An unusual function for the medial pterygoid muscle in the guinea pig

ArchsoralViol. Vol. 31.No. II,pp. 781-783,1986

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SHORT COMMUNICATION AN UNUSUAL

FUNCTION FOR THE MEDIAL MUSCLE IN THE GUINEA PIG

PTERYGOID

E-KWAN CHEN and SUSAN W. HERRING Departments of Oral Anatomy and Anatomy, University of Illinois at Chicago, Health Sciences Center, 801 S. Paulina St, Chicago, IL 60612, U.S.A. muscle has some physiological properties resembling those of jaw-opening muscles. Biomechanical analysis showed that, because of the anatomical peculiarities of the feeding apparatus in

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caviid rodents, the action of the muscle changes from closing to opening as the mandibular condyles are protruded in the groove-like mandibular fossa. This unusual changing function may be useful in producing the ventral, medial and anterior power stroke of mastication.

When mammalian jaw muscles are classified functionally, the masseter, temporalis and medial pterygoid are invariably grouped together as jaw elevators or closers, whereas the digastric and usually the lateral pterygoid are considered jaw depressors or openers. This classification is based on biomechanical (e.g. Turnbull, 1970) and electromyographic (reviewed by Gorniak, 1985) analyses. We have re-assessed the action of the medial pterygoid in the guinea pig (Cuuiu porceh) and related rodents. Neurophysiological studies have shown that in the guinea pig this muscle has reflexes which are characteristic of jaw-opening muscles as well as the expected jaw-closing reflexes (Tal and Goldberg, 1981). Further, electromyographic studies of mastication in the guinea pig (Byrd, 1981, unilateral cycles; Byrd, 1984) and in Kerodon rupestris, a cavy from Brazil, have indicated that, in contrast to other elevating muscles, the medial pterygoid is often electrically active during jaw opening as well as during closing. In other mammals, opening activity in the medial pterygoid is rare and when it occurs is associated with mediolateral deviations rather than opening per se (Kallen and Gans, 1972; Weijs and Dantuma, 1975; Hiiemae and Crompton, 1985). Because the results from caviid rodents conflict with the traditional view of the medial pterygoid as an adductor, we re-examined the biomechanics of this muscle. Three guinea pig and two Kerodon rupestris heads were dissected to determine the architecture and attachment sites of the muscle. These observations were used to estimate lines of action for different parts of the muscle and for the muscle as a whole in various positions of the mandibular condyles relative to the skull. These lines of action were evaluated in lateral and dorsal views in order to determine the effect of muscle contraction on jaw movement. To facilitate this, guinea pig skulls were manipulated and rubber bands were used to simulate the jaw muscles. Anterior, middle and posterior portions of the medial pterygoid were simulated by three separate rubber bands; jaw movements were observed as each rubber band was pulled individually. As in other mammals, the medial pterygoid of the

guinea pig pulls the mandible superiorly, anteriorly (in lateral view, about 65” from the tooth row) and medially (in occlusal view, about 50” from the tooth row). However, the structure of the masticatory apparatus in the guinea pig, other members of the Caviidae, and the closely-related capybara (Hydrochaeridae) is highly specialized, even among caviomorph rodents. The occlusal plane slants from dorso-lateral to ventro-medial, requiring a masticatory power stroke that is directed ventrally as well as medially and anteriorly (Fig. 1; Byrd, 1981). This inclination of approx. 40” makes the pull of the medial pterygoid essentially perpendicular to the occlusal plane. As is typical for mammals with a strong propalinal component of the power stroke, the small condyle makes substantial antero-posterior excursions along the elongated mandibular fossa. The long angular process of the mandible carries the insertion of the medial pterygoid posteriorly, and the extension of the muscle’s origin through the pterygoid fossa to the orbit prolongs the origin anteriorly (Landry, 1957; Woods, 1972). The biomechanics of the medial pterygoid are best considered in three different occlusal positions (Fig. 2): posterior occlusion, as in the beginning of the

Fig. 1. Anterior view of the mandible of Kerodon rupestris (actual intercondylar breadth, 2.5 cm). The arrow shows the ventral and medial components of movement during the power stroke of mastication. 781

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E-KWAN CHEN and SUSAN W. HERRING

Fig. 2. Line of action of the medial pterygoid muscle, seen from a medial view of the molar toothrow in a sagittallysectioned guinea-pig skull (tracings from prints). The occlusal plane is perpendicular to the page. The action line is drawn from the centre of the muscle’s origination from the pterygoid fossa to the centre of the insertion on the angular process. The stippled area shows the outline of the whole muscle. The point of the arrow indicates the location of the articular surface of the condyle. The expanded area behind the condyle, most clearly visible in (C), is the postcondylar process for the insertion of the lateral pterygoid and posterior masseter. a-auditory bulla; bbraincase; -medial pterygoid plate; s-symphyseal articulation. The absolute anterior excursions of the mandible are 4 mm between (A) and (B) and 3 mm between (B) and (C), for a total of 7mm, which is the same excursion recorded by Byrd (I 98 I, 1984) in naturally chewing animals, (A) Posterior occlusion. The whole muscle passes in front of the jaw joint and thus has a closing moment in addition to a large anterior vector. (B) Full occlusion. The action line passes just in front of the joint, so the muscle as a whole has only a small closing moment. If differential fibre contraction is possible, the anterior fibres will have a closing moment and the posterior tibres an opening moment. (C) Anterior occlusion. The action line passes behind the joint so that the muscle as a whole has a small opening moment, although the anterior fibres still have a small closing moment. The anterior component of pull is decreased.

power stroke; full apposition of upper and lower molars, as during the power stroke; and anterior occlusion, as at the end of the power stroke when opening begins. The frame of reference is provided by the occlusal plane; because this plane is inclined in relation to the body axes, a true medial view of the molars, such as in Fig. 2 where the plane is viewed on end, provides a ventro-medial view of the mandible and cranium. In posterior occlusion (Fig. 2A) the muscle’s moment arm for closing was maximized because the condyle (the fulcrum) was situated posteriorly, away from the origin at the pterygoid fossa. Nevertheless, the moment arm of the medial pterygoid was short compared to those of the masseter and temporalis. This position also increased the anterior-pulling component of muscle force relative to its medial component. The muscle as a whole thus pulled upward and forward at the beginning of the power stroke. As the power stroke progressed, the mandible moved forward, decreasing the muscle’s moment arm as well as its anterior vector. In full occlusion (Fig. 2B) the action line of the muscle passed just in front of the craniomandibular joint, so there was even a smaller closing moment. The posterior fibres, in fact, passed behind the joint and hence had a small opening moment. At the end of the power stroke (Fig. 2C) the anterior vector was minimized and the muscle as a whole had a small opening moment. Only the most anterior fibres passed in front of the joint. In summary, the medial pterygoid begins the power stroke as an adductor with a strong protrusive component but ends it as a depressor with a weak protrusive component. The weakening protrusive component during the course of the power stroke also caused a change in the muscle’s action with respect to side-to-side movements (rotations around a vertical axis, not shown in Fig. 2). The initially strong tendency of the workingside medial pterygoid to move the mandible toward the balancing side by a rotation around the balancing side condyle decreased as the mandible was moved anteriorly. Thus, both the analysis and the manipulations indicated that under certain circumstances contraction of the medial pterygoid can open the jaw. Although the medial pterygoid may never contract by itself naturally, these findings indicate that the contribution of the muscle to overall jaw movement varies according to condylar position. If selective contraction of different parts of the muscle is possible, then the muscle may have an even greater functional range. For example, selective contraction of the posterior fibres could aid in mandibular depression in all but the most retruded positions of the condyle. In guinea pigs the closing stroke of mastication begins with the mandible depressed and retruded and involves elevation, protrusion and some deviation toward the balancing side (Byrd, 1981) all actions for which the working-side medial pterygoid is well suited at the beginning of the movement. During the power stroke the mandible continues to move toward the balancing side and anteriorly, but rather than elevation, the slope of the occlusal plane requires slight depression (Fig. I). At the end of the power

Medial pterygoid muscle function stroke the protrusive and contralateral movements are reversed while depression becomes dominant (Byrd, 1981). Because of the changing action of the medial pterygoid, it remains well suited to effect movements at each point during the stroke. The implication that the medial pterygoid first closes and then opens the jaw during the masticatory cycle provides a plausible explanation for the occurrence of jaw-opening reflexes (Tal and Goldberg, 1981) and electromyographic activity (Byrd, 1981, 1984). Also, the small and decreasing closing moment of the media1 pterygoid during the power stroke helps to explain how this movement, with its element of depression, can be powered by the closing musculature. Although the resultant force of the contraction must still be directed upwards (in order to perform work on the food), the upward vector need not be great. The inclined plane of the upper molars assures that if a medial vector is supplied, the required depression will take place. The media1 vector is, in fact, supplied by the working-side medial pterygoid and masseter and the balancing-side temporalis effecting a rotation toward the balancing side (Byrd, 1981; Gorniak, 1985). Toward the end of the power stroke, the medial pterygoid can help directly with depression by lessening the overall closing vector with its opening moment. As guinea pigs are primarily grass eaters, the anteriorly directed, shearing power stroke is as significant as the closing force for comminution of the bolus; a closing force need only keep the molars approximated. Although the changing action of the medial pterygoid muscle is at this stage a biomechanical hypothesis, a partial test can be made using available electromyographic data. The test involves the participation of the medial pterygoid in incisal biting. As in other rodents, incision requires protrusion of the condyles (to a position like that shown in Fig. 2C). Under these circumstances, the medial pterygoid is expected to have a negligible or opening moment; therefore it should not be recruited. Unfortunately, the incisors of caviid rodents are gracile and used more for picking up vegetation than for gnawing, so information on true incisal biting is lacking. However, Byrd (1981) induced guinea pigs to gnaw on thin pieces of carrot, resulting in cycles in which the mandible closed in a protruded position and then

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retruded before opening and protruding once again. The associated electromyographic recordings showed that the two medial pterygoids were the only adductors to begin their activity during opening (and protrusion), and that their activity ceased before full closure was attained. The results are consistent with the hypothesis that the media1 pterygoid has an opening vector when the condyles are protruded in the mandibular fossae. Acknowledgemenrs-We

thank the Division of Mammals, Field Museum of Natural History, Chicago, for the loan of skulls, and Mr W. Winn for DhotoaraDhv.We are grateful to Dr H. Barghusen, Dr J. cleall,-D; \ir. Greaves, Dr T. Lakars and Dr R. Scapino for their comments on various versions of this manuscript and to an anonymous reviewer for suggesting the test of the hypothesis. REFERENCES

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