The pattern of occlusion of teeth is highly variable in the human, and in orthodontics there is an increasing tendency to attribute an abnormal pattern of masticatory movements as well as hyperactivity of the jaw-closing muscles to malocclusion of the teeth. For example, it has been suggested that in normal occlusion of the teeth chewing movements are simple and well coordinated, and the periodontal mechanoreceptors provide information reflexly to the trigeminal motor neurons to determine the muscle force and jaw movements required in each masticatory cycle [78, 93, 171]. On the contrary, subjects with malocclusion of the teeth may have an irregular, complicated pattern of chewing movements due to disruption of the function of the “central pattern generator” (CPG) which regulates the rhythm of the cyclic masticatory movements [78, 93]. There is now convincing evidence that specific patterns of malocclusion of the teeth can alter the symmetrical bilateral function of the masticatory muscles. For instance, while in normal occlusion the amplitudes of the responses recorded from the left and right masticatory muscles are very similar, in subjects with unilateral posterior crossbite malocclusion of the teeth there is asymmetry between the left and right masticatory muscles, suggesting abnormality of the bilateral chewing function [125], which may be due to abnormal coupling of hyperneurons of the left and right cerebral hemispheres (see Sect. 4.2).

The amplitude of an action potential is determined by the number of motor neurons which are activated by the stimulus. If the stimulus is constant, then the increase in the action potential that is recorded in the muscles of the unilateral posterior crossbite side, suggests an increase in the number of facilitatory impulses to the trigeminal motor neuron nuclei of the crossbite side, thereby increasing the number of motor neurons required in the response [125]. This increased excitability of muscles of the crossbite side may come from many sources. For example, the muscle spindles may signal to the brain that the jaw-closing muscles on one side of the head are a slightly longer, since the mandible is displaced from the mid-line of the face towards the crossbite side [125]. In the masticatory system the proprioception of the mandible (the sense of position in space of the mandible relative to the maxilla) arises primarily from the muscle spindles in the jaw-closing muscles, as well as from other mechanoreceptors in and around the mouth. Muscle spindle receptors are exquisitely sensitive to stretch, and hence are able to signal the length of the jaw-closing muscles and, by inference, the vertical position of the mandible relative to the maxilla [78].

The phenomenon of uncoordinated masticatory movements in malocclusions of teeth has been attributed to the presence of occlusal interference or premature contacts of the teeth, which modify the activity of muscles through the receptors of the muscles, skin, and oral mucosa, as well as of periodontal receptors (see Chap. 8). When interferences are present, muscle responses (especially the peaks) occur earlier prior to intercuspation of the teeth, presumably through an increase in the excitability at least of the periodontal reflexes [172]. When there is displacement of the mandible during occlusion of the teeth, the muscle activity on the displaced side is altered, as for instance in the unilateral posterior crossbite malocclusion [124, 125].

Occlusal morphology plays some part in determining the precise pattern of movement in chewing, and therefore will determine the demands upon the different muscles, and upon motor units within the muscles. With a stable occlusion in the intercuspal position, it is reasonable to consider that the average work-load of individual motor units may remain constant over a period [78, 98, 173].

The presence of occlusal irregularities and premature contacts, however, has often been considered to be a key factor in the dysfunction of the mandibular muscles of both sides as a whole, and of individual motor units. Such circumstances may reasonably be regarded as producing local and/or central hyperactivity of muscles associated with abnormal movements. Although there is no direct cause and effect relationship between dysfunction and a defective occlusal pattern, it is reasonable to consider the possibility that the occlusal change initiates the functional difficulty, causing redistribution of muscle activity, which results in damage to the muscles. On the other hand, there is evidence which suggest that painful jaw muscle dysfunction occurs in spasm, implying excessive continuous contraction, which is likely to be associated with emotional states of anxiety [173].

However, the neuromuscular dysfunction associated with occlusal interferences or premature dental contacts may be related to sensory deprivation of the cerebral cortex through the affected ascending reticular activating system (ARAS), which regulates the sensory input to the cortex. This breakdown of the central reference of arousal of the brain for attention and awareness (a preconscious state of the brain, which correlates with the emergence of the “conscious electromagnetic field” instructing the muscles how to contract), by which the organism–mouth guides its correction strategies in perceiving, cognizing, reacting, and manipulating the environment, may make it increasingly difficult for the cerebral cortex to discriminate sensory inputs and to emit motor responses to muscles (see Sects. 3.4, 4.8).

Chewing is a cyclical activity of the jaw-opening and jaw-closing muscles occurring at a rate determined by the CPG. In chewing the CPG programs four phases of the masticatory cycle. The first phase is the preparatory phase, which begins with the opening of the mouth, through activation of the jaw-opening muscles, in order to receive food. The second phase is the food contact phase, which occurs by switching off the activity of the jaw-opening muscles and activating the jaw-closing muscles to produce the initial closing movement. It is in this phase of the cycle that the periodontal reflexes may assist in grasping the food in the correct position between the teeth, ready to be bitten through. The third phase is the food crushing phase, in which the output from the CPG to the jaw-closing muscles forces the teeth through the food bolus, producing the chewing strokes. In the last phase of the chewing cycle, the tooth contact phase, activation of the jaw-closing muscles continues as the opposing teeth come into contact, and while they slide into the intercuspal position for the final movement grinding the food into a paste. A reflex emanating from the periodontal mechanoreceptors initiates and controls this final grinding movement in the masticatory cycle [78].

It has been suggested that the periodontal receptors are directionally sensitive and modulate the activity of the jaw-closing muscles to ensure that the final grinding phase of the chewing cycle occurs in the correct direction, i.e., from the working side towards the intercuspal position. The periodontal mechanoreceptors activated by contact of the teeth, signal to the brain that the correct pattern of contact of the opposing teeth has occurred before the grinding movement is reflexly initiated. In other words, if the cusps of the upper teeth do not meet the occlusal surfaces of the lower teeth in the correct relationship, the grinding movement is not initiated. This suggests that the central nervous system issues motor commands to the jaw-closing muscles, through the CPG, in proportion to the total periodontal afferent discharge evoked by the number of teeth in contact [78, 93