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Nervous System
Each of the Schwann cells along a peripheral nerve
fiber is encased in a continuous tube of basement mem-
brane, which in turn is surrounded by a multilayered
sheath of loose connective tissue known as the
endoneu-
rium
(see Fig. 34-2). The endoneurial sheath, which is
essential to the regeneration of peripheral nerves, pro-
vides a collagenous tube through which a regenerating
axon can again reach its former target. The endoneu-
rial sheath does not penetrate the CNS. The absence of
the endoneurial sheaths is thought to be a major factor
in the limited axonal regeneration of CNS nerves com-
pared with those of the PNS.
The endoneurial sheaths are bundled with blood
vessels into small clusters of nerves called
fascicles
.
In the nerve, fascicles consisting of bundles of nerve
fibers are surrounded by another protective covering
called the
perineurium
. Usually, several fascicles are
further surrounded by the heavy, protective
epineu-
rial sheath
of the peripheral nerve. The protective lay-
ers that surround the peripheral nerve processes are
continuous with the connective tissue capsule of the
sensory nerve endings and the connective tissue that
surrounds the effector structures, such as the skeletal
muscle cell. Centrally, the connective tissue layers con-
tinue along the dorsal and ventral roots of the nerve
and fuse with the meninges that surround the spinal
cord and brain.
Satellite cells are a type of neuroglial cell that sur-
round the cell body of neurons in the PNS, including
those with ganglia. They have been found to serve a
variety of roles, including control over the microen-
vironment of sympathetic ganglia. Satellite cells are
thought to have a role similar to that of astrocytes in the
CNS. They supply nutrients to the surrounding neurons,
and they also act as protective, cushioning cells.
Neuroglial Cells of the Central Nervous System
The neuroglial cells of the CNS consists of four types of
cells: oligodendrocytes, astrocytes, microglia, and epen-
dymal cells (Fig. 34-3). The
oligodendrocytes
form the
myelin in the CNS. Instead of forming a myelin cover-
ing for a single axon, these cells reach out with several
processes, each wrapping around and forming a multi-
layered myelin segment around several different axons.
As within the PNS, the covering of axons in the CNS
increases the velocity of nerve conduction.
Astrocytes
are the largest and most numerous of neu-
roglia and are particularly prominent in the gray mat-
ter of the CNS. They form a network within the CNS
and communicate with neurons to support and modulate
their activities. Astrocytes have many processes, some
stretching their processes from blood vessels to neurons
and others filling most of the intercellular space within
the CNS. It is now thought that astrocytes play an impor-
tant role in the movement of metabolites and wastes to
and from neurons and regulate ionic concentrations in
the intercellular compartment. In addition, astrocytes
take up neurotransmitters from synaptic zones after their
release and thereby help regulate synaptic activity. They
also have a role in maintaining the tight junctions of cap-
illaries that form the blood-brain barrier. Astrocytes are
also the principal cells responsible for repair and scar
formation in the brain. They can fill their cytoplasm with
microfibrils (i.e., fibrous astrocytes), and masses of these
cells form the special type of scar tissue that develops in
the CNS when tissue is destroyed, a process called
gliosis
.
A third type of neuroglia, the
microglia,
the microglial
cells are the resident macrophages of the central nervous
system. They are available for cleaning up debris after
cellular damage, infection, or cell death. The fourth type
Capillary
Astrocyte
Ependymal
cells
Cerebrospinal
fluid
Oligodendrocyte
Neuron
Microglial cell
FIGURE 34-3.
The supporting
neuroglial cells of the central
nervous system (CNS).
Diagrammatic view of
relationships between the
glial elements (astrocyte,
oligodendrocyte, microglial cell,
and ependymal cells), capillaries,
cerebrospinal fluid, and cell
bodies of CNS neurons.
