PERIPHERAL NERVE FIBERS
Figure 8­18
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Holdings,
A bundle, or fascicle, of nerve fibers (A) in the peripheral nervous system contains a combination of both afferent
(sensory) fibers and efferent (motor) fibers and, therefore, is a mixed nerve. One cannot distinguish between the
LLC.
two types of fibers by routine histologic methods for light microscopy. The cell bodies of these nerve fibers are loc-
ated either in collections of neurons called ganglia in the peripheral nervous system or nuclei in the central nervous
Not
system.
to
be
redistributed
The fascicles of nerve fibers are enclosed by, and invested in, connective tissue. The layer of connective tissue that
ensheathes the entire group of fascicles is known as the epineurium (arrow), which is the fibrous connective tissue
that carries blood vessels that supply the fascicles. The epineurium extends from the outermost regions of the nerve
inwardly to fill in the spaces between the individual fascicles to bind them together (double arrow).
or
modified
In addition, each fascicle is also encased within a specialized, densely stained tissue called the perineurium (ar-
rowhead). This arrangement of connective tissue and specialized tissue associated with peripheral nerves is different
in
from that investing skeletal muscle and its fascicles. Recall that the epimysium surrounds the entire muscle and is
any
continuous with the perimysium around the fascicles. In nerve fibers, the perineurium is a tissue that differs from
way
the epineurium.
without
Each nerve fiber is enclosed by a Schwann cell, a glial or nonneuronal cell derived from neural crest tissue, which
produces the myelin sheath of some of the larger fibers. The smaller nerve fibers may not be myelinated but will
permission.
have a Schwann cell covering them. The Schwann cell and its nerve fiber are enclosed by loose connective tissue,
the endoneurium, which contains collagen fibrils and capillaries.
This image shows a transverse section through a peripheral nerve containing three darkly stained fascicles. Blood
vessels (B) can be identified in the epineurium between the individual fascicles.
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