C h a p t e r 3 4
Organization and Control of Neural Function
823
the dendrites and the axon. The proteins and other mate-
rials used by the axon are synthesized in the cell body
and then flow down the axon through its cytoplasm.
The cell body of the neuron is equipped for a high
level of metabolic activity. This is necessary because the
cell body must synthesize the cytoplasmic and mem-
brane constituents required to maintain the function of
the axon and its terminals. Some of these axons extend
for a distance of 1 to 1.5 m and have a volume that is
200 to 500 times greater than the cell body itself. Two
axonal transport systems, one slow and one rapid, move
molecules from the cell body through the cytoplasm of
the axon to its terminals. Replacement proteins and
nutrients slowly diffuse from the cell body, where they
are transported down the axon, moving at the rate of
approximately 1 mm/day. Other molecules, such as neu-
rosecretory granules (e.g., neurotransmitters and neuro-
hormones) or their precursors, are conveyed by a rapid,
energy-dependent active transport system, moving at
the rate of approximately 400 mm/day. For example,
antidiuretic hormone (ADH) and oxytocin, which are
synthesized by neurons in the hypothalamus, are car-
ried by rapid axonal transport to the posterior pituitary,
where the hormones are released into the bloodstream.
A reverse (retrograde) axonal transport is responsible
for moving molecules destined for degradation from the
axon back to the cell body, where they are broken down.
Neuroglial Cells
The neuroglial, or supporting cells, which outnumber
neurons 10 to 1, provide support and protection for
neurons in both CNS and PNS. Although they do not
participate directly in the short-term communication of
information through the nervous system, the support-
ing cells segregate the neurons into isolated metabolic
compartments, which are required for normal neural
function.
The neuroglial cells in the PNS include the Schwann
cells and satellite cells. The CNS has four types of glial
cells: astrocytes, oligodendrocytes (oligodendroglia),
microglia, and ependymal cells. Two of these cell types
share a similar function: The Schwann cells of the PNS
and the oligodendrocyte of the CNS wrap nerve axons
in multiple layers, producing myelin sheaths that serve
to increase the velocity of nerve impulse conduction in
axons (see Fig. 34-1B, inset). Myelin has a high lipid
content, which gives it a whitish color, and hence the
name
white matter
is given to the masses of myelin-
ated axons of the spinal cord and brain. Besides its role
in increasing conduction velocity, the myelin sheath is
essential for the survival of larger neuronal processes,
perhaps by secreting neurotrophic compounds. In
some pathologic conditions, such as multiple sclero-
sis in the CNS and Guillain-Barré syndrome involving
the PNS, the myelin may degenerate or be destroyed,
leaving a section of the axonal process without myelin
while leaving the nearby Schwann or oligodendroglial
cells intact. Unless remyelination takes place, the axon
eventually dies.
Neuroglial Cells of the Peripheral
Nervous System
The Schwann cells and satellite cells provide support and
protection for the PNS.
Schwann cells
produce the myelin
sheath that isolates nerve axons in the PNS from the sur-
rounding extracellular compartment, ensuring rapid con-
duction of nerve impulses. They also aid in cleaning up
PNS debris and guide the regrowth of PNS nerve fibers.
During the process of myelination, the Schwann cell wraps
around each nerve fiber several times in a “jelly roll” fash-
ion (Fig. 34-2). Successive Schwann cells are separated by
short extracellular fluid-filled gaps, called the
nodes of
Ranvier,
where the myelin is missing and voltage-gated
sodium channels are concentrated. The nodes of Ranvier
increase the speed of nerve conduction by allowing the
impulse to jump from node to node through the extracel-
lular fluid in a process called
saltatory conduction.
In this
way, the impulse can travel more rapidly than it could if
it was required to move systematically along the entire
nerve fiber. This increased conduction velocity greatly
reduces reaction time, or time between the application of
a stimulus and the subsequent motor response. The short
reaction time is especially important in peripheral nerves
with long distances for conduction between the CNS and
distal effector organs.
Schwann cell nucleus
Layers of myelin
Axon
Node of Ranvier
Schwann cell
Epineurium
Perineurium
Endoneurium
FIGURE 34-2.
Section of a peripheral nerve containing both
afferent (sensory) and efferent (motor) neurons. Schwann
cells form a myelin sheath around the larger nerve fibers in
the peripheral nervous system. Successive Schwann cells are
separated by short extracellular fluid gaps called the nodes of
Ranvier, where the myelin is missing and the voltage-gated
sodium channels are concentrated.
