NERVE FIBER

DEGENERATION IN THE NEURON
Degenerative changes take place at three levels:
1. Changes in the distal stump
2. Changes in the proximal stump (at the site of injury)
3. Changes in the cell body

Changes in the Distal Stump (Wallerian Degeneration)
• When a peripheral nerve is cut, the part of the nerve separated from the cell body shows a series of chemical and physical degenerative changes. These constitute Wallerian degeneration.
• They appear within few hours and continue for several days.
• Degenerative changes also appear in the proximal part and in the cell body. These changes are called retrograde degeneration.   Regeneration starts from the proximal stump and proceeds distally at a slower rate. proceeds distally at a slower rate. The degenerative period may be divided into:

A. Early phase
B. Late phase

A. Early phase (1-7″ day): During an early phase. only functional changes appear without physical changes. This phase lasts for about a week. The following changes appear during an early phase in a sequential order:

1. Changes in the enzymatic activity of choline acetylase and ACh esterase.
2. Changes in the activity of ionic channels (decrease).
3. Change in amplitude and conduction velocity of nerve impulse appears within 2-3 days
4.Ultrastructural changes in the organelle appear.
5.Failure of conduction of nerve impulse by 5th day .
6. No gross structural changes take place until 7 day

B. Late phase (8-32 days): During this phase, histochemical and physical changes appear. The changes are:
1. Neurofibrils swell and then disappear
2.Axis cylinder breaks up into short lengths
3. Myelin sheath disintegrates into droplets of fat
4. Axon debris and disintegrated myelin are removed by macrophages
5.Neurilemmal sheath with its nucleus and endoneural tubes are left intact at the end of 32nd day.
The entire degenerative process is completed by 4 weeks.

Degenerative Changes in the Proximal stump

In the central stump (proximal stump), where the nerve fibers are still attached to their cell bodies, degeneration is usually confined to a centimeter or so next to the point of section. These changes are very similar to the changes that takes place in the distal stump.
(Schwann cells do not undergo degeneration during axonal degeneration)

Degenerative Changes in the Cell Body(Soma) Degenerative changes also appear in the cell body.
These changes depend on:
1.Severity of injury
2.Proximity of lesion to the cell body

The changes are:

1. Chromatolysis: NissIe’s granules which represent endoplasmic reticulum and ribosomes break up into fine dust and lose their staining reaction. Hence the name chromatolysis.
This process is a reaction to injury and is necessary for synthesis of proteins required for neuronal survival.
2. Golgi apparatus, mitochondria and neurofibrils disappear.
3. The cell draws in more fluid and swells up.
4. The nucleus is pushed to one side. In severe cases, it may be totally extruded leading to the death of the neuron.

These changes , which are associates with an alteration in the excitability of the cells begins on the first day and reach the maximum in the third week, after which the cell regains its normal appearance.