Physiology of the Oral Cavity PDF

SECTION 3 Oral Cavity 86 86 Physiology of the Oral Cavity Mohamedkazim M. Alwani, Fawaz M. Makki, K. Thomas Robbins...

SECTION 3 Oral Cavity 86 86 Physiology of the Oral Cavity Mohamedkazim M. Alwani, Fawaz M. Makki, K. Thomas Robbins KEY POINTS Somatosensory innervation of the oral cavity is provided define each function. Coordination of muscle activity by cranial nerve V (trigeminal nerve), second and third occurs among functions, among motor groups, and within divisions, and cranial nerve IX (glossopharyngeal nerve): the muscles themselves and is affected by peripheral (1) V2—maxillary, hard, and soft palates; oral mucosa of feedback mechanisms. the maxillary vestibule; and maxillary teeth, gingivae, and The main function of the masticatory muscles is to break periodontal ligaments; (2) V3—mandibular, oral mucosa of down solid food into amounts small enough to be the cheek and mandibular vestibule; anterior two thirds swallowed. These strong muscles that open and close the of tongue; mandibular teeth, gingivae, and periodontal jaw generate significant forces across short distances and ligaments; and (3) IX—oropharynx and posterior one third apply them via teeth. These forces must be controlled of the tongue. precisely and effectively—by cortical preprogrammed Receptors on the periodontal ligament—the ligament that movement patterns, reflex stimulation, and peripheral separates tooth from bone in healthy, functioning teeth— input/feedback loops—to allow for smooth movements and initiate oral reflexes of jaw opening and salivation and, successful deglutition. together with receptors in the temporomandibular joint, Recent molecular and functional data show that no specific contribute to interdental force discrimination and oral tongue “map” exists for taste buds; responsiveness to the stereognosis. Therefore, bite force is lessened with a five tastes—sweet, sour, bitter, salty, and umami—is present corresponding decrease in these periodontal ligament in all areas of the tongue supplied with taste buds. receptors (i.e., with periodontal disease, tooth extraction, and consequent denture construction). Sweet taste sensations are associated with simple carbohydrates, sour taste is generated by weak organic Dental pain is mediated by C fibers (dull, burning) and A acids, salty taste is stimulated mostly by sodium chloride delta fibers (sharp, bright) located in the pulp chamber, the (sodium ions), bitter taste arises from stimulation by plant “nerve” of a tooth, and extending a short distance into the alkaloids (potential toxins), and umami taste is associated dentinal tubules (A delta only). In the hydrodynamic with amino acids and peptides. theory of dental pain, when enamel is breached by decay, fracture, or wear, the fluid in the tubules responds to The biology of taste perception is complex, mediated by stimuli and activates these nerve endings. taste receptor cells in the taste buds and innervated by cranial nerves VII, IX, and X; however, there is no Chewing, swallowing, and breathing are produced by argument that the clinical recognition of taste affects our brainstem central pattern generators that control the survival. fundamental rate and pattern of muscle contractions that The oral cavity is a complex organ that comprises muscle, glands, addition, several recent articles have reviewed oral pain17,18 and teeth, and specialized sensory receptors. For most animals, the taste dysfunction.19 orosensory and oromotor apparatus is critical for successful defense, This chapter provides a concise overview of orosensory and reproduction, exploration, nutrition, and vocalization.1 In humans, oromotor function. A brief synopsis of orosensory function describes vocalization has evolved into complex speech production, but other the innervation and sensitivity of the oral cavity and a summary human behaviors depend less on the mouth and tongue than on the of central pathways; a section on sensorimotor function includes eye and hand. In all animals, however, the mouth is essential for the a discussion of masticatory, lingual, and autonomic reflexes followed ingestion of nutrients. The incorporation of nutrients by mastica- by a discussion of mastication and the oral phase of deglutition. tion and drinking involves a high degree of coordination within The sense of taste is treated separately. and among different oral motor systems. Chewing requires the reciprocal activation of antagonist trigeminal muscles to open and close the jaws and the tongue to position food between the teeth. SENSORY FUNCTION A diverse array of highly specialized sensory systems guides these complex oromotor responses and initiates secretion of digestive Oral Somesthesia enzymes. Mechanoreceptors in the tongue, palate, and periodontal Somatosensory innervation of the oral cavity is provided by the ligament (PDL) all contribute to a three-dimensional stereognostic maxillary and mandibular branches of the trigeminal nerve and perception of the oral cavity.2 The sense of taste serves in both food by the glossopharyngeal nerve. selection and protection from ingesting potentially toxic substances. Recent reviews provide comprehensive coverage of specific Mandibular nerve: oral mucosa of the cheek, anterior two thirds aspects of oral function; these include mastication,3–8 swallow- of the tongue, mandibular dentition, PDL, gingiva, and anterior ing,9,10 oral mechanoreception,11,12 and the sense of taste.13–16 In mandibular vestibule. 1213 Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados.  CHAPTER 86 Physiology of the Oral Cavity1213.e1 Abstract 86 This chapter examines the functions and mechanisms of the oral cavity, a complex organ that comprises muscle, glands, teeth, and specialized sensory receptors. It provides a brief synopsis of orosensory function by describing the somatosensory innervation of the oral cavity by the lower cranial nerves followed by the organization of central sensory pathways. It also provides a basis to understand specific patterns of sensitivity achieved through specialized oral tissues including the lips, teeth, periodontal liga- ment, tongue, and palate. A section on oromotor function is also presented, which includes a discussion of mastication and the oral components of deglutition and respiration. This section also explains masticatory, lingual, and autonomic reflexes in addition to the complex interplay and coordinated activity of masticatory, lingual, facial, and infrahyoid muscles to accomplish chewing, swallowing, and respiration. The sense of taste is treated separately, and gusta- tory sensitivity is thus distinguished from chemesthesis. The chapter affords the reader a vivid description of gustatory structures with a focus on taste receptor cells followed by a discussion about gustatory physiology, transduction, and central taste pathways that are fundamental in realizing the special sensation of taste. Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados. 1214 PART VI Head and Neck Surgery and Oncology Maxillary nerve: hard and soft palates, the oral mucosa of (Fig. 86.2). On the basis of their small receptive fields and low the maxillary vestibule, and the maxillary dentition, gingiva, thresholds, lingual fibers could be divided into: and PDL. 1. Fibers innervating superficial (mucosal) surface of the tongue: the Glossopharyngeal nerve: back of the tongue and oropharynx. majority of the superficial fibers are rapidly adapting—a characteristic they have in common with other highly sensitive Although the entire oral cavity is densely innervated with sensory structures used in exploratory activity (e.g., the hand) fibers, considerable evidence indicates that the innervation is not 2. Fibers innervating deeper muscle tissue of the tongue: the deeper uniform. Specialized oral tissues—including the lips, teeth, PDL, receptors were all slowly adapting, and these deeper receptors tongue, and palate—each display specific patterns of sensitivity. provide information about the position of the tongue. Although specific parts of the oral cavity rival the hand in terms of absolute psychophysical thresholds for tactile and thermal Trigeminal nerve endings that mediate somesthetic and thermal sensitivity, the structure and function of somesthetic correlations sensitivity of the tongue and palate can be any of the following: so painstakingly deduced for the hand have little predictive veracity a. Free nerve endings in the mouth. b. An intermediate group of “semiorganized” endings25 Overall, the anterior oral cavity displays greater tactile sensitivity c. More highly organized endings variously referred to as Krause than does the posterior oral structure.1,20 The tip of the tongue end bulbs,26 mucocutaneous end organs,25 or coiled terminations.27 is particularly sensitive, with a discriminative capability equivalent to that of the digits (Fig. 86.1). The midline of the palate and All investigators agree that no Pacinian corpuscles exist in the tongue are more sensitive than lateral regions, and a similar pattern oral mucosa. Based on ultrastructural criteria, Munger28 referred of sensitivity applies to the teeth.21 Adults with complete dentition to many highly organized oral mucosa endings as Meissner corpuscles, could detect a 1-g von Frey hair applied to the anterior (midline) similar to those found in glabrous skin of the hand. However, teeth, but they would require nearly 10 g to detect stimulation despite all this variation in nomenclature, many of the illustrations of the first molar. The sensitivity to warm and cold stimuli also of the specialized endings are quite similar and show “finely wound varies widely across oral tissues. Sensitivity to warm stimuli is nonmyelinated fibers” without a clearly defined capsule.25 Ultra- relatively high on the tip of tongue but not particularly so on structural studies further reveal that some of these organized either the palate or buccolabial surfaces.22,23 In contrast, the sensitiv- endings in the palate, but not in the lingual epithelium, send ity to a cool stimulus is less differentiated within the oral cavity, axonal processes into the overlying epithelial pegs and are associated and the sensitivity of the tongue tip, palate, and buccolabial surfaces with Merkel cells.28,29 In the hand, Merkel cells are correlated is essentially equal. In general, the sensitivity to cool stimuli is physiologically with slowly adapting mechanoreceptors; however, greater compared with warm stimuli. a similar correlation has not been made in the palate, and their Recording from single human lingual fibers that innervate the apparent absence in the lingual epithelium does not preclude slowly anterior tongue24 confirms the small receptive fields and high adapting mechanoreceptors in this structure (see Fig. 86.2). Thus, sensitivity to low-threshold forces perceived psychophysically unlike the hand, a correlation between the morphology of oral receptor endings and their response properties as rapidly or slowly adapting has not been demonstrated. Mechanoreceptors in the PDL have been studied in some detail.30,31,276 In addition to detecting forces directed against the 11.7 5 5 11.1 Receptive field (mm2) 5-9 5-9 12 7.8 23.4 RA 24.4 SA irregular 6 SA regular 0 RA SA irr SA reg B Indentation threshold (mN) 1 40-100 164.2 272.8 0 116.4 RA SA irr SA reg A C 11.5 Fig. 86.2 Receptive field properties of superficial 14.9 mechanoreceptive afferents recorded from the human lingual 8.9 nerve. (A) Size and location of receptive fields of three types of mechanoreceptors: rapidly adapting (RA), slowly adapting regular (SA reg), and slowly adapting irregular (SA irr). (B) Receptive field area. Fig. 86.1 Spatial discrimination of tactile detection thresholds (C) Receptive field threshold. Small squares indicate corresponding from a number of studies. Numbers represent mean threshold in data from human median nerve. Vertical bars indicate standard error. millimeters. (From Rath EM, Essick GK: Perioral somesthetic (From Trulsson M, Essick GK: Low-threshold mechanoreceptive sensibility: do the skin of the lower face and the midface exhibit afferents in the human lingual nerve, J Neurophysiol 77:737–748, comparable sensitivity? J Oral Maxillofac Surg 48:1181–1190, 1990.) 1997.) Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados.  CHAPTER 86 Physiology of the Oral Cavity 1215 teeth, PDL receptors initiate oral reflexes of jaw opening and The central termination of these mesencephalic force salivation and, together with receptors in the temporomandibular detectors includes inhibitory connections to trigeminal 86 joint (TMJ), contribute to interdental discrimination and oral jaw-closer motoneurons via the supratrigeminal area.39 stereognosis.8,30 As many as six varieties of receptor morphology Thus these receptors serve a protective role in preventing are found in the PDL, ranging from complex branched endings potentially damaging tooth contact during mastication. to free nerve endings.32,33 The cell bodies for PDL receptors are located peripherally in the trigeminal ganglion and centrally in Although mechanoreceptors in the PDL are not encapsulated, the mesencephalic trigeminal nucleus.34 Mesencephalic trigeminal their response characteristics are influenced by the elastic properties innervation of the PDL is primarily in the apical region near the of the ligament. When the attachment of the ligament is com- root and consists mostly of small, myelinated, Ruffini-like endings.35 promised, such as during periodontitis that loosens the connective Trigeminal ganglion innervation extends from the apical region attachments of the ligament, a corresponding loss in interdental to the more superficial region and includes small unmyelinated force discrimination is observed.40 Periodontal receptors also nerve endings. contribute to the regulation of bite force. Individuals with dentures Both rapidly and slowly adapting mechanoreceptors are found cannot bite as hard as normal dentulous subjects and cannot perceive in the PDL, and it is likely that the location of the receptor in variations in their own bite force.2,41 Similar results were obtained the ligament determines its response characteristic. Because the by anesthetizing the inferior alveolar nerve.42 In contrast, anesthetiz- tooth rotates about its fulcrum, forces directed laterally to the ing the TMJ does not affect bite-force discrimination, but it does crown will translate to greater stretch at the root of the tooth impair jaw-positioning performance. Thus different populations compared with the fulcrum. Thus it is perhaps not surprising that of oral receptors may regulate sensation of jaw position and control lower-threshold fibers are found near the root and that they tend of bite force during mastication. to be slowly adapting compared with receptors located near the fulcrum.36 In addition, individual Ruffini endings are not uniformly distributed around the tooth, and thus they display directional Common Chemical Sense sensitivity to the force required to activate them. Recordings from Stimulation of the oral cavity with high concentrations of salts, human nerves (microneurography) demonstrate the directional acids, alkaloids, and other compounds elicits intense taste sensations sensitivity of PDL receptors37 (Fig. 86.3) and further indicate but also evokes nontaste sensations that range from stinging and mechanical coupling between the teeth. Single fibers respond to burning to warm, cool, and painful. This sensitivity of the oral stimulation of multiple (adjacent) teeth; however, no anatomic cavity, mediated by nonspecialized free nerve endings and shared evidence suggests that individual fibers innervate multiple teeth.38 by all mucosal membranes, is referred to as the common chemical The PDL is innervated by two different receptors that have sense, or chemesthesis; this should not be confused with taste. Although functional significance: free nerve endings respond to many traditional gustatory stimuli, they typically display a much lower sensitivity. Electrophysiologic A. The Trigeminal Ganglion Receptors: recordings from the lingual nerve, for example, indicate that single Include slowly adapting mechanoreceptors (position detec- fibers require concentrations of sodium chloride (NaCl) a thousand tors) and high-threshold C fibers (nociceptors) in addition times higher than those necessary to elicit a response from a to rapidly adapting mechanoreceptors. gustatory fiber in the chorda tympani nerve.43 However, much Because these periodontal receptors from the trigeminal lower concentrations of other types of chemical stimuli, such as ganglion terminate centrally in the sensory trigeminal menthol,10 are adequate to elicit a response in trigeminal nerve complex, the source for the ascending sensory pathway to fibers. The types of chemical stimuli that elicit low-threshold the thalamus and cortex, they provide information about responses in trigeminal fibers suggest that one function of the tooth displacement and dental pain to the forebrain. common chemical sense is to protect the oral cavity. Responses B. The Mesencephalic Trigeminal Nucleus Receptors: to common chemical stimuli include reflex salivation and coughing Are primarily medium and rapidly adapting receptor types, that function to diffuse and remove offending stimuli from the and many have directional sensitivity. mouth. The common chemical sense is not purely protective, however. Spices such as horseradish, ginger, and red pepper are effective stimuli for trigeminal afferent fibers and contribute to the flavor of food. In 2001, one of the receptors for chemesthetic Li La stimulation was cloned.44 A member of the transient receptor potential (TRP) family of G-protein–coupled receptors, the vanilloid receptor termed VR1 responds to both noxious heat and low concentrations of protons in addition to vanilloid compounds Me Di such as capsaicin, found in chili peppers. Stimulation of this receptor results in the opening of a cation channel and thus depolarizes the afferent fiber. 250 mN Do Up Dental Pain People usually describe dental pain as either dull and burning or sharp.45 Sensations of dull burning pain are associated with stimula- tion of C fibers that terminate in the pulp chamber, whereas sharp, 1s “bright” dental pain is associated with A delta fiber innervation Fig. 86.3 Responses of a single human periodontal afferent fiber to a that extends a short distance into the dentinal tubules matrix force (∼250 mN) from various directions. The cell responded best to a interposed between the pulp chamber and the enamel covering force from the distal (Di) direction. Directions: lingual (Li), labial (La), of the tooth (Table 86.1).46,277 Unmyelinated C fibers constitute mesial (Me), downward (Do), upward (Up). (From Trulsson M, the majority of pulpal innervation (50% to 75%); however, endings Johansson RS: Orofacial mechanoreceptors in humans: encoding within the pulp chamber may be unmyelinated terminals of A characteristics and responses during natural orofacial behaviors, delta (myelinated) afferent fibers. Polymodal C fibers that innervate Behav Brain Res 135:27–33, 2002.) the pulp chamber respond to thermal stimuli and, in particular, Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados. 1216 PART VI Head and Neck Surgery and Oncology TABLE 86.1 Comparison of C-Fibers and A-delta Fibers the exposed dentinal tubules and the air pressure stimulus. When the smear layer was intact and covered the dentinal tubules, it C-Fibers took more air pressure to induce the perception of sharp pain 1 Mediate dull burning pain than when the smear layer was dissolved. 2 Unmyelinated fibers constitute the majority of pulpal innervation (50%–75%) 3 Terminate in the pulp chamber Central Projections of the Trigeminal System 4 Respond to thermal stimuli and to inflammatory mediators (e.g., histamine, bradykinin) Afferent fibers of the trigeminal nerve enter the brainstem in the 5 Contain and release neuropeptides upon activation (e.g., pons, bifurcate, and either terminate in the principal sensory nucleus substance P, calcitonin gene-related peptide), which augments or descend to terminate in the spinal trigeminal complex in the pain and may reduce inflammation and promote recovery medulla. The bifurcation of the trigeminal nerve at the level of 6 Peripheral sensitization: the release of neuropeptides produces the pons reflects a tendency toward a segregation of function.53 local vasodilation increasing the pressure within the pulp, In general, low-threshold mechanoreceptors predominate in the which further augments C fiber activation principal trigeminal sensory nucleus, indicative of a tactile dis- A-delta Fibers criminative function. In contrast, considerable evidence implicates the subnucleus caudalis in orofacial pain mechanisms, and many 1 Mediate sharp, “bright” dental pain 2 Myelinated fibers neurons in the subnucleus caudalis respond to noxious stimuli 3 Extend 0.2–0.3 mm into the dentinal tubules that encase the pulp applied to the head and neck.17 These neurons include those chamber specifically activated by noxious stimuli (nociceptive-specific 4 Respond to heat, mechanical, and osmotic stimuli applied to the neurons) and wide-dynamic-range neurons responsive to both distal end of the dentinal tubules low- and high-intensity stimulation. 5 Supports the “hydrodynamic” theory of dental pain, which offers Because the receptive fields for many nociceptive neurons in an explanation of dental hypersensitivity the subnucleus caudalis are large and include responses to nocicep- 6 Hydrodynamic theory: dentinal tubules are filled with a fluid, the tive stimuli applied to the masticatory muscles, tooth pulp, and fluid transmits mechanical, thermal, and osmotic stimuli to the TMJ, a role for these neurons in referred pain has been suggested.54 proximal end of the dentinal tubules, where the nerve endings are located generating a sharp pain stimulus Anatomic studies confirm that afferent fibers that innervate the oral cavity, tooth pulp, oropharynx, TMJ, masticatory muscles, and superficial skin all converge in the subnucleus caudalis.55,56 In many patients, lesions in mandibular teeth have a high likelihood to inflammatory mediators that include histamine and bradykinin, of producing referred pain to the maxillary region, cheek, and ear endogenous factors associated with pulp pathology. C fibers that in addition to the mandible itself.57 Likewise, lesions in the maxillary innervate the pulp chamber contain neuropeptides such as substance teeth are often referred to the mandible and to the maxilla, temple, P and calcitonin gene-related peptide.47 The peripheral release and orbital region. of these neuropeptides on C fiber activation produces local In addition to the subnucleus caudalis, other parts of the sensory vasodilation and thus increases the pressure within the rigid pulp trigeminal complex are also involved in trigeminal pain. Nociceptive chamber and further augments C fiber activation (i.e., peripheral responses have been obtained from extensive areas of the sensory sensitization). The release of substance P in infected teeth has trigeminal complex, and destruction of the subnucleus caudalis been directly measured in human patients using microdialysis, does not prevent all trigeminal pain function.54 Case studies of and patients with irreversible pulpitis had significantly higher levels patients who have undergone trigeminal tractotomy for intractable of substance P in the pulp chamber of infected teeth compared pain associated with cancer are completely analgesic on the face, with noninfected teeth.48 Although the release of neuropeptides but pulpal pain is intact.58 Likewise, when the principal trigeminal augments pain, evidence suggests that it may also reduce inflam- nucleus and subnucleus oralis were damaged after a stroke, oral mation and promote recovery. In experiments with animals, and perioral pain sensitivity was diminished, as was normal tactile elimination of the afferent terminal release of neuropeptides by sensitivity from these structures.59 denervation of the teeth reduced wound healing after lesions were Neurons in both the rostral sensory trigeminal complex experimentally induced.49 (subnucleus oralis) and the subnucleus caudalis may also form a Sharp pain is mediated by A delta fibers that extend 0.2 to substrate for “central sensitization,” in which central neurons in 0.3 mm into the dentinal tubules that encase the pulp chamber.50 the pain pathway have their response characteristics magnified as These nerve fibers respond to heat, mechanical, and osmotic stimuli a result of peripheral stimulation.60 These changes can last a variable applied to the distal end of the dentinal tubules that become amount of time and potentially contribute to both short-term exposed to environmental stimuli when the enamel layer is hyperalgesia and long-term chronic pain. Fundamental to the breached.51 Because the dentinal tubules are filled with a fluid, concept of central sensitization is that some neurons initially the fluid transmits mechanical, thermal, and osmotic stimuli to responsive to only high-threshold (nociceptor) input become the proximal end of the dentinal tubules, where the nerve endings responsive to low-threshold, nonnociceptive input. The increased are located. This “hydrodynamic” theory of dental pain is supported responsiveness is thought to be mediated by A-β (nonnociceptive) by a growing body of anatomic, physiologic, and psychophysical input that becomes functionally only active after intense peripheral evidence and further offers an explanation of dental hypersensitivity. nociceptor input. One neural mechanism for the nascent response When the dentinal tubules are exposed by a cavity or other lesion, to nonnociceptive input has been studied in great detail. An intense patients report sharp pain in response to innocuous stimuli such afferent barrage of nociceptor input following peripheral tissue as mild temperature or osmotic stimuli (e.g., sweet compounds). damage or inflammation “sensitizes” a central neuron via structural However, the theory predicts that if the tubules are covered, thus modification of an N-methyl-D-aspartate (NMDA) glutamate limiting exposure to environmental stimuli, stimulated pain should receptor. NMDA receptors are voltage sensitive and will not pass be reduced. This had been experimentally assessed in human ions, even in the presence of a ligand, unless the cell is sufficiently volunteers, in whom a small cavity in a tooth scheduled for removal depolarized. However, the central release of a neuropeptide such was prepared, and a conical chamber was positioned over the as substance P by nociceptor afferents may provide sufficient cavity through which regulated air pressure could be delivered.52 depolarization to modify NMDA glutamate receptors via intracel- Creating a smear layer of amorphous tooth particles in the cavity lular signaling pathways, thereby allowing glutamate released by or dissolving it away with solvents controlled the interface between nonnociceptive (A-β) input to activate central neurons; this Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados.  CHAPTER 86 Physiology of the Oral Cavity 1217 activation thus provides a neural mechanism for allodynia, and coordination. Bolus formation during mastication requires the similar mechanisms have been demonstrated in the brainstem coordinated activity of masticatory, lingual, and facial muscles, 86 sensory trigeminal complex and may provide a substrate for chronic which are innervated by motoneuron groups highly segregated oral and facial pain.17 Experimental studies demonstrate that in the brainstem. Although the jaw and tongue can function neuropharmacologically blocking NMDA receptors prevents TMJ independently,69 often they appear inextricably “linked.”3,70 The and tooth pulp afferents from inducing hyperactivity in central nature of this linkage and whether it relies on interactions between trigeminal neurons (i.e., central sensitization).61,62 central pattern generators, reflex control, or peripheral mechanical Somatosensory information reaches the ventrobasal complex linkage represents a significant problem in oral motor control. of the thalamus from all major subdivisions of the trigeminal In addition to the complexity of coordination between functions sensory complex.17 Many cells in the ventrobasal complex respond and coordination between motor groups, another level of complexity to low-intensity stimulation, indicative of a tactile discriminatory in oral motor control can be found within the muscles themselves. function; however, other neurons require high-intensity stimulation. Individual masticatory and lingual muscles are not homogeneously The small receptive fields of both types of neurons suggest a role functioning units; muscles are often composed of multiple compart- in localization. Other nuclei, including the posterior thalamic ments, with muscle fibers oriented in multiple directions, and thus nuclei and the nucleus submedius, respond preferentially to high- different parts of a muscle can be more or less active during a intensity stimulation and may be involved in affective components given behavior.71 Further adding to the complexity of oral mus- of pain.63 Both nociceptive and nonnociceptive trigeminally culature are the multiple isoforms of myosin heavy chain (MHC) activated neurons from the thalamus project to the somatosensory proteins that form the contractile elements of the muscle fibers. cortex. Electrophysiologic mapping studies in primates indicate The differential distribution of MHCs within different muscles a complex, sometimes discontinuous somatotopic map of the facial and muscle compartments imparts additional degrees of freedom and oral region.64 In general, the face is represented medially on to motor output. the cortical surface adjacent to the representation of the hand, A myriad of “simpler” oral reflexes serve protective functions with successively lateral representations of the teeth and tongue. and contribute to complex rhythmic output. Muscle spindles in Magnetic resonance imaging in humans confirms this somatotopic jaw-closer muscles, for example, may contribute to load regulation representation.65 during chewing, and oral reflexes may assist coordination between the jaw and tongue. Autonomic oral reflexes modulate salivation and initiate digestive processes. Several recent reviews of oral MOTOR FUNCTION reflex function are available.4,69,72 Oral motor functions include mastication, swallowing, respiration, and vocalization. This review will focus on mastication and the oral components of swallowing and respiration. One of the Muscles of Mastication and Reflex Control dominant concepts in oral motor physiology is central pattern The muscles of mastication can be divided into jaw openers and generation. Chewing, swallowing, and breathing are each produced jaw closers. However, human jaw movement is more complex, by brainstem central pattern generators that control the funda- even during stereotyped rhythmic mastication. During opening, mental rate and pattern of muscle contractions that define each the jaw translates forward; during closing, it translates backward.73,74 function. Although sensory pathways from the mouth play an A given muscle is not isomorphic with a single movement. The intimate role in oral motor function, fundamental to the concept masseter, temporalis, and medial pterygoid muscles and the superior of central pattern generation is that afferent activity is not necessary head of the lateral pterygoid muscle have major jaw-closing to evoke rhythmic activity, and it does not provide the critical (mandible elevation) functions, but contraction of the masseter timing information for coordinated motor output.66 Although and lateral pterygoid protrudes the mandible, whereas contraction organized in the brainstem, central pattern generators for chewing, of the temporalis muscle retracts the mandible. Contraction of swallowing, and respiration are influenced by descending inputs the anterior belly of the digastric opens and retrudes the jaw; from virtually all major regions of the neuraxis. Detailed reviews contraction of the inferior head of the lateral pterygoid lowers of oromotor central pattern generation can be found in works by and laterally directs the mandible. Contraction of the mylohyoid Nakamura and Katakura,67 Rekling and Feldman,68 and Jean.9 muscle also depresses the mandible, as does contraction of the Transection studies that relied on electrical stimulation to induce geniohyoid muscle, a muscle innervated by the hypoglossal nucleus fictive jaw movements have localized the central pattern generator (Table 86.2). for mastication to the medial core of the reticular formation. More Individual muscle fibers are physiologically classified as slow recent studies using reversible pharmacologic lesion techniques (S), fast fatigue-resistant (FR), or fast fatigable (FF) and correlate in awake, freely moving (feeding) animal preparations indicate to a high degree with specific isoforms of MHC contractile that a necessary substrate for rhythmic lingual/masticatory move- proteins.75 Thus S-fibers express the MHC-I isoform, FR units ments is in the lateral reticular formation in a region that overlaps express the MHC-IIA isoform, and FF units express the MHC-IIB substantial populations of preoromotor interneurons.122,123 This region of the brainstem reticular formation is also the target of descending projections from metabolic integrative substrates in the hypothalamus and from the motor cortex.67,124 TABLE 86.2 Functional Role of Muscles of Mastication in Fundamental to oral motor function is the complex interplay Mandibular Motion among behaviors that compete for the same muscles. Chewing, swallowing, and respiration all require the coordinated activity of Muscle Masticatory Motion of Mandible masticatory, lingual, facial, and infrahyoid muscles. Swallowing Masseter Elevation and protrusion and respiration further depend on pharyngeal and abdominal Temporalis Elevation and retraction muscles, and motor coordination takes place on multiple levels. Medial pterygoid Elevation At a behavioral or molar level, swallowing and respiration must Lateral pterygoid (superior head) Elevation and protrusion be coordinated to prevent aspiration of food into the airway. How Lateral pterygoid (inferior head) Depression and lateral this coordination is achieved is only beginning to be understood, displacement Anterior belly of digastric Depression and retraction but it likely involves both peripheral feedback and interactions Mylohyoid Depression among central pattern generators as explained later. However, Geniohyoid Depression individual oral motor functions also require a high level of Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados. 1218 PART VI Head and Neck Surgery and Oncology isoform. Individual fibers frequently contain more than one MHC isoform (i.e., hybrid isoforms),76 and overall, masticatory muscles contain a larger proportion of hybrid isoforms compared with MesV limb and trunk muscles.77 Moreover, masticatory muscles express mV MHC isoforms not found in the limb and trunk, specifically MHC-fetal and MHC-cardiac-α. Thus muscle fibers in jaw-closing + + muscles (masseter, temporalis, and pterygoid) include numerous + C O – hybrids that express MHC-I combined with MHC-fetal and MHC-cardiac-α. Muscle fibers in jaw-opening muscles differ from those of their jaw-closing counterparts.77 Overall, jaw-opening + CNS muscles have fewer hybrid fibers and a different distribution and relative weighting of the constitutive isoforms. In the anterior PNS and posterior bellies of the digastric, mylohyoid, and geniohyoid muscles, MHC-I, MHC-fetal, and MHC-cardiac-α were more scarce compared with jaw-closing muscles, but more MHC-IIA was present. The presence of both MHC-fetal and MHC-cardiac-α isoforms may reflect developmental factors, but functionally, hybrid + isoforms confer intermediate contraction speeds and thus provide greater flexibility in motor output.75 Differences in MHC isoform reflect different motor demands + on jaw-opening and jaw-closing muscles. The preponderance of slow and hybrid fiber isoforms in jaw-closing muscles reflects a JC JO muscle that contracts slowly and requires flexibility against a load Fig. 86.4 Circuit diagram for jaw-closing and unloading reflexes. during mastication. In contrast, jaw opening is more ballistic and Mesencephalic neurons innervate muscle spindles in jaw-closer does not normally work against a load. Even the relative distribution neurons and monosynaptically excite jaw-closer motoneurons and of MHC isoforms in different muscle compartments reflects interneurons with inhibitory connections to jaw-opener motoneurons. functional specialization. Jaw-closing muscles that are particularly C, Closer motoneuron; CNS, central nervous system; JC, jaw-closer active during mastication (e.g., the anterior temporalis and the muscle; JO, jaw-opener muscle; MesV, mesencephalic trigeminal deep masseter muscles) have more MHC-I fibers than the posterior nucleus; mV, motor trigeminal nucleus; O, opener motoneuron; temporalis and the superficial masseter, which are less active. The PNS, peripheral nervous system. (From Orchardson R, Cadden SW: cross-sectional size of individual masticatory motor units also Mastication. In Linden RWA, editor: The scientific basis of eating, imparts additional degrees of freedom in muscle control. Individual Basel, Switzerland, 1998, Karger.) efferent axons from the motor trigeminal nucleus innervate a relatively small area of the target muscle, on the order of 5%, compared with innervation patterns of the limb and trunk, which are much greater.75 Small cross-sectional innervation patterns allow input during unloading, thereby allowing the background excitation specific areas of a muscle to be differentially controlled. to dominate jaw-opener motoneuron activity. Damaging occlusal Jaw-opener and jaw-closer muscles differ in their investment forces would thus be mitigated by a simultaneous lack of excitation with muscle spindles and hence differ in reflex function. Muscle to jaw closers and released excitation to jaw openers (see Fig. spindles, found only in jaw-closer muscles, are involved in multiple 86.4). The cell bodies for muscle spindle afferent fibers are located reflexes.4,5,78 A “jaw-jerk” reflex that elevates the mandible (jaw centrally in the mesencephalic trigeminal nucleus. Monosynaptic closing) can be elicited by a rapid depression of the mandible (i.e., projections to jaw-closer motoneurons are well characterized; tapping on the chin). This reflex, analogous to the patellar knee however, the location of inhibitory interneurons to jaw-opener reflex, is mediated by muscle spindle afferents that respond to the motoneurons is more speculative.79 rapid stretching of jaw-closing muscles and monosynaptically excite Jaw-opening muscles do not have muscle spindles; thus during jaw-closing motoneurons. During mastication, these muscle spindle the jaw-closing phase of mastication, the corresponding lengthening afferents may play an important role. During jaw closing against of the jaw-opener muscles does not itself provide an afferent signal a food bolus, resistance to the load results in intrafusal fibers in for a reciprocal reflex. However, stimulation of mechanoreceptors the spindle that are momentarily shorter than the extrafusal motor located in the PDL, tongue, and other soft tissues of the mouth fibers in which they are embedded. The consequent stretch of can initiate reflexive jaw opening, at least in many nonprimate the spindle afferent adds excitatory drive to closer motoneurons mammals.4,5,78 The jaw-opening reflex is, at a minimum, disynaptic and thus compensates for the load. This reflex action is termed through neurons in the trigeminal sensory complex, and it may the jaw-closing reflex.5 well involve additional interneurons (Fig. 86.5). Although the Muscle spindle afferents can also mediate a protective unloading reflex can be elicited by nonnoxious stimulation, it is generally reflex. When the jaws unexpectedly break through hard or brittle thought to serve a protective function by protecting soft tissues food, the rapid downward movement differentially shortens the (e.g., the tongue) against potentially damaging occlusal forces. extrafusal fibers, compared with the intrafusal fibers, and muscle The existence of a jaw-opening reflex in humans is still in doubt. spindle afferent activity is decreased, thus producing a “silent Although it cannot be as readily demonstrated with sensory stimuli period” in the jaw-closer muscle that limits excessive, potentially sufficient to produce it in experimental animals, robust electrical damaging forces directed against the teeth. These muscle spindle stimulation delivered to the upper lip produces electromyographic afferents can also indirectly potentiate jaw opening during the activity in the anterior digastric muscle of humans.79 It is of long unloading reflex through indirect, polysynaptic pathways. In latency, consistent with a polysynaptic substrate as suggested by addition to a monosynaptic excitatory synapse on jaw-closing animal studies. motoneurons, these afferents can inhibit jaw-opening motoneurons through an inhibitory interneuron (Fig. 86.4). Thus during the jaw-closing phase of mastication, if there is concurrent excitation Lingual Muscles and Reflexes to jaw-opener motoneurons from a central pattern generator, jaw The tongue is composed of both intrinsic and extrinsic muscles opening could be disinhibited by a lack of muscle spindle afferent innervated by the hypoglossal nerve.80 Extrinsic lingual muscles Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados.  CHAPTER 86 Physiology of the Oral Cavity 1219 role, either for the tongue during mastication or for the airway sV mV during swallowing. 86 To further compound the complexity of interpreting lingual + + reflexes, observations suggest that reflex excitation of the tongue rarely influences a single lingual muscle, and contraction of a CNS single lingual muscle can move the tongue in more than one plane.91 For example, although a primarily retrusive movement PNS of the tongue is produced by electrical stimulation of the lingual TG nerve, both protruder and retractor hypoglossal motoneurons are excited. Electrical stimulation of the glossopharyngeal nerve that innervates mechanoreceptors on the posterior aspect of the tongue and oropharynx also elicits tongue movement. Similar to the lingual nerve, stimulation of the glossopharyngeal nerve excites both protruder and retractor motoneurons, and the movement of the + tongue is primarily retrusive. The simultaneous activation of the glossopharyngeal nerve afferent fibers by electrical stimulation, however, may mask a more complex reflex organization. Lowe JO has suggested that stimulation of lingual receptors innervated by Fig. 86.5 Circuit diagram for jaw-opening reflex. Nociceptor and the glossopharyngeal nerve elicits a primarily retrusive movement nonnociceptor afferents in oral mucosa, muscle, and ligament of the tongue, in contrast to the lingual protrusion produced by terminate on interneurons in the sensory trigeminal complex to form a stimulating pharyngeal regions innervated by the glossopharyngeal dyssynaptic pathway to jaw-opener motor trigeminal neurons. CNS, nerve.69 Thus both lingual and glossopharyngeal reflexes may Central nervous system; JO, jaw-opener muscle; mV, motor trigeminal protect the tongue during the occlusal phase of mastication with nucleus; PNS, peripheral nervous system; sV, sensory trigeminal a retrusive movement. nuclei; TG, trigeminal ganglion. (From Orchardson R, Cadden SW: In contrast, electrical stimulation of the superior laryngeal nerve Mastication. In Linden RWA, editor: The scientific basis of eating, that innervates laryngeal mechanoreceptors depolarizes protruder Basel, Switzerland, 1998, Karger.) motoneurons and produces a protrusive action of the tongue. Thus mechanoreceptors in the oropharynx and larynx innervated by the superior laryngeal and the glossopharyngeal nerve may preserve airway patency during a swallow with a protrusive tongue include the major tongue protrudor muscle, or genioglossus muscle, movement. Lingual reflexes also play a protective role in respiration. the major tongue retractor muscles, or styloglossus and hyoglossus When normal respiration is suppressed by hypoxia, a normal muscles, and the palatoglossus muscle. Intrinsic lingual muscles breathing pattern, eupnea, is replaced by gasps.91,92 Gasps are consist of the vertical, transverse, superior, and inferior longitudinal associated with the coactivation of lingual protruder and retractor muscles. The geniohyoid often functions with the lingual muscles muscles that enlarge the upper airway.93–95 during tongue protrusion,81,82 and most lingual movements involve both extrinsic and intrinsic muscles. The hydrostatic model of lingual function, in which the tongue is modeled as a closed bag, Jaw-Tongue Reflexes postulates that during tongue protrusion by contraction of the Oromotor reflexes can involve multiple motor systems. Electrical extrinsic genioglossus and geniohyoid muscles, the tongue is further stimulation of either the masseteric or anterior digastric nerves, lengthened by the simultaneous contraction of the intrinsic vertical for example, suppresses genioglossus activity, which suggests and horizontal intrinsic muscles.83 Likewise, shortening of the that proprioceptive or nociceptive signals from the trigeminal tongue during retraction is augmented by contraction of the musculature inhibit lingual protrusion.96 In contrast, passive longitudinal muscles together with the extrinsic hyoglossus and depression of the mandible in cats has been shown to excite the styloglossus muscles. Coactivation of different combinations of genioglossus muscle, which suggests that lingual protrusion may intrinsic muscles can curl or deviate the tongue. be reflexively assisted during jaw opening, when the tongue is not Expression of different MHC isoforms varies across different subject to occlusal force.97 Further evidence that masticatory muscle human lingual muscles.77,84,85 Intrinsic muscles of the anterior proprioceptive afferents influence hypoglossal motoneuron activity tongue have a large proportion of type MHC-IIA fast fibers in comes from experimental lesions of the Probst tract. Ishiwata and contrast to the posterior tongue, in which MHC-I (slow) and colleagues98 showed that such lesions, which destroy descending hybrid MHCs predominate. As with other suprahyoid muscles, mesencephalic projections, suppressed hypoglossal activity induced the geniohyoid has a large proportion of MHC-I fibers. The by passive jaw opening but left intact hypoglossal activity induced distribution of type II (fast) fibers in the anterior tongue is consistent by stimulating a cutaneous oral sensory nerve. A jaw-tongue with a role in fast, flexible movements compared with posterior reflex in humans may also be mediated by masticatory muscle tongue activity. proprioceptive afferents.99 Stimulation of the hypoglossal nerve, Although lingual muscles contain muscle spindles,69,86,87 it is which contains some afferent fibers, inhibited the masseteric unclear whether there are any monosynaptic inputs from sensory (jaw-closing) reflex.89 afferents onto hypoglossal motoneurons.80 Rather, muscle spindle afferents travel in the ansa cervicalis and hypoglossal nerve and terminate in either the sensory trigeminal complex or the nucleus Autonomic Reflexes of the solitary tract (NST). Electrical stimulation of the hypo- In addition to somatomotor reflexes, stimulation of the oral cavity glossal nerve elicits synaptic responses not only in hypoglossal elicits numerous autonomic responses. Gustatory and mechanical motoneurons but also in facial88 and trigeminal motoneurons.89 stimuli are highly effective in eliciting the flow of saliva during Lingual reflexes can also be elicited by stimulation of virtually any mastication.100 The stimulation of receptors in the PDL may be of the afferent nerves that innervate the oral cavity. Depending one source for reflex salivation. In both rabbits and humans, a on the site of stimulation, either a protrusive or retractive move- high correlation was found between parotid flow and mandibular ment of the tongue is produced. An overview by Lowe90 on the movement, especially on the working, ipsilateral side. In humans, clinical significance of lingual reflexes emphasizes a protective selective anesthetization of the nerves that innervate the PDL Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados. 1220 PART VI Head and Neck Surgery and Oncology significantly reduced the amount of saliva elicited from crushing a food stimulus held between the teeth.101 Appetitive Consummatory Both location and stimulus modality influence the release of saliva.100,102 Stimulating the anterior part of the tongue is most effective for evoking salivation from the sublingual and subman- dibular glands, but posterior tongue stimulation is more effective Ingestion T1 Mastication T2 Deglutition for producing parotid gland flow. Aversive gustatory stimuli, such as from sour acids or bitter quinine hydrochloride (QHCl), are more effective for eliciting saliva than is stimulation with weak salt or sucrose solutions. In animal experiments, sweet stimuli FC SC SO FO Oral Phar Esop were the most effective stimuli for the release of the enzyme amylase from the parotid gland.103 Fig. 86.6 Schematic representation of different stages and substages Mechanical and chemical stimulation of the oral cavity also of ingestive sequence. Esop, Esophageal; FC, fast closure; FO, fast initiates the release of digestive enzymes. These cephalic-phase open; SC, slow closure; Phar, pharyngeal; SO, slow open; T1 and T2, responses include the release of gastric acid, insulin, glucagon, first and second intraoral transfer. and pancreatic polypeptide.104,105 An increase in gastric motility and emptying of the gallbladder also occurs. Although cephalic- phase insulin release (CPIR) is a highly variable response that of each stage of feeding is variable and highly species specific and does not occur in all individuals, insulin levels rise, on average, dependent on what is being ingested. Fluid consumption does 25% above baseline within 2 minutes of oral stimulation. This not require mechanical breakdown by mastication and thus has release is neurally mediated, and it does not occur in the absence only three stages. In humans, drinking uses the same muscles as of an intact vagus nerve.106 In animal studies, sweet stimuli— mastication, but the coupling among the facial, trigeminal, and particularly glucose—are most effective in triggering CPIR107,108 lingual muscles is different. The orbicularis oris muscle contracts but are somewhat less effective in humans.105 Rather, “palatable” to form a tight seal during human drinking and sucking but relaxes stimuli appear more effective in general. Although CPIR accounts during mastication. for perhaps only 1% of the total insulin release associated with a The movements of mastication can be further subdivided. meal, this amount underestimates its potential importance in glucose Kinematic measurements during mastication indicate that rhythmic metabolism. Experimental studies in which CPIR is bypassed by masticatory movements of solid food typically involve several intragastric infusions show that it can result in both hyperinsu- distinct components.113,114 Beginning the masticatory cycle with linemia and hyperglycemia.109,110 One possible mechanism for this an open mandible, the jaw closes rapidly and then slows down. is that vagally mediated insulin release acts as a signal on hepatic The transition from fast closure to slow closure occurs when the receptors to further regulate glucose metabolism; that is, it is teeth make contact with solid food, and it is thought to involve acting as a signal to initiate metabolic events rather than simply sensory feedback from the PDL. More detailed analysis of the to convert glucose.105 Other cephalic-phase responses represent opening phase of mastication indicates additional complexity. After larger fractions of total meal responses. In humans, cephalic-phase the slow-closure phase, when the teeth make maximal intercuspa- gastric acid secretion can reach 50% of total meal release, antral tion, the masticatory cycle continues with a slow-opening phase motor activity can reach 70% of that achieved during a meal, and followed by a fast-opening phase. gallbladder emptying can reach 50% of total meal response.104 Feeding sequences analyzed using combined sirognathography Although cognitive factors and other sensory stimuli, such as and electromyography or videofluorography present a modified sight and sound, can elicit cephalic-phase responses, oral stimuli picture of the human ingestive sequence.115,116 Human jaw move- are usually the most effective. Oral stimuli signals carried by the ments associated with eating natural foods do not show obvious gustatory and trigeminal nerves influence preganglionic parasym- changes in the rate of opening and closing during rhythmic pathetic vagal neurons located in the dorsal motor nucleus of the mastication. Thus the stages of fast closing, slow closing, and slow vagus.111 Oropharyngeal receptors innervated by the superior opening were not evident as in animal studies. Nevertheless, the laryngeal nerve may also influence digestive functions.111 Diuresis ingestive sequence could be divided into three stages, beginning is increased in response to drinking a saline solution compared with biting and transport of the bolus to the molars, chewing, and with the intragastric infusion of the same volume of fluid.112 “clearance” (swallowing). A distinct stage from chewing to bolus formation for swallowing was not obvious, and swallowing occurred during mastication and as a terminal event. Mastication Although mastication involves coordinated activity of the jaws, The orosensory apparatus of the mouth and perioral region is an hyoid apparatus, and tongue,3 the majority of electromyographic integral part of the regulation of food and fluid intake. In general, studies of mastication have focused on the jaw musculature. Jaw the sensory receptors in the mouth are specialized for ingestion, opening during mastication is associated with activity in the anterior and they play an important role in the sensory evaluation of food digastric muscles and the inferior head of the lateral pterygoid and in the sensory control of mastication and deglutition. muscle.114,117 The closing phase of mastication begins with contrac- Food consumption through the oral cavity can be character- tion of the masseter muscle, followed by the temporalis and medial ized as a series of stages or phases (Fig. 86.6). Different stages of pterygoid muscles and superior head of the pterygoid muscle, ingestion have been defined by placing small metal markers in which are recruited during the power stroke (slow closure). Food the jaws, hyoid, and tongue. These markers can be detected with is typically chewed unilaterally. Although the trigeminal musculature high-speed cine-fluorographic techniques that allow the move- is bilaterally activated during mastication, the ipsilateral (working) ments of the internal oral apparatus to be monitored during the side is active earlier. entire ingestive sequence of the awake preparation. The division Food consistency is one factor that affects the masticatory of feeding into five dynamic stages by Hiiemae and Crompton3 is rhythm. In a study of the effects of hardness on chewing, Plesh indicated on the second tier of Fig. 86.6. The first stage of putting and colleagues118 observed that most subjects chewed hard gum food into the mouth (ingestion) is followed by intraoral transport at a slightly slower rate than soft gum. The decreased frequency and the positioning of food between the molars (second stage) for of chewing was associated with significantly longer opening and mastication (third stage). Intraoral transport to the back of the occlusal phases of chewing, rather than with the closing phase, tongue (fourth stage) initiates deglutition (fifth stage). The duration despite the significantly greater electromyographic activity in the Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados.  CHAPTER 86 Physiology of the Oral Cavity 1221 masseter muscle. Age is another factor that affects the masticatory geniohyoid, and mylohyoid) during swallows of solid food in rhythm.119 Older subjects chewed at the same frequency as younger contrast to fluid swallows.126 Similarly, individual variation was 86 subjects (∼1.4 Hz), but the structure of the rhythm was different. observed in the activation sequence of the suprahyoid muscles The older subjects opened and closed their mouths at a slower and the genioglossus muscle during voluntary swallows in humans.127 velocity but achieved the same overall chewing rate by not opening In summary, the overall movement of a bolus from the dorsal their mouths as far. Movement irregularities during chewing were surface of the posterior tongue to the pharynx characterizes the also observed during the jaw-opening and jaw-closing phases of oral phase of swallowing. The precise motor sequence of individual mastication in patients diagnosed with temporomandibular pain.120 muscles during the oral phase of deglutition can vary, depending Unlike the smooth, uninterrupted alteration between opening on both the individual and sensory characteristics of the bolus. and closing seen in healthy persons, patients with temporoman- Contact of the bolus with sensory receptors in the oropharynx dibular pain frequently started reopening their mouths during triggers peristaltic contractions of the pharyngeal musculature. the closing phase of mastication or reclosed them during the Like mastication, swallowing can be evoked from electrical opening phase. stimulation of central structures in the absence of peripheral As denoted earlier, experimental studies indicate that the (muscular) feedback and is thus thought to be controlled by a masticatory rhythm is centrally programmed; that is, a peripheral central pattern generator.9,10 The location of the central pattern stimulus is not necessary to initiate the masticatory rhythm, nor generator for swallowing involves the caudal region of the NST is feedback from the active muscles necessary to sustain the and the medullary reticular formation adjacent to the nucleus response.117 Fictive mastication evoked by central stimulation ambiguus. Cortical pathways that reach these medullary regions in a paralyzed experimental animal preparation indicates that through descending pathways mediate voluntary swallowing. neither the afferent limb of the jaw-opening reflex nor that of Swallowing takes precedence over both respiration and mastica- the jaw-closing reflex is necessary to generate the masticatory tion, causing a brief disruption of rhythm. Respiratory apnea rhythm. Thus the alternating activation of a jaw-opening reflex associated with swallowing shows considerable variation across followed by a jaw-closing reflex does not explain the origins of the individuals, and in one study it ranged from 0.61 to 3.83 seconds.128 masticatory rhythm. For some individuals, the apneic period increases with the volume Nevertheless, both the jaw-opening and jaw-closing reflexes swallowed; for others it decreases. Spontaneous swallows tend to are functionally entwined in rhythmic oral behavior, and the be associated with a shorter apneic period.129 Swallows do not excitability of these reflexes varies as a function of jaw position occur randomly throughout respiration. Rather the majority of during rhythmic opening and closing.121 In general, the jaw-opening swallows occur during expiration or late inspiration and subse- reflex is attenuated during rhythmic masticatory movements quently reset the respiratory rhythm (i.e., no modification of the compared with a stationary mandible. In particular, low-threshold postswallow rhythm compensates for the swallow-induced apnea).129 mechanical stimuli are less effective than high-threshold stimuli Sensory information from the oropharynx could reach the central in producing a jaw-opening reflex when applied during rhythmic pattern generator for respiration via the NST to mediate adapt- masticatory movements. Thus during the occlusal phase of mastica- ability of the respiratory rhythm to bolus size.128 Alternatively, tion, a protective jaw-opening reflex can be initiated in the presence oropharyngeal afferents could influence the central deglutition of unexpected mechanical forces directed against the teeth or soft substrate to modify respiration129; that is, there could be an interac- tissues, but innocuous mechanical stimulation associated with tion between central pattern generators for respiration and chewing will not interrupt the masticatory rhythm. swallowing.130 Furthermore, mastication has been shown to increase Although the basic neural circuitry necessary for the rhythmic respiratory rate and at the same time decrease inspiratory and alternating contraction of jaw-opening and jaw-closing muscles expiratory time.132 In addition to possible metabolic demands of does not require sensory input, intraoral sensory receptors are mastication on respiration (i.e., physical exertion), mastication can critical for regulating bite force during mastication. Efficient eating increase upper airway resistance by more closely apposing the requires that food be reduced in size for swallowing. This requires tongue with the palate. In humans, there is a small but significant determining both the hardness and size of the food and correctly tendency for inspiration to occur during masticatory jaw opening.131 positioning food between the occlusal surfaces of teeth. Psycho- Although swallowing affects the masticatory and licking rhythm physical studies in humans indicate that receptors in both the in animals, it has only a minimal effect on human mastication.131,132 PDL and the TMJ contribute to the interdental discrimination Swallows most often occur during the early jaw-opening phase required during eating.2 The loss of PDL receptors associated of mastication and significantly prolong the masticatory cycle. In with complete dentures results in impaired interdental discrimina- rodents, the prolonged lick cycle associated with a swallow equals tion, as does anesthetization of the dentition in individuals with the increased duration of tongue protrudor and retractor muscle natural teeth. Receptors in the TMJ also contribute to size dis- contractions; that is, the increased cycle duration was used to crimination in the mouth. When the TMJ is anesthetized, accommodate the participation of the tongue in swallowing.82 interdental discrimination decreases. SPECIALIZED SENSORY SYSTEMS: TASTE Oral Phase of Deglutition After mastication and the intraoral transport of food to the back Gustatory Sensitivity of the tongue, deglutition is initiated. The oral phase of deglutition In contrast to the common chemical sense, taste sensations are consists of an upward movement of the tongue against the soft evoked by relatively low concentrations of chemical stimuli when palate to force the bolus in the direction of the pharynx.9,10 The applied to the specialized gustatory receptor cells. Most investigators precise nature of the stimulus that triggers the pharyngeal stage agree that there are a discrete number of taste sensations; the of deglutition is unknown. Both the volume and the rate of bolus most common and easily recognizable are sweet, salty, sour, and accumulation interact to trigger swallows in experimental animals.125 bitter. Some contend that there is a fifth taste, known as umami When the rate of licking (intraoral transport) increased in response (heavenly), associated with the taste of monosodium glutamate to increased stimulus delivery, the volume per swallow also (MSG).133 More recent research has suggested that there may also increased. Moreover, the physical nature of the bolus can influence be a sixth taste for fats, or lipids.278 The sensations of flavor while the sequence and recruitment of individual muscles involved in eating are more diverse than those of pure taste and result from the buccal phase of swallowing. In monkeys, the masseter muscle the interaction of taste with the smell and texture of food. The was recruited with the suprahyoid muscles (the anterior digastric, confusion between taste and flavor is well documented in taste Descargado para Diana Salgado ([email protected]) en Pontifical Catholic University of Ecuador de ClinicalKey.es por Elsevier en octubre 16, 2024. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2024. Elsevier Inc. Todos los derechos reservados. 1222 PART VI Head and Neck Surgery and Oncology and smell clinics. Self-reports of chemosensory dysfunction are are predominantly affected in the pathophysiology of black hairy highly unreliable; on testing, many individuals who report loss of tongue.282,283 The rest of the papillae are distributed in a specific taste are frequently found to have impaired olfactory function pattern over the tongue. A single row of circumvallate papilla is with no loss in taste sensitivity.134 located in the posterior oral tongue anterior to the sulcus terminalis. In addition to a sensory quality dimension with four distinct Fungiform papillae are present on the anterior tongue, and foliate tastes, taste stimuli can be categorized on a hedonic dimension papillae on the lateral sides of the tongue. with stimuli divided into those that are preferred and those that The innervation of all taste buds is via three cranial nerves (VII, are disliked, most commonly divided into a 9-point hedonic scale.279 IX, X). The chorda tympani branch of the facial nerve innervates The hedonic attribute of taste is concentration dependent and two to five taste buds on each of approximately 400 fungiform spans the different submodalities of sweet, sour, salty, and bitter. papillae on the anterior aspect of the tongue.139 Fungiform papillae Low and medium concentrations of salt are preferred, but salt density is greatest at the tip of the tongue and decreases along the becomes aversive at high concentrations. Although there is a strong dorsal and dorsolateral edges of the tongue. No fungiform papillae genetic component to the hedonic values associated with gustatory are found along the midline. Taste buds on the posterior aspect of stimuli, taste preferences are clearly modifiable by experience.135 the tongue are innervated by the glossopharyngeal nerve, and they Human neonates find bitter solutions strongly aversive, but adults are located either in tightly packed clusters distributed along the learn to enjoy coffee, alcohol, and other bitter-tasting substances. walls of the trenches surrounding 7 to 10 circumvallate papillae or The hedonic attributes of taste are also subject to metabolic state. in the inner folds of the foliate papillae located along the lateral edges of the posterior part of the tongue. Foliate papillae taste buds also receive innervation from the chorda tympani branch of Gustatory Structures the facial nerve.278,280,284 The 2400 taste buds in the circumvallate Gustatory receptor cells are taste receptors located within the papillae and the 1300 taste buds in the foliate papillae constitute taste buds and they are responsible for the transduction of taste the largest percentage in the human oral cavity. A third large stimuli. Approximately 7900 gustatory receptor cells in the human subpopulation of gustatory receptors located in the pharynx and mouth are grouped into distinct subpopulations defined by their larynx numbers approximately 2400 in humans. These taste buds are intraoral location, gross morphology, and innervation.136 Gustatory not associated with distinct papillae; however, the bud morphology subpopulations differ in sensitivity to chemical stimuli; however, is similar to that found on the tongue. Taste buds of the pharynx the overall morphology of the taste bud structure within each are innervated by the glossopharyngeal nerve, and the superior subpopulation is similar. Taste buds are mainly present in the laryngeal nerve branch of the vagus innervates those in the larynx. tongue and palate; however, to a lesser extent they are also present A smaller subpopulation of taste buds (∼400 in humans) is found in the epiglottis, pharynx, and larynx. There are between 2000 on the soft palate. These taste receptors, also not associated with and 5000 taste buds in the human oral cavity, distributed on the distinct papillae, are innervated by the greater superficial petrosal tongue and the palate.280 In the tongue, taste buds are contained nerve branch of the facial nerve.140 within the structure of the papillae. There are four different types Taste buds in the palate and tongue respond to sweet, salty, of tongue papillae: circumvallate, fungiform, foliate, and filiform sour, bitter, and umami, however, they differ in their sensitivities (Table 86.3). All except filiform papillae contain taste buds and to these different tastes.278,280 Each taste bud contains 50 to 150 are referred to as taste papillae, whereas filiform papillae contain neuroepithelial cells arranged in spindle-like clusters. Additionally, no taste buds and are referred to as a nongustatory papillae.281 the specific pattern of innervation of taste buds by a peripheral Filiform papillae are covered by partially keratinized stratified nerve has been characterized for the fungiform papillae on the epithelium and they are distributed over most of the oral dorsal front of the tongue. Single fibers of the chorda tympani nerve tongue surface. Their function is to increase friction between the synapse on multiple receptor cells within a single taste bud and tongue and food and move particles within the oral cavity. They on receptor cells in adjacent taste buds.141 There are four different types of cells in each taste bud.278,280 1. Type I glial-like supporting cells: they express Na+ channels and TABLE 86.3 Characteristic Features of Various Types of are thought to participate in sensing sodium (salt taste) Tongue Papillae 2. Type II gustatory receptor cells: they extend microvilli into a non-keratinized “pore” region on the apical surface of the bud. Tongue Papillae They respond to sweet, bitter, and umami via G-protein–coupled Circumvallate Single row of 7–10 papillae anterior to the sulcus receptors. They do not communicate with afferent gustatory terminals nerve fibers via synapses. They secrete ATP as a neurotransmitter, Taste buds innervated by the glossopharyngeal