Abstract
We investigated functional roles of the superior and inferior heads of the human lateral pterygoid muscle (SUP. LPT and INF. LPT, respectively). In 6 subjects EMG activities were recorded from the both heads of the lateral pterygoid muscle and other masticatory muscles. The movements of the incisor and condyle points and biting force were measured. Three-dimensional structure of the temporomandibular joint (TMJ) was examined in 12 cadavers. The findings are summarized as follows.
(1) Articular disk can rotate around the condyle backward, but not forward, on the axis of the lateral and medial poles of the condyle to which the articular disk is tightly attached.(2) The SUP. LPT was active primarily in the jaw closing phase and the INF. LPT in the jaw opening phase during chewing. The SUP. LPT and INF. LPT showed reciprocal EMG activities in various basic jaw movements.(3) The SUP. LPT showed the stretch reflex but the INF. LPT did not.(4) The SUP. LPT muscle activity increased rapidly at a lower biting force level, whereas the relationship between the biting force and the activity of the masseter and temporalis muscles changed in a manner quite different from that in SUP. LPT.(5) The SUP. LPT maintains a considerable activity level at the rest position and increases its activity during swallowing, chewing and clenching of the teeth.
