122
UNIT 2
Basic Tissues
Peripheral Nervous System
CLINICAL CORRELATION
Figure 7-5C.
Hereditary Sensory Motor Neuropathy,
Type III (HSMN III).
Paragon stain of epon section,
3
680;
inset 1
, paragon,
3
1,150;
inset 2
, toluidine blue,
3
907
The profound loss of large myelinated axons, shown here
in a
hereditary
demyelinating neuropathy
, is best appreci-
ated when compared with the density of large myelinated
axons in a normal peripheral nerve (
inset 2
). Surviving
large axons are remyelinated, encircled by thin sheaths of
compact myelin, which are in turn surrounded by layers
of Schwann cell processes such as the layers of an onion
(
onion bulb
,
inset 1, arrows
). Note the Schwann cell nuclei,
marked by sparse central chromatin (open chromatin) and
large nucleoli.
HSMN III
is both autosomal dominant and
recessive. Affected children are
ataxic
and have difF
culty
walking. They may have
scoliosis
, a curvature of the spine.
Sometimes peripheral nerves become so hypertrophic that
they can be palpated beneath the skin.
2
1
2
C
Figure 7-5A.
Cross section of a peripheral nerve.
Trichrome,
3
68; inset toluidine blue
3
336
Peripheral nerve
± bers
carry motor, sensory, and autonomic
nerve F bers. Peripheral nerves are surrounded by a sheath of
dense, irregular connective tissue, the
epineurium
. Blood vessels
that run with peripheral nerve trunks typically lie in the epineu-
rium. The axons in a nerve are arranged into clusters called
fas-
cicle
s
(
red dashed line
)
. Each fascicle is surrounded by a layer of
connective tissue, the
perineurium
, containing many fl attened
F broblasts. These cells are connected with each other, forming
a
blood-nerve barrier
similar to the blood-brain barrier. Within
fascicles, a loose, delicate connective tissue, the
endoneurium
,
surrounds each axon. The
inset
shows a small branch of a
motor nerve within a skeletal muscle. Many of the axons in this
branch are surrounded by a dense layer of myelin (
M
). There
are no separate fascicles within this small nerve branch but only
myelinated axons each surrounded by endoneurium.
M
Endoneurium
Endoneurium
Perineurium
Perineurium
Fascicle
Fascicle
Epineurium
Epineurium
Epineurium
Fascicle
M
Perineurium
Endoneurium
A
Figure 7-5B.
Posterior root ganglion.
H&E,
3
146
Posterior root ganglia
(
sensory
ganglia
) are enlargements in
the posterior peripheral nerve roots of the spinal cord (±ig.
7-4) and contain the cell bodies of
unipolar sensory neurons
(±ig. 7-1C) and their axons. The cell bodies are generally
round in shape with centrally located nuclei. There is a wide
range of sizes of neuron cell bodies, with the largest having
axons that are heavily myelinated and carry touch or muscle
stretch information and the smallest having axons that are
lightly myelinated or unmyelinated and that carry pain and
temperature information. Small glialike cells,
satellite cells
,
surround the neuron cell bodies and regulate the
extracellular
ionic environment. Schwann cells provide myelin for the
myelinated axons. The posterior root contains only sensory
neurons, whereas the anterior root contains axons of motor
neurons. In contrast to autonomic ganglia (see below), there
are no synapses in posterior root ganglia.
Satellite cell
Satellite cell
nucleus
nucleus
Blood
Blood
vessel
vessel
Axons
Axons
Unipolar
Unipolar
neuron
neuron
Satellite cell
nucleus
Unipolar
neuron
Axons
Blood
vessel
B
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