RESPONSE OF NERVE TISSUE TO INJURY

ØA. Damage to the Cell Body: Because mature neurons cannot divide, dead neurons cannot be replaced. Neurons not connected with otherfunctioning neurons or end organs are useless, and mechanisms have evolved to dispose of them. Thus, if a neuron makes synaptic contact with Only one other neuron and the latter is destroyed, the former undergoes autolysis, a process termed transneuronal degeneration. Most neurons, however, have multiple connections.

ØB. Damage to the Axon: Regeneration can occur in axons injured or severed Far enough from the soma to spare the cell. Such injuries are followed by partial degeneration and then regeneration. Nerve system

1. Degeneration. A crushed or severed axon degenerates both distal and proximal to the injury. Distal to the site Of injury, both the axon and myelin sheath undergo complete degeneration connection with the soma has been lost. During this Wallerian, descendent, or secondary degeneration, whichusually lakes about 2-3 days, nearby Schwann cells proliferate, phagocytose degenerated tissue, and invade the remaining endoneurial channel. Proximal to the site of injury, degeneration of the axon and myelin sheath is similar but incomplete. This retrograde, ascendent, orprimary degeneration proceeds for about 2 internodes before the injured axon is sealed. The cell body also changes in response to injury. The perikaryon enlarges; chromatolysis, or dispersion of Nissl substance, occurs; and the nucleus moves to an eccentric position. Proximal degeneration and cell body changes fake about 2 weeks.

2. Regeneration. This begins in the third week after the injury. As the perikaryon gears up for increased protein synthesis, the Nissl bodies 'eappear. The axon's proximal stump gives off a profusion of smaller processes called neurites; one of these encounters and grows into the endoneurial channel, while the others degenerate. In the channel, the neurite grows 3-4 mm/d, guided and then myelinated by the Schwann cells. Growth is maintained by orthograde axoplasmic transport of material synthesized in the soma. When the tip of the neurite reaches its termination, it connects with its end organ or another neuron in the chain. If the cut ends of a severed nerve are matched by by fascicle size and arrangement and sutured together by their epineurial sheaths within 34 weeks after injury, sensory and motor innervation can often be restored. If the gap between the cut ends is too wide, the neurites may fail to find endoneurial sheaths to grow into and may grow out in a potentially painful disorganized swelling called a neuroma. Target organs deprived of innervation often atrophy.