Differential effects on tonic and phasic reflex mechanisms produced by vibration of muscles in man.

Abstract

Vibration of muscle in human subjects depressed phasic reflexes, such as tendon jerks and the H-reflex, while tonic contraction slowly developed. These effects are hypothetically attributed to repetitive trains of group la afferent impulses elicited by stretch-sensitive muscle spindle receptors responding to vibration. Such afferent discharge depresses excitability of homonymous alpha motor neurons concerned with phasic monosynaptic reflexes and activates a polysynaptic pathway terminating on tonic alpha motor neurons. Depression was partly overcome by reinforcement and occurred below the level of a spinal cord lesion. Recovery of the H-reflex occurred in a time period similar to that for recovery following afferent tetanization of the muscle nerve at the same frequency as vibration. Tonic contraction was diminished by procaine infiltration of the muscle belly; absent on the areflexic side in patients with unilateral nerve root compression and bilaterally below the level of spinal cord lesions; diminished or absent on the side affected by unilateral cerebellar lesions in 2 patients; abolished by the intravenous injection of thiopentone and Ciba 28,882-Ba; and produced by tetanic contraction of the medial popiteal nerve in the absence of H or M responses. The reflex depends upon supraspinal centers, possibly the cerebellum. Separate tonic and phasic mechanisms in man may be of importance in explaning the dichotomy between muscle tone and tendon jerks observed in certain clinical states. In addition the differential effects of muscle vibration on these mechanisms may aid in the analysis of motor disorders.