Neuronal Signaling and the Structure of the Nervous System
139
axons. In the peripheral nervous system, cells called
Schwann
cells
form individual myelin sheaths at regular intervals along
the axons. The spaces between adjacent sections of myelin
where the axon’s plasma membrane is exposed to extracellular
fl uid are the
nodes of Ranvier.
The myelin sheath speeds up
conduction of the electrical signals along the axon and con-
serves energy.
To maintain the structure and function of the cell axon,
various organelles and other materials must move as far as one
meter between the cell body and the axon terminals. This
movement, termed
axonal transport,
depends on a scaffold-
ing of microtubule “rails” running the length of the axon
(Chapter 3) and specialized types of “motor proteins” known
as
kinesins
and
dyneins
(
Figure 6–3
). At one end, these
double-headed motor proteins bind to their cellular cargo,
while the other end uses energy derived from the hydrolysis
of ATP to “walk” along the microtubules. Kinesin transport
mainly occurs from the cell body toward the axon terminals
(
anterograde
), and is important in moving nutrient mol-
ecules, enzymes, mitochondria, neurotransmitter-fi lled vesi-
cles, and other organelles. Dynein movement is in the other
direction (
retrograde
), carrying recycled membrane vesicles,
growth factors, and other chemical signals that can affect
the neuron’s morphology, biochemistry, and connectivity.
Retrograde transport is also the route by which some harmful
agents invade the central nervous system, including tetanus
toxin and the herpes simplex, rabies, and polio viruses.
Functional Classes of Neurons
Neurons can be divided into three functional classes: afferent
neurons, efferent neurons, and interneurons (
Figure 6–4
).
Afferent neurons
convey information from the tissues and
organs of the body
into
the central nervous system.
Efferent
neurons
convey information from the central nervous system
out
to effector cells like muscle, gland, or other nerve cells.
Interneurons
connect neurons
within
the central nervous
system. As a rough estimate, for each afferent neuron entering
the central nervous system, there are 10 efferent neurons and
200,000 interneurons. Thus, the great majority of neurons are
interneurons.
At their peripheral ends (the ends farthest from the cen-
tral nervous system), afferent neurons have
sensory recep-
tors,
which respond to various physical or chemical changes in
their environment by generating electrical signals in the neu-
ron. The receptor region may be a specialized portion of the
plasma membrane or a separate cell closely associated with the
neuron ending. (Recall from Chapter 5 that the term
recep-
tor
has two distinct meanings, the one defi ned here and the
other referring to the specifi c proteins a chemical messenger
combines with to exert its effects on a target cell.) Afferent
neurons propagate electrical signals from their receptors into
the brain or spinal cord.
Afferent neurons are unusual because they have only a
single process, usually considered an axon. Shortly after leav-
ing the cell body, the axon divides. One branch, the periph-
eral process, begins at the receptors. The other branch, the
central process, enters the central nervous system to form
Figure 6–2
Myelin formed by Schwann cells (a) and oligodendrocytes (b) on
axons. Electron micrograph of transverse sections of myelinated
axons in brain (c).
(c)
Myelin
sheath
Axon
Axon
Cell body
Terminal
Schwann cell
nucleus
Myelin
Oligodendrocyte
Node of
Ranvier
(a)
(b)
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