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PubMed1987

A review of masticatory muscle function.

Current reports of the actions of the major muscles of mastication are reviewed for clenching tasks in centric occlusion and eccentric jaw positions, mandibular opening, and unilateral chewing. The relationship of the mandible to the maxillae in the intercuspal position and, consequently, the relationship of the condyle to the fossa of the temporomandibular joint may be controlled by the clinician. Similarly, the number and site of tooth contacts in the intercuspal position and in eccentric jaw positions may be altered. These alterations all produce alteration of muscle activity during functional and parafunctional acts. Some of these alterations are now predictable. Nevertheless, much more study needs to be done before the outcomes of morphologic changes on the muscular system can be accurately predicted. Even now, however, clinicians must understand the nature of clenching and chewing muscle activity and how it can be altered to a more favorable situation for each individual. This review indicates that: The elevator muscles demonstrate maximum activity when even bilateral occlusal contacts occur during clenching in the intercuspal position. The elevator muscles are activated together in the intercuspal zone of tooth contact during chewing when the occlusal contacts are balanced bilaterally in this intercuspal position. Increasing the number of eccentric tooth contacts increases the muscle activity during both chewing and clenching. The medial pterygoid muscle is variable in its action. This variability appears to be related to the chewing pathway and the type of occlusion. Its action is enhanced during laterally directed chewing actions of the mandible. The inferior head of the lateral pterygoid muscle has a reciprocal role with the medial pterygoid muscle during chewing and contributes to forward and lateral bracing of the condyle of the mandible. Although the superior head of the lateral pterygoid muscle appears to be active during mandibular closing, the significance of this finding is not fully understood.

Electromyography↗
PubMed1985

On the bilateral innervation of masticatory muscles: a study with retrograde tracers.

Using horseradish peroxidase and fluorescent compounds as retrograde tracers, the myotopical arrangement and axon trajectories of the motoneurons innervating the supramandibular and the suprahyoidal muscles were studied in the rat. Only motoneurons ipsilateral to the injection site were labelled. In the trigeminal motor nucleus, motoneurons of the superficial masseter muscle were found laterally, of the anterior deep masseter muscle centrally, and of the temporalis muscle dorsally and medially. Motoneurons of the anterior and posterior digastric muscle were located in the accessory motor nucleus of the trigeminal and facial nerve respectively. Evidence is provided that jaw-opening muscles are innervated from a separate motor complex.

Animals↗
PubMed1986

Changes in jaw relations and activity of masticatory muscles in patients with immediate complete upper dentures.

A longitudinal radiographic cephalometric and electromyographic investigation was performed on eighteen subjects provided with an immediate complete upper and a partial lower denture. The observation stages were: before final extractions and 7 weeks, 6 months and 1 year after denture insertion. The cephalometric analysis was based on measurements from lateral head films taken in centric occlusion. The EMG activity of the anterior temporal and masseter muscles was studied in postural position, light tapping and in maximal clench. The changes in jaw and occlusal relationships due to resorption of the edentulous maxillary ridge were slight, the mean decrease in occlusal vertical dimension during one year being 0.7 mm. The EMG clench activity at the pre-extraction stage, when the patients occluded on a residual anterior dentition, showed low mean values round 100 microV, and a further decrease was observed at the post-insertion stage. At the 1-year stage all muscles displayed significant increases to above the post-insertion level. The jaw muscle activity in light tapping and in postural position generally showed no significant mean changes during the observation period. The results indicate that preservation of a residual dentition in the lower jaw prevents marked changes in jaw and occlusal relationships and resulting changes in muscle activity.

Adult↗