Peripheral Nervous System
Hereditary Sensory Motor Neuropathy,
Type III (HSMN III).
Paragon stain of epon section,
, toluidine blue,
The profound loss of large myelinated axons, shown here
, is best appreci-
ated when compared with the density of large myelinated
axons in a normal peripheral nerve (
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
inset 1, arrows
). Note the Schwann cell nuclei,
marked by sparse central chromatin (open chromatin) and
is both autosomal dominant and
recessive. Affected children are
and have difF
walking. They may have
, a curvature of the spine.
Sometimes peripheral nerves become so hypertrophic that
they can be palpated beneath the skin.
Cross section of a peripheral nerve.
68; inset toluidine blue
carry motor, sensory, and autonomic
nerve F bers. Peripheral nerves are surrounded by a sheath of
dense, irregular connective tissue, the
. Blood vessels
that run with peripheral nerve trunks typically lie in the epineu-
rium. The axons in a nerve are arranged into clusters called
red dashed line
. Each fascicle is surrounded by a layer of
connective tissue, the
, containing many ﬂ attened
F broblasts. These cells are connected with each other, forming
similar to the blood-brain barrier. Within
fascicles, a loose, delicate connective tissue, the
surrounds each axon. The
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 (
are no separate fascicles within this small nerve branch but only
myelinated axons each surrounded by endoneurium.
Posterior root ganglion.
Posterior root 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,
surround the neuron cell bodies and regulate the
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.