Internal Organization in the Human Jaw Muscles

Abstract

The human jaw muscles are essential to mastication and play an important part in craniofacial growth. They contribute to dental and articular forces, deform the mandible, and, like other tissues, are subject to disorders, often manifested as pain. The literature describes how their contraction is controlled by the nervous system, and how their general structure and function contribute to craniofacial biology, but there has been little appraisal of their internal organization. Most of these muscles are not simple; they are multipennate, complexly layered, and divided by aponeuroses. This arrangement provides substantial means for differential contraction. In many ways, jaw muscle fibers are intrinsically dissimilar from those found in other skeletal muscles, because they are arranged in homogeneous clusters and generally reveal type I or type II histochemical profiles. Most are type I and are distributed preferentially in the anterior and deeper parts of the jaw closers. Additionally, most motor unit (MU) territories are smaller than those in the limbs. There is circumstantial evidence for intramuscular partitioning based in part on innervation by primary muscle nerve branches. During normal function, MU recruitment and the rate coding of MU firing in human jaw muscles follow the general principles established for the limbs, but even here they differ in important respects. Jaw muscle MUs do not have stable force recruitment thresholds and seem to rely more on rate coding than on sequential unit recruitment to grade the amplitude of muscle contraction. Unlike those in the limbs, their twitch tensions correlate weakly with MU fatiguability and contraction speed, probably because there are so few slow, fatigue-resistant MUs in the jaw muscles. Moreover, the type I fibers that are present in such large numbers do not contract as slowly as normally expected. To complicate matters, estimation of jaw MU twitch tensions is extremely difficult, because it is affected by the location used to measure the twitch, the background firing rate, muscle coactivation, and regional, intramuscular mechanics. Finally, there have been very few systematic studies of jaw MU reflex behavior. The most recent have concentrated on exteroceptive suppression and suggest that MU inhibition following intra- and perioral stimulation depends on the location of the MU, its background firing rate, the timing of the stimulus, and the task used to drive the unit. Task dependency is a common feature of human jaw MU behavior, reflecting interaction between peripheral sensory information from orofacial and muscle afferents and corticobulbar drive. In summary, several lines of evidence, including intramuscular structure, the disposition and physiological behavior of the intrinsic MUs, strongly suggest that human jaw muscles are uniquely organized internally. Both structural and functional attributes need to be incorporated into a hypothetical model of each muscle in order to explain how it produces local tensions and displacements during normal use and the circumstances under which any disordered biomechanical events might be induced in it.