138
Chapter 6
The nervous system is composed of trillions of cells distributed in a network throughout the brain,
spinal cord, and periphery. These cells communicate with each other by electrical and chemical signals.
They maintain homeostasis by coordinating the functions of internal organs, as well as mediating
sensation, controlling movements, and encoding the fabulous complexity that is the human mind. In
this chapter, we discuss the structure of individual nerve cells, the chemical and electrical mechanisms
underlying nerve cell function, and the basic organization and major divisions of the nervous system.
SECTION A
Neural Tissue
The various structures of the nervous system are intimately
interconnected, but for convenience we divide them into two
parts: (1) the
central nervous system (CNS),
composed of
the brain and spinal cord, and (2) the
peripheral nervous
system,
consisting of the nerves that connect the brain or spi-
nal cord with the body’s muscles, glands, and sense organs.
The basic unit of the nervous system is the individual
nerve cell, or
neuron.
Neurons operate by generating electri-
cal signals that move from one part of the cell to another part
of the same cell or to neighboring cells. In most neurons, the
electrical signal causes the release of chemical messengers—
neurotransmitters
—to communicate with other cells. Most
neurons serve as
integrators
because their output refl ects the
balance of inputs they receive from thousands or hundreds of
thousands of other neurons that impinge upon them.
Structure and Maintenance
of Neurons
Neurons occur in a wide variety of sizes and shapes, but all
share features that allow cell-to-cell communication. Long
extensions, or
processes,
connect neurons to each other and
perform the neuron’s input and output functions. As shown in
Figure 6–1
, most neurons contain a cell body and two types
of processes—dendrites and axons.
As in other types of cells, a neuron’s
cell body
(or
soma
)
contains the nucleus and ribosomes and thus has the genetic
information and machinery necessary for protein synthesis.
The
dendrites
are a series of highly branched outgrowths of
the cell body. They and the cell body receive most of the inputs
from other neurons, with the dendrites taking a more impor-
tant role in this regard than the cell body. The branching den-
drites increase the cell’s surface area—some neurons may have
as many as 400,000 dendrites! Thus, dendrites increase a cell’s
capacity to receive signals from many other neurons.
The
axon,
sometimes also called a
nerve fi
ber,
is a long
process that extends from the cell body and carries output to
its target cells. Axons range in length from a few microns to
over a meter. The region where the axon connects to the cell
body is known as the
initial segment
(or
axon hillock
). The
initial segment is the “trigger zone” where, in most neurons,
the electrical signals are generated. These signals then propa-
gate away from the cell body along the axon or, sometimes,
back along the dendrites. The axon may have branches, called
collaterals.
Near their ends, both the axon and its collater-
als undergo further branching (see Figure 6–1). The greater
the degree of branching of the axon and axon collaterals, the
greater the cell’s sphere of infl uence.
Each branch ends in an
axon terminal,
which is respon-
sible for releasing neurotransmitters from the axon. These
chemical messengers diffuse across an extracellular gap to the
cell opposite the terminal. Alternatively, some neurons release
their chemical messengers from a series of bulging areas along
the axon known as
varicosities.
The axons of many neurons are covered by
myelin
(
Figure 6–2
), which usually consists of 20 to 200 layers of
highly modifi ed plasma membrane wrapped around the axon
by a nearby supporting cell. In the brain and spinal cord,
these myelin-forming cells are the
oligodendrocytes.
Each
oligodendrocyte may branch to form myelin on as many as 40
Figure 6–1
(a) Diagrammatic representation of a neuron. The break in the axon
indicates that axons may extend for long distances; in fact, they may
be 5000 to 10,000 times longer than the cell body is wide. This
neuron is a common type, but there are a wide variety of neuronal
morphologies, one of which has no axon. (b) A neuron as observed
through a microscope. The axon terminals cannot be seen at this
magnifi cation.
Dendrites
Cell body
Initial segment
Axon
collateral
Axon
Axon
terminals
(b)
(a)
previous page 166 Vander's Human Physiology The Mechanisms of Body Function read online next page 168 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off