innervation o the intrafusal muscle fibers (Grass et al. 1993), a secondary rise ii muscle tone is brought about by activation of the afferent fibers a the muscle spindles and the efferent α-motoneurons (Schwarze 1993).
Sympathetic Innervation of the Temporomandibular Joint
The sympathetic innervation of the temporomandibular joint comes from the superior cervical ganglion (Biaggi 1982, Widenfalk and Wiberg 1990). Neurons with the neuropeptides CGRP (calcitonin gene-related peptide) and SP (substance P), that are associated with the sensory nervous system, are found chiefly in the anterior part of the joint capsule (Kido et al. 1993). Sympathetic fibers containing neuropeptide A (NA), Y (NPY), or VIP (vasoactive intestinal peptide) are more numerous in the posterior part of the joint. The ratio of sympathetic to sensory nerve fibers is approximately 3:1 in the temporomandibular joint Schwarzer 1993). Sympathetic neurons serve primarily for monitoring the vasomotor status. This monitoring allows optimal adjustment of the blood volume in the genu vasculosum during excursive and incursive condylar movements. There is evidence that, in addition to the vasomotor effect, the sympathetic nervous system also plays a role in pain perception (Roberts 1986, Jähnig 1990, McLachlan et al. 1993). Both NA and SP effect the release of prostaglandins, which heighten the sensitivity of pain receptors (Levine et al. 1986, Lotz et al. 1987).
75 Effects of the sympathetic nervous system on the temporomandibular joint
Certain neuropeptides can increase the sensitivity of nociceptors and thereby directly influence pain perception. Special importance has been attributed to the neuropeptides SP and CGRP in the production of synovial cells (Shimizu et al. 1996). Bone remodeling processes are likewise guided by neuropeptides. A nonphysiological increase of pressure within the genu vasculosum caused by a sympathetic or hormonal malfunction during incursive condylar movement results in an anteriorly directed force on the articular disk (Ward et al. 1990. Graber 1991) and this can contribute to an anterior disk displacement. (Revised drawing from Schwarzer 1983.)
76 Afferent paths of the trigeminal nerve and conections of neurons in the brain stem
Schematic representation of the interrelationships between afferent fibers of the trigeminal nerve and the so-called nociceptive specific (NS) neurons and/or the wide dynamic range (WDR) neurons (Dubner and Bennett 1983, Sessle 1987a. b) in the region of the cervical spinal column. The specific connections (A-D) in the sensory trigeminal nucleus in the brain stem can result in identical perceptions in the cortex regardless of where the pain was first perceived.
Muscles of Mastication
Anatomically the muscles of mastication can be divided into simple and complex muscles (Hannam 1994, 1997). The lateral pterygoid and the digastric muscles are counted among the simple muscles. These muscles work through a favorable lever arm relative to the joint and so do not have to produce a great deal of force to bring about functional mandibular movements. The parallel muscle fibers in these muscles have their sarcomeres arranged in series, and these are responsible for the adequate muscle contraction. During contraction, the diameter of each muscle increases and is at its greatest near the midpoint of the muscle.
In contrast, the complex muscles include the temporal, masseter, and medial pterygoid muscles with their many aponeuroses and varying sizes. During function the aponeuroses can shift and become deformed (Langenbach et al. 1994). The muscle fibers in this group run obliquely and increase their angle to one another during contractions. A complex muscle can produce a force of approximately 30-40 N per cm2 of cross-section (Korioth et al. 1992, Weijs and van Spronsen 1992). The orientation of the muscle fibers and their facultative activation during various mandibular movements is one of the reasons that muscle symptoms can be reproducibly provoked by loading in one certain direction but not in others. Although there are recurrent principles in muscle architecture (Hannam and McMillan 1994), variations in the areas of muscle attachment and differences in intramuscular structure have an effect on craniofacial development (Eschler 1969, Lam et al. 1991, Tonndorf 1993, Holtgrave and Müller 1993, Goto et al. 1995).
The motor units in the muscles of mastication are small and seldom extend beyond the septal boundaries (Tonndorf and Hannam 1997). The “red” muscle fibers (with higher myoglobin content) contract slowly. They maintain postural positions and are slow to become fatigued. The “white” fibers (with lower myoglobin content) have fewer mitochondria and can contract more rapidly, but they fatigue sooner because of their predominantly anaerobic metabolism. The muscles of mastication are composed of varying mixtures of fibers of types I, IIA, IIB, IIC, and IM (Mao et al. 1992, Stal 1994).
Next, the four chewing muscles proper (temporal, masseter, medial pterygoid, and lateral pterygoid) and the suprahyoid and perioral musculature will be described in preparation for the clinical examination that will be addressed later.
77 Muscles of mastication
Drawing of the muscles of mastication. In the narrowest sense these include only the temporal, masseter, medial pterygoid, and lateral pterygoid muscles. The suprahyoidal musculature is also shown here because it is of interest in the diagnosis and treatment of functional disturbances. The sternocleidomastoid muscle is not included here because it belongs to the musculature of the neck.
78 Function and structural adaptation of the muscles of mastication
Antagonistic muscle activity serves not only to execute mandibular movements, but also helps stabilize the joints. Functional demands can bring about changes in tonus, response to stimuli, and muscle length. Adaptation depends upon the combination of fibers present. Chronic overloading may lead to inflammation, ruptures, or ossification.
Temporal Muscle
The temporal muscle is a compartmentalized muscle that arises from the superior and inferior temporal lines of the temporal bone. It inserts on the coronoid process and on the anterior edge of the ascending ramus of the mandible. Three functional parts can be distinguished (Zwijnenburg et al. 1996). The anterior part has muscle fibers that pull upward and serve as elevators (Moller 1966). The middle part effects closure of the jaws and, with a posterior vector, retrusion (Blanksma and van Eijden 1990). According to DuBrul (1980) the posterior part is involved primarily with jaw closure and only to a minimal extent with retrusion. Nevertheless, experimental studies have revealed a distinct retrusion when the posterior part is activated (Zwijnenburg et al. 1996). During normal opening and closing movements, as well as during tooth clenching, the activity in all three parts is at a nearly equal high level. During chewing, however, there are great differences between the anterior and posterior parts. The activity is greater on the working side than on balancing side (Blanksma and van Eijden 1995). During lateral jaw movement, EMC activity is markedly lower where there is canine guidance than where there is group function (Manns et al. 1987).
79 Macroscopic anatomical preparation
The pars anterior and pars media of the temporal muscle consist of approximately 47% fatigue-resistant type-I muscle fibers with a low threshold of stimulation (Eriksson and Thornell 1983). The content of thinner type-IIB fibers is about 45%, leading to a higher concentration of fibers in the muscle (Stalberg et al. 1986). Type-NA muscle fibers are not present at all and those of type IIC and/or IM account for only about 4% (Ringqvist 1974).
