The Influence of the Gamma Motor System on Jaw Movements during Speech: A Theoretical Framework and Some Preliminary Observations

Abstract

A selective anesthesization technique was applied to the mandibular branch of the trigeminal nerve in man, and jaw activity was observed during speech under conditions of gamma motor blockade. The basic procedure involves special application of a conventional nerve block whereby it appears possible to selectively block the gamma efferent fibers of a motor nerve while leaving the alpha fibers unaffected. Measurement and analysis of jaw displacement, velocity, and acceleration under both normal and gamma block conditions revealed systematic effects for two experimental subjects. The experimental changes in jaw activity were interpreted to suggest that the spindle afferent facilitation of alpha motoneurons, thougnt to optimize the initiation of movement under normal conditions, was absent or disrupted. Thus, the jaw musculature was unable to produce adequate opening and closing forces with normal temporal control. Furthermore, from these data, under normal conditions, the spindle motor system would appear to operate most clearly under conditions of movement where large values of acceleration, velocity, and displacement were demanded. Finally, these findings were accepted as support for spindle motor system operation in oral-facial movements during the production of speech.