Estimation of the rising phase of EPSP analyzed by computer simulation of the coding process

Based on data obtained from intracellular recordings of cat alpha-motoneurons in the stretch reflex, the firing process of these motoneurons was computer-simulated. The impulse response EPSP (IR-EPSP) was simulated to correspond to a monosynaptic mass EPSP elicited by a spindle afferent volley, while the returning potential was simulated to correspond to a potential gradient rising toward an augmenting depolarization of the membrane (augmentative EPSP) after motoneuronal spike generation. The IR-EPSPs were generated by input at random intervals and added to each other, linearly, on the returning potential. As soon as the resultant potential attained the critical threshold level, Vth, motoneuron firing occurred. Then IR-EPSPs were again added to the returning potential until another motoneuron firing occurred. This process was repeated continuously, and the time relation between input and output, lag-time distribution (PT(T], was determined Distribution of the bias potential, PV(V), from which the motoneuron spike triggering EPSP started to rise, was also calculated. The relations between PT(T), PV(V) and a waveform of the IR-EPSP were obtained analytically. The relation indicated that the shape of PT(T) corresponds to a time derivative of the rising phase of the IR-EPSP if the PV(V) distribution is uniform. In this study, we investigated the possibility of making the PV(V) distribution uniform.