Relationship between EPSP shape and cross-correlation profile explored by computer simulation for studies on human motoneurons

We used a computer model (making several simplifying assumptions) to explore the relationship between the characteristics of an excitatory postsynaptic potential (EPSP) and the profile of the change in firing probability that occurred when that EPSP was delivered to a rhythmically discharging neuron. In circumstances applicable to studies on human motoneurons we found that the magnitude of the period of increased firing probability in the cross-correlation (produced by the rising phase of the EPSP) was related to the number of stimuli and to the proportion of the interspike interval that the EPSP was within reach of threshold. The interstimulus interval and the statistical distribution of the motoneuron interspike intervals were of little consequence. For this model the subsequent period of reduced firing probability was proportional to the amplitude of the EPSP and not to the duration of its falling phase.