364
UNIT THREE
axons in the CNS lack neurilemmae, and the myelin-
producing oligodendrocytes do not proliferate following
injury. Consequently, the proximal end of a damaged axon
that begins to grow has no tube of sheath cells to guide it.
Therefore, regeneration is unlikely.
If a peripheral nerve is severed, it is important that the two cut ends
be connected as soon as possible so that the regenerating sprouts
of the axons can more easily reach the tubes formed by the base-
ment membranes and connective tissues on the distal side of the
gap. When the gap exceeds 3 millimeters, the regenerating axons
may form a tangled mass called a neuroma. It is composed of sensory
axons and is painfully sensitive to pressure. Neuromas sometimes
complicate a patient’s recovery following limb amputation.
PRACTICE
4
What are neuroglia?
5
Name and describe four types of neuroglia.
6
What are some functions of neuroglia?
7
Explain how an injured peripheral axon might regenerate.
8
Explain why functionally signiF
cant regeneration is unlikely in
the CNS.
Neuroglia and Axonal Regeneration
Injury to the cell body usually kills the neuron, and because
mature neurons do not divide, the destroyed cell is not
replaced unless neural stem cells become stimulated to pro-
liferate. However, a damaged peripheral axon may regener-
ate. For example, if injury or disease separates an axon in a
peripheral nerve from its cell body, the distal portion of the
axon and its myelin sheath deteriorate within a few weeks.
Macrophages remove the fragments of myelin and other cel-
lular debris. The proximal end of the injured axon develops
sprouts shortly after the injury. Infl
uenced by nerve growth
factors that nearby neuroglia secrete, one of these sprouts
may grow into a tube formed by remaining basement mem-
brane and connective tissue. At the same time, any remaining
Schwann cells proliferate along the length of the degenerat-
ing portion and form new myelin around the growing axon.
Growth of a regenerating axon is slow (3 to 4 millime-
ters per day), but eventually the new axon may reestablish
the former connection
(f g. 10.10)
. Nerve growth factors,
secreted by neuroglia, may help direct the growing axon.
However, the regenerating axon may still end up in the
wrong place, so full function often does not return.
If an axon of a neuron within the CNS is separated from
its cell body, the distal portion of the axon will degenerate,
but more slowly than a separated axon in the PNS. However,
FIGURE 10.10
If a myelinated axon is injured, the following events may occur over several weeks to months: (
a
) The proximal portion of the axon
may survive, but (
b
) the portion distal to the injury degenerates. (
c
and
d
) In time, the proximal portion may develop extensions that grow into the
tube of basement membrane and connective tissue cells that the axon previously occupied and (
e
) possibly reestablish the former connection. Nerve
growth factors that neuroglia secrete assist in the regeneration process.
Skeletal
muscle fiber
Motor neuron
cell body
Axon
Site of injury
Proximal end of injured axon
regenerates into tube of sheath cells
Schwann cells
Distal portion of
axon degenerates
Schwann cells
degenerate
Former connection
reestablished
(a)
(b)
(c)
(d)
(e)
Schwann cells
proliferate
Changes
over time
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