Chapter 5 - Jaw Movement and Its Control

Abstract

The jaw muscles move the jaw in a complex three-dimensional manner during jaw movements. There are three jaw-closing muscles (masseter, temporalis, and medial pterygoid) and two jaw-opening muscles (lateral pterygoid and digastric). The basic functional unit of muscle is the motor unit. The internal architecture of the jaw muscles is complex, with many exhibiting a complex pennate (feather-like) internal architecture. Within each of the jaw muscles, the central nervous system (CNS) appears capable of activating separate compartments with specific directions of muscle fibers. This means that each jaw muscle is capable of generating a range of force vectors (magnitude and direction) required for a particular jaw movement. In the generation of any desired movement, the CNS activates motor units in different muscles. Movements are classified into voluntary, reflex, and rhythmical. Many parts of the CNS participate in the generation of jaw movements. The face motor cortex is the final output pathway from the cerebral cortex for the generation of voluntary movements, such as opening, closing, protrusive, and lateral jaw movements. Reflexes demonstrate pathways that aid in the refinement of a movement and can be used by the higher motor centers for the generation of more complex movements. Mastication or chewing is a rhythmical movement that is controlled by a central pattern generator in the brainstem. The central pattern generator can be modified by sensory information from the food bolus and by voluntary commands from higher centers.