Modification of the recovery-cycle of human median nerve by ischemia.

Variations in excitability have been shown to follow single activation of sensory and motor axons in human peripheral nerve, with a sequence of refractoriness, supernormal and subnormal excitability ("recovery cycle"). Modifications of the recovery cycle of nerve excitability by ischemia have been investigated in the present study in sensory fibers of human median nerve. During ischemia, there is an initial fall in the nerve threshold to a brief electric pulse, followed by a progressive rise in threshold until the nerve becomes inexcitable. These ischemia-induced threshold changes interact with the impulse-induced variations in nerve excitability during the recovery period, leading to a characteristic alteration of the recovery cycle with a progressive reduction of the magnitude and duration of the supernormal period. Conduction velocity becomes progressively reduced, probably because of a slowed conduction of the action potential in the fastest conducting fibres, due to ischemic depolarization of the resting membrane potential. In the postischemic phase the magnitude and duration of supernormality are increased, probably because of the hyperpolarization of the resting membrane potential in this condition. Conduction velocity only returns to normal values after 12-25 min.