Analysis of synaptic efficacy in spinal motoneurones from 'quantum' aspects.

1. Synaptic responses of triceps surae motoneurones of the cat following stimulation of single afferent fibres were examined by intracellular recording techniques.2. The mean quantum content (m) of monosynaptic excitatory postsynaptic potentials (EPSPs) was independent of the type of motoneurone recorded and of the afferent fibre stimulated. There was no significant difference in m value between homonymous and heteronymous synapses.3. A positive correlation was found between the amplitude of unit EPSPs and the input resistance of motoneurones. The difference in the amplitude of unit EPSPs appears to be responsible for the higher synaptic efficacy in slow-conducting motoneurones than in fast-conducting motoneurones.4. There was no significant difference in the time course of monosynaptic EPSPs evoked by impulses from homonymous and heteronymous afferent fibres.5. The ratio of monosynaptic connexions from a given afferent fibre was significantly greater on to homonymous than to heteronymous motoneurones. It is concluded that the difference in efficacy between homonymous and heteronymous synaptic transmission is due to the difference in the number of afferent fibres converging upon these motoneurones.