spiraling many times around the axon (Figure 12.8a).
Eventually, multiple layers of glial plasma membrane surround the
axon, with the Schwann cell’s cytoplasm and nucleus forming the out-
ermost layer. The inner portion, consisting of up to 100 layers of
Schwann cell membrane, is the myelin sheath. The outer nucleated
cytoplasmic layer of the Schwann cell, which encloses the myelin
sheath, is the neurolemma (sheath of Schwann) (noo′-rō-LEM-ma). A
neurolemma is found only around axons in the PNS. When an axon is
injured, the neurolemma aids regeneration by forming a regeneration
tube that guides and stimulates regrowth of the axon. Gaps in the
myelin sheath, called nodes of Ranvier (RON-vē-ā), appear at inter-
vals along the axon (Figure 12.8; see also Figure 12.2). Each Schwann
cell wraps one axon segment between two nodes.
In the CNS, an oligodendrocyte myelinates parts of several axons.
Each oligodendrocyte puts forth about 15 broad, flat processes that,
spiral around CNS axons, forming a myelin sheath. A neurolemma is
not present, however, because the oligodendrocyte cell body and nu-
cleus do not envelop the axon. Nodes of Ranvier are present, but they
are fewer in number. Axons in the CNS display little regrowth aft er
injury. This is thought to be due, in part, to the absence of a neuro-
lemma, and in part to an inhibitory influence exerted by the oligoden-
drocytes on axon regrowth.
The amount of myelin increases from birth to maturity, and its
presence greatly increases the speed of nerve impulse conduction. An
infant’s responses to stimuli are neither as rapid nor as coordinated as
those of an older child or an adult, in part because myelination is still
in progress during infancy.

Collections of Nervous Tissue

The components of nervous tissue are grouped together in a variety of
ways. Neuronal cell bodies are oft en grouped together in clusters. The
axons of neurons are usually grouped together in bundles. In addi-
tion, widespread regions of nervous tissue are grouped together as
either gray matter or white matter.
Clusters of Neuronal Cell Bodies Recall that a ganglion
(plural is ganglia) refers to a cluster of neuronal cell bodies located
in the PNS. As mentioned earlier, ganglia are closely associated
with cranial and spinal nerves. By contrast, a nucleus is a cluster of
neuronal cell bodies located in the CNS.

Q What is the functional advantage of myelination?

Bundles of Axons Recall that a nerve is a bundle of axons
that is located in the PNS. Cranial nerves connect the brain to the
periphery, whereas spinal nerves connect the spinal cord to the
periphery. A tract is a bundle of axons that is located in the CNS.
Tracts interconnect neurons in the spinal cord and brain.
Gray and White Matter In a freshly dissected section of the
brain or spinal cord, some regions look white and glistening, and others
appear gray (Figure 12.9). White matter is composed primarily of
myelinated axons. The whitish color of myelin gives white matter its
name. The gray matter of the nervous system contains neuronal cell