Muscles And Nerves Of The Masticatory System
The muscles and nerves nf the masticatory system are extensively reviewed elsewhere and are only briefly discussed here for the purpose of understanding the mechanisms involved. Appropriate references are provided for further reading.
The muscles of mastication consist principally of two groups: the elevator muscles and depressor muscles. The muscles responsible for elevating the mandible are the masseter, internal pterygoid, and much of the temporal muscle. The posteriorly oriented fibers of the temporal muscle also ret rude the mandible. I he superfk ial muscle bundle of the masseter muscle may also assist in protruding the mandible, while the deeper bundle serves to stabilize the condylar head against the articular eminence. Juxtaposed with the masseter muscle, the medial pterygoid forms a muscular support tor the mandible at its angle. Although the primary timet ion of this muscle is elevation of the mandible, it is also active during protrusion. 10 The lateral pterygoid muscle is now known to function as two distinct muscles, the inferior and superior lateral pterygoid muscles, having independent and nearly opposite functions." 11 Ihe inferior lateral pterygoid muscle depresses and protrudes the mandible. The superior lateral pterygoid muscle does not contrac t during depression ol the inaudible but rather contracts along with the elevator muscles, bracing the condyle an-teromediallyr.....
Physiologic mandibular posture and movement are products of harmonious muscular contraction among masticatory and supportive muscles. The neurologic input to produce synergy of complementary and antagonistic muscles is extremely complex. Motor and sensory innervation of the TMJs and the rest ol the masticatory system are provided by structures of the trigeminal nerve. Mechanoreceptors in the skin, muscle, and ligamentous structures, especially the periodontal ligament, discern pressure differences a I sensitive degrees ol discrimination. Painful stimuli are perceived by nociceptors and result in both pain perception and reflex responses. The innervation of both the capsular ligaments and discal ligaments provide essential proprioceptive input with regard to joint position. Kfferent or motor neurons cause muscle contraction in response to central cortical stimulation and in response to afferent stimuli in rellex ac-tivity.:MU 19,64
Fig. 25-1 Lateral view of cross-section through the temporomandibular joint. /, Posterior slope of the articular eminence ol the temporal bone; 2, head of the condyle; 3, disc (note biconcave shape); A, superior lateral pterygoid muscle (note attachment to both the head of the condyle and disc); 5, inferior lateral pterygoid muscle; 6, synovial tissue; /, retrodiscal tissue; 8, discal ligament attachment to the posterior surlace of the head of the condyle (Modified from Dawson PE: Evaluation, Diagnosis, and Treatment of Occlusal Problems, ed 2. St Louis, Mosby, 1989.)
Fig. 25-1 Lateral view of cross-section through the temporomandibular joint. /, Posterior slope of the articular eminence ol the temporal bone; 2, head of the condyle; 3, disc (note biconcave shape); A, superior lateral pterygoid muscle (note attachment to both the head of the condyle and disc); 5, inferior lateral pterygoid muscle; 6, synovial tissue; /, retrodiscal tissue; 8, discal ligament attachment to the posterior surlace of the head of the condyle (Modified from Dawson PE: Evaluation, Diagnosis, and Treatment of Occlusal Problems, ed 2. St Louis, Mosby, 1989.)
Fig. 25-2 Temporomandibular joint (anterior view), showing collateral ligaments. The following are identified: At), articular disc; C/, capsulai ligament; /C, inferior joint cavity; tDI, lateral discal ligament; MDt, medial discal ligament; SC, superior joint cavity. (From Okeson JP: Management ol Temporomandibular Joint Disorders and Occlusion, ed 4. St Louis, Mosby, 1998.)
Fig. 25-2 Temporomandibular joint (anterior view), showing collateral ligaments. The following are identified: At), articular disc; C/, capsulai ligament; /C, inferior joint cavity; tDI, lateral discal ligament; MDt, medial discal ligament; SC, superior joint cavity. (From Okeson JP: Management ol Temporomandibular Joint Disorders and Occlusion, ed 4. St Louis, Mosby, 1998.)
Sensory input from the periodontal ligament (Pl>l > oilers the potential to be an important component ol the complex neurologic management of the masticatory system. Currently, little evidence of the existence ol proprioceptive sensory organs within the neuroanatomy ol Ihe PDI is available, although it was once considered likely. Pain perception causes the nociceptive reflex to rapidly open the mouth through contraction ol depressor muscles and suppression of elevator muscles, consistent with other protective reflexes within the musculoskeletal system.11" Protective reflexes may he suppressed in individuals experiencing chronic occlusal parafunction (clenching or grinding of their teeth).-''"1 Pressure perception is «1 function of the numerous mechanoreceptors within the PD1. ol teeth in contact. Discrimination within the dentition based on specific teeth in contact, direction of force, and intensity of force and their in 11 uenee on muscle activity have been demonstrated in human study populations and animal studies.22, istJUMX-,L' Both research and clinical observations suggest that elevator muscle contraction is suppressed when ¿interior teeth promote disclusion or separation of posterior teeth during excursive mandibular movements.1,,<I Implicit in the experience of loss of attachment due to periodontitis is the loss of some mechanoreceptors. Patients with significant bone loss, significant inflammatory disruption of the integrity of tlie PDl. or chronic occlusal parafunction may experience compromised regulation of muscle activity.*
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