EMG of the digastric muscle in gibbon and orangutan: Functional consequences of the loss of the anterior digastric in orangutans

Abstract

Unlike all other primates, the digastric muscle of the orangutan lacks an anterior belly; the posterior belly, while present, inserts directly onto the mandible. To understand the functional consequences of this morphologic novelty, the EMG activity patterns of the digastric muscle and other potential mandibular depressors were studied in a gibbon and an orangutan. The results suggest a significant degree of functional differentiation between the two digastric bellies. In the gibbon, the recruitment pattern of the posterior digastric during mastication is typically biphasic. It is an important mandibular depressor, active in this role during mastication and wide opening. It also acts with the anterior suprahyoid muscles to move the hyoid prior to jaw opening during mastication. The recruitment patterns of the anterior digastric suggest that it is functionally allied to the geniohyoid and mylohyoid. For example, although it transmits the force of the posterior digastric during mandibular depression, it functions independent of the posterior digastric during swallowing. Of the muscles studied, the posterior digastric was the only muscle to exhibit major differences in recruitment pattern between the two species. The posterior digastric retains its function as a mandibular depressor in orangutans, but is never recruited biphasically, and is not active prior to opening. The unique anatomy of the digastric muscle in orangutans results in decoupling of the mechanisms for hyoid movement and mandibular depression, and during unilateral activity it potentially contributes to substantial transverse movements of the mandible. Hypotheses to explain the loss of the anterior digastric should incorporate these functional conclusions.

Phase II jaw movements and masseter muscle activity during chewing in Papio anubis

“…As the jaw opens slightly during phase II, asymmetric activity in the anterior suprahyoid muscles and the posterior digastric may produce some of the medial movement (Moller, 1966;McNamara, 1973;Hylander et al, 1987;Wall et al, 1994). Further study of the activity patterns of these muscles and of the working-side lateral pterygoid muscle (which may act unilaterally to produce transverse movement of the corpus) is needed to evaluate this idea.…”

The instantaneous center of rotation during human jaw opening and its significance in interpreting the functional meaning of condylar translation

“…Indeed, the inferior head of the lateral pterygoid muscle is known to show such behavior in some primate species. In gibbons, for example (Wall et al, 1994), during the wide opening stroke the EMG levels of the inferior head of the lateral pterygoid muscle are found to be generally much more pronounced than those of the digastric muscle. The same study also shows that the inferior head of the lateral pterygoid muscle is more active in the late fast opening phase than the anterior digastric muscle during mastication.…”

The relationships among jaw‐muscle fiber architecture, jaw morphology, and feeding behavior in extant apes and modern humans

“…Compared with extant apes, modern humans exhibit a reduction in masseter PCSA relative to condyle-M 1 length but retain relatively long fibers, suggesting humans may have sacrificed relative masseter muscle force during chewing without appreciably altering muscle excursion/ contraction velocity. Despite their functional significance, we know relatively little about the functional anatomy of the jaw muscles in hominoids (Gregory, 1950;G€ ollner, 1982;Wall et al, 1994). These findings underscore the difficulty of accurately estimating jaw-muscle fiber architecture from craniometric measures and suggest models of fossil hominin and hominoid bite forces will be improved by incorporating architectural data in estimating jaw-muscle forces.…”

“…V C 2013 Jaw muscles are functionally important for generating the masticatory movements and occlusal forces associated with ingestion, biting and chewing. Despite their functional significance, we know relatively little about the functional anatomy of the jaw muscles in hominoids (Gregory, 1950;G€ ollner, 1982;Wall et al, 1994). Among the structural characteristics of muscle, fiber architecture is an important determinant of muscle function.…”