Changes in Nerve cell Body

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