Masticatory muscles and the skull: a comparative perspective

Abstract

Masticatory muscles are anatomically and functionally complex in all mammals, but relative sizes, orientation of action lines, and fascial subdivisions vary greatly among different species in association with their particular patterns of occlusion and jaw movement. The most common contraction pattern for moving the jaw laterally involves a force couple of protrusor muscles on one side and retrusors on the other. Such asymmetrical muscle usage sets up torques on the skull and combines with occlusal loads to produce bony deformations not only in the tooth-bearing jaw bones, but also in more distant elements such as the braincase. Maintenance of bone in the jaw joint, and probably elsewhere in the skull, is dependent on these loads.

Figure 1

Anatomy and EMG of the masseter muscle in the caviomorph rodent, Kerodon rupestris. A. Superficial view; the arrow indicates the posterior masseter, which inserts on the condylar process. B. Overlying layers of the masseter removed to show the posterior masseter (asterisk). C. EMG of right-side muscles during ingestion and mastication. The posterior masseter is mainly active in protrusion during ingestion (open arrows) and acts with the digastric in opening the jaw. MedPt: medial pterygoid; Mlat: lateral masseter; Mmain: main (middle) masseter; Mrefl: pars reflexa of superficial masseter; Mpost: posterior masseter; Dig: digastric muscle.

Figure 2

Coronal μCT slices through the mandibular condyles of a rabbit (Oryctolagus cuniculus) that had received an injection of type A botulinum toxin into one masseter 29 days before sacrifice. A. Intact side condyle. B. Paralyzed side condyle. The relative volume of trabecular bone on the paralyzed side is 17% less than on the intact side.