Large reductions in composite monosynaptic EPSP amplitude following conditioning stimulation are not accounted for by increased postsynaptic conductances in motoneurons.

A compartmental neuronal model was used to show that increased compartmental leak conductances distal to the site of excitatory postsynaptic potential (EPSP) generation have little effect on EPSP amplitude but decrease half-width markedly. Using intracellular recording from cat hindlimb motoneurons, reductions of composite Ia EPSP amplitude by up to 70% unaccompanied by reductions in half-width were seen following conditioning stimuli to hindlimb nerves. Appropriate condition-test intervals produced large reductions in EPSP amplitude that were unaccompanied by detectable increases in motoneuron conductance. These observations suggest that presynaptic inhibition and not increased postsynaptic motoneuron conductances is responsible for large EPSP amplitude reductions following conditioning stimulation.