Simultaneously active and inactive synapses of single Ia fibres on cat spinal motoneurones.

Abstract

A technique is described for recording large numbers of individual or single-fiber excitatory postsynaptic potentials (EPSP) from single motoneurons by spike-triggered averaging. The cable properties of the motoneurons were calculated from the decay time course of a voltage transient in the motoneuron following a current pulse applied to the soma. From this response a theoretical shape index curve was calculated. Most individual or single-fiber EPSP elicited by impulses in different I a fibers had simple decay time courses and shape indices that fitted the theoretical shape index curve of the motoneuron from which they were recorded very well. The active terminals of these afferent fibers apparently were located within limited post-synaptic areas. In a few cases the original amplitude, latency and shape of individual EPSP changed dramatically when they were re-averaged 40 min later after the membrane potential had decreased, but was still at an acceptable level. EPSP with simple decay time changed to EPSP with composite decay time courses, presumably due to activation of previously silent synapses. Impulses conducted in a single afferent fiber from a muscle spindle do not necessarily activate all of the synapses which the fiber forms on a motoneuron, but may repeatedly fail to activate some endings during prolonged periods of spike-triggered averaging, while consistently activating others. Evidence regarding the site of transmission failure and the possible mechanism of its relief is discussed.