Fluctuations in Time of Onset of la-Motoneuron EPSPs in the Cat

Synaptic delay between Ia fiber volley and motoneuron EPSP was originally estimated at about 0.3-0.4 msec. When averaged individual EPSPs evoked by the action of single Ia fibers were recorded, it became evident that this delay was not constant but varied from synapse to synapse. In general, EPSPs with short rise times were initiated with less delay than EPSPs with long rise times, and this was ascribed to electrotonic conduction time in motoneuron dendrites in accord with the prediction of Rall. All of this work was done using averaged individual EPSPs. Here, we show that the synaptic delays associated with a given Ia-motoneuron synapse exhibit fluctuations, and we examine the properties of these EPSPs with long and short delays. Some evidence from this variability is already available at this synapse.