Tonic and phasic spinal cord mechanisms in man

Abstract

Sustained vibration of muscle normally suppresses phasic reflexes and provokes a reflex tonic contraction in the muscle vibrated. Evidence is presented that vibration stimulates the muscle spindle. The diminution of phasic rei'iexes by vibration is best explained by occlusion of most Ia afferent fibers, occupied by asynchronous vibration-induced activity. This is supported by the demonstration of the same single motor unit participating in voluntary contraction, vibration-induced activity, and the H reflex. The effect of various drugs on tonic contraction is studied. Vibration-induced tonic contraction can be diminished or eliminated in nerve root lesions, neuritis, chronic spinal transverse lesions, upper motor neurone lesions, cerebellar disorders, and dystonia. Vibration induced a 'red nucleus tremor1 in a patient with a mid-brain thrombosis. The tonic contraction is normal in Parkinsonism.