ACTIVITY-DEPENDENT CONDUCTION IN SINGLE MOTOR UNITS

Vertebrate sensory and motor axons vary in their responses to submaxim al stimuli as a function of time since prior activation. When two equa l but submaximal stimuli are presented in pairs, the response to the s econd stimulus may be greater or less than the response to the first s timulus, depending on the interstimulus interval (ISI). We studied bot h the supernormal period (ISI between 6 and 25 msec) and the subnormal period (ISI between 25 and 100 msec) under conditions where only sing le motor axons were stimulated. Twenty single motor units from eight n ormal subjects were studied. The behavior of single units was very sim ilar to that observed in compound motor action potentials, with the su pernormal period lasting approximately 20 msec, followed by a subnorma l period lasting at least 80 msec. Surprisingly, a supernormal period could be evoked by a stimulus that did not produce a response in the m otor unit being studied; however, the presence of subnormality was dep endent on an action potential being generated in response to the first stimulus. Based on these results, we conclude that the supernormal pe riod does not require the opening of voltage-dependent ion channels, i n contrast to the later occurring subnormal period.