Influence of post-synaptic properties on the time course of synaptic potentials in different types of cat lumbar...

Shape indices of excitatory post-synaptic potentials (EPSPs) have been calculated on compartmental models assembled using average properties obtained from two motoneuron groups classified as fast and slow, on the basis of rheobase current and input conductance. The calculated EPSP time courses differed considerably between the two models, the rise-time and half-width being more prolonged in the slow model. With a conductance change distributed uniformly among compartments 3-6 in a 10-compartment model, the resulting shape indices in the slow and fast model, respectively, were quite similar to the apparent average values previously observed experimentally for composite Ia EPSPs in types S (slow) and F (fast) motoneurons. The results of the calculations suggest that differences in EPSP shape indices observed between F and S motoneurons arise from systematic differences in motoneuron postsynaptic properties (specific membrane resistivity and dendritic geometry) rather than differences in dendritic location of synaptic input. The results also suggest that changes in EPSP time course, following section of the motor axon, may similarly be related to changes in motorneuron postsynaptic properties.