Synchronous afferent discharge from a passive muscle of the cat: significance for interpreting spike-triggered averages.

Evidence is presented for the existence of synchrony between the spike trains of muscle afferents of the passive cat medial gastrocnemius muscle held at fixed length. Synchrony between the spike trains of a population of muscle afferents was quantified by means of a synchronization index (Is), derived from spike-triggered averages of the muscle-nerve neurogram and the rectified neurogram. A previously used test based solely upon the neurogram average (Watt, Stauffer, Taylor, Reinking & Stuart, 1976) is shown to be invalid. The differences between experimentally derived Is values and theoretical Is values derived for the condition of asynchrony were compared to estimated confidence limits for those differences. This comparison revealed that twenty-two of fifty-three muscle-afferent spike trains whose rectified averages satisfied certain conditions for interpreting the Is were synchronized with the discharge of other afferents. The form of the rectified averages of another eight afferents suggested that these afferents might also have been synchronized. Synchrony in the discharge of muscle afferents was found in three experiments in which the neurogram was recorded from a single nerve branch to medial gastrocnemius, as well as in the data of experiments in which the whole muscle nerve was used. The degree of synchrony was similar for Ia, spindle group II and Ib afferents. The magnitude of the synchrony found in these experiments was judged by comparison to the results of analog simulations and the increase in Is values resulting from the application of small, quick stretches to the medial gastrocnemius muscle. The degree of synchrony found on average was approximately equivalent to that of a single spike occurring once for every four discharges of the reference spike train. Simulations were performed to determine the distortion of monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) obtained by spike-triggered averaging which would be produced by synchrony between the spike trains of Ia and spindle group II afferents of the magnitude found in this study. These simulations indicate that the apparent amplitude would be increased by approximately 4 microV on average. Both the 10-90% rise time and the half-width would increase, the effects being greater for smaller e.p.s.p.s. Consequently, the synchrony found in this study is of most concern in the study of small post-synaptic potentials, such as those due to spindle group II afferents.