© Springer-Verlag Berlin Heidelberg 2009
Margaritis Z. PimenidisThe Neurobiology of Orthodontics10.1007/978-3-642-00396-7_6
6. Mastication in Man
Marathonos Street 22, 152 33 Halandri, Athens, Greece
Margaritis Z. Pimenidis
6.1 The Central Pattern Generator
Current authors view mastication as a cyclical movement, like breathing or walking. The basic rhythm of cyclical movements is driven by a program that is hard-wired in the “central pattern generator” which is located in the reticular formation of the midbrain at a site strategically placed near the trigeminal nuclei and the nuclei reticularis pontis oralis and caudalis [78, 172, 219]. The central pattern generator is probably programmed by the conscious electromagnetic field generated by the synchronous firing of cortical neurons (see Sect. 4.8).
Like voluntary movements, cyclical movements are normally fine-tuned by sensory signals from the masticatory muscles and oral receptors acting through the action of reflexes. There is very good experimental evidence that cyclical masticatory movements are neither purely voluntary nor purely reflex. Mastication manifests itself as a patterned cycle of muscle contractions, which in turn produce the characteristic three-dimensional envelope of jaw movement, coordinated tongue and cheek movement, and compressive and shearing forces between the teeth to comminute the food bolus making the act of chewing as we know it [78, 172].
It should be emphasized, however, that the central pattern generator is principally a timing mechanism for creating rhythmicity coupling the movement of the jaw and tongue. As such it does not produce mastication or chewing in the true sense, in which the development of specific paths of movement, the altered timing sequences, and the interocclusal forces are characteristic additions to rhythmicity per se [78, 218]. During normal rhythmical chewing movements all the jaw-closing muscles on both sides are activated at about the same time through the coupling of the hyperneurons in the left and right hemispheres (see Sect. 4.2). During the opening movement only the jaw-opening muscles are active. When chewing occurs on the right side of the mouth the activity of the left masseter during the chewing stroke is less than the activity in the right masseter .
In sum, the important components of the central pattern generator include the motor cortex, which activates the premotor neurons, which in turn activate the trigeminal motor nucleus, which cause the rhythmical chewing movements. The rhythmical chewing movements generate sensory signals through activation of the oral sensory receptors the input of which activates the premotor neurons following the sensorimotor integration function in the somesthetic cerebral cortex. Thus, the activity of the central pattern generator can be triggered both from the cortex and from sensory signals from the masticatory system. It is noted, however, that feedback signals from the masticatory apparatus are not necessary for continued activity of the central pattern generator. In other words the chewing center can continue to send out its signals to the motor nerves of the muscles even when the rhythmical chewing movements of the muscles are pharmacologically prevented [78, 218].
For example, in this particular experiment, rabbits were not only given a general anesthetic but were also paralyzed with a curare-like drug that blocked the neuromuscular transmission of action potentials to the masticatory muscles without, however, interfering with the action potentials that traveled out from the brain along the motor nerves. The curare-like drug stopped the action potentials from activating the chewing muscles. Hence, the drug prevented the jaw from moving in the normal rhythmical masticatory pattern when the animal’s brain was stimulated. It was concluded that the central pattern generator does not depend on sensory feedback to continue the rhythmical chewing movements [78, 219].
The central pattern generator, however, is unable by itself to adjust the muscle force required to break down food whose texture may be unpredictable and changes from one chewing stroke to another. For this reason the masticatory system is powerfully modulated by reflexes, which automatically fine-tune the centrally generated masticatory movements in order to control the force of the muscle contraction. The most important of these reflexes is the stretch reflex and the reflexes emanating from the periodontal mechanoreceptors, which are described below and in Chap. 8, respectively. It is interesting to note that reflexes emanating from the mechanoreceptors of the skin may also modulate masticatory muscle function. For example, recordings from different nerves innervating the mechanoreceptors of the skin of the face, lips and oral mucosa stimulated by voluntary movements of the jaw through deformation of the tissues have indicated that there are tactile receptors activated only by jaw-closing movements, suggesting that proprioceptive signals from the face and mouth may specifically control the opening and closing rhythm of the mouth during chewing [23, 78]. The reader is reminded that the jaw-opening muscles as well as the facial muscles have no muscle spindles and their function may be substituted by the mechanoreceptors of the face and oral mucosa (see Sect. 8.12).
It is also interesting to note that a number of authors have suggested that the function of mastication does not gradually develop from the suckling function, although the phasic alternation of cyclical masticatory movements are reminiscent of the cyclic function of the act of suckling. Rather, the development of the mature patterns of biting and chewing appear as new learned functions that may be triggered by eruption of the teeth, reflecting coincident maturation in oral sensory experience and in the progressions of encephalization [1, 65] (see Chap. 10).
6.2 The Trigeminal Stretch Reflex
The stretch reflex in the trigeminal system functions to adjust the force exerted by the jaw-closing muscles to compensate for changing resistance to closing that occurs during chewing. It also plays a role in maintaining the posture of the jaw in the “rest position” during vigorous head movements, for instance during running. The stretch reflex, like other somatic reflexes in the body, consists of a sensory receptor, an afferent or sensory pathway, an integrating center in the central nervous system, where the sensory neurons synapse with the motor neurons, and an output pathway consisting of the axon channels of pyramidal motor neurons, which innervate the masticatory muscles through the motor nuclei of the trigeminal nerve. The receptors for the stretch reflexes are muscle spindles, which are located in the muscles in parallel with the muscle fibers. The jaw-closing muscles, but not the jaw-opening muscles, are richly endowed with muscle spindles [78, 217].
The sensory nerves that carry the signal from the muscle spindles to the brain are the largest myelinated nerve fibers found anywhere in the body, the so-called group Ia afferent neurons. The trigeminal Ia afferents pass into the brainstem via an anatomically unique path. Although they are sensory fibers, they enter the brain in the same nerve trunk as the motor nerves that are leaving the brain, rather than with other sensory neurons as is the case in the spinal cord. Their cell bodies lie within the brain, in the mesencephalic trigeminal nucleus, which in turn sends fibers to the trigeminal motor nuclei where they form excitatory synapses with the alpha motor neurons that innervate the jaw-closing muscles [78, 217].
Because both the nerve fibers that carry the action potentials from the muscle spindles to the mesencephalic nucleus of brainstem, and the fibers that bring action potentials from the brainstem back out to the masticatory muscles are large and myelinated, they conduct action potentials at very high velocities (about 50 m s−1). Also, because the distances over which action potentials must travel in this reflex are quite short (about 90 mm in each direction), the stretch reflex occurs very quickly (about 7–8 ms). The speed of the stretch reflex response enables the muscles to react extremely quickly to the changes in the resistance of the food being chewed. The chin-tap reflex is used by neurologists to test the normal function of the sensory and motor nerves in the trigeminal system .
During any normal skeletal muscle contraction, the brain also sends signals along the gamma motor neurons, which innervate the ends of the muscle spindles causing them to contract too, through their own actin-myosin contractile system. The contraction of both ends of the spindle stretches and hence activates the middle part of the spindle, where the sensory receptor (the primary or annulospiral ending) is located, causing action potential signals to be sent along the large myelinated Ia sensory fibers to the mesencephalic nucleus of the trigeminal nerve. Then the action potentials reach the jaw-closing muscles causing them to contract (to twitch), through the motor nuclei of the trigeminal nerve [78, 217].
It should be noted that the muscle spindles also have “secondary” sensory endings, which are anatomically different from the primary endings. The primary endings are sensitive to changes in muscle length and are rapidly adapting receptors. The secondary endings give a conscious signal related to muscle length and, therefore, are slowly adapting receptors .
6.3 Isometric Contraction of Jaw-Closing Muscles
The isometric tension developed by a muscle during maximal voluntary effort corresponds closely to that produced by tetanic stimulation of its motor nerve. Therefore, activity of the elevator jaw muscles during maximal voluntary clench of teeth may indicate their maximal strength .
The strength of voluntary contraction is increased both by an increase in the rate of individual motor unit discharges and also by recruitment of more motor units. Since each action potential represents activation of a motor unit, a correlation can be expected between muscle force and its pattern of electrical activity . In order to understand how isometric contraction controls biting force, consider what happens when you bite down onto brittle food such as a raw carrot.
The brain sends signals along the alpha trigeminal motoneurons to the jaw-closing muscles. This causes the muscle to contract and hence, to shorten and move the lower teeth towards the food. At the same time, the brain is also sending signals along the gamma motor nerves innervating the muscle spindles in the jaw-closing muscles. These cause both ends of the spindle to contract, so that each spindle shortens at the same rate as the skeletal muscle fibers, keeping the spindle under steady tension. When the teeth touch the food, the resistance to closing movement suddenly increases and the muscles stop shortening. However, the muscle fibers are still actively contracting, although they are prevented from shortening. The contraction of the muscles now becomes isometric, meaning that they are contracting under constant length. When, however, the resistance of the food during chewing increases, the gamma motor neurons of the brain continue to send signals that further shorten the spindle and hence further stretch the elastic center where the sensory receptor lies. This increases the activation of the receptor which sends action potentials to the mesencephalic nucleus, which in turn are projected to the trigeminal motor nucleus causing the jaw-closing muscles to contract more strongly, resulting in an additional biting force on the food.
All this happens in a small fraction of a second and automatically, in other words without any conscious intervention. Thus, the resistance of the food being chewed informs the brain through the muscle spindle receptors of the masticatory muscles that the food is tough, and the brain rapidly learns about the texture of food and automatically resets the reflex response accordingly [78, 219].
Numerous studies have demonstrated a rectilinear relationship between electrical and mechanical activity during isometric contraction. This implies that the electrical and mechanical activity recorded during maximal clenching of the teeth are alternative measures of the strength of the jaw-closing muscles. However, the question of whether the maximal bite force in fact represents the maximal strength of the jaw-closing muscles (i.e., the output derived from the motor units available working at their maximal rate of discharge) may be raised . Rasmussen and Moller  tested the maximal clenching of the teeth in the intercuspal position by determining the electrical activity in the anterior temporal and masseter muscles, and found that the maximal bite force does not represent the maximal strength of these muscles. They then suggested that there exists for each individual a maximal level of attainable muscular strength. Below this level the actual strength varies with the degree of physical activity. The effect of training on isometric endurance and maximal voluntary contraction supports this suggestion. Isometric endurance is related to fiber type composition both before and after training, but the effect of training is not. Therefore training does not level out differences between individuals with respect to endurance.
Functional disorders of the chewing apparatus are characterized by tenderness and pain of the muscles of mastication. The endurance and strength of the masticatory muscles may determine susceptibility to tenderness and pain. For example, nonfunctional activities such as bruxism (clenching and grinding of the teeth) may be responsible for tenderness and pain in the muscles of mastication. Indeed the comparison of the strength of the temporal and masseter muscles between subjects with and without such symptoms has shown that patients with functional disorders of the masticatory muscles have significantly lower values of maximal bite force compared to normal subjects. The affected and unaffected sides do not differ in patients with unilateral localization of the disorder. Reduction of signs and symptoms due to treatment is followed by an increase in maximal bite force .
6.4 The Unloaded Reflex
A dramatic example of biting on a very hard brittle object, for instance a nut shell, is the unloaded reflex. In this instance, the brain is sending ever increasing signals to the trigeminal alpha motor neurons that innervate the jaw-closing muscles, as well as to the contractile part of the muscle spindles through the gamma motor fibers, resulting in harder contraction of muscles. When the nut shell cracks the resistance of the jaw-closing muscles suddenly falls to a low level or disappears altogether. At this point, the jaw-closing muscles are still active and contracting hard, and there is the potential for the teeth to cause a lot of damage to oral structures. However, this does not happen.
First, because when the nut shell cracks the jaw begins to close as the muscles, now unresisted, begin to shorten at high speed. This shortening of the jaw-closing muscles removes the tension from the middle of the spindle and hence, the primary sensory receptor stops sending action potentials through the Ia afferents to the trigeminal mesencephalic nucleus, and the muscles are deactivated. This is the unloaded reflex, which protects the oral structures from damage. The second reason is, that when the jaw-closing muscles bite on the nut shell they are making powerful isometric contractions. This activates the jaw-opening muscles. Then, when the nut shell cracks and the jaw begins to close unresisted, the tension in the tonically active jaw-opening muscles keeps the mandible from springing too far upwards .
6.5 Voluntary Control of Masticatory Muscles
The main focus of the discussion so far has been on the control of the masticatory muscles by the central pattern generator through central motor commands, and by the various peripheral reflexes that fine-tune the output of the central pattern generator. The masticatory muscles, however, like most other skeletal muscles are also subject to highly precise voluntary control, following patterned oral experiential sensory input to the cerebral cortex [85, 189]. This implies that the neural development of the mouth is concomitant with the process of maturation of the decoding of information and learning mechanisms in the cerebral cortex [43, 67].
In the voluntary movement plan the output of the motor cortex bypasses the central pattern generator to reach directly the trigeminal motor neurons, which send action potential signals to the masticatory muscles. Then voluntary masticatory movements and forces can be generated without invoking rhythmicity. This is essentially the process that controls other skeletal muscles as well, with an important difference. While it is common to move, for instance, one finger on one hand, the masticatory muscles of both sides are activated simultaneously during almost every voluntary jaw movement through coupling of the left and right hyperneurons (see Sect. 4.2). We can voluntarily start chewing movements of the mouth, yet in most circumstances of normal mastication a conscious or voluntary effort is not required to maintain the masticatory activity .
6.6 The Closed-Loop Theory of Motor Control
A prevailing problem in motor coordination is how the motor systems are organized and regulated. In this context, there are two antithetical mechanisms that adjust the muscle force required during chewing. One of the foremost views, and until recently the favored position, is that peripheral receptors are necessary and sufficient for motor control through learning. This “closed-loop” theory proposes that sensory feedback, error detection and error correction are necessary requirements for movement regulation. The motor command that specifies the response to be generated is executed. Feedback through the muscle spindle and joint receptors checks for discrepancies between the output specified and the movement actually produced (error detection). The errors are evaluated and the central nervous system attempts to minimize the size of the error (error correction). Typically, slower movement sequences, for instance slow movements involved in speech learning to pronounce words as well as other sounds, are under closed-loop control .