Neural Control

2.5.1. Central nervous system (CNS) control

Mastication is regarded as an unconscious and automatic behaviour. It is regulated by a central pattern generator (CPG) in the hindbrain (Dellow & Lund, 1971; Hiiemae, 2004; Lucas, et al., 2004; Lund, 1991; Nakamura & Katakura, 1995). The CPG needs external triggers. Once the motor output starts, it produces a fixed movement with a constant rhythm (Lucas, et al., 2004).

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The mastication CPG is subdivided into two neuronal groups by function: one group generates masticatory rhythm, which means giving a time signal to alter the rhythm of jaw closing and jaw opening; the other group generates a spatio-temporal pattern of the activities of the jaw, tongue and facial muscles (Nakamura & Katakura, 1995). The former neuronal group regulates the cyclical movements of feeding: each cycle has closed and open phases in humans. Each open and closed phase includes antero-

posterior and medio-lateral elements (Hiiemae, 2004).It is assumed that the heart of the

CPG is located between the Vth and VIIth nuclei in adults. Part of the brainstem between the rostral poles of the trigeminal (NVmot) and facial motor nuclei (NVII) can produce rhythmical movements in the jaw muscles even when separated from the rest of the brain (Kogo, Funk, & Chandler, 1996; Nakamura, Katakura, Nakajima, & Liu, 2004).

A series of studies were carried out to determine the role of the facial primary motor area (MI) in the cerebral cortex with different oral behaviour environments. Facial MI was found to play an important role in elemental and learned motor behaviours and in certain aspects of chewing and swallowing (Sessle et al., 2007).

The CPG receives inputs from higher centres of the brain, especially from the inferio- lateral region of the sensorimotor cortex and from sensory receptors. Mechanoreceptors in the lips, oral mucosa, muscles, and in the periodontal ligaments around the teeth roots have particularly powerful effects on movement parameters. Besides controlling motoneurons to regulate the jaw, tongue and facial muscles, the CPG also modulates reflex circuits, and these brainstem circuits are believed to participate in the control of human speech. During mastication some reflexes are suppressed, while the amplitude of

others is regulated in phase with mastication. The ipsilateral and contralateral cortical

representations of the tongue are under analogous inhibitory and facilitatory control, possibly by the same intracortical network (Muellbacher, Boroojerdi, Ziemann, & Hallett, 2001).

Sakamoto et al. (2008) conducted a systematic review of research into the activated

regions in the tongue secondary somatosensory cortex presentation (SII) following stimulation of the tongue. They found that the tongue areas are considered to occupy a small region in SII with insufficient spatial separation to differentiate anterior from posterior areas using magnetoencephalography, which has a higher spatial resolution

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than EEG. They found that the tongue primary somatosensory cortex (SI) lay more laterally and anteriorly than the hand or foot SI. And the location of the tongue SI in the contralateral hemisphere was significantly different from that of the tongue SII. SII has been speculated to serve a higher level of cognitive function in somatosensory processing (Sakamoto, Nakata, & Kakigi, 2008).

The CPG was proposed to control mastication and swallowing several decades ago (Baessler, 1986; Selverston, 1980). Recently, it was proposed that the CPG for mastication controls soft palate motion during mastication and oral food transport, but not swallowing (Matsuo, et al., 2005). Another study found that subjects can consciously inhibit food to the valleculae in stage II transport during mastication and decide when to swallow. Individual decisions can alter the position of a food bolus in the oral cavity at swallow onset (Palmer, Hiiemae, Matsuo, & Haishima, 2006). This study also indicated that mastication and swallowing are located in different brain areas.

From another viewpoint, it also indicated that mastication is not completely

unconsciously controlled; individual decisions do affect mastication, especially in food transport and bolus formation.

The final motor pattern is determined by the coordinated activity of all motoneurons in the Vth, VIIth and XIIth nuclei that are fired within each cycle of mastication. There are interactions among CPGs controlling them, between mastication and swallowing, swallowing and respiration, but not between mastication and respiration (Lund & Kolta,

2006).

2.5.2. Peripheral nerve control

2.5.2.1. Tongue innervations

The peripheral nerve control of the tongue is quite complex involving several cranial nerves. First of all, most muscles of the tongue are innervated by the hypoglossal nerve (cranial nerve XII); only the palatoglossal muscle is innervated by the pharyngeal plexus, a branch of the Vagus nerve (cranial nerve X) (Gest & Schlesinger, 1995). Secondly, sensory innervation of the tongue is divided into taste sensation and general sensation. For the anterior two-thirds of the tongue, which are referred to as the oral part, general sensations and taste sensations are carried via different nerves. Somatic sensations travel from the tongue through the lingual nerve — a main branch of the

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mandibular nerve, which emerges from the trigeminal nerve (cranial nerve V). General sensation from the areas of the oral mucosa and the gingiva of the lower teeth is also delivered by this nerve, while the taste sensation of the oral part of the tongue is carried to the facial nerve (cranial nerve VII) through the chorda tympani (Ross, 2007). The posterior third of the tongue is the pharyngeal part, which is innervated simply, as the taste sensation and general sensation are both carried by the glossopharyngeal nerve (Gest & Schlesinger, 1995) (Figure 2-3.).

2.5.2.2. Peripheral feedback

Based on peripheral nerve control, all kinds of sensory receptors in the oral cavity collect sensory information and send them to the central nervous system (CNS) through peripheral nerves (Martini, 1988), which impact and adjust the CNS control during oral processing (Bailey, Rice, & Fuglevand, 2007; Chicharro, et al., 1998). In this process four kinds of papillae, rapid adapting receptors and deep receptors collect all taste and general sensations and deliver peripheral feedback to affect masticatory behaviour (Brown, Langley, Martin, & Macfie, 1994; Lassauzay, Peyron, Albuisson, Dransfield, & Woda, 2000).

Many studies have shown that peripheral feedback does exist. Foster et al. (2006)

hypothesised a dual theory: firstly, a cortical-brain stem preprogrammed mechanism to adapt the shape of the jaw movements to the rheological properties of the food; secondly, a brain stem mechanism with mainly sensory feedback from the mouth to adapt muscle force to food hardness. Lowe (1980) found that local anesthesia had been applied to oral tissue and the temporomandibular joint (TMJ), rhythmic coordinated masticatory movements did not change in humans, but the tongue was often injured; this indicates that tongue protective reflexes are disrupted when the peripheral sensory feedback is blocked. The tongue protective function during mastication has been attributed to the lingual nerve in humans (Lowe, 1980).

The feedback includes signals about the physical properties of food which change progressively during mastication producing a concomitant change in the muscle work and cycle speed (Brown, et al., 1994; Lassauzay, et al., 2000).

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Figure 2-3. Left: Tongue innervations by cranial nerves. Right: Mapping of areas sensory innervation on

the tongue (Colbert, B. J., Ankney, J. J., Lee, K. T., 2010)

2.5.2.3. Blocking tongue function

In animal experiments, researchers have found that periodontal pressure receptors and muscle spindles provide positive feedback to jaw-closing muscles during mastication (Bilt, Engelen, Pereira, Glas, & Abbink, 2006). However, human topical anaesthesia

experiments have shown that rhythmic chewing activity can still be evoked in the cortex,

after elimination of superficial sensory feedback from peripheral receptors. This means that neither periodontal input nor muscle spindle input is essential for basic rhythmic mastication activity (van der Bilt, et al., 2005). Possibly, rhythmic chewing activity after topical anaesthesia could be attributed to deficient anaesthesia.

In experiments in which topical anaesthesia was applied to tongue dorsum and palate, no significant difference was found in size perception of food particles (Engelen, van der Bilt, & Bosman, 2004). Engelen, et al. (2005) found that oral perception of the size of small spheres was underestimated, the sizes of large spheres were overestimated, and that topical anaesthesia also reduced spatial acuity (such as two-point discrimination). It was assumed that two-point discrimination only stimulates the superficial receptors which can be blocked by anaesthetic, while perception of spheres or irregular particles might stimulate more deep receptors, which are important to mastication performance

and swallowing (Engelen, et al., 2004). All these experiments only blocked the

superficial receptors in the oral cavity, as the method used was invasive. However, some interesting findings emerged from studies with participants with hemiplegia. Those with hemiplegia of the right side cannot chew on the left unparalysed side, though the left side mandibular muscles are still healthy, because the muscles of the right half of the

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tongue cannot throw food to the left side of teeth. People with cancer of the tongue experience a similar impairment; they only masticate on the affected side after recovery from operation (Abdelmalek, 1955).