Characteristics of reflex excitation in close synergist muscles evoked by muscle vibration.

The tonic vibration reflex evoked in the vibrated medial gastrocnemius muscle was compared with that induced simultaneously in an unvibrated close synergist, the lateral gastrocnemius muscle, in the unanesthetized decerebrated cat. The time course of the force rise and fall in the synergist muscle was found to be markedly different from that produced in the vibrated muscle. In addition the magnitude of force produced in the synergist was only weakly modulated with increasing vibration frequency, in that the synergist uniformly displayed much flatter force-frequency relations than the vibrated muscle, and even showed overt force saturation in several preparations. Comparable differences in reflex responses arose when the lateral gastrocnemius served as the unvibrated synergist. In a parallel series of experiments, low intensity electrical stimulation of the proximal end of the sectioned medial gastrocnemius muscle nerve at different frequencies weakly modulated the amount of reflex force induced in the lateral gastrocnemius muscle. High frequency electrical stimulation caused the synergist force to saturate in a similar way to that seen in the vibrated preparations. A variety of possible mechanisms are discussed, with particular emphasis on "bistable" properties of motoneurons and Ia excitatory interneuronal contributions.