belly inserts in the digastric fossa on the inner side of the mandible. When the hyoid bone is braced by the infrahyoid musculature, contraction of the digastric muscle tends to open and retract the jaw. The mylohyoid muscle extends from the step-like myohyoid line on the inner aspect of the body of the mandible to the body of the hyoid bone. The geniohyoid muscle arises at the inner side of the mandibular symphysis, and from there its parallel fibers run directly to the body of the hyoid bone. The pair of geniohyoid muscles can function together as jaw openers, but they can also lift the floor of the mouth and pull the hyoid bone forward. The stylohyoid muscle runs from the styloid process of the temporal bone to the greater horn of the hyoid and serves to stabilize the hyoid.
88 Mylohyoid muscle
Anatomical preparation of a right mylohyoid muscle (1). The anterior and middle fibers of the mylohyoid muscle insert in the center at the mylohyoid raphe. The fibers of the posterior portion insert on the hyoid bone under the geniohyoid muscle (2). These muscles are active primarily during jaw opening, swallowing, and protraction of the tongue, but they also show some activity during lateral jaw movements (Vitti and Basmajian 1977).
89 Suprahyoid musculature from a left posteroinferior view
The digastric muscle with its anterior belly (1) and posterior belly (2) can be seen from this perspective. The mylohyoid (3) and stylomandibular (4) muscles are also visible. When the mandible is stabilized by the closing muscles (elevators), contraction of the suprahyoid musculature lifts the hyoid bone. This is an important part of the act of swallowing.
90 Suprahyoid musculature
preparation of the left half of the jaw showing the posterior belly of the digastric (1), stylohyoid (2), styloglossus (3), and stylopharyngeus (4) muscles as well as the stylomandibular ligament (5). Except for the digastric muscle, all these structures have their origin on the stylohyoid process (6). Muscle fibers of type I, type IIA, and type IIB each make up one-third of the digastric muscle (Eriksson et al. 1982). From the collection of B. Tillmann (Figures 85, 87, 88, 90)
Lateral Pterygoid Muscle
The lateral pterygoid muscle is made up of two functionally different parts (McNamara 1973): an upper (superior) head and a lower (inferior) head. In 12% of the specimens studied. Abe et al. (1993) could also identify a third, intermediate muscle belly.
The upper head arises on the greater wing of the sphenoid bone. It always has an insertion in the upper portion of the pterygoid fovea, but it may also insert to varying degrees into the disk-capsule complex (Chissin 1906, Troiano 1967, Porter 1970, Mahan et al. 1983, White 1985, Meyenburg et al. 1986, Dauber 1987, Carpentier et al. 1988, Merida Velasco et al. 1993, Bade et al. 1994). In 30-40% of temporomandibular joints, the upper head inserts exclusively on the condyle (Moritz and Ewers 1987. Wilkinson 1988, Luder and Bobst 1991, Heylings et al. 1995, Naidoo and Juniper 1997).
The lower head arises on the lateral face of the lateral lamina of the pterygoid process and inserts in the pterygoid fovea. It is possible that fibers from the upper and lower head are intermingled, but in any event, fibers of the lower head can radiate into the disk-capsule complex (Dauber 1987).
91 Lateral pterygoid muscle
Left: Lateral view of an anatomical preparation. This muscle is characterized by relatively long fiber bundles with small cross-sections (van Eijden et al. 1995) in its upper head (1) and lower head (2). The upper head always inserts on the condyle, and in 60% of specimens it also inserts on the disk-capsule complex.
From the collection of B. Tillmann
Right: Schematic drawing.
1 Upper head
2 Lower head
92 EMC activity of the muscles of mastication at rest and during jaw opening
After juniper 1984
Left: In the relaxed mandibular position only minimal EMC activity can be detected from the upper head of the lateral pterygoid muscle.
Right: During jaw opening the upper head is active and serves to protrude the condyles. Activity of the suprahyoid musculature was not measured in this study.
93 EMC activity during grinding of the teeth
After juniper 1984.
Left: During tooth grinding in maximum occlusion, the two large jaw-closing muscles and the upper head of the lateral pterygoid show a high level of EMC activity.
Right: During grinding of the anterior teeth, the lower head holds the mandible forward and the activity of the elevators decreases because of the reflex inhibition from the periodontium (Widmalm et al. 1987).
Fibers of the lateral pterygoid muscle were found to insert into the periphery of the disk in 22% of the joints studied. In these cases 88% of the muscle fibers were from the upper head and 12% from the lower head (Abe et al. 1993). The smaller the area of attachment of the lateral pterygoid muscle to the condyle, the greater the tendency for disk displacement (Dreger 1994). The upper and lower heads have an antagonistic action. Numerous EMG studies (Molin 1973; Cibbs et al. 1984; Juniper 1983, 1984; Widmalm et al. 1987; Yoshida 1995) have revealed that the lower head is always active during excursive mandibular movements (jaw opening, protrusion, mediotrusion), whereas the upper head is active during incursive movements (jaw closing, retrusion, laterotrusion), serving to hold the disk-condyle complex continuously against the slope of the eminence and to restrain it during incursive movements (Wood et al. 1986). The high proportion (80%) of type-l muscle fibers (low stimulation threshold, fatigue-resistant) is also indicative of a continuous holding action with a low level of force (Eriksson et al. 1981, Mao et al. 1992). Because the upper head becomes longer during its holding action, this can be referred to as eccentric muscle activity (Wilkinson 1988).
94 MRI of a left lateral pterygoid muscle in the sagittal plane
According to Abe et al. (1993), in 12% of individuals the lateral pterygoid muscle divides into three parts as seen here. According to Ögüt-cen-Toller and Juniper (1994) the musde becomes segmented into three parts in week 12 of embryonic development to form an upper (1), middle (2), and lower (3) head.
