Jaw movement responses to electrical stimulation of different parts of the human temporalis muscle.

Previous EMG studies have provided indications for the differential activation 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 study was to examine functional differences among various parts of the human temporalis muscle by means of recording the incisal point movement response to electrical stimulation of parts of the muscle. Five healthy 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 depths of the stimulation electrodes were determined by means of magnetic resonance images. Stimulation was performed in four jaw positions (resting 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. The movement responses were expressed in polar coordinates. The variation in the direction of the jaw movement response was partly explained by the factors 'stimulation location' and 'jaw position' (ANOVA, p < 0.001). When the stimulation location shifted in an antero-posterior direction, the response changed from a vertical-lateral incisal point movement to a lateral-posterior movement with a smaller vertical component. The jaw position during stimulation also influenced the movement response. A functional subdivision of the temporalis muscle into at least three parts is favored.