Electromyography and mechanics of mastication in the albino rat

Abstract

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.

Jaw, Tongue and Hyoid Movement — A Question of Synchrony? Discussion Paper

“…The major components of the pattern of EMG activity found in the cat (part of the pattern has also been described by Gorniak & Gans 1980) are similar to those reported in other animals (e.g. Herring & Scapino 1973, Crompton et al 1977, Weijs & Dantuma 1975 and correspond to the periods of muscle shortening, the pattern of which has been described . However, EMG activity occurs at times other than during muscle shortening.…”

“…Although a considerable amount of background work on oral functiont hag already been carried out on a variety of animals, most studies have still concentrated upon quite limited aspects of oral activity, although to a lesser extent than human studies. The animal studies have been diverse, using for example, the mudskipper (Sponder & Lauder 1981), the salamander , the lizard (Throckmorton 1980, Smith 1982, 1984, the bat (Kallen & Gans 1972), the opossum (Crompton et al 1977), the rat (Hiiemae & Ardran 1968, Weijs & Dantuma 1975, the rabbit (Ardran et al 1958b, Weijs & Dantuma 1981, the pig (Herring & Scapino 1973, Herring 1976, the cat (Hiiemae et al 1978, Thexton et al 1980, 1982 and the monkey (Luschei & Goodwin 1974, Byrd et al 1978, Franks et al 1981. Although the details of the oral mtisculoskeletal system may vary considerably between the animals studied and may also differ from niafi, the essential functional mechanisms remain the same.…”

Temporomandibular joint model: anatomical comparison between human and rat

“…The rounded condyle travels in a trough-like temporal fossa and the power stroke is in the protrusive direction with only a minor medial component1. Since the mandibular symphysis is fibrous, rotation of the long axis may occur around this mobile symphysis [6,7].…”

Quaternary Murid Rodents of Timor Part I: New Material of Coryphomys buehleri Schaub, 1937, and Description of a Second Species of the Genus

“…There is a substantial body of work on the functional anatomy of the masticatory muscles of rodents, especially of Rattus norvegicus (e.g., Hiiemae, 1971;Hiiemae and Houston, 1971;Weijs and Dantuma, 1975), and a growing body of comparative work on other groups of muroids (Bekele, 1983;Satoh, 1997Satoh, , 1999Satoh and Iwaku, 2004. In contrast, osteological features that relate to the attachment of tendons and aponeuroses are generally given light treatment in standard anatomical sources.…”

“…Enlargement of the fossa for the deep portion of the transverse mandibular muscle is particularly noteworthy. In laboratory rats this muscle is particularly active during adduction of the mandible (Weijs and Dantuma, 1975), with muscular contraction causing spreading of the anterior and superior borders of the symphysis and relieving compression on the symphyseal cartilage (Beecher, 1979).…”