Inhibition of extensor gamma motoneurons by anatagonistic primary and secondary spindle afferents.

About 2/3 of the gamma efferents isolated from the medial gastrocnemius nerve were inhibited by longitudinal high-frequency vibration applied to the tendons of the non-contracting pretibial flexors (decerebrate cats). The inhibition appeared at 15-25 mum amplitude of vibration and increased up to maximum at nearly 100 mum. Increasing the frequency of vibration from 100 to 300 Hz increased the inhibition. The reflex effects elicited by muscle vibration corresponded well in incidence and magnitude with those evoked by tetanization of the deep peroneal nerve at group I stimulus strength. The reflex disappeared when the nerve supply of the vibrated muscles was cut. The sensitivity of some pretibial proprioceptors to vibration was also tested. It is concluded that primary spindle endings of the pretibial flexors inhibit the extensor gamma motoneurons. Some findings hint at a spinal pathway involving Ia inhibitory interneurons. In addition, an inhibitory action of pretibial group II afferents, probably secondary spindle endings, on extensor gamma efferents was demonstrated. The described fusimotor inhibition by antagonistic muscle spindle afferents is a further example of alpha-gamma-linkage.