390
UNIT 3
Regulation and Integration of the Body
11
glucose. Tey cannot survive for more than a few minutes
without oxygen.
Although neurons vary in structure, they all have a
cell body
and one or more slender
processes
(Figure 11.4)
.
Neuron Cell Body
Te
neuron cell body
consists of a spherical nucleus with a con-
spicuous nucleolus surrounded by cytoplasm. Also called the
perikaryon
(
peri
5
around,
kary
5
nucleus) or
soma
, the cell
body ranges in diameter from 5 to 140 μm. Te cell body is the
major
biosynthetic center
of a neuron and so it contains the usual
organelles needed to synthesize proteins and other chemicals.
Te neuron cell body’s protein- and membrane-making ma-
chinery, consisting of clustered free ribosomes and rough endoplas-
mic reticulum (ER), is probably the most active and best developed
in the body. Tis rough ER, also called the
chromatophilic sub-
stance
(
chromatophilic
5
color loving) or
Nissl bodies
(nis
9
l), stains
darkly with basic dyes. Te Golgi apparatus is also well developed
and forms an arc or a complete circle around the nucleus.
Mitochondria are scattered among the other organelles.
Microtubules and
neurofibrils
, which are bundles of interme-
diate filaments (
neurofilaments
), are important in maintaining
cell shape and integrity. Tey form a network throughout the
cell body.
Te cell body of some neurons also contains pigment in-
clusions. For example, some contain a black melanin, a red
iron-containing pigment, or a golden-brown pigment called
lipofuscin
(lip
0
o-fu
9
sin). Lipofuscin, a harmless by-product of
lysosomal activity, is sometimes called the “aging pigment” be-
cause it accumulates in neurons of elderly individuals.
Te cell body is the focal point for the outgrowth of neuron
processes during embryonic development. In most neurons, the
plasma membrane of the cell body also acts as
part of the recep-
tive region
that receives information from other neurons.
Most neuron cell bodies are located in the CNS, where they
are protected by the bones of the skull and vertebral column.
Clusters of cell bodies in the CNS are called
nuclei
, whereas
those that lie along the nerves in the PNS are called
ganglia
(gang
9
gle-ah;
ganglion
5
“knot on a string,” “swelling”).
Neuron Processes
Armlike
processes
extend from the cell body of all neurons. Te
brain and spinal cord (CNS) contain both neuron cell bodies
and their processes. Te PNS consists chiefly of neuron pro-
cesses. Bundles of neuron processes are called
tracts
in the CNS
and
nerves
in the PNS.
Te two types of neuron processes,
dendrites
and
axons
(ak
9
sonz), differ in the structure and function of their plasma
membranes. Te convention is to describe these processes using
a motor neuron as an example. We shall follow this practice, but
keep in mind that many sensory neurons and some tiny CNS
neurons differ from the “typical” pattern we present here.
Dendrites
Dendrites
of motor neurons are short, tapering, dif-
fusely branching extensions. ±ypically, motor neurons have hun-
dreds of twiglike dendrites clustering close to the cell body. Virtu-
ally all organelles present in the cell body also occur in dendrites.
Oligodendrocytes
Tough they also branch, the
oligodendro-
cytes
(ol
0
ĭ-go-den
9
dro-sīts) have fewer processes (
oligo
5
few;
dendr
5
branch) than astrocytes. Oligodendrocytes line up
along the thicker nerve fibers in the CNS and wrap their pro-
cesses tightly around the fibers, producing an insulating cover-
ing called a
myelin sheath
(Figure 11.3d).
Neuroglia in the PNS
Te two kinds of PNS neuroglia—
satellite cells
and
Schwann
cells
—differ mainly in location.
Satellite cells
surround neuron cell bodies located in the
peripheral nervous system (Figure 11.3e), and are thought to
have many of the same functions in the PNS as astrocytes do in
the CNS. Teir name comes from a fancied resemblance to the
moons (satellites) around a planet.
Schwann cells
(also called
neurolemmocytes
) surround all
nerve fibers in the PNS and form myelin sheaths around the
thicker nerve fibers (Figure 11.3e and 11.4a). In this way, they
are functionally similar to oligodendrocytes. (We describe the
formation of myelin sheaths later in this chapter.) Schwann cells
are vital to regeneration of damaged peripheral nerve fibers.
Check Your Understanding
3.
Which type of neuroglia controls the extracellular fluid
environment around neuron cell bodies in the CNS? In the PNS?
4.
Which two types of neuroglia form insulating coverings
called myelin sheaths?
For answers, see Appendix H.
Neurons
Define neuron, describe its important structural
components, and relate each to a functional role.
Differentiate between (1) a nerve and a tract, and (2) a
nucleus and a ganglion.
Explain the importance of the myelin sheath and describe how
it is formed in the central and peripheral nervous systems.
Te billions of
neurons
, also called
nerve cells
, are the structural
units of the nervous system. Tey are typically large, highly spe-
cialized cells that conduct messages in the form of nerve impulses
from one part of the body to another. Besides their ability to con-
duct nerve impulses, they have three other special characteristics:
Neurons have
extreme longevity
. Given good nutrition, they
can function optimally for a lifetime (over 100 years).
Neurons are
amitotic
. As neurons assume their roles as com-
municating links of the nervous system, they lose their ability
to divide. We pay a high price for this feature because neu-
rons cannot be replaced if destroyed. Tere
are
exceptions to
this rule. For example, olfactory epithelium and some hip-
pocampal regions contain stem cells that can produce new
neurons throughout life. (Te hippocampus is a brain region
involved in memory.)
Neurons have an exceptionally
high metabolic rate
and re-
quire continuous and abundant supplies of oxygen and
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