492
UNIT 3
Regulation and Integration of the Body
13
CNS Fibers
Unlike peripheral nerve fibers, most CNS fibers never regen-
erate. Consequently, damage to the brain or spinal cord has
been viewed as irreversible. Tis difference in regenerative ca-
pacity seems to have less to do with the neurons themselves
than with the “company they keep”—oligodendrocytes, their
supporting cells.
Oligodendrocytes are studded with growth-inhibiting pro-
teins. Consequently, the neuronal growth cone collapses and
the fiber fails to regrow. Moreover, astrocytes at the site of in-
jury form scar tissue rich in chondroitin sulfate (see p. 127) that
blocks axonal regrowth.
For the clinician treating spinal cord injury, this means that
multiple inhibitory processes need to be blocked simultaneously
to promote axon regrowth. ±o date, experimentally neutralizing
the myelin-bound growth inhibitors, blocking the receptors for
the inhibitory proteins, or enzymatically destroying chondroi-
tin sulfate have yielded promising results.
Check Your Understanding
6.
What are ganglia?
7.
What is in a nerve besides axons?
8.
Bill’s femoral nerve was crushed while clinicians tried to control
bleeding from his femoral artery. This resulted in loss of function
and sensation in his leg, which gradually returned over the
course of a year. Which cells were important in his recovery?
For answers, see Appendix H.
Cranial Nerves
Name the 12 pairs of cranial nerves; indicate the body
region and structures innervated by each.
±welve pairs of
cranial nerves
are associated with the brain
(Figure 13.6)
. Te first two pairs attach to the forebrain, and
the rest are associated with the brain stem. Other than the vagus
nerves, which extend into the abdomen, cranial nerves serve
only head and neck structures.
In most cases, the names of the cranial nerves reveal either
the structures they serve or their functions. Te nerves are also
numbered (using Roman numerals) from the most rostral to
the most caudal.
We will begin with a brief overview of each cranial nerve
and then describe the composition of all cranial nerves (p. 501).
A²er that, you will be ready to tackle
Table 13.2
(p. 494), which
provides detailed descriptions of the cranial nerves. Notice that
it describes the pathways of the purely or mostly sensory nerves
(I, II, and VIII) from the receptors to the brain, and the path-
ways of the other nerves in the oppositive direction (from the
brain distally).
An Overview
I.
Olfactory.
Tese are the tiny sensory nerves (filaments) of
smell, which run from the nasal mucosa to synapse with the ol-
factory bulbs. Note that the olfactory bulbs and tracts, shown in
Endoneurium
Droplets of
myelin
Fragmented
axon
Schwann cell
Schwann cell
Single enlarging
axon filament
Fine axon sprouts
or filaments
Macrophage
Aligning Schwann cells form
regeneration tube
New myelin
sheath forming
Schwann cells
Site of nerve damage
The axon
becomes
fragmented at
the injury site.
Macrophages
clean out the dead
axon distal to the
injury.
Axon sprouts,
or filaments, grow
through a
regeneration tube
formed by
Schwann cells.
The axon
regenerates and a
new myelin sheath
forms.
4
3
2
1
Figure 13.5
Regeneration of a nerve fiber in a peripheral
nerve.
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