Trends in the Actions of Mammalian Masticatory Muscles

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Abstract
The actions of the masticatory muscles of a variety of mammals in which feeding behavior and the configuration of the masticatory apparatus differ have been reported. The most common approach used in these studies involves (1) obtaining a good anatomical perception of the musculature, (2) deriving a theoretical model of the actions of these muscles during jaw movement, and (3) testing this model by recording muscle activity and jaw movements simultaneously. A catalogue of the activity patterns in eleven species of mammals during food reduction reveals certain trends in the actions of the masticatory muscles. Horizontal jaw movements are generated primarily by differential activities of the deep temporalis, superficial masseter, and medial pterygoid. Vertical movements and the maintenance of tooth to food contact apparently are produced by action of the superficial temporalis, deep masseter, and zygomaticomandibularis. Thus, horizontal movements are seemingly generated by muscles having fibers arranged in marked anteroposterior direction, whereas vertical movements are generated by muscles having more or less vertically arranged fibers. The asymmetry of jaw movement and the muscular activity generating it suggest that mastication involves an interactionbetween an unbalanced and flexible functional unit (muscles) and a balanced and stable structural unit (skull and teeth). Thus, any unbalancing of the structural unit results in a further unbalancing of the masticatory process.

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