The influence of stimulus trains applied to single I a axons on the firing behaviour of single motoneurones was assessed in anaesthetized cats. The change in motoneurone firing probability associated with a single I a afferent spike was measured from short-latency peaks in peristimulus time histograms or cross-correlograms. Some synapses showed frequency-dependent depression of the short-latency peak, which is consonant with the frequency-dependent depression reported for the I a-motoneurone excitatory postsynaptic potential (EPSP). 2. Where they could be measured, EPSPs superimposed on the depolarizing ramps of potential recorded from motoneurones as they fired repetitively showed frequency-dependent changes in amplitude that parallelled those of the simultaneously recorded histograms. 3. Thus it appears that at synapses with small EPSPs, which are typical in the mammalian CNS, modulation of the EPSP should result in similar modulation of cell firing.
