Observations on the Local Response in Single Medullated Nerve Fibers

Abstract

A humped local response to stimulation with subrheobasic rectangular currents of long duration is demonstrated in single medullated fibers of the phalangeal nerve preparation, which in many respects behaves like the humped responses of single giant fibers. Under the exptl. conditions the hump first becomes apparent when the stimulus attains a strength of .85 to .90 rheobase; it has a definite though variable latency for rheobasic stimuli which has ranged roughly between 1 and 2 msec, has a duration of about 2 msec, and a variable maximum height in the same and in different prepns., with a maximum of 10% that of the spike. Terminating the stimulating current pulse during the hump does not stop its completion. Within the limit of error electrotonic potential underlying the hump develops linearly with the applied current up to .5 - .6 rheobase when non-linear but continuous components appear. When at "threshold" a spike eventuates, it invariably rises out of the crest of the hump with an upward inflection of the latter. Previously it had been shown, with multifibered preparations stimulated as in this research, that excitability rises to a plateau from which it falls abruptly along a curve concave upwards. This abrupt decrease in excitability has the same latency and threshold as the present humps, and may be indicative of the association of a degree of refractoriness with the process that gives rise to the humps. When in successive records a long rectangular current elicits repetitive spikes, some of the latter may be replaced or followed by humps. The latter often are diphasic, signifying that they may not arise at the lead and polarizing electrode and that they then travel toward the latter in the fiber made relatively refractory by the preceding response. A sufficient condition for excitation appears to involve only the attainment of a critical electrotonic potential at the cathode without regard to its time rate of change, provided that accommodation can be neglected. When this local nonlinear potential attains a critical height it leads to instability and initiates either a local or a propagated response.