Degeneration and regeneration of peripheral nerves

Question 1

Neurones

Answer 1

Specialised cells of the nervous system, responsible for conducting nerve impulses. Neurones in the PNS connect sensory receptors to effector organs via the CNS, which integrates information.

Question 2

Nerves

Answer 2

Composed of bundles of neurones, their connective tissue sheaths, and their blood and lymphatic vessels.

Question 3

Causes of nerve degeneration

Answer 3

• injury
• genetics
• aging

Question 4

Descending pathways

Answer 4

Relay chains of neurones through which the CNS communicates with effectors.

Question 5

Vulnerable features of a nerve

Answer 5

• epineurium
• perineurium
• endoneurium
• myelin sheath
• axon

Question 6

Neuropraxis

Answer 6

Occurs when a neurone temporarily loses its ability to function, with injury only at the level of the myelin sheath eg. temporary compression. Restoration of function would be complete upon recovery.

Question 7

Axonotmesis

Answer 7

Occurs when the axons of a nerve together with their myelin sheaths are damaged, but the connective tissue sheaths remain intact eg. severe crush injury. Restoration of function can be expected to return fully, as new nerve sprouts are guided by the ensheathing membranes to re-supply the target effector.

Question 8

Neurotmesis

Answer 8

Occurs when the entire nerve is completely severed, resulting in damage to the axon, myelin sheath and connective tissue layers. Restoration of function does not occur.

Question 9

Minutes after a nerve injury

Answer 9

• the neurone immediately stops conducting APs beyond the site of injury
• the two cut ends of the axon begin leaking intracellular fluid, as axonal transport is bidirectional
• the cut ends pull apart, sealing themselves and swelling at the same time

Question 10

Hours after a nerve injury

Answer 10

• the synaptic terminal degenerates
• neurofilaments and vesicles accumulate
• astrocytes surround the terminal and pull away from the postsynaptic cell

Question 11

Wallerian degeneration

Answer 11

The distal segment of a severed neurone soon dies as a result of loss of nutritional support from the soma. The axonal stump is digested by microglial cells in phagocytosis.

Question 12

Chromatolysis

Answer 12

The proximal segment of a severed neurone may begin to regenerate. The soma becomes extremely active, producing lots of proteins for cell repair. Its volume increases, and the nucleus is pushed to the periphery from its central position.

Question 13

Neuroma

Answer 13

The injured nerve soon seals the wounded axonal stump on the proximal segment to form a neuroma. This segment of nerve does not die, and may regenerate to re-innervate peripheral structures.

Question 14

Innervation

Answer 14

The normal state of nerve supply to a muscle or other target.

Question 15

Denervation

Answer 15

The deprivation of nerve supply to a muscle.

Question 16

Re-innervation

Answer 16

The regrowth of a nerve to resupply a muscle. It is not always successful, as most nerves often re-innervate an effector organ that is different from it’s original target.

Question 17

Short term effects of denervation

Answer 17

When a muscle is denervated, it is immediately paralysed, becoming flaccid and areflexic. The muscle may begin to twitch involuntarily, called fasciculation.

Question 18

Long term effects of denervation

Answer 18

As fasciculations subside, a muscle will lose bulk (atrophy). It will eventually die and be replaced with connective tissue, called fibrosis.