Chapter 13
The Peripheral Nervous System and Reflex Activity
491
13
(Tese are the
dorsal root ganglia
described in Chapter 12.) Gan-
glia associated with
efferent
nerve fibers mostly contain cell bod-
ies of autonomic motor neurons. We will describe these more
complicated ganglia in Chapter 14.
Regeneration of Nerve Fibers
Damage to nervous tissue is serious because, as a rule, mature
neurons do not divide. If the damage is severe or close to the
cell body, the entire neuron may die, and other neurons that are
normally stimulated by its axon may die as well. However, if the
cell body remains intact, cut or compressed axons of peripheral
nerves can regenerate successfully.
PNS Fibers
What happens if a peripheral axon is severed or crushed? As
shown in
Figure 13.5
:
1
The axon fragments.
Almost immediately, the separated
ends seal themselves off and then swell as substances be-
ing transported along the axon accumulate in the sealed
ends. Within a few hours, the axon and its myelin sheath
distal to the injury site begin to disintegrate because they
cannot receive nutrients from the cell body. Tis process,
Wallerian degeneration
, spreads distally from the injury
site, fragmenting the distal part of the axon.
2
Macrophages clean out the dead axon.
Within a week, mac-
rophages generally degrade the entire axon distal to the injury,
disposing of the debris. However, the myelin sheath remains
intact within the endoneurium. Te macrophages also release
chemicals that stimulate the Schwann cells to divide.
3
Axon filaments grow through a regeneration tube.
Sur-
viving Schwann cells proliferate in response to the mitosis-
stimulating chemicals, and migrate into the injury site. Once
there, they release growth factors and begin to express cell
adhesion molecules (CAMs) that encourage axonal growth.
Additionally, they form a
regeneration tube
, a system of cel-
lular cords that guide the regenerating axon “sprouts” across
the gap to their original contacts.
4
The axon regenerates and a new myelin sheath forms.
Te
same Schwann cells protect, support, and remyelinate the
regenerating axons.
Changes also occur in the neuronal cell body aFer the axon
has been destroyed. Within two days, its rough endoplasmic
reticulum breaks apart, and then the cell body swells as protein
synthesis revs up to support regeneration of its axon.
Axons regenerate at the approximate rate of 1.5 mm a day. Te
greater the distance between the severed ends, the less the chance
of recovery because adjacent tissues block growth by protruding
into the gaps, and axonal sprouts fail to find the regeneration tube.
Neurosurgeons align cut nerve ends surgically to promote success-
ful regeneration, and scaffolding devices can help guide the axons.
Whatever the measures taken, post-trauma axon regrowth
never exactly matches what existed before the injury. Patients
must oFen “retrain” the nervous system to respond appropri-
ately so that stimulus and response are coordinated.
Blood
vessels
Fascicle
Fascicle
Epineurium
Perineurium
Perineurium
Endoneurium
Epineurium
Axon
Myelin sheath
(b)
(a)
Nerve
fibers
Endoneurium
Blood
vessel
Figure 13.4
Structure of a nerve.
(a)
Scanning electron
micrograph of a cross section of a portion of a nerve (90
3
).
(b)
Three-dimensional view of a portion of a nerve, showing
connective tissue wrappings.
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