Changes in Nerve Cell Body
Changes in
the nerve cell body commence within 48 hours after the section of nerve. The changes
are:
i. First,
the Nissl granules disintegrate into fragments by chromatolysis
ii. Golgi
apparatus is disintegrated
iii. Nerve
cell body swells due to accumulation of fluid and becomes round
iv.
Neurofibrils disappear followed by displacement of the nucleus towards the
periphery
v.
Sometimes, the nucleus is extruded out of the cell.
In this
case, death of the neuron occurs and regeneration of the injured nerve is not
possible.
Changes in Axon
Proximal to Cut End
In the
axon, changes occur only up to first node of Ranvier from the site of injury.
Degenerative changes
that occur
in proximal cut end of axon are similar to those changes occurring in distal
cut end of the nerve
fiber.
TRANSNEURONAL
DEGENERATION
If an
afferent nerve fiber is cut, the degenerative changes occur in the neuron with
which the afferent nerve fiber synapses. It is called transneuronal
degeneration.
Examples:
i.
Chromatolysis in the cells of lateral geniculate body occurs
due to sectioning of optic nerve
ii.
Degeneration of cells in dorsal horn of spinal cord occurs when the posterior
nerve root is cut
iii.
Degeneration of cells in ventral horn of spinal cord occurs when there is tumor
in cerebral cortex.
REGENERATION OF NERVE FIBER
The term
regeneration refers to regrowth of lost or destroyed
part of a tissue. The injured and degenerated nerve fiber can regenerate. It starts as early as 4th day after injury, but becomes more effective only after 30days and is
completed in about 80 days.
CRITERIA FOR
REGENERATION
Regeneration
is possible only if certain criteria are fulfilled by the degenerated nerve
fiber:
1. Gap
between the cut ends of the nerve should not exceed 3 mm
2.
Neurilemma should be present; as neurilemma is absent in CNS, the regeneration
of nerve does not
occur in CNS
3. Nucleus
must be intact; if it is extruded from nerve cell body, the nerve is atrophied
and the re generation does not occur
4. Two cut
ends should remain in the same line. Regeneration does not occur if any one end
is moved away.
STAGES OF
REGENERATION
1. First,
some pseudopodia like extensions grow from the proximal cut end of the nerve.
These extensions are called fibrils or regenerative
sprouts. The number of fibrils is up to 100.
2. Fibrils
move towards the distal cut end of the nerve fiber
3. Some of
the fibrils enter the neurilemmal tube of distal end and form axis cylinder
4. Schwann
cells line up in the neurilemmal tube and actually guide the fibrils into the
tube. Schwann
cells also
synthesize nerve growth factors, which attract the fibrils form proximal
segment.
5. Axis
cylinder is fully established inside the neurilemmal tube. These processes are
completed
in about 3
months after injury.
6. Myelin
sheath is formed by Schwann cells slowly. Myelination is completed in 1 year.
7.
Diameter of the nerve fiber gradually increases. However, the degenerated nerve
fiber obtains only
80% of
original diameter. Newly formed internodes are also shorter than the original
ones.
8. In the
nerve cell body, first the Nissl granules appear followed by Golgi apparatus
9. Cell
looses the excess fluid; nucleus occupies the central portion
10. Though
anatomical regeneration occurs in the nerve, functional recovery occurs after a
long period
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