© Springer-Verlag Berlin Heidelberg 2009
Margaritis Z. PimenidisThe Neurobiology of Orthodontics10.1007/978-3-642-00396-7_8
8. Applied Neurophysiological Concepts in Orthodontics
Marathonos Street 22, 152 33 Halandri, Athens, Greece
Margaritis Z. Pimenidis
Diagnosis of malocclusion of the teeth associated with evident neuromuscular dysfunction is a specialized professional skill requiring at least a basic knowledge of the mechanisms of the brain that underlie sensation, perception, learning, memory, and motor control of muscles. A thorough analysis of these mental events is presented in Chap. 4. It is the specific intent of this chapter to present certain neurophysiological concepts that could be used as adjunctive therapeutic modalities within the framework of a comprehensive approach geared to the management of oral neuromuscular dysfunction. Comprehensive management includes analysis of the oral–facial sensorimotor system and evaluation of physiological effects that the proposed biomechanical treatment could have on the oral neuromuscular behavior. The proper use of one or more adjunctive therapy modalities can significantly enhance achievement of the orthodontic treatment goals.
8.2 Oral Perceptual–Motor Dysfunction in Children
The oral–facial region may be regarded as a major sensory–perceptual–motor system of the body surrounded by a complex musculature [67, 154]. Stimulation of the sensory receptors of the skin and oral mucosa by experience gives rise to a patterned neural input and processing of information in the cerebral cortex, resulting in sensation, perception, learning, and memory of experience, as well as motor control of muscles and of salivary glands. Thus, the senses interacting with the environment give rise to a number of functions through transformations in the brain, depending on the stimulus [15, 38, 58, 85].
To interact with the environment, we must know about the environmental stimulation of sensory receptors and about attributing meaning to the response and the use of the stimuli by the brain. This point is so basic that it is easy to overlook it. Just because there is an environment acting on the child and that his/her receptors seem to be responding to stimulation, does not mean that the cerebral cortex is really interpreting the world in a meaningful manner, or with the same meaning that we might expect. One of the first things we need to learn to do is to avoid taking the functions of the cerebral cortex for granted [69, 71, 81, 82]. Perceptual or motor functions are not an either/or matter, but one of degree, just as any ability varies by degrees. Because the motor acts are so closely associated with perception of our mouth structure and because we so often express our interpretations of our tangible environment in motor acts, the perception and motor processes must be considered together. This reasoning accounts for the use of the term perceptual–motor [63, 69, 85].
Let us further analyze the behavior dimension we are discussing. Let us begin with the motor component. We know how the mouth is constructed (we are talking about the mechanical aspects) and how it moves, by seeing it and feeling it. The major types of sensory receptors through which this information comes are visual, tactile, proprioceptive, and vestibular. Although the vestibular sense is proprioceptive, because it is a specialized sense it is considered separately from other proprioception. When we refer to proprioception, we are referring to all those sensations, which arise from muscles, tendons, joints, fascia, and related tissues. It is noted that taste is not a pure sensory modality, but rather a mixed sensation composed of gustatory, tactile, pressure, cold, heat, and olfactory impressions. Taste is not a pure modality because the tongue perceives not only sweet and saline, but also the weight, fluidity, roughness or smoothness, temperature, viscosity, and volatility of food .
Our task then in orthodontics is to understand the nature of the central nervous system’s mechanisms, which integrate the oral–facial sensory input and create the appropriate output. The more careful the evaluation of these essential processes and the better the insight we have of the perceptual–motor problem, the more effective will be the treatment. An orthodontist who seeks treatment procedures only, without a firmly based understanding of the sensorimotor dysfunction requiring treatment, is advised not to treat at all.
For example, in unilateral posterior cross-bite malocclusion of teeth, there is dissociation of the normal bilateral activity of the masticatory muscles [124, 125]. The patients, however, do not seem to perceive a difference between the activities of the left and right masticatory muscles. It is interesting that the orthodontic literature gives very little account of how this central sensorimotor–perceptual dysfunction affects the bilateral firing (and contraction) of the masticatory muscles.
Recent neurophysiological studies, however, suggest that thalamocortical neurons generating synchronous alpha and theta cortical activity in the electroencephalogram (EEG), are linked by gap junctions (electrical synapses) in the thalamus forming a giant neuron or hyperneuron , so that thalamocortical projections (or trans-corpus callosum pathways) could couple both hemispheres in hyperneurons to account for bilateral synchronous activity . Gap junction-connected neurons are the cellular level of the neural correlate of perception or consciousness. A key feature of gap junction-connected neurons is continuity of dendritic membranes of neurons that depolarize coherently. Another key feature is continuity of cytoplasmic interiors .
Thus, abnormal coupling of cerebral hemispheres in hyperneurons seems to underlie the dissociation of the activity of the left and right masticatory muscles in unilateral posterior cross-bite malocclusion of teeth. This might suggest that the normal bilateral masticatory function has never been consciously learned and hence no memory record for bilateral function of muscles has ever been inscribed in the left and right motor cortices. This relationship between perception and cognition is very well established [15, 58]. In fact, motor learning and perception form a continuum .
Judging by those aspects we have considered so far, we may assume that we are concerned with the child’s defective manipulation of his sensory environment. In this view, the most obvious and significant changes in the electrical and chemical activity of the brain occur not necessarily with specific oral stimuli, but are dependent upon the meaning of the stimuli to the child’s cerebral cortex [69, 82]. If the stimulus is to warn the brain of danger and to help mobilize it for defense by producing a high level of arousal for alerting the brain (fight or flight response) then the very cortical processes that are essential for perception and learning are inhibited. Thus, over-arousal of the brain impedes discrimination of senses, resulting in perceptual deficit and inability to learn the experiences provided by the senses [63, 64]. This means that if during a critical period of development a child complains of oral–facial pain, tooth ache, or another oral disorder causing over-arousal of the brain, then learning of new experiences may be inhibited.
Similarly, if the central nervous system is interpreting the orthodontic treatment stimuli (tactile, pressure, proprioceptive, etc) as danger signals, then the ability of the cortex to discriminate and process these stimuli will be suppressed, resulting in no control of the expected orthodontic changes. The implications of these cortical reactions in orthodontic practice are obvious. We must try to provide the kind of sensory and motor experiences to patients that will normalize the central nervous system’s ability to process the sensory input that the treatment provides. In other words, if we can inhibit the over-protective response of the brain, which calls for fight or flight reaction, then we will likely facilitate discrimination of sensory–perceptual processes, learning and memory abilities, and motor control of muscles. Thus, the important point is to inhibit the irrelevant stimuli. Our objective is to accomplish this on a neurophysiological level, the way the normal brain does it, rather than by simply preventing the stimulation of receptors. The reticular formation and the ascending reticular activating system (ARAS) play a critical role in inhibiting distracting information, as well as in facilitating the relevant stimuli reaching the cerebral cortex, enhancing then learning and perception ability .
Survival value is one of the strongest principles of sensorimotor functions. The protective system predominates in early life and often whenever development deviates from the normal. Predominance of the protective system inhibits manifestation of the discriminative system. Activation of the discriminative system inhibits the protective system. Maturation inhibits the protective system and enhances the discriminative system, probably through maturation of inhibitory nerve fibers and inhibitory mechanisms of the brain .
8.3 The Scheme of the Mouth
The question that arises is what type of development must occur before the mouth of a child is able to react effectively to environmental stimuli. An obvious answer is that the child must learn just how his/her mouth is constructed and just how it moves. In other words, the child must build an adequate oral scheme. The word “learn” does not refer to cognitive learning. Little is to be gained by teaching a child with oral dysfunction that he/she has, for instance, twenty teeth, in the same way that you would teach in the same child the concept “twenty.” The scheme of the mouth is not acquired in that manner. It is acquired through the sensory receptors and, it is hypothesized, through reinforcement from the results of purposeful movements. If the movement accomplishes some desired result the brain “notes” that certain directions are effective in bringing about certain movements, and that the same directions may be used again in a comparable situation. For example, the balanced action of the paired genioglossus muscles is required to protrude the tongue straight out of the mouth (the base of the tongue is pulled forward by the simultaneous contraction of the genioglossus muscles). If one genioglossus is inactive the action of the intact muscle is unopposed, resulting in deviation of the tongue towards the inactive muscle, suggesting damage to the hypoglossal nerve (lower motor neuron lesion).
Research in apraxia (perceptual–motor disorder or syndrome affecting especially the skilled motor functions) suggests that the role of receptors deserves particular note. While one might assume that the kinesthetic (proprioceptual) receptors such as muscle spindles might be critical for providing information about their position and movement, scientific data force us to give even more attention to the tactile receptors in the development of the ability to motor plan. It is the tactile receptors that provide us with much of the information about the nature of the external world that we are manipulating with our fingers and tongue . We all know how hard it is to handle coins with gloves or to put lipstick on after a Novocain shot from the dentist.
We must be careful to distinguish here between mere awareness of sensory stimuli and the ability to interpret those stimuli, especially as to their spatial elements. If we touch a child’s cheek when he is not looking and he indicates that he felt the stimulus, his response in no way indicates that he knew where he was touched. Discriminative interpretation of the spatial and temporal elements of tactile stimuli are essential to the development of the scheme of the mouth. Note that there is a difference between the concept of the scheme of the mouth and the idea that one needs tactile information at the same time as execution of the motor task. While the latter is an important process, motor planning, as an antecedent of execution, is dependent upon meaningful organization of previous tactile experiences. The scheme of the mouth is a neurological organization of previous tactile, proprioceptive and probably vestibular stimuli in association with planned movement.
The major questions facing us now are what kind of developmental processes must occur to assure the perception of the spatial elements of tactile stimuli, and what central nervous system mechanisms might be responsible for these processes? To answer these questions it is helpful to conceptualize the tactile functions of the nervous system. It has been suggested that there are two tactile systems: one designed primarily for warning the organism’s mouth against impending danger and one designed to convey information about the environment for better interpretation of the danger. The two systems can be called “protective” and “discriminative”, respectively. Their existence is finding increasing support from current research. It is the adequate function of the discriminative tactile system which is required for praxis or motor planning [79, 164].
Survival value is one of the strongest principles of sensorimotor function. The protective system predominates in early life and often whenever development deviates from normal. Predominance of the protective system inhibits manifestation of the discriminative system. Activation of the discriminative system inhibits the protective system, most likely through maturation of the inhibitory nerve fibers and inhibitory mechanisms [37, 64].
In this context, we might need to advance new methods to improve oral perception in cases of oral–facial neuromuscular dysfunctions associated with some form of oral agnosia, namely through enhanced stimulation of the tactile receptors in such a way that it will inhibit the protective system and facilitate the discriminative system. Stimuli should precede and be used in conjunction with motor tasks.
For example, there is good neurophysiological evidence that tactile stimuli from the face, lips, and oral mucosa can be activated during voluntary movements of the jaw-closing muscles, through deformation of these tissues, suggesting that the proprioceptive signals of tactile receptors may be specific to the control of the opening and closing rhythm of the mouth during chewing [23, 78]. Some contraindications for using tactile receptors to enhance oral perception and movement are the presence of hypertonicity (muscle spasm), pain, and a strong negative reaction on the part of the patient. The younger the patient, the more careful we need to be.
8.4 Oral Sensory and Motor Somatotopic Coincidence
Another neurophysiological detail to be considered is that in the sensorimotor cortex the oral tactile and motor patterns coincide somatotopically. This means that the tactile impulses that arise when one holds an object between the teeth go to the part of the cortex that when stimulated causes motion of the mouth. A reasonable explanation of somatotopic coincidence would be that the distinction between the motor (precentral gyrus) and the sensory (postcentral gyrus) areas in the cerebral cortex is not clearly delineated. Both the precentral and postcentral gyrus are actively involved in sensory and motor processes . This anatomical arrangement in the cortex may suggest that the oral tactile stimuli facilitate the discharge of the cortical motor neurons that are involved in the motion of the mouth, and vice versa that movements of the mouth stimulate the tactile receptors of the mouth. Thus the therapeutic scheme involving tactile stimulation in conjunction with motor tasks may be justified on the basis of oral sensory and motor somatotopic functional coincidence. The cerebral cortex, of course, is considered the major mediator of oral skilled motor functions that are so vulnerable to cortical dysfunction of “agnosia of teeth” that is discussed next.
8.5 Agnosia of Teeth
The density of the tactile innervation of the periodontal ligament and of the oral–facial region in general and its possible implications for tongue posture and the position of the teeth (mechanisms that are described later in this chapter) is reflected in the enormous size of the trigeminal and facial nerves and their large representation in the primary somesthetic cortex. Roughly half of the somesthetic cortex processes information from the oral, facial, periodontal, and pharyngeal regions. The large oral–facial and pharyngeal representation in the cerebral cortex explains our heightened awareness of the face, mouth, tongue and teeth, and why agnosia of these structures, such as agnosia of teeth (the brain is not aware of the presence of the teeth), is one aspect of inadequate oral perceptual–motor scheme disorder or oral apraxia, as discussed below.
A 21-year-old male patient with severe class III open bite malocclusion of the teeth who was not aware without looking of the presence of the incisor teeth in his mouth has recently been reported. The agnosia of teeth was associated with oral motor dysfunction including tongue thrust and impairment of coordinated chewing movements. However, after closure of the open bite and the establishment of normal occlusal and skeletal relationships through orthodontic therapy the patient became conscious of the full complement of his teeth . Accordingly, it has been suggested that normal occlusion of the teeth is essential in forming the basis of normal perceptual awareness of the scheme of the mouth (see Sect. 3.3). Clinical studies support this conclusion and suggest that normal occlusion of the teeth is necessary to maintain the complete normal sensibility of the teeth. In an individual with malopposed teeth sensibility is more or less disturbed, and the individual will have a poorer ability to discriminate the physical characteristics of objects held between the teeth .
The role of receptors involved in oral agnosia deserves particular note. While one might assume that the kinesthetic (proprioceptive) receptors, such as muscle spindles, might be critical for providing information about position and movement, scientific data on limb apraxia suggest more attention to the tactile receptors in the development of motor planning ability. It is the tactile receptors that provide much of the information about the nature of the external world that we manipulate with our fingers and tongue . The tactile receptors of the finger tips have neurophysiological properties similar to those of the tactile receptors of the tip of the tongue [93, 157]. It is mainly the tongue tip that is used to manipulate and explore objects and structures in the mouth, including the teeth. It is then most likely that it is the tactile receptors of the tip of the tongue, as well as the periodontal mechanoreceptors, that are affected in agnosia of teeth. It is noted that all the basic five sensory systems of the mouth need to be working simultaneously and cooperatively for higher skill acquisition and performance. In fact, if any one system does not work properly either by itself or in conjunction with others, a sensory dysfunction of some kind may result .
Thus the discriminative interpretation of the spatial and temporal elements of the tactile stimuli at the tip of tongue and of the periodontal ligament are essential for the development of the scheme of the mouth, including awareness of the presence of the teeth. Note that the scheme of the mouth is a neurological organization of previous tactile, proprioceptive, and probably vestibular stimuli in association with planned movement .
8.6 Oral Apraxia
As orthodontic specialists dealing with neurophysiological problems of oral motor dysfunctions, we study in greater depth the central nervous system’s function in order to have a better understanding of the perceptual–motor problems of the masticatory system. Oral apraxia is one type of oral perceptual–motor disorder or syndrome. It is a disorder of skilled motor function—a disorder of the ability to plan coordinated masticatory movements of the mouth, as opposed to a disorder of executing the motor plan, which involves a lesion of the neurons of the precentral motor cortex or their axon channels (upper motor lesion).
Some degree of developmental oral apraxia should be suspected in, for example: an orally clumsy child with uncoordinated chewing movements; a child with dissociation of the left and right masticatory muscle function (as occurs in unilateral posterior cross-bite malocclusion of the teeth), with difficulty in jaw and tongue postural position or with difficulty in recognizing the shape of objects put in the mouth without the use of vision; a child who is not aware of the presence of teeth in the mouth without the use of vision; a child with disordered taste or salivation; a child who is a picky eater (dislikes certain textures, tastes or temperatures); a child who does not know when his mouth is “full”; a child who vomits or gags easily; or a child who is clumsy with fine motor activities (eating, drinking, etc) . The condition is especially evident when the child is faced with a new oral task which is unfamiliar and which requires considerable focus of attention for learning, as for instance touching the nose with the tip of the tongue.
The clumsy child who has difficulty learning to manipulate objects in the mouth could be helped with tactile and pressure stimuli in conjunction with motor tasks followed by assumption of postural patterns. These procedures may activate the discriminative system while at the same time inhibiting the protective system. Remember that in the ontogenetic process, the child first learns to plan gross motor actions, gradually refining his/her skill until, for instance, he/she writes with ease and bites and chews with finesse. Most of us forget that we all once motor planned walking and sitting in a chair. We no longer have to plan them.
For a task to be therapeutic the child must have some success in the task which should demand that he/she motor plan. Any gross motion frequently repeated, such as walking, does not make a demand for planning. Activities on which we must focus our attention do require planning. This means that the stimuli must cause arousal of the brain for attention and awareness which correlate with conscious learning, memory, and motor planning (see Chap. 4). Thus walking backwards or putting the tongue in specified spots in the mouth may cause arousal of the brain for learning new experiences. Motor planning should be taught, taking the child passively through the motions, for it is the sensory stimuli we learn when we learn to motor plan.
In addition, crossmodal influences, which appear instantly and disappear rapidly during early development in the primary and secondary cortices serving multisensory tasks, may be involved in learning ability . For example, a recent functional magnetic resonance imaging (fMRI) study has identified a region of the brain which is activated specifically during a combined visual–auditory task . In another study neurons in the secondary auditory cortex tuned to precise frequencies of sound also responded to a tactile input . Similarly, it has been found that visual stimuli enhance tactile activity through modulation of responses in primary and secondary somatosensory cortices serving multisensory components .
In this view, oral stereognosis (the recognition of form of objects put in the mouth, without the benefit of vision) is a multisensory experience; it involves at least the integration of an oral kinesthetic (proprioceptive) sensory modality providing information about the position in space and the tactile and pressure receptors providing information on the form of objects, and can be enhanced with the conscious motions of the mouth in early age. One of the directions of evolutionary development has been intersensory experience, that is multisensory integration coming from the same sensory system, for instance the mouth, and crossmodal experiences involving the integration of two or more sensory systems. Crossmodal experiences involving visual–auditory–tactile information are involved in the development of speech (see Chap. 5).
8.7 Enhancing Oral Perception: Pacini Receptors
The orthodontic practice is making demands for inclusion in our treatment of oral apraxia, the development of the discriminative and learning ability of the mouth, through the motor system, which activates in addition to other receptors, the Pacini corpuscles, one of the major proprioceptor mechanoreceptor, known to send information to the cerebral cortex for conscious perception of the moving tongue and jaw. These receptors are found in the masticatory muscles and temporomandibular joints. The Pacini corpuscles are rare or lacking in most areas of the facial skin and oral mucosa, the ventral side of the tongue being a noted exception. These sensory endings have a higher threshold to stimulation than many other proprioceptors. This suggests that heavy muscular exertion is more beneficial in activating the Pacini corpuscles, which convey the modalities of pressure (deep sensibility), stereognosis and of course proprioception. The Pacini corpuscles respond to muscle stretch and contraction, as well as to the movements of the temporomandibular joints [44, 93]. The use of heavy class II intermaxillary elastic forces in orthodontics to protrude the mandible also apparently leads to stimulation of the Pacini receptors of the patient’s muscles and temporomandibular joints.
It has been suggested that the Pacini corpuscles of the temporomandibular joints play an important role in the conscious perception of the angular position of the mandible [162, 163]. The ability of the Pacini corpuscles to monitor mandibular angular position is impaired by severe malocclusion of the teeth with a reduced anterior vertical dimension of the lower face . These studies might substantiate the orthodontic restoration of the vertical dimension of the lower face in patients with deep overbite . In this view, mandibular angular proprioceptive perception and motor control could be enhanced through muscle contraction, especially postural patterns, against resistance, stimulating the high-threshold Pacini corpuscles in the masticatory muscles and temporomandibular joints.
Functional jaw orthopedic appliances which reposition the mandible to alter the muscle forces against the teeth and craniofacial skeleton may stimulate the Pacini receptors in addition to muscle spindles. Functional jaw orthopedic appliances are used where neuromuscular dysfunction has played a role in the etiology of the malocclusion and/or where enhancement or alteration of normal functional activities may provide optimal conditions for growth and development of the jaws and occlusion of the teeth.
8.8 Enhancing Oral Perception: Muscle Spindles and Tactile Receptors
Another approach which might eventually be more effective than facilitating the perception of moving oral structures lies in the function of the muscle spindles. However, there seems to be little neurophysiological support from these receptors in the oral–facial region. There is little evidence to indicate cortical receipt of information from the spindles of jaw-closing muscles. The jaw-opening muscles have no spindles . Therefore, in order to understand the proprioception of the moving mandible relative to the maxilla we need to extrapolate from studies of the stretch reflex in spindles of the limb muscles.
The proprioception of the position of moving limbs relative to other structures of the body arises largely from muscle spindles, although inputs from tactile mechanoreceptors of the skin that covers the muscle can be even more effective, particularly in the fingers and in the oral–facial region. These tactile receptors provide discriminative interpretation of the spatial and temporal elements of tactile stimuli, which is essential for the development of the ability to plan the movement of the mouth. In fact, it is the tactile receptors of the skin and of the oral mucosa that provide much of the information about the nature of the external world that we experience with our hands, mouth, and body [78, 93].
There is good neurophysiological evidence that tactile stimuli from the face, lips, and oral mucosa may be activated during voluntary movements of the jaw-closing muscles through deformation of these tissues, suggesting that the proprioceptive signals of the tactile receptors may be specific to controlling the opening and closing rhythm of the mouth during chewing [23, 78].
The effect of tactile stimulation may have a direct effect on the transmission of kinesthetic (proprioceptive) impulses to the cerebral cortex. The effect may come through centrifugal influences on the kinesthetic receptors and their relay stations. Another mechanism through which tactile stimuli enhance kinesthetic and visual perception is through the ARAS .
8.9 Enhancing Oral Perception: Centrifugal Influences
This seems an appropriate place to introduce another neurophysiological process which undoubtedly has a profound influence on perception, but the real functional role of which is little understood at present. It is not only the external environment which acts on the sensitive receptors of the oral–facial region, but also the central nervous system itself. Through descending axon channels, the brain influences the flow of sensory input at each relay station and at each sensory receptor . The principle is seen and understood most clearly in reference to the muscle spindle. One of the most effective methods of influencing motor function is by controlling the sensory input from the muscle spindle . The sensory input in turn is influenced by the motor task confronting the patient, in accordance with the principle that most of the oral sensory input is generated principally by the motions of the mouth and vice versa, i.e., the sensory stimulation generates the motion of the mouth . The centrifugal influences serve the purpose of making the receptors and pathways more capable of sending information or preventing the normal deluge of sensory impulses from reaching the cortex. Thus, selection of the incoming sensory information in the central nervous system seems to be the basic principle  (see Sect. 2.6).
8.10 Affecting Oral Perception and Learning
The ARAS and its anatomically and functionally close associate, the diffuse thalamic projection system, are concerned with arousal of the brain for attention and learning purposes. The ARAS allows selective attention, augmenting meaningful and inhibiting irrelevant information. Sometimes, however, the purpose is to produce a high level of arousal or to alert the brain to danger and help mobilize it for defense (fight or flight reaction). In this instance, the very cortical process that is essential for perception and learning is inhibited because the brain’s priority is the danger [63, 64].
For example, if the central nervous system is interpreting a tactile stimulus (and perhaps other stimuli, such as an auditory stimulus) as a danger signal, then at least two major mechanisms will affect perception. The tactile perception essential to motor planning will be depressed, and the ability of the nervous system to discriminate, separate out, augment, or inhibit stimuli will be reduced. If we can inhibit this overprotective response of the brain, then we will likely facilitate the perceptual process. The only method for which we have any assurance of success is through tactile stimulation accompanied by pressure (deep sensibility).
Thus, the reticular formation plays a role in inhibiting distracting information input to the brain. It also plays the companion role of increasing the information input to enhance perception. Activation of the primary, specific sensory pathways alone, such as the spinothalamic or medial lemniscal pathway, is not a sufficient process to enhance perception. In addition to receipt of information over these specific pathways, perception requires generalized nonspecific activation of the cortex from the diffuse effects of the reticular formation. All five sensory modalities contribute to the activation of the diffuse system through crossmodal stimulation. For instance, visual stimuli enhance tactile acuity through modulation of responses in the primary and secondary somatosensory cortex [15, 136].
A general therapeutic procedure which occupational therapists use to enhance perception of any sensory modality is to precede a specific perceptual task with general stimulation (tactile, proprioceptive, vestibular, olfactory, etc) which, however, is not such as to cause over-arousal and the protective defense response of the brain, but is to be received by the central nervous system as a comfortable and pleasant stimulation. The general arousal effect can be expected to last for a number of minutes.
8.11 Affecting Oral Perception: Emotional Influences
Another vital point concerning the centrifugal influences in oral sensory–perception–motor functions lies in the fact that each sensory receptor is also served by an autonomic system nerve. The discharge of muscle spindle receptors and cutaneous receptors is increased by sympathetic nerve stimulation. The sympathetic nerves exert a facilitatory effect on reflexes arising from afferent stimulation, including the enhancement of muscle contraction . On the other hand, the sympathetic system is responsive to emotions. Accordingly, since perception is related to cognitive (learning ability) and emotional development  whose mutual developmental depends upon adequate patterned sensory input , it is not unlikely that the degree and kind of emotional and cognitive involvement manifested by a child will, through centrifugal influences acting on sensory reception and transmission, have a direct and specific effect on the development of perceptual processes . This reasoning leads to the assumption that the motivated child will receive more benefit from the orthodontic therapy than the nonmotivated one. Our goal, however, in emotional involvement is not merely to elicit cooperation or to provide a pleasant experience during the therapy, but to alter the sensory–perceptual–motor function of the oral–facial region through the appropriate response of the cortex.
Under certain conditions, which we will assume to be associated with brain dysfunction, the balance of the protective and discriminative system is tilted in the direction of protective system predominance. In this instance, centrifugal influences lower the threshold of the oral receptors and the sensory impulse transmission favors a nociceptive interpretation of tactile stimuli. Specifically, it is light touch stimuli that are most apt to elicit the negative, defensive response. What protective system predominance does to the affect is only conjecture. It is suggested that we watch for elements of a basic fighting response or one of fear and flight. Heightened annoyance, “unreasonable” anger or belligerence may appear. Some of the elements of the physiological basis of the emotional reaction may be seen in the responsiveness of the brainstem reticular activating system to adrenaline. It is to this level of the reticular formation that is delegated the responsibility for alerting the organism to potential danger. Fear produces adrenaline and adrenaline not only enhances readiness for fight or flight but also lowers the threshold of some tactile receptors.
Thus, here we see the possible neurophysiological basis of the effect of emotions on perpetual–motor function. There is undoubtedly an interaction effect on a neurophysiological level between perception and emotions. The nervous system geared to fight or flight is at the same time inhibiting the very cortical recruitment process essential to perception and learning. This level of perception and learning is not restricted to the highly cognitive levels. Depression of the perceptual process relating to somatic (mouth) factors also likely occurs.
This theoretical formulation provides us at least with a direction for a physiological basis for orthodontic treatment. Instead of simply shielding a child from distracting stimuli, let us aim toward providing the kind of sensory and emotional environment which will encourage a better balance between protective and discriminative systems. It is suggested that we try pressure touch as opposed to light touch cutaneous stimulation, applying the stimulus especially to those parts of the body richly supplied with tactile receptors. These areas are hands, face, and forearms , and are discussed next.
8.12 Proprioception of Muscles of Facial Expression
The muscles of facial expression are also involved in proprioceptive perception. These muscles, however, lack proprioceptive mechanoreceptors, such as muscle spindles or Golgi tendon organs. Instead, the proprioceptive function of these muscles is served by tactile mechanoreceptors, which are embedded in the skin, lips, and oral mucosa. The muscles of facial expression, innervated by the facial nerve, insert directly into the skin of the face and their contraction deforms the skin in very specific patterns, causing the tactile receptors to discharge vigorously in a way that can be interpreted meaningfully by the central nervous system. The precise nature of the neurophysiological response of the tactile mechanoreceptors indicates that in addition to touch and pressure response, they also serve as proprioceptors, signaling detailed information about the consequences of activation of the muscles of facial expression in the perioral and lip tissues. Thus, the action potential of the tactile mechanoreceptors in the oral–facial region, elicited through deformation of the skin during voluntary movement of the muscles of facial expression, is particularly important for the proprioceptive control of these muscles [93, 155, 156].
Surface electromyograms from the lips during mastication suggest that the lips are activated most vigorously during the opening phase of the chewing cycle (primary activity) in order to produce a complete or partial anterior oral seal preventing the food from leaving the oral cavity. This is a consistent finding of electromyographic studies in subjects of all ages. Fine wire electrodes have localized this activity to the oral sphincter, the orbicularis oris muscle. Whether the primary activity of the lips results in a complete anterior seal or merely prevents the lips from being separated as much as the jaws is of little importance. From a neurophysiological point of view the significant finding is that the activity causing the lips to approach increases with morphological traits that tend to impede lip closure. For example, one of the most frequent abnormal lip functions which tends to impede lip closure is tongue protrusion in swallowing. The mylohyoid muscles act strongly when the tongue protrudes by raising the floor of the mouth and stabilizing the hyoid bone .
The degree of primary activity in the lower lip during mastication is related to facial prognathism. In subjects with retrognathism of both jaws the lips are usually sufficient and the anterior seal demands slight activity; with prognathism the lower lip may be taut at rest and chewing requires strong activity. The relationship between lip function and facial morphology exemplifies an important feature: the lips adjust the oral cavity to the particular function being performed (mastication, swallowing, speech) and are primarily activated to produce their own movements. The adaptive function seems to depend more on recognition of the shape of the supporting hard tissues than on the tension produced. The tactile mechanoreceptors of the perioral skin are in keeping with this assumption .
The lips are also active during the closing movement of the jaw (secondary activity). Recordings made with wire electrodes from individual muscles moving the mandible (for example, the right and left digastric, the right and left posterior temporal, the right and left lateral pterygoid) show that the secondary activity in the upper lip originates from its upper levator muscle for the purpose of withdrawal. The activity during jaw closing is especially pronounced in subjects with deep overbite and retroclination of the upper incisors, reflecting a more acute demand for active withdrawal of the upper lip during closure .