OBSERVATIONS ON REPETITIVE RESPONSES IN AXONS

Abstract

Repetitive responses of individual axons to constant current stimulation are studied by means of the cathode ray oscillograph in the pharyngeal nerve of Rana pipiens. Under comparable conditions the constituent fibers may respond in an individual manner with only a make spike or with repetition which may be fast, and then immediate or delayed, or, very rarely, slow. Fibers that do not repeat yield typical excitability curves rising to a plateau and then declining; in delayed repetition the excitability curves attain a correspondingly delayed maximum; the excitability curve of immediate repetition is intermediate in form. Immediate repetition can be converted into delayed repetition, with a corresponding alteration in the excitability curve, by applying the cathode of the rectangular current through the anode of a current running continuously. Consequently the individual behavior of the fibers of a nerve that respond repetitively to the rectangular current alone, is attributed to local anodal polarization by demarcation currents. A fiber that will repeat under a rectangular current may repeat when that current is below the repetition threshold in response to a shock, even if it be subthreshold, to a transmitted action potential, etc. A sheathed phrenic nerve responds repetitively to a rectangular current, but not when desheathed. The former behaves in this respect, and in respect to its excitability, as does an anodally polarized nerve. A repetitive response of a fiber may consist of one or more repetitive bursts. The spacing and height of the spikes of a burst indicate that the interspike time at first is shorter than the time to maximum excitability following a response, and that the burst ends when they become equal in duration. Only those fibers that repeat in a superrheobasic current recover from the make response to a somewhat weaker current through a series of 3 periodic excitability oscillations, of which the first probably is the supernormal phase; the oscillation and repetition periods are about the same. Recovery via supernormality is one of the conditions that make for repetition of the rapid type. A repetitive response often presents gaps which seem to be due to failure of one or more continuing oscillations in excitability to elicit responses. Apparently, slow repetition can occur only when the excitability of supernormal recovery does not rise too high. Then, presumably spontaneous variations in excitability determine repetition in fibers whose level of excitation is adequately high.