Responses of human motoneurons to Ia inputs: effects of background firing rate.

The effects of synchronous Ia volleys on the firing probability of repetitively firing human motoneurons were examined at fast and slow firing rates. Ia afferents of either the median or the posterior tibial nerve were stimulated, while single motor unit activity was recorded from the homonymous muscles. Motoneuron responses to the Ia inputs were quantified by measurement of the magnitude of the short latency excitatory peak in peristimulus time histograms (PSTHs). When the stimuli were given at random with respect to the times of motor unit spikes, the magnitude of the PSTH peak (response probability) was significantly lower at a faster firing rate. In the "triggered" mode of stimulation, stimuli were given at various known times during the interspike interval. In this mode the response probability to the input increased monotonically as the stimuli were delivered progressively later during the interspike interval. The response probability at a fixed delay with respect to the triggering spike was higher at the faster firing rate. The results obtained with the two modes of stimulation are not in contradiction and both may be explained by the nature of membrane voltage trajectories and ionic conductances during the interspike interval described for repetitively firing cat motoneurons.