Postcontractile motoneuronal discharge produced by muscle afferent activation

Muscle afferent activity is markedly enhanced following isometric contractions in hindlimb musculature of various vertebrates. Such increased and persistent postcontractile sensory discharge (PCSD) consists for the most part of potentiated firing of muscle spindle primary efferents. To determine if a positive feedback function is served, involving recruitment of homonymous and synergistic motor units, 62 motoneurons were isolated in L₇ ventral filaments of cats categorized according to their reflex response to triceps surae stretch, and monitored before and after isometric contraction for variations in spike frequency. Motoneurons found reflexly coupled to triceps surae stretch receptors significantly increased (P<.01) their firing frequency during PCSD. Tonic motoneurons, believed to be primarily of alpha origin, accounted for most of the observations. This latter finding is consistent with known effects of spindle afferent input on motoneuronal recruitment. Functionally, this autogenetic control circuit should enhance 'muscle stiffness' and stabilize the muscle against minor perturbations.