Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
393
11
Classification of Neurons
Classify neurons by structure and by function.
Neurons are classified both structurally and functionally. We
describe both classifications here but use the functional clas-
sification in most discussions.
Structural Classification
Neurons are grouped structurally ac-
cording to the number of processes extending from their cell
body. Tree major neuron groups make up this classification:
multipolar (
polar
5
end, pole), bipolar, and unipolar neurons.
(
Table 11.1
is organized according to these three neuron types,
and the top row shows their structures.)
Multipolar neurons
have three or more processes—one axon
and the rest dendrites. Tey are the most common neuron type
in humans, with more than 99% of neurons belonging to this
class. Multipolar neurons are the major neuron type in the CNS.
Bipolar neurons
have two processes—an axon and a
dendrite—that extend from opposite sides of the cell body.
Tese rare neurons are found in some of the special sense
organs. Examples include some neurons in the retina of the eye
and in the olfactory mucosa.
Unipolar neurons
have a single short process that emerges
from the cell body and divides ±-like into proximal and distal
branches. Te more distal process, the
peripheral process
, is
oFen associated with a sensory receptor. Te
central process
enters the CNS (±able 11.1). Unipolar neurons are more accu-
rately called
pseudounipolar neurons
(
pseudo
5
false) because
they originate as bipolar neurons. Ten, during early embry-
onic development, the two processes converge and partially fuse
to form the short single process that issues from the cell body.
Unipolar neurons are found chiefly in ganglia in the PNS, where
they function as sensory neurons.
Te fact that the fused peripheral and central processes of
unipolar neurons are continuous and function as a single fiber
might make you wonder whether they are axons or dendrites.
Te central process is definitely an axon because it conducts
impulses away from the cell body (one definition of axon).
However, the peripheral process is perplexing. Tree facts favor
classifying it as an axon: (1) It generates and conducts an im-
pulse (functional definition of axon); (2) when large, it is heavily
myelinated; and (3) it has a uniform diameter and is indistin-
guishable microscopically from an axon. However, the older
definition of a dendrite as a process that transmits impulses
to-
ward
the cell body interferes with that conclusion.
So which is it? In this book, we have chosen to emphasize
the newer definition of an axon as generating and transmitting
an impulse. ²or
unipolar neurons
, we will refer to the combined
length of the peripheral and central process as an axon. In place
of “dendrites,” unipolar neurons have
receptive endings
(sensory
terminals) at the end of the peripheral process.
Functional Classification
Tis scheme groups neurons ac-
cording to the direction in which the nerve impulse travels
relative to the central nervous system. Based on this criterion,
there are sensory neurons, motor neurons, and interneurons
(±able 11.1, last row).
much like gauze wrapped around an injured finger. Tis tight
coil of wrapped membranes is the myelin sheath, and its thick-
ness depends on the number of spirals. Te nucleus and most of
the cytoplasm of the Schwann cell end up as a bulge just exter-
nal to the myelin sheath. Tis portion of the Schwann cell, next
to the exposed part of its plasma membrane, is called the
outer
collar of perinuclear cytoplasm
(formerly known as the
neuri-
lemma
) (²igure 11.5b).
Plasma membranes of myelinating cells contain much less
protein than the plasma membranes of most body cells. Chan-
nel and carrier proteins are notably absent, a characteristic that
makes myelin sheaths exceptionally good electrical insulators.
Another unique characteristic of these membranes is the pres-
ence of specific protein molecules that interlock to form a sort
of molecular Velcro between adjacent myelin membranes.
Adjacent Schwann cells along an axon do not touch one
another, so there are gaps in the sheath. Tese
myelin sheath
gaps
, or
nodes of Ranvier
(ran
9
vē-ā
0
), occur at regular intervals
(about 1 mm apart) along a myelinated axon. Axon collaterals
can emerge from the axon at these gaps.
Sometimes Schwann cells surround peripheral nerve fibers
but the coiling process does not occur. In such instances, a sin-
gle Schwann cell can partially enclose 15 or more axons, each
of which occupies a separate recess in the Schwann cell surface.
Nerve fibers associated with Schwann cells in this manner are
said to be
nonmyelinated
and are typically thin fibers.
Myelination in the CNS
Te central nervous system contains
both myelinated and nonmyelinated axons. However, in the
CNS, it is the oligodendrocytes that form myelin sheaths (²ig-
ure 11.3d).
Unlike a Schwann cell, which forms only one segment of a
myelin sheath, an oligodendrocyte has multiple flat processes
that can coil around as many as 60 axons at the same time. As
in the PNS, myelin sheath gaps separate adjacent sections of
an axon’s myelin sheath. However, CNS myelin sheaths lack an
outer collar of perinuclear cytoplasm because cell extensions
do the coiling and the squeezed-out cytoplasm is forced back
toward the centrally located nucleus instead of peripherally.
As in the PNS, the smallest-diameter axons are nonmyelin-
ated. Tese nonmyelinated axons are covered by the long exten-
sions of adjacent glial cells.
Regions of the brain and spinal cord containing dense collec-
tions of myelinated fibers are referred to as
white matter
and are
primarily fiber tracts.
Gray matter
contains mostly nerve cell
bodies and nonmyelinated fibers.
Check Your Understanding
5.
Which part of the neuron is its fiber? How do nerve fibers
differ from the fibers of connective tissue (see Chapter 4)
and the fibers in muscle (see Chapter 9)?
6.
How does a nucleus within the brain differ from a nucleus
within a neuron?
7.
How is a myelin sheath formed in the CNS, and what is its
function?
For answers, see Appendix H.
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