Recovery of degenerating mammalian nerve after prolonged stimulation

Abstract

When large numbers of closely succeeding stimuli are applied to a mammalian nerve with intact blood supply, the conduction time of the compound action potential is increased. The return of the conduction time to normal and the effect of the blood supply on this recovery have been reported in previous papers (Causey & Schoepfle, 1951; Causey & Stratmann, 1953 a, b). The purpose of the present investigation was to study the behaviour of degenerating nerves under the same conditions.In previous work (Causey & Stratmann, 1953b;Causey & Palmer, 1952 we have related failure of conduction and the centrifugal course of this failure to the increasing area of the non-myelinated axoplasm at the node of Ranvier. The recovery of degenerating nerve fibres after prolonged repetitive stimulation indicates that the ability of a nerve to recover is not greatly changed until such time as it ceases to conduct. METHODSRabbits of about 2-5 kg were used for all experiments. The operation of sectioning the tibial nerve in the region of the pelvic notch, under aseptic conditions, while leaving the blood supply intact, has been fully described (Causey & Stratmann, 1953b). The animals were allowed to recover. After periods of 24-60 hr the animals were anaesthetized with urethane, and the degenerating portion of the tibial nerve was stimulated in the middle, the action potentials being recorded at the two ends in vivo by the method previously described (Causey & Stratmann, 1953a). The nerves were stimulated for 2 hr at a frequency of 600/sec. The stimulus strength was supramaximal at the beginning of each experiment. One hour was allowed for recovery during which time the response of the two ends of the nerve to single stimuli was examined periodically.As before (Causey & Stratmann, 1953a) the change of conduction time of the fastest fibres, measured from the end of the stimulus artifact to the initial rise from the base-line of the negative wave of the transmitted compound action potential, was used as an index of fatigue and recovery. The changes of conduction time are expressed as percentages of the initial conduction time at the beginning of each experiment. In all experiments the greatest care was taken to ensure as little interference as possible with the blood supply to all parts of the nerve under examination.