Responses of Mammalian Nerve to Strong Shocks

Abstract

In peripheral mammalian nerve, differential depression may be produced by strong tetanic shocks from an induction coil, which at proper strength block conduction in all myelinated fibers and allow all non-myelinated fibers still to conduct. The mammalian vagus nerve may be arranged, still attached peripherally to the body, on 3 sets of electrodes; stimulating electrodes next the body, a pair of lead electrodes at the cut central end, and a pair of blocking electrodes between. The ascending action currents due to respiratory inspiration, recorded at 500 mm./mv. amplification, are 5 to 10 mm. in amplitude (cat). These may be blocked temporarily and reversibly, to record base-line noise. The responses may be rendered monophasic by application of 1% cocaine to the killed end of the nerve.

Single strong shocks are then applied. If the myelinated fibers are blocked, only single C waves are recorded. If they are not blocked, each stimulus is usually followed not only by the A, B, and C waves characteristic of this nerve's normal action potential, but by a further random discharge that may last for several tenths of a second. These discharges resemble the respiratory discharges in form of the individual waves, which appear to be too brief to be assignable to non-myelinated fibers. Respiratory rate or depth does not change. The effects of successive shocks are summed, until after 5 or 10, at 1/8 second intervals, the discharge may last several seconds, and become greater in amplitude and in apparent frequency than the respiratory discharge itself. In some cases shocks slightly above the maximal for the C wave block the nerve at the stimulating electrode to the extent that for many seconds no respiratory discharges can be recorded, but this random discharge following each shock persists.