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Neural Tissue
Did you know...?
Sixty percent of the sensory neurons innervate
the forehead and hands.
Learning Outcomes
After completing this chapter, you should be able to do the following:
12-1
Describe the anatomical and functional divisions of the nervous
system.
12-2
Sketch and label the structure of a typical neuron, describe the
functions of each component, and classify neurons on the basis
of their structure and function.
12-3
Describe the locations and functions of the various types of
neuroglia.
12-4
Explain how the resting potential is created and maintained.
12-5
Describe the events involved in the generation and propagation
of an action potential.
12-6
Discuss the factors that affect the speed with which action
potentials are propagated.
12-7
Describe the structure of a synapse, and explain the mechanism
involved in synaptic activity.
12-8
Describe the major types of neurotransmitters and
neuromodulators, and discuss their effects on postsynaptic
membranes.
12-9
Discuss the interactions that enable information processing to
occur in neural tissue.
Clinical Notes
Rabies p. 390
Tumors p. 395
Demyelination p. 397
387
Chapter 12
Neural Tissue
An Introduction to Neural Tissue
The
peripheral nervous system (PNS)
includes all the
neural tissue outside the CNS. The PNS delivers sensory in-
formation to the CNS and carries motor commands to periph-
eral tissues and systems. Bundles of axons, or
nerve fibers
,
carry sensory information and motor commands in the PNS.
Such bundles, with associated blood vessels and connective
tissues, are called
peripheral nerves
, or simply
nerves
. Nerves
connected to the brain are called
cranial nerves
; those at-
tached to the spinal cord are called
spinal nerves
.
This chapter considers the structure of neural tissue and in-
troduces the basic principles of neurophysiology. The nervous
system includes all the neural tissue in the body.
l
p. 141
The basic functional units of the nervous system are individ-
ual cells called
neurons
. Supporting cells, or
neuroglia
(noo-
ROG-le-uh or noo-ro-GLE-uh;
glia
, glue), separate and
protect the neurons, provide a supportive framework for neu-
ral tissue, act as phagocytes, and help regulate the composi-
tion of the interstitial fluid. Neuroglia, also called
glial cells
,
far outnumber neurons.
Neural tissue, with supporting blood vessels and connec-
tive tissues, forms the organs of the nervous system: the brain;
the spinal cord; the receptors in complex sense organs, such
as the eye and ear; and the
nerves
that link the nervous sys-
tem with other systems. In the
Systems Overview
section
(pp. 148–156), we introduced the two major anatomical di-
visions of the nervous system: the central nervous system and
the peripheral nervous system.
The Functional Divisions of the
Nervous System
The PNS is divided into afferent and efferent divisions. The
afferent division
(
ad
, to
ferre
, to carry) of the PNS brings
sensory information to the CNS from receptors in peripheral
tissues and organs.
Receptors
are sensory structures that ei-
ther detect changes in the internal environment or respond to
the presence of specific stimuli. There are complex receptor
organs, such as the eye or ear; at the cellular level, receptors
range from the dendrites (slender cytoplasmic extensions) of
single cells to complex organs. Receptors may be neurons or
specialized cells of other tissues.
l
p. 168
The
efferent division
(
effero
, to bring out) of the PNS
carries motor commands from the CNS to muscles, glands,
and adipose tissue. These target organs, which respond by
doing
something, are called
effectors
. The efferent division
has both somatic and autonomic components.
Tips
&
Tricks
To distinguish between afferent and efferent, associate the
“a” in
a
fferent with the “a” in
a
ccessing, and the “e” in
e
fferent with the “e” in
e
xiting.
12-1
The nervous system has
anatomical and functional
divisions
In this section we provide an overview of the nervous system.
We begin with anatomical and functional perspectives.
The Anatomical Divisions of the
Nervous System
Viewed anatomically, the nervous system has two divisions:
the central nervous system and the peripheral nervous sys-
tem. The
central nervous system (CNS)
consists of the
spinal cord and brain. These are complex organs that include
not only neural tissue, but also blood vessels and the various
connective tissues that provide physical protection and sup-
port. The CNS is responsible for integrating, processing, and
coordinating sensory data and motor commands. Sensory
data convey information about conditions inside or outside
the body. Motor commands control or adjust the activities of
peripheral organs, such as skeletal muscles. When you stum-
ble, for example, the CNS integrates information regarding
your balance and the position of your limbs and then coordi-
nates your recovery by sending motor commands to appro-
priate skeletal muscles—all in a split second and without
conscious effort. The CNS—specifically, the brain—is also
the seat of higher functions, such as intelligence, memory,
learning, and emotion.
•
The
somatic nervous system (SNS)
controls skeletal
muscle contractions.
Voluntary
contractions are under
conscious control. For example, you exert conscious
control over your arm as you raise a full glass of water to
your lips.
Involuntary
contractions may be simple,
automatic responses or complex movements, but they are
controlled at the subconscious level, outside your
awareness. For instance, if you accidentally place your
hand on a hot stove, you will withdraw it immediately,
usually before you even notice any pain. This type of
automatic response is called a
reflex
.
•
The
autonomic nervous system (ANS)
, or
visceral motor
system
, provides automatic regulation of smooth muscle,
cardiac muscle, and glandular secretions at the
subconscious level. The ANS includes a
sympathetic
division
and a
parasympathetic division
, which commonly
388
Unit 3
Control and Regulation
12-2
Neurons are nerve cells
specialized for intercellular
communication
have antagonistic effects. For example, activity of the
sympathetic division accelerates the heart rate, whereas
parasympathetic activity slows the heart rate.
Now that we have completed a brief orientation on the
nervous system as a whole, we can examine the structure of
neural tissue and the functional principles that govern neural
activities. We begin by considering neurons, the basic func-
tional units of the nervous system.
In this section, we will examine the structure of a representa-
tive neuron before considering the structural and functional
classifications of neurons.
The Structure of Neurons
CHECKPOINT
1. Identify the two anatomical divisions of the nervous
system.
2. Identify the two functional divisions of the peripheral
nervous system, and cite their primary functions.
3. Identify the two components of the efferent division of
the PNS.
4. What would be the effect of damage to the afferent
division of the PNS?
See the blue Answers tab at the end of the book.
Figure 12–1
shows the structure of a representative neuron.
Neurons have a variety of shapes. The one shown is a
multipolar neuron
, the most common type of neuron in the
central nervous system. Each multipolar neuron has a large
cell body
that is connected to a single, elongate
axon
and sev-
eral short, branched
dendrites
.
The Cell Body
The
cell body
, or
soma
(plural,
somata
), contains a rela-
tively large, round nucleus with a prominent nucleolus
(
Figure 12–1
). The cytoplasm surrounding the nucleus con-
stitutes the
perikaryon
(per-i-KAR-e-on;
peri,
around
Dendrites
Perikaryon
Cell body
Nucleus
Dendritic branches
Telodendria
Axon
Nissl bodies (RER
and free ribosomes)
(a) Regions of a neuron
Mitochondrion
Axon hillock
Initial segment
of axon
Axolemma
Axon
Telodendria
Golgi apparatus
Neurofilament
Synaptic
terminals
Nucleus
Nucleolus
See Figure 12-2
Dendrite
PRESYNAPTIC CELL
POSTSYNAPTIC CELL
(b) Structural components of a neuron
Figure 12–1
The Anatomy of a Multipolar Neuron. (a)
The general structure of a neuron and its primary components.
(b)
A more detailed
view of a neuron, showing major organelles.
389
Chapter 12
Neural Tissue
karyon
, nucleus). The cytoskeleton of the perikaryon con-
tains
neurofilaments
and
neurotubules
, which are similar to
the intermediate filaments and microtubules of other types of
cells. Bundles of neurofilaments, called
neurofibrils
, extend
into the dendrites and axon, providing internal support for
these slender processes.
The perikaryon contains organelles that provide energy
and synthesize organic materials, especially the chemical
neurotransmitters that are important in cell-to-cell communi-
cation.
l
p. 304
The numerous mitochondria, free and
fixed ribosomes, and membranes of rough endoplasmic retic-
ulum (RER) give the perikaryon a coarse, grainy appearance.
Mitochondria generate ATP to meet the high energy demands
of an active neuron. The ribosomes and RER synthesize pro-
teins. Some areas of the perikaryon contain clusters of RER
and free ribosomes. These regions, which stain darkly, are
called
Nissl bodies
, because they were first described by the
German microscopist Franz Nissl. Nissl bodies account for
the gray color of areas containing neuron cell bodies—the
gray matter
seen in gross dissection.
Most neurons lack centrioles, important organelles in-
volved in the organization of the cytoskeleton and the move-
ment of chromosomes during mitosis.
l
p. 102
As a result,
typical CNS neurons cannot divide; thus, they cannot be re-
placed if lost to injury or disease. Although neural stem cells
persist in the adult nervous system, they are typically inactive
except in the nose, where the regeneration of olfactory
(smell) receptors maintains our sense of smell, and in the
hippocampus
, a portion of the brain involved with memory
storage. The control mechanisms that trigger neural stem cell
activity are now being investigated, with the goal of prevent-
ing or reversing neuron loss due to trauma, disease, or aging.
Dendrites and Axons
A variable number of slender, sensitive processes known as
dendrites
extend out from the cell body (
Figure 12–1
). Typi-
cal dendrites are highly branched, and each branch bears fine
0.5- to 1-mm-long studded processes called
dendritic spines
.
In the CNS, a neuron receives information from other neu-
rons primarily at the dendritic spines, which represent 80–90
percent of the neuron’s total surface area.
An
axon
is a long cytoplasmic process capable of propagat-
ing an electrical impulse known as an
action potential
.
l
p. 304
The
axoplasm
(AK-so-plazm), or cytoplasm of the
axon, contains neurofibrils, neurotubules, small vesicles, lyso-
somes, mitochondria, and various enzymes. The axoplasm is
surrounded by the
axolemma
(
lemma
, husk), a specialized
portion of the plasma membrane. In the CNS, the axolemma
may be exposed to the interstitial fluid or covered by the
processes of neuroglia. The base, or
initial segment
, of the
axon in a multipolar neuron is attached to the cell body at a
thickened region known as the
axon hillock
(
Figure 12–1
).
An axon may branch along its length, producing side
branches collectively known as
collaterals
. Collaterals en-
able a single neuron to communicate with several other cells.
The main axon trunk and any collaterals end in a series of fine
extensions, or
telodendria
(tel-o-DEN-dre-uh;
telo
-, end
dendron
, tree) (
Figure 12–1
). The telodendria of an axon end
at
synaptic terminals
.
The Synapse
Each synaptic terminal is part of a
synapse
, a specialized site
where the neuron communicates with another cell (
Figure 12–2
).
Every synapse involves two cells: (1) the
presynaptic cell
, which
Telodendrion
Synaptic knob
Endoplasmic
reticulum
Mitochondrion
Synaptic
vesicles
Presynaptic
membrane
Postsynaptic
membrane
Synaptic
cleft
Figure 12–2
The Structure of a Typical Synapse.
A diagrammatic view (at left) and a micrograph (at right) of a typical synapse between
two neurons. (TEM, color enhanced,
×
222,000).
390
Unit 3
Control and Regulation
includes the synaptic terminal and sends a message; and (2) the
postsynaptic cell
, which receives the message. The communica-
tion between cells at a synapse most commonly involves the re-
lease of chemicals called
neurotransmitters
by the synaptic
terminal. These chemicals, released by the presynaptic cell, affect
the activity of the postsynaptic cell. As we saw in Chapter 10, this
release is triggered by electrical events, such as the arrival of an
action potential. Neurotransmitters are typically packaged in
synaptic vesicles
.
The presynaptic cell is usually a neuron. (Specialized re-
ceptor cells may form synaptic connections with the dendrites
of neurons, a process that will be described in Chapter 15.)
The postsynaptic cell can be either a neuron or another type
of cell. When one neuron communicates with another, the
synapse may occur on a dendrite, on the cell body, or along the
length of the axon of the receiving cell. A synapse between a
neuron and a muscle cell is called a
neuromuscular junction
.
l
p. 304
At a
neuroglandular junction
, a neuron controls
or regulates the activity of a secretory (gland) cell. Neurons
also innervate a variety of other cell types, such as adipocytes
(fat cells). We will consider the nature of that innervation in
later chapters.
The structure of the synaptic terminal varies with the type
of postsynaptic cell. A relatively simple, round
synaptic knob
occurs where the postsynaptic cell is another neuron.
1
At a
synapse, a narrow
synaptic cleft
separates the
presynaptic
membrane
, where neurotransmitters are released, from the
postsynaptic membrane
, which bears receptors for neuro-
transmitters (
Figure 12–2
). The synaptic terminal at a neuro-
muscular junction is much more complex. We will primarily
consider the structure of synaptic knobs in this chapter, leav-
ing the details of other types of synaptic terminals to later
chapters.
Each synaptic knob contains mitochondria, portions of
the endoplasmic reticulum, and thousands of vesicles filled
with neurotransmitter molecules. Breakdown products of
neurotransmitters released at the synapse are reabsorbed and
reassembled at the synaptic knob, which also receives a con-
tinuous supply of neurotransmitters synthesized in the cell
body, along with enzymes and lysosomes. These materials
travel the length of the axon along neurotubules, pulled along
by “molecular motors,” called
kinesin
and
dynein
, that run on
ATP. The movement of materials between the cell body and
synaptic knobs is called
axoplasmic transport
. Some mater-
ials travel slowly, at rates of a few millimeters per day. This
transport mechanism is known as the “slow stream.” Vesicles
containing neurotransmitters move much more rapidly, trav-
eling in the “fast stream” at 5–10 mm per hour.
Axoplasmic transport occurs in both directions. The flow
of materials from the cell body to the synaptic knob is
anterograde
(AN-ter-o-grad;
antero-
, forward)
flow,
carried by
kinesin. At the same time, other substances are being trans-
ported toward the cell body in
retrograde
(RET-ro-grad)
flow
(
retro
, backward), carried by dynein. If debris or unusual chem-
icals appear in the synaptic knob, retrograde flow soon delivers
them to the cell body. The arriving materials may then alter the
activity of the cell by turning appropriate genes on or off.
CLINICAL NOTE
Rabies
Rabies
is perhaps the most dramatic example of a
clinical condition directly related to retrograde flow. A bite
from a rabid animal injects the rabies virus into peripheral
tissues, where virus particles quickly enter synaptic knobs
and peripheral axons. Retrograde flow then carries the
virus into the CNS, with fatal results. Many toxins
(including heavy metals), some pathogenic bacteria, and
other viruses also bypass CNS defenses by exploiting
axoplasmic transport.
The Classification of Neurons
Neurons can be grouped by structure or by function.
Structural Classification of Neurons
Neurons are classified as anaxonic, bipolar, unipolar, or mul-
tipolar on the basis of the relationship of the dendrites to the
cell body and the axon (
Figure 12–3
):
•
Anaxonic
(an-ak-SON-ik)
neurons
are small and have no
anatomical features that distinguish dendrites from
axons; all the cell processes look alike. Anaxonic neurons
are located in the brain and in special sense organs. Their
functions are poorly understood.
•
Bipolar neurons
have two distinct processes—one
dendritic process that branches extensively at its distal
tip, and one axon—with the cell body between the two.
Bipolar neurons are rare, but occur in special sense
organs, where they relay information about sight, smell,
or hearing from receptor cells to other neurons. Bipolar
neurons are small; the largest measure less than 30 mm
from end to end.
•
In a
unipolar neuron
,or
pseudounipolar neuron
, the
dendrites and axon are continuous—basically, fused—and
the cell body lies off to one side. In such a neuron, the
initial segment lies where the dendrites converge. The rest
of the process, which carries action potentials, is usually
considered to be an axon. Most sensory neurons of the
peripheral nervous system are unipolar. Their axons may
extend a meter or more, ending at synapses in the central
1
The term
synaptic knob
is widely recognized and will be used throughout this
text. However, the same structures are also called terminal buttons, terminal bou-
tons, end bulbs, or neuropods.
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