Degeneration and Regeneration of Nerve Fibers
All these
changes are together called the degenerative changes.
Causes for Injury
Injury to
nerve fiber occurs due to following causes:
1.
Obstruction of blood flow
2. Local
injection of toxic substances
3.
Crushing of nerve fiber
4.
Transection of nerve fiber.
DEGREES OF INJURY
Sunderland
had classified the injury to nerve fibers into five
categories depending upon the order of severity.
FIRST DEGREE
First
degree injury is the most common type of injury to the nerves. It is caused by applying
pressure over a nerve for a short period leading to occlusion of blood flow and hypoxia. By first degree of injury, axon is not
destroyed but mild demyelination occurs. It is not a true degeneration. Axon
looses the function temporarily for a short time, which is called conduction
block. The function returns within few hours to few weeks. First degree of
injury is called Seddon neuropraxia.
SECOND DEGREE
Second degree is due to
the prolonged severe pressure, which causes Wallerian
degeneration (see
below).
However, the endoneurium is intact. Repair and restoration of function take
about 18 months. Second degree of injury is called axonotmesis.
THIRD DEGREE
In this
case, the endoneurium is interrupted. Epineurium and
perineurium are intact. After degeneration,
the
recovery is slow and poor or incomplete. Third, fourth and fifth degrees of
injury are called neurotmesis.
FOURTH DEGREE
This type
of injury is more severe. Epineurium and perineurium are also interrupted.
Fasciculi of nerve
fibers are
disturbed and disorganized. Regeneration is poor or incomplete.
FIFTH DEGREE
Fifth
degree of injury involves complete transaction of the nerve trunk with loss of continuity. Useful
regeneration is not possible
unless the cut ends are rearranged and approximated quickly
by surgery.
DEGENERATIVE CHANGES IN THE NEURON
Degeneration refers to
deterioration or impairment or pathological changes of an
injured tissue. When
a
peripheral nerve fiber is injured, the degenerative changes occur in the nerve
cell body and the nerve fiber of same neuron and the adjoining neuron.
Accordingly,
degenerative changes are classifiedinto three types:
1.
Wallerian degeneration
2.
Retrograde degeneration
3.
Transneuronal degeneration.
WALLERIAN
DEGENERATION OR ORTHOGRADE DEGENERATION
Wallerian degeneration is
the pathological change that occurs in the distal cut end
of nerve fiber (axon). It
is named after the
discoverer Waller. It is also called orthograde
degeneration. Wallerian degeneration starts within 24 hours of injury. Change
occurs throughout the length of distal part of nerve fiber simultaneously.
Changes in Nerve
i. Axis
cylinder swells and breaks up into small pieces. After few days, the broken
pieces appear as debris
in the
space occupied by axis cylinder.
ii. Myelin
sheath is slowly disintegrated into fat droplets. The changes in myelin sheath
occur from
8th to
35th day.
iii.
Neurilemmal sheath is unaffected, but the Schwann cells multiply rapidly.
Macrophages invade from
outside
and remove the debris of axis cylinder and fat droplets of disintegrated myelin
sheath. So,
the
neurilemmal tube becomes empty. Later it is filled by the cytoplasm of Schwann
cell. All these changes take place for about 2 months from the day of injury.
RETROGRADE
DEGENERATION
Retrograde
degeneration is the pathological changes, which occur in the nerve cell body
and axon proximal to the cut end.
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