109 Influence of head and body posture on the mandible
Schematic drawing showing the muscular suspension of the mandible and its relation to the muscles of the neck. As an example, the sternocleidomastoid muscle is always active during bruxism; however, an activity level of 50% is necessary in the masseter muscle before a 5% fevel is reached in the sternocleidomastoid muscle (Clark et al. 1993).
110 Influence of a functional disturbance in the cervical spinal column upon centric position
Changes in condylar position in the horizontal plane following isolated orthopedic treatment of the upper Red: Initial findings Blue: Control findings after 6 weeks with no treatment Lilac: Findings after 3 weeks of treatment Green: Control findings after 3 months In each group the difference from habitual occlusion (HO) is given in millimeters.
111 Influence of the occlusion upon body posture
Left: Recording of changes in the upright body posture (orthostasis). In this case voluntary clenching on an unstable occlusion resulted in a repeatable posterior shift in the body’s center of gravity.
Right: Clinical arrangement with the patient on the measuring platform that detects even minor changes in the patient’s center of gravity projected horizontally.
Peripheral and Central Control of Muscle Tonus
Muscle spindles and Golgi tendon organs are the two types of receptors in muscles that are involved in controlling muscle activity. Peripheral monitoring occurs primarily through the monosynaptic proprioceptive reflex and the polysynaptic stretch reflex (Guyton and Hall 1996). Increases in length are reported from the muscle spindles over la fibers directly to the corresponding α-motoneuron and can then be corrected (phasic proprioceptive reflex). Golgi receptors read the muscle force and transmit this information over Ib fibers to an inhibitory intermediate neuron. This again inhibits the α-motoneurons (Johnson 1998). The resting tonus is a continuous background level of contraction that is maintained primarily through a segmental spinal reflex (Graber 1989). Supraspinal centers can both inhibit and promote muscle tone. Here the limbic system plays an essential role. It is the complex circuitry of numerous pathways and nuclei that make it possible for individuals to control their emotions and behavior. Muscle tone is influenced predominantly by the γ-motoneurons (Johnson 1998).
112 Limbic system
After Nieuwenhuys et al. 1980.
1 Mamillotegmental tract
2 Fasciculus telencephalicus med.
3 Inferior longitudinal fasciculus
4 Hypothalamus
5 Nucleus medialis dorsalis thalami
6 Anterior thalamic nucleus
7 Septal area
8 Amygdaloid body
9 Hippocampus formation
10 Mamillary body
11 Habenula
12 Ncl. Interpendicularis
13 Dorsal tegmental nucleus
14 Olfactory bulb
15 Mamillothalamic tract
113 Central and peripheral neuromuscular control
Muscle activity can be influenced by both peripheral and central nerves. Emotional factors increase muscle tonus through γ-neurons.
1 Periodontal receptors
2 Joint capsule receptors
3 Muscle spindles (stretching)
4 Motor end plates
5 Trigeminal nerve (semilunar ganglion)
6 Trigeminal nerve (semilunar ganglion)
7 Trigeminal nerve (semilunar ganglion)
8 Mesencephalic tract nucleus
9 Superior sensory nucleus (n.V)
10 Inferior sensory nucleus (n.V)
11 Motor nucleus (n. V)
12 Thalamus
13 Descending paths from cortex (α-motor = “voluntary motor”)
14 Descending paths from limbic system (γ-motor = “psychovegetative impulses”
15 Ascending paths to cortex (“consciousness”)
Physiology of the Jaw-Opening Movement
Jaw opening is accomplished through the action of the suprahyoid muscles (rotation) and the lateral pterygoid muscles (translation). In centric condylar position the elastic fibers in the temporomandibular joint are in equilibrium. The initial phase of an opening movement is primarily a rotation that always progresses with a translationl component (Merlini and Palla 1988, Maeda et al. 1992, Ferrario et al. 1996a). The opening rotation of the condyle always causes the disk to lie against a more posterior region of the condyle where it is more stable. During translation the disk is passively carried along in an anterior direction (Sicher 1964, Roth et al. 1984. Osborn 1985). During jaw opening, tension increases in the superior stratum and in the lower anterior wall of the joint capsule. While the superior stratum can restrict anterior movement of the disk (Dauber 1987), it cannot restrict jaw opening. This is limited by the joint capsule and the lateral ligament. During jaw opening the genu vasculosum expands to approximately four or five times its original volume (Rees 1954, Wilkinson et al. 1994), so that during excursions a negative pressure arises within it (Finlay 1964, Ward et al. 1990).
Jaw-opening movements
114 Initial phase
Illustration or the structural loading during the initial opening movement. The condyle makes a rotational movement with a small translational component, changing its position relative to the fossa only slightly. Because of the condylar rotation, the disk moves posteriorly part of the lateral pterygoid muscle that is active is its lower head (1). The elastic fibers are brought out of equilibrium only minimally.
115 Intermediate phase
In this phase the condyle executes a definite translation. The disk moves anteriorly relative to the fossa, but posteriorly