Mandibular movements in response to electrical stimulation of superficial and deep parts of the human masseter muscle at different jaw positions

Anatomical and electromyographical evidence suggests a compartmentalized fu nction of the human jaw-closing muscles during both static and dynamic moto r tasks. However, the voluntary nature of these tasks hampers unequivocal i nterpretation of this evidence, because it is impossible to activate volunt arily a single part of a muscle exclusively. Activation of discrete, locali zed regions can be accomplished with electrical stimulation. A previous stu dy confirmed a functional subdivision of the temporalis muscle into at leas t three parts. Here, differences in the direction of the lower incisal-poin t (IP) movement in response to electrical stimulation of four different par ts of the masseter muscle were examined in five healthy men. The deep masse ter muscle and the anterior, middle, and posterior parts of the superficial masseter muscle were stimulated with monopolar wire electrodes in four dif ferent jaw positions (resting position; 50% maximum mouth opening; and 10-m m right and left lateral excursions, both with respect to resting position) . Electrode-insertion depth was measured from magnetic resonance images. Mo vement responses to stimulation were recorded with the OKAS-3D jaw-movement analysis system. The variation in the direction of the IP movement in resp onse to stimulation of parts of the masseter was partly explained by the ef fects of stimulus location and jaw position. The response to stimulation of the deep masseter was mainly laterovertically directed, whereas the respon se to stimulation of each of the superficial parts had a mainly anteroverti cal direction, the responses being most pronounced with the mandible in its resting position. These results provide further evidence for a functional subdivision of the masseter into a superficial part and a deep part, but no t for a further subdivision of the superficial part into an anterior, middl e, and posterior part. (C) 1999 Elsevier Science Ltd. All rights reserved.