Degeneration and Regeneration of Nerve Fibers

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.