the collection of B. Tillmann
80 Schematic drawing of the right temporal muscle
The muscle comprises a pars anterior (1), pars media (2), and pars posterior (3). Although the sarcomere lengths are the same in the various parts, there are significant differences in the lengths of the muscle-fiber bundles (21.7-28.9 mm) which indicates different functional demands (van Eijden et al. 1996).
81 Insertion of the temporal muscle on the disk-capsule complex
Left: Medial view. Some of the horizontal fibers (arrows) insert onto the middle and lateral third of the middle and lateral third of the disk (Merida Velasco et al. 1993, Bade et al. 1994).
Right: Insertion of the temporal muscle viewed from above. Easily identified is the tendon (*) of the pars posterior, which inserts on the lateral portion of the disk-condyle complex.
Masseter Muscle
The masseter muscle consists of a superficial and a deep part. The origin of the superficial part is on the zygomatic arch and its insertion is on the lateral masseteric tuberosity at the angle of the mandible. The deep part also arises on the zygomatic arch but inserts on the lateral surface of the ascending ramus. Portions of the deep part also insert on the joint capsule and the disk (Frommer and Monroe 1966, Meyenberg et al. 1986, Dauber 1987). In this way the masseter can influence the capsule receptors by changing the capsule tension. The lowest EMG activity and the greatest chewing force in this muscle can be demonstrated at a jaw opening of 15-20 mm (Manns et al. 1979, Lindauer et al. 1993, Morimoto et al. 1996). Seventy-four percent of the masseter’s muscle spindles are to be found in the deep part (Eriksson and Thornell 1987). These muscle spindles have large diameters and a four-fold higher concentration of intrafusal fibers. From this it can be deduced that there are special functions for different areas of the muscle. The masseter muscle shows no significant difference in EMG activity between subjects with canine guidance and those with group function (Borromeo et al. 1995).
82 Pars superficialis of the masseter muscle
Left: Schematic drawing of the pars superficial. The masseter has a higher concentration of capillaries relative to the diameter of the fibers than all the other skeletal muscles (Stal et al. 1996).
Right: Macroscopic anatomical preparation of the masseter muscle. The resultant force of the pars superficialis is in an anterosuperior direction. The posterior part of the pars superficialis is composed of up to 45% type-IIB fibers (Eriksson and Thornell 1983), which have a high threshold and are fatigue-resistant.
83 Pars profunda
Left: Schematic drawing.
Right: The pars profunda revealed in an anatomical dissection. The muscle has a relatively broad insertion on the zygomatic arch from which it pulls on the lateral surface of the ascending ramus of the mandible. The posterior part of the pars profunda also inserts into the lateral third of the disk-capsule complex (Merida Velasco et al. 1993, Bade et al. 1994). Only approximately 25% of the muscle fibers of the pars profunda are of type IIB.
84 Origin and insertion of the masseter muscle
Schematic drawing showing the areas of origin and insertion of the masseter muscle. The origin of the pars superficialis is on the inferior surface of the zygomatic arch anterior to that of the pars profunda. The insertion of the pars superficialis lies on the lateral surface of the angle of the mandible. At the inferior border of the mandible it is continuous with the medial pterygoid muscle on the inner surface of the mandible. The pars profunda inserts above the masseteric tuberosity on the ascending ramus of the mandible.
Medial Pterygoid Muscle
The medial pterygoid muscle, together with the temporal and masseter muscles, are the jaw-closing muscles. This muscle has its origin in the pterygoid fossa of the pterygoid process of the sphenoid bone. From there it extends interiorly, posteriorly, and laterally to the inner side of the angle of the mandible, where it connects with the masseter to form a muscle sling. The course of the medial pterygoid muscle closely parallels that of the pars superficialis of the masseter. The medial pterygoid muscle functions primarily during jaw closure, but also takes part in protrusive movements. Unilateral contraction results in mediotrusion. Because of its oblique course in the frontal plane, this muscle also influences the transverse position of the condyle. Unlike the temporal and masseter muscles, the medial pterygoid cannot be adequately palpated except for its insertion. Its activity in protrusive position increases with the number and size of tooth contacts (MacDonald and Hannam 1984, Wood 1986). Tooth gnashing in a posterior direction is accompanied by a greater increase in EMG activity than in anteriorly directed gnashing (Vitti and Basmajian 1977).
85 Posterior view of the medial pterygoid muscles in the frontal plane
This preparation clearly reveals the relationship of the medial pterygoid muscles (1) to the lateral pterygoids (2) and the masseters (3) and the muscular slings they form with the latter. The strength of a muscle depends on its cross-sectional area (Sasaki et al. 1989). In patients with dentitions damaged by periodontal disease, the chewing force was found to be reduced by 25% in the masseters and 10% in the medial pterygoids (Gilbert and Newton 1997).
86 Schematic drawing of the medial pterygoid muscle
Like the pars superficialis of the masseter, the composition of the anterior part of the medial pterygoid is 64% type-I muscle fibers (low threshold, fatigue-resistant) and 27.5% type-IIB (high threshold, fatigue-resistant) (Eriksson and Thornell 1983). In the posterior part, on the other hand, 44% of the fibers are of type I and 45% are of type IIB.
87 Medial view of an anatomical dissection of a right medial pterygoid muscle (1)
The muscle runs inferiorly and posteriorly from the fossa of the pterygoid process to the inner side of the angle of the mandible. As in the masseter, the muscle fibers are extensively interlaced with no uniform direction of alignment. They are comparatively short, but have a relatively large cross-section (van Eijdenetal. 1995).
Suprahyoid Musculature
The suprahyoid muscles, together with the lateral pterygoids, make up the jaw-opening muscles. The suprahyoid musculature includes the digastric, mylohyoid, geniohyoid, and stylohyoid muscles. The digastric muscle has two bellies. The posterior belly has its origin medial to the mastoid process and extends anteriorly, inferiorly, and medially to the hyoid bone; here it attaches to the bone and connects with the anterior belly by means of an intermediate tendon. The anterior
