5.4 Physiology of Mastication and Deglutition

4. Physiology of Mastication and Deglutition

Mastication (Chewing)

Mastication is a coordinated neuromuscular process that reduces food particle size, mixes food with saliva to form a coherent, lubricated bolus, and initiates chemical carbohydrate digestion.

• Neural Control: Rhythmic jaw movements are governed by a Central Pattern Generator (CPG) located in the pontine reticular formation near the motor nucleus of the trigeminal nerve (CN V). The CPG is continuously modulated by sensory feedback from:
  1. Periodontal Mechanoreceptors (PMRs): Ruffini-like endings located in the periodontal ligament that detect both the magnitude and three-dimensional direction of occlusal bite forces, triggering reflexive jaw unloading (jaw-opening reflex) to protect against tooth fracture upon striking unexpected hard objects.
  2. Muscle Spindles: Located within the jaw-closing muscles (masseter, temporalis, medial pterygoid), mediating the monosynaptic jaw-jerk stretch reflex.
  3. Temporomandibular Joint (TMJ) Mechanoreceptors: Monitor condylar translation and articular loading.
• The Chewing Cycle: Comprises the preparatory opening phase (lateral pterygoid, digastric, mylohyoid), closing phase (temporalis, masseter, medial pterygoid), and the intercuspal power stroke (shearing and crushing forces).

Deglutition (Swallowing)

Deglutition is a stereotypic sequence transporting a food bolus from the oral cavity into the stomach while protecting the lower respiratory tract. It occurs ~600 to 1,000 times per day and comprises three distinct phases:

Phase 1: Oral Phase (Voluntary)   ──▶ Bolus formed and propelled posteriorly by tongue (CN XII)
                                      │ [Contact with palatoglossal arch / anterior tonsillar pillar]
                                      ▼
Phase 2: Pharyngeal Phase (Reflex)──▶ Medullary Swallowing Centre (NTS & Nucleus Ambiguus)
                                      Velopharyngeal closure (CN X) ──▶ Laryngeal elevation
                                      Epiglottic tilt & Vocal cord adduction (Swallowing Apnoea)
                                      Pharyngeal constrictor contraction ──▶ Cricopharyngeus relaxes
                                      │
                                      ▼
Phase 3: Oesophageal Phase        ──▶ Primary & secondary peristaltic waves propel bolus (CN X)

1. Oral Phase (Voluntary):
   • Oral Preparatory Stage: Chewing, salivary wetting, and bolus consolidation on the anterior dorsal tongue.
   • Oral Propulsive Stage: Voluntary movement initiated by the tongue tip pressing against the anterior hard palate. The intrinsic and extrinsic tongue muscles (genioglossus, styloglossus, hyoglossus; innervated by CN XII) create a sequentially expanding anterior-to-posterior peristaltic pressure wave, propelling the bolus into the oropharynx. The phase terminates when the bolus triggers sensory receptors on the palatoglossal arches (anterior tonsillar pillars) and posterior pharyngeal wall.
2. Pharyngeal Phase (Involuntary, Reflexive):
   • Extremely rapid (~0.7 to 1.0 second). Triggered by afferent sensory signals via the glossopharyngeal (CN IX) and internal branch of the superior laryngeal nerve (CN X) projecting to the nucleus tractus solitarius (NTS) in the medulla. The NTS coordinates motor output through the nucleus ambiguus:
     • Step 1: Velopharyngeal Closure: The levator veli palatini and tensor veli palatini contract, elevating and tensing the soft palate against the posterior pharyngeal wall (Passavant's ridge), completely sealing the nasopharynx to prevent nasal regurgitation.
     • Step 2: Pharyngeal Transit: The palatopharyngeal folds approximate, forming a vertical slit that selects properly masticated boluses.
     • Step 3: Hyolaryngeal Elevation: Suprahyoid muscles (geniohyoid, mylohyoid, digastric) pull the hyoid bone and larynx anterosuperiorly beneath the base of the tongue.
     • Step 4: Airway Protection: Laryngeal elevation retroflexes the epiglottis downward over the laryngeal inlet. Simultaneously, the true vocal cords, false vocal cords (ventricular folds), and aryepiglottic folds contract tightly (adduct) via the recurrent laryngeal nerves (CN X). Central respiration is momentarily inhibited, termed swallowing apnoea.
     • Step 5: Pharyngeal Peristalsis: The superior, middle, and inferior pharyngeal constrictor muscles contract in rapid craniocaudal succession.
     • Step 6: Cricopharyngeal Relaxation: The upper oesophageal sphincter (cricopharyngeus muscle) relaxes, and the advancing peristaltic wave propels the bolus into the cervical oesophagus.
3. Oesophageal Phase (Involuntary):
   • Takes ~6 to 10 seconds. Coordinated primary peristaltic waves driven by the vagus nerve (striated muscle in the upper third; smooth muscle in the lower third) propel the bolus toward the stomach. The lower oesophageal sphincter (LOS) relaxes to allow entry into the gastric fundus.

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Neural Control and Clinical Relevance

Mastication is a rhythmic, largely automatic motor programme generated by a central pattern generator in the brainstem and modulated by peripheral feedback from periodontal mechanoreceptors, muscle spindles in the elevator muscles and receptors in the temporomandibular joint and oral mucosa. Periodontal mechanoreceptors are exquisitely sensitive and allow discrimination of particle sizes of a few tens of micrometres, which is why patients detect a high restoration that a clinician cannot see on articulating paper. Complete denture wearers lose this input entirely and rely on mucosal and muscle receptors, so their masticatory efficiency and their ability to detect occlusal error are substantially reduced — a point examiners use when asking why denture patients tolerate occlusal discrepancies poorly.

Dysphagia and the Dental Team

Swallowing is examined through its failures. The oral preparatory and oral phases are voluntary and can be disrupted by pain, xerostomia, missing teeth, ill-fitting dentures or a neurological deficit affecting the tongue. The pharyngeal phase is a reflex, triggered from the oropharyngeal mucosa and coordinated in the medulla; its failure risks aspiration. Warning features that should prompt referral include coughing or a wet voice after swallowing, food sticking, unexplained weight loss and recurrent chest infections. Dysphagia that is progressive and associated with weight loss requires urgent medical referral because of the possibility of oesophageal malignancy. For the dental team, the practical implications are careful use of rubber dam and high-volume aspiration in patients at risk of aspiration, upright positioning, and recognition that a patient with an unsafe swallow is at risk from irrigation fluids and from small instruments and restorative materials.