Recurrent inhibition of firing motoneurones in man

Abstract

The effects of antidromic stimulation of large motoneurones on firing small motoneurones of soleus muscle have been studied. Against the background of rhythmic firing of small motor units (MUs) activated during weak voluntary muscle contraction, thick efferents of the tibialis posterior nerve were selectively stimulated and an M response was evoked in which small MUs were not involved. This provided a means of avoiding antidromic stimulation of the motoneurone under study and, thus, analysing the effect of stimulation without its summation with after-hyperpolarization. The background firing rate of MUs was 4.5–9.2/sec. PSTHs revealed a distinct inhibitory effect with a latency of 35–40 msec (slightly exceeding the latency of monosynaptic reflex) and duration 5–30 msec. It was concluded that the short-latency inhibition could be identified as recurrent inhibition. The effectiveness of recurrent inhibition evaluated by the lengthening of the interspike interval was shown to depend on the arrival time of the volley in the interval and on the background firing rate of the motoneurone. When the inhibitory volley arrived at the beginning of the interspike interval it was ineffective. This indicates that in the investigated range of firing rates the motoneurone is unable to exert an inhibitory effect on its own firing via recurrent collaterals. The inhibitory volley became highly effective at the end of an interspike interval, when the membrane potential approached threshold. The lengthening of interspike interval was more marked at a lower firing rate of the motoneurone. An increase in the background firing rate reduced the extent of recurrent inhibition (at a rate above 10/sec up to its complete ineffectiveness). Various methods for evaluating recurrent inhibition in a firing motoneurone are compared.