The Independent Functions of the Two Heads of the Lateral Pterygoid Muscle JAMES A. McNAMARA, JR. Department of Anatomy and Center for Human Growth and Development, The University of Michigan, Ann Arbor, Michigan 48104 ABSTRACT Investigations on the role of the lateral pterygoid muscle in mandibular movements have been limited due to difficulties in obtaining con- sistent 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, depend- ing 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 dur- ing opening movements. Electromyographic activity of the superior head, an- tagonistic to the suprahyoid muscles, was observed during such closing move- ments 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 in- ferior 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 man- ness of the articular eminence and the dibular movement has historically been position of the pterygoid plates indicate a one of the most difficult to determine of more horizontal pull by the lateral ptery- all masticatory muscle functions. The goid in the monkey than in man. Schu- positions of the lateral pterygoid muscle macher ('61) and Grant ('73b) have within the infratemporal fossa has ham- demonstrated that orientation of the two pered human electromyographic studies heads of the lateral pterygoid in the mon- because electrode placement frequently key is similar to that in man, but that produced pain and regional hematomas they function at less oblique angles. In (Carlsoo, '52; Ekholm and Siirila, '60; man, a more oblique downward pull by the Griffin and Munro, '69). inferior head and a more oblique upward The rhesus monkey (Macaca mulatta) pull by the superior head is observed offers a substitute experimental animal. (Sicher, '60). However, in both man and However, specific differences exist in the the rhesus monkey, fibers from the supe- anatomy of the temporomandibular joint rior head are attached to the articular disc in man and the rhesus monkey. The flatter and capsule as well as to the condylar articular eminence, shallower glenoid head (fig. 2). fossa and larger postglenoid spine (fig. 1) The purpose of this study was to moni- have been noted in the rhesus monkey by tor and describe the functions of the lateral previous workers (Parsons, 1899; Zielin- pterygoid muscle in the rhesus monkey. ski, '65, and Zimmermann, '71). Angel 1This research was supported in part by United States public Health Service grants HD-02272 and ('48) has suggested that the relative flat- DE-03610. AM. J. ANAT., 138: 197-206. 197 198 JAMES A. McNAMARA, JR. Fig. 1 Photomicrograph of a sagittal section of the temporomandibular joint. PGS, post- glenoid spine; C, mandibular condyle; D, articular disc; AE, articular eminence; F, superior portion of the glenoid fossa. Hematoxylin and eosin. x 8. MATERIALS AND METHODS electrodes, 1-3 mm apart. In addition, Thirty-three Macaca mulatta, age six pairs of 8 mm bipolar platinum electrodes months to eight years, were monitored in were inserted aseptically into the anterior 113 electromyographic recording sessions, temporal, posterior temporal, anterior bor- consisting of one to two and one-half hours der of the superficial head of the masseter, each. Initially, the animal was anesthetized superior portion of orbicularis oris, and by an injection of 6-10 mg/kg of Keta- the suprahyoid group of muscles including mine HCl, a short-acting, dissociative the anterior digastric, mylohyoid and genio- anesthetic which has only a minor and hyoid muscles (fig. 3). The latter three transient effect upon the neuromuscula- muscles were regarded as a single muscle ture (McNamara and Kuroda, '71; Kuroda group because of the difficulty in separat- and McNamara, '72). ing them anatomically during positioning The anesthetized monkey was then of electrodes. The anterior heads of the placed in a sound-proof, electrically shield- two digastric muscles in the rhesus monkey ed chamber and chaired in a primate re- are not separated into two discrete bellies, straining device especially designed for as is typical in man. Rather, the anterior electromyographic procedures (McNamara, heads fuse at the midline and, in addition, '73). The head of the animal was oriented often blend with fibers of the mylohyoid in a fixed position by means of a Plexiglas muscle (Bosma, '56; Howell and Straus, headholder to allow for normal jaw func- '69). All recordings were taken from the tion in an unanesthetized condition. musculature on the left side. Electromyo- The approach to the lateral pterygoid graphic activity was recorded on Kodak muscle was made extra-orally through the Linagraph direct print paper using a Beck- sigmoid notch of the mandible using two man Type R Dynograph amplifier and a 25 mm Teflon insulated bipolar needle Honeywell Model 1108 Visicorder. FUNCTIONS OF THE LATERAL PTERYGOID HEADS 199 Fig. 2 Photomicrograph of a medial s8agittal section of the attachment of the laded pterygoid muscle into the amticular disc, joint capsule, and mandibular condyle. Hematox- ylin and oosin. x 8. After verification of electrode placement, goid muscle. Sometimes pattern A was re- the animal was left isolated in the sound- corded, sometimes pattern B was recorded, proof room for a minimum of 30 additional and in a few instances a third pattern of minutes to allow for complete recovery activity was observed. It was soon obvious from the anesthetic. After this time, re- that the latter pattern represented a super- cordings of posture, oral reflexes, and imposition of pattern A and pattern B in random jaw movements were initiated the electromyographic records. The occur- while the animal was still in isolation. Re- rence of each pattern of lateral pterygoid flex salivary swallows were recorded at muscle activity depended upon the specific this time. Later, the investigator entered location of the pair of electrodes within the sound-proof room and generated water the muscle. The location of electrode swallows and subsequent clearing swal- placement in 12 animals was examined lows through the administration of 0.5 ml through anatomical dissection and the two tap-water by a syringe into the animal’s patterns of activity (patterns A and B) mouth. Chewing was elicited by feeding were related to the inferior and superior the animal small chunks of sugar and bits heads of the lateral pterygoid muscle, re- of apple. spectively. A drop of India ink was in- jected at the placement site of electrode OBSERVATIONS tips, the animal was sacrificed immedi- Initially no attempt was made to differ- ately, these areas were examined, and the entiate between the functions of the two precise location of the electrodes within heads of the lateral pterygoid muscle. the muscle noted. However, during the recording sessions, Pattern A (the inferior head). The lat- three patterns of muscle activity were ob- eral pterygoid muscle acted synergistically served in the region of the lateral ptery- with the suprahyoid muscle group in 200 JAMES A. McNAMARA, JR. opening movements of the mandible (figs. and contralaterally during opening move- 4, 5). The lateral pterygoid did not appear ments of the mandible. to initiate opening movements because the Pattern B (the superior head). This onset of activity in the suprahyoid usually pattern of activity was distinctly different preceded lateral pterygoid firings. How- from that of pattern A. The firings of pat- ever, the duration of suprahyoid activity tern B occurred during a different phase before the initiation of lateral pterygoid of mandibular movement than did those function varied, ranging from 25-350 of the suprahyoid musculature and thus no msec. The lateral pterygoid muscle was activity was recorded in the superior head never active during the closing movements of the lateral pterygoid during opening of the mandible, such as clenching and the movements of the mandible. Discharges closing phase of mastication, nor did it from the superior head were observed dur- function in salivary, water, or masticatory ing closing movements (figs. 4, 6). The swallows (fig. 5). onset of lateral pterygoid function was Anatomical dissection confirmed that usually concurrent with that of the eleva- pattern A originated from the inferior head tor musculature during the closing phase of the lateral pterygoid muscle. This head of mastication and during clenching of presumably assisted in the translation of teeth. the condylar head downward, anteriorly, The superior head of the lateral ptery- Fig. 3 Animal positioned in head-holder. Needle electrodes are in the anterior and posterior portions of the temporal muscle, the anterior portion of the superficial head of the masseter muscle, the orbicularis oris muscle, the lateral pterygoid muscle and the supra- hyoid muscle group. FUNCTIONS OF THE LATERAL PTERYGOID HEADS 20 1 5 sec A. TEMP - I1 f? TEMP I MASS. v.L. ,L A. TEMP -t-, f? TEMP L ORBIC. ORIS. s SUP. HY LATPT (SUP)--" Fig. 4 Electromyographic recordings of the activity of the lateral pterygoid muscle dur- ing random jaw movement. (a) The activity of the inferior head (Pattern A) was usually associated with suprahyoid function during opening movements of the mandible and was antagonistic to the elevator muscles.