Interactions between motor units in modulating discharge patterns of primary muscle spindle endings.

Discharge patterns of group Ia afferents of muscle spindles in the (isometric) cat semitendinosus muscle were recorded when subjected to the influence of contractions of three motor units elicited by stimulation of three alpha-efferents with different regular or stochastic stimulus patterns. In order to study conditioning interactions between motor unit contractions, defined time constellations of stimuli to different motor units were isolated from the stimulus trains by electronic means. This technique was also applied to discharge patterns of two Ia fibres to isolate "correlated discharges." The effects of defined time constellations of stimuli upon discharge patterns of single Ia afferents or "correlated discharges" of pairs of Ia afferents were assessed by poststimulus time histograms (PSTHs). It turned out that the influence exerted by one motor unit on spindle discharges was more or less strongly modulated by actions of the other motor units, dependent on the relative timing of their contractions. These effects could in part be explained by the relative locations of motor units and spindles within the muscle. These results are discussed with respect to possible mechanisms of low-amplitude physiological tremor.