Fundamentals of the Nervous System and Nervous Tissue
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.
Neurons are grouped structurally ac-
cording to the number of processes extending from their cell
body. Tree major neuron groups make up this classiﬁcation:
end, pole), bipolar, and unipolar neurons.
is organized according to these three neuron types,
and the top row shows their structures.)
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.
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.
have a single short process that emerges
from the cell body and divides ±-like into proximal and distal
branches. Te more distal process, the
oFen associated with a sensory receptor. Te
enters the CNS (±able 11.1). Unipolar neurons are more accu-
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
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
, we will refer to the combined
length of the peripheral and central process as an axon. In place
of “dendrites,” unipolar neurons have
terminals) at the end of the peripheral process.
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
collar of perinuclear cytoplasm
(formerly known as the
) (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
nodes of Ranvier
), 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
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-
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
primarily ﬁber tracts.
contains mostly nerve cell
bodies and nonmyelinated ﬁbers.
Check Your Understanding
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)?
How does a nucleus within the brain differ from a nucleus
within a neuron?
How is a myelin sheath formed in the CNS, and what is its
For answers, see Appendix H.