Introduction:
Mandibular movements during normal function and during parafunction (bruxism) involve complex neuromuscular patterns with their origin as a pattern generator in the brainstem and modified by influences from higher centers, mainly by the coordination of the cerebral cortex and basal ganglia, and also as a result of peripheral influences (for example, the periodontium, facial and masticatory musculature, etc.). Oral motor behavior refers to the function and parafunction of the mouth and associated structures. More generally, behavior includes observable actions ranging from simple movements such as retrusion or protrusion to more complex movements such as chewing. To accomplish complex behavior, sensory-motor systems consisting of muscles and neural processes are required for the initiation, programming, and execution of motor functions.
The mandible, which holds the lower teeth, comprises the majority of the lower third of the skeleton in the maxillofacial region of the area and is of pivotal functional importance for any voluntary movement in this region. Complex mandibular movements are always produced by the actions of these muscles i.e., the masseter, temporalis, medial pterygoid, lateral pterygoid muscles, and temporomandibular joints.
The upper jaw or maxilla acts as a stationary unit; hence all the effects of mastication, i.e., chewing and swallowing food, the masticatory movements crucial to food bolus formation are only dependent on the movements of the lower jaw. In certain populations, masticatory function related to dental status is also considered a significant determinant of nutritional status because the individual's nutrition constitutes a significant role in the systemic health of an individual.
What Is the Musculature Involved Behind Jaw Movements?
The muscles involved in the jaw movements are listed below:
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The digastric, mylohyoid, and geniohyoid muscles are active during jaw opening, either slowly or maximally, against resistance.
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No activity occurs in the temporalis and masseter muscles when the mouth is opened slowly or maximally, although some activity may occur in the medial pterygoid muscle.
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When the jaw is opened against resistance, the temporalis muscle remains silent. During opening movements, the lateral pterygoid muscles show initial and sustained activity.
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In forced depression, the digastric muscle is activated almost as soon as the lateral pterygoid muscle is. Generally, the activity of the anterior digastric muscle follows that of the lateral pterygoid muscle.
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Voluntary mandibular retrusion with the mouth closed is brought about by contraction of the posterior fibers of the temporalis muscle and by the suprahyoid and infrahyoid muscles.
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Protrusion of the mandible without occlusal contract results from the contraction of the lateral and medial pterygoid muscles and also masseter muscles.
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Lateral movement of the mandible to the right side (without occlusal contract) is achieved by ipsilateral contraction of primarily the posterior fibers of the temporalis muscle.
What Are the Musculatures Involved Behind Chewing and Swallowing?
Chewing movements depend on complex integrative neural processes of the central nervous system that may be initiated by either internal or external influences, including innate drives, emotional states, and instructions to patients. During chewing, a large amount of proprioceptive (muscle sense) and exteroceptive (tactile sense) information is fed to the central nervous system (for example, the cerebral cortex, brainstem, basal ganglia, and spinal cord). Rhythmic movements such as chewing are largely programmed or preprogrammed and involve learning, which reduces the need for peripheral sensory input. However, inputs from muscle, joint, tendon, and periodontal receptors still have important functions, especially in relation to learning new experiences and protective reflexes. Neuronal mechanisms must be present to provide for modification of reflexes and continued updating of masticatory movements by information about such factors as occlusal forces and the state and location of the bolus.
When the food is taken into the mouth, the lips, tongue, and periodontium function to estimate size, hardness, and other characteristics that must be correlated with previous behavior required for chewing; thus, the information sets the chewing program in the pattern generator, including subsequences that relate to central and peripheral influences already in progress.
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Chewing is a highly complex oral motor behavior usually seen in the frontal plane in a simple form. No archetypal chewing cycle exists. The dimensions of the chewing cycle are between 16 and 20 mm for vertical movements and between 3 and 5 mm for lateral movements. The duration of the cycle varies from 0.6 to one second, depending on the type of food. The speed of masticatory movements varies within each cycle according to the type of food, individuals' occlusion, and presence of dysfunction.
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Swallowing involves most of the tongue muscles and buccal musculature. In the initial stage of swallowing, the bolus moves from the mouth to the fauces. The bolus is then moved from the fauces to the esophagus and finally through the esophagus to the stomach. When saliva is swallowed, total participation of the suprahyoid muscles occurs, with the marked activity of the digastric and mylohyoid muscles, followed by moderate activity in the geniohyoid muscles. The medial pterygoid muscle is often active; the temporalis and masseter muscles are less active with occlusal contact.
What Is the Importance of Oral Motor Behavior in Dentistry?
Occlusion, or instead simply put as the individual's correct bite, is the articulation between the mandibular and maxillary dental arches for the act of chewing and grinding. Understanding these static and dynamic occlusal contact relationships is always case-dependent (that is, it varies from individual to individual depending on the alignment). But this gives the dental surgeon or the orthodontist an insight into the ability to design and place dental implants, implement orthodontic braces, and fabricate and correct existing dentures that eventually aid in the re-establishment of masticatory function, and restore a patient’s correct bite in cases of maxillofacial trauma as well.
Conclusion:
The range and complexity of orofacial movements require sophisticated neural circuitries that provide for the coordination and control of these movements and their integration with other motor patterns such as those associated with breathing and walking. To conclude, the masticatory forces and even facial movements, including protrusion, extrusion, and lateral movements of the jaw and chewing and swallowing actions, result from oral motor behavior that the dentist understands in detail for establishing the ideal occlusion and jaw functionality.
