The mechanism of "Silent period" preceding the act in rapid voluntary movements.

Abstract

It has been observed that the tonic discharge disappears for some extent immediately preceding the act in rapid voluntary movements. To reveal the mechanism of this phenomenon, a series of experiment was conducted in healthy normal subjects. The action potentials were led-off by bipolar surface electrode, simultaneous mechanical response was recorded with resistance strain gauge transducer. While a subject made a slight voluntary contractions and maitained it steadily, he was asked to respond with a rapid voluntary movements as quick as possible to a visual or acoustic stimulus. The latency of this silent period in elbow extension was found to be around 40 msec at minimum. It was also found that the most consistent results were obtained by using upward jumping movement. The silent period preceding the act usually occurred in the musle synergist; however, antagonist showed the same silent period simultaneously. This finding suggested this phenomenon was not to be reciprocally innervated. The silent period in the triceps brachii and the rectus femoris were simultaneously found in the electromyogram of the whole body movements such as the starting in sprint running. It was supposed from these results that the center in the brain or brain stem sent some inhibitory discharge to the motoneurons in the ventral horn of the spinal cord.