The independent functions of the two heads of the lateral pterygoid muscle.

Investigations on the role of the lateral pterygoid muscle in mandibular movements have been limited due to difficulties in obtaining consistent neuromuscular recordings in human subjects. The rhesus monkey was used as a substitute experimental animal. Thirty-three Macaca mulatta were monitored in 113 electromyographic recording sessions. Two distinct functional patterns were identified from the region of the lateral pterygoid muscle, depending upon the location of the electrodes within this muscle. Through anatomical dissection of areas of electrode placement in 12 animals, the two patterns of activity were related to the inferior and superior heads of the lateral pterygoid muscle. The inferior head acted synergistically with the suprahyoid muscle group in opening movements of the mandible. No activity was noted in closing movements, or in swallowing. In contrast, the superior head was not active during opening movements. Electromyographic activity of the superior head, antagonistic to the suprahyoid muscles, was observed during such closing movements as chewing and clenching of the teeth and during deglutition. The superior head presumably positioned or stabilized the condylar head and disc against the articular eminence during closing movements of the mandible, while the inferior head assisted in the translation of the condylar head downward, anteriorly, and contralaterally during opening movements. Thus, the two heads of the lateral pterygoid can be considered as two functionally distinct muscles. The role of the lateral pterygoid in mandibular movement has historically been one of the most difficult to determine of all masticatory muscle functions. The positions of the lateral pterygoid muscle within the infratemporal fossa has hampered human electromyographic studies because electrode placement frequently produced pain and regional hematomas (Carlsoo, '52; Ekholm and Siirila, '60; Griffin and Munro, '69). The rhesus monkey (Macaca mulatta) offers a substitute experimental animal. However, specific differences exist in the anatomy of the temporomandibular joint in man and the rhesus monkey. The flatter articular eminence, shallower glenoid fossa and larger postglenoid spine (fig. 1 ) have been noted in the rhesus monkey by previous workers (Parsons, 1899; Zielinski, '65, and Zimmermann, '71). Angel ('48) has suggested that the relative flatAM. J. ANAT., 138: 197-206. ness of the articular eminence and the position of the pterygoid plates indicate a more horizontal pull by the lateral pterygoid in the monkey than in man. Schumacher ('61) and Grant ('73b) have demonstrated that orientation of the two heads of the lateral pterygoid in the monkey is similar to that in man, but that they function at less oblique angles. In man, a more oblique downward pull by the inferior head and a more oblique upward pull by the superior head is observed (Sicher, '60). However, in both man and the rhesus monkey, fibers from the superior head are attached to the articular disc and capsule as well as to the condylar head (fig. 2). The purpose of this study was to monitor and describe the functions of the lateral pterygoid muscle in the rhesus monkey. 1This research was supported in part by United States public Health Service grants HD-02272 and DE-03610.