A new way of using modelling to estimate the size of a motoneurone's EPSP.

Earlier modelling of a noisy motoneurone has been extended to provide a new measure of excitability. The distance-to-threshold estimate of an MN's AHP, derived from its interval histogram, is used to create a simplified daughter model to mimic the behaviour of its parent and determine a new measure of an MN's response to a stimulus. This Estimated Potential (EP) provides a linear measure of the size of the parent's underlying EPSP, irrespective of its firing rate, and thereby improves on the classic firing index from which it is derived. The EP is applicable with both random and spike-triggered stimulation. It is emphasized that in the presence of noise a tonically firing MN's average responsiveness at a given time during its AHP depends upon what may be termed the "survivor's mean trajectory", rather than upon the "distance to threshold" AHP found in the absence of noise; these differ because noise-induced spiking eliminates individual trajectories when they reach threshold.