Process:
i)
Chromatolysis : 24 to 48 hrs after injury. Nissl bodies break up into fine granular masses.
Wallerian degeneration : 3 rd to 5 th day after injury. Part of the axon distal to the injured region becomes slightly swollen nd then breaks up into fragments and the myelin sheath deteriorates. The neurolemma remains.
ii)
iii)
Clean-up of debris by macrophages
iv)
RNA and protein synthesis
v) Regeneration tube formation: Schwann cells on either side of the injured site multiply by mitosis, grow toward each other, and may form a regeneration tube across the injured area. This tube guides the growth of a new axon from the proximal area across the injured area into the distal area previously occupied by the original axon. New axons cannot grow if the gap at the site of injury is too large or if the gap is filled with collagen fibres. vi) B uds of regenerating axons fill regeneration tube: buds at the proximal area grows at about 1.5mm per day across the area of damage, find their way into the distal regeneration tubes, and grow toward the distally located receptors and effectors. Then some sensory and motor connections are re-established restoring some function. In time, the Schwann cells form a new myelin sheath.
Reflexes and Reflex Arcs
Reflex : a fast, automatic, unplanned sequence of actions that occurs in response to a particular stimulus. Some are inborn (pulling hand away from hot surface) while others are learned or acquired (slamming on the brakes of a car). When integration takes place in the spinal cord gray matter, the reflex is a Spinal Reflex (the patellar reflex or knee jerk reflex). If integration occurs in the brain stem, the reflex is called a Cranial Reflex (tracking movements of eyes as you read this). Somatic Reflexes involve the contraction of skeletal muscles. Autonomic (Visceral) Reflexes are generally not perceived but involve responses of smooth muscle, cardiac muscle and glands (heart rate, digestion, urination, defecation). The pathway of nerve impulses into, through, and out of the CNS is called a Reflex Arc . Reflexes provide valuable information about the health of the nervous system.
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