FREQUENCY-DEPENDENT SYNAPTIC DEPRESSION MODIFIES POSTSYNAPTIC FIRING PROBABILITY IN CATS

1. The influence of stimulus trains applied to single Ia axons on the firing behaviour of single motoneurones was assessed in anaesthetized cats. The change in motoneurone firing probability associated with a s ingle Ia afferent spike was measured from short-latency peaks in peris timulus time histograms or cross-correlograms. Some synapses showed fr equency-dependent depression of the short-latency peak, which is conso nant with the frequency dependent depression reported for the Ia-moton eurone excitatory postsynaptic potential (EPSP). 2. Where they could b e 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 simultan eously recorded histograms. 3. Thus it appears that at synapses with s mall EPSPs, which are typical in the mammalian CNS, modulation of the EPSP should result in similar modulation of cell firing.