Reflex responses of sternocleidomastoid muscle induced by mechanical stimulation of upper anterior tooth and forehead

Abstract

Electromyographic coactivation of the sternocleidomastoid muscle (SCM) during masticatory function was observed. To analyze the mechanism of the coactivation, we studied the reflex responses of the SCM induced by mechanical stimulations and tooth-tapping movements. Six healthy subjects with an average age of 26 years were asked to make various kind of efforts using SCM and masseter muscles, then the mechanical taps were applied to a forehead and an upper anterior tooth, and the surface EMG from the SCM and masseter were recorded. The head position during the recordings was controlled in the upright or the rotation position. The gingiva of the tapped teeth was anesthetized in two subjects. EMG signals were rectified and digitized and then analyzed using the signal average program on a computer (NEC SANEI 7T17). Results: 1. Reflex responses of the SCM with a latency of ca. 30 ms were induced by mechanical taps to the forehead and to the anterior tooth. It can result from a stretch reflex. 2. During the tooth-tapping movement at 1 Hz, the SCM activated at ca. 90 ms after teeth contacts. It is Possible that the activity was derived from a central control mechanism or from another reflex other than the stretch reflex.