JAW MOVEMENT RESPONSES TO ELECTRICAL-STIMULATION OF DIFFERENT PARTS OF THE HUMAN TEMPORALIS MUSCLE

Previous EMG studies have provided indications for the differential ac tivation of the human temporalis muscle. However, in these studies the contribution of different parts of the temporalis muscle could not be separated from the contributions of other muscles, since contraction was performed by voluntary activation. Therefore, the aim of this stud y was to examine functional differences among various parts of the hum an temporalis muscle by means of recording the incisal point movement response to electrical stimulation of parts of the muscle. Five health y male subjects participated in this study. Three locations (anterior, middle, and posterior temporalis muscle) were stimulated, by means of monopolar wire electrodes and rectangular pulses. The insertion depth s of the stimulation electrodes were determined by means of magnetic r esonance images. Stimulation was performed in four jaw positions (rest ing position, 50% maximum mouth opening, 1 cm to the left, and 1 cm to the right). Movement responses to stimulation of the different muscle parts were recorded with the OKAS-3D jaw movement analysis system. Th e movement responses were expressed in polar coordinates. The variatio n in the direction of the jaw movement response was partly explained b y the factors 'stimulation location and 'jaw position' (ANOVA, p < 0.0 01). When the stimulation location shifted in an antero-posterior dire ction, the response changed from a vertical-lateral incisal point move ment to a lateral-posterior movement with a smaller vertical component . The jaw position during stimulation also influenced the movement res ponse. A functional subdivision of the temporalis muscle into at least three parts is favored.