Recovery of Mammalian Nerve Fibres in Vivo

Abstract

An isolated nerve fibre, as it recovers from the condition of absolute refractoriness which follows activity, is known under suitable conditions to pass through 3 successive phases, relative refractoriness, supernormality, and subnormality. 1 , 2 , 3 We have attempted to determine the course of recovery of nerve fibres in as nearly as possible physiological conditions, i. e., those conditions existing in the body of a healthy, intact animal. Etherized or decorticated rabbits were used, and induction shocks were applied either to the oculomotor nerve in situ, 4 or to the sciatic nerve at the level of the trochanter major of the femur. The greatest care was taken to prevent hemorrhage, tissue injury or temperature change. Responses were recorded by means of a cathode ray oscillograph from the internal rectus muscle, the nerve to the inferior oblique muscle, the tibialis or gastrocnemius muscle or the deep peroneal nerve, according to the nerve stimulated. Thirty experiments were performed.

The recovery curves obtained often included the three phases of relative refractoriness, supernormality and subnormality, but there was considerable variation from nerve to nerve, and in the same nerve from time to time. Apparently the nerve fibers are extremely sensitive to metabolic changes taking place in the body, so that the recovery curve varies continuously. The relatively refractory period usually ended 2-5 msec. after the conditioning shock, and was succeeded by a supernormal period lasting until 10-20 msec. after the shock, with a peak of excitability at 4-7 msec. At the peak, the maximum change in excitability (as measured by the change in strength of stimulus required to bring the response to normal height) was 5-10%. There was no striking difference between the oculomotor and the sciatic nerves in regard to the refractory and supernormal periods, but the subnormal period was much more prominent in the latter.