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

A technique is described for recording large numbers of individual or single-fibre excitatory post-synaptic potentials (e.p.s.p.s) from single motoneurones by means of spike-triggered averaging. The cable properties of the motoneurones were calculated from the decay time course of a voltage transient in the motoneurone following a current pulse applied to the soma. From this response a theoretical shape index curve was calculated. Most individual or single-fibre e.p.s.p.s elicited by impulses in different Ia fibres had simple decay time courses and shape indices that fitted the theoretical shape index curve of the motoneurone from which they were recorded very well. This suggested that the active terminals of these afferent fibres were located within limited post-synaptic areas. In a few cases the original amplitude, latency and shape of individual e.p.s.p.s changed dramatically when they were re-averaged 40 min later after the membrane potential had decreased, but was still at an acceptable level. E.p.s.p.s with simple decay time courses changed to e.p.s.p.s with composite decay time courses, presumably due to activation of previously silent synapses. The results suggest that impulses conducted in a single afferent fibre from a muscle spindle do not necessarily activate all of the synapses which the fibre forms on a motoneurone, 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.