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.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of mandibular loading and bite force in mammals during mastication

“…This hypothesis predicts that variation in strain magnitude will be positively correlated with variation in strain rate, and not with variation in load time, chewing duty factor, or cycle time. Theoretical support for this hypothesis derives from considerations of the consequences of orderly recruitment of motor units, the primary mechanism for force modulation at low force amplitudes during mastication (Goldberg and Derfler, 1977;Hannam and McMillan, 1994;Scutter and Türker, 1998) and locomotion (Fournier and Sieck, 1988;Hennig and Lomo, 1987;Tansey et al, 1996). If the small motor neurons, which innervate small motor units consisting of slow twitch fibers, are recruited first, followed by progressively larger motor neurons and motor units consisting of faster fiber types, then increases in bite force will necessarily be achieved in a constant or decreasing time period.…”

“…In mammalian masticatory muscles, as in the locomotor muscles, force modulation at low force amplitudes appears to be predominantly via muscle fiber recruitment rather than rate modulation (Goldberg and Derfler, 1977;Hannam and McMillan, 1994;Scutter and Türker, 1998). Mammalian masticatory muscles are not uniform in their fiber types (Anapol and Herring, 2000;Herring, 1994;Maxwell et al, 1979;Wall et al, 2006;Wall et al, 2005), and a large number of studies suggest that smaller, slower motor units are recruited before larger, faster motor units (Clark et al, 1978;Desmedt and Godaux, 1979;Goldberg and Derfler, 1977;LevTov et al, 1993;Lund et al, 1979;Miles and Türker, 1986;Miles et al, 1987;van Eijden and Turkawski, 2001;Van Wessel et al, 2005;Wall et al, 2006;Wall et al, 2005;Yemm, 1977).…”

Expression of muscle-specific integrins in masseter muscle fibers during malocclusion disease

“…The masseter muscle shows a very high ATPase activity for contracting very quickly and forcefully participating in a wide variety of functional activities of the stomatognathic system including mastication, swallowing and speech (28,29). This diversity of functions requires coordination of motor output elements of the neuromuscular system with appropriate activation of tongue, facial and oropharyngeal muscles (23).…”

Modeling the Mechanical Behavior of the Jaws and Their Related Structures By Finite Element (Fe) Analysis

“…Van Eijden et ai., 1988) (which themselves are also oversimplified and do not account for their full mechanical effect due to complex internal architecture; Hannam and McMillan, 1994). Due to the spatial anatomic complexity of the masticatory muscles and the dental as well as temporomandibular joint (TMI) surfaces, the mandibular force system can be defined as being non-coplanar and non-concurrent-that is, a system in which the force vectors neither lie in the same plane nor intersect at a unique location.…”