Depression of Reciprocal Ia Inhibition of Crural Motoneurons during Rhythmical Jaw Movement in Rabbit

The purpose of this study was to investigate how the reciprocal inhibition of the ankle-flexor motoneuron by the ankle-extensor spindle afferents is altered during cortically-induced rhythmical jaw movement (RJM) in urethane-anesthetized rabbit. The monosynaptic reflex (MSR) was induced by test shock of the common peroneal nerve (CPN) and recorded from a distal point of the nerve. The conditioning shock applied to the tibial nerve (TN) at the intensity below 1.2 times the threshold caused a significant inhibition of the MSR in the CPN. The inhibition tended to slightly decrease with an increase in amplitude of the test MSR with either application of stronger test shock or during RJM. The decrease of the reciprocal inhibition significantly exceeded the degree that can be ascribed to the increase of the amplitude of the MSR during RJM. The reciprocal inhibition was significantly reduced during fictive as well as actual RJM. It was concluded that (1) the reciprocal Ia inhibition of the crural motoneurons in the rabbit is reduced with RJM and (2) the intraoral sensory input is not essential for this reduction. This study indicated that the oral motor activity generally exerts a strong influence on the bodily motor function.