Oral Cavity

The oral cavity is comprised of the lips, mandible, maxilla, hard palate, soft palate, tongue, cheeks, and floor of mouth muscles. The posterior boundary of the oral cavity is typically delineated at the anterior faucial pillars [2]. Spaces in between the mandible/maxilla and the cheeks are the lateral sulci, and the space between the mandible/maxilla and the lips is the anterior sulci. The oral cavity is separated from the pharynx by a ring of tissue that includes the circumvallate papillae, lingual tonsils (at the tongue base), and soft palate [3].

Pharynx

The pharynx is divided into three subregions: the nasopharynx, oropharynx, and hypopharynx. The nasopharynx connects the nasal cavity with the oropharynx and hypopharynx and serves as a conduit for air and nasal/paranasal sinus secretions. The boundaries of the nasopharynx are the posterior surface of the nasal turbinates (anterior), the skull base (posterior), the adenoids (superior), and the soft palate (inferior). The oropharynx follows posteriorly from the oral cavity, bounded by the anterior faucial pillars (anterior), posterior pharyngeal wall (posterior), the soft palate (superior), and the tip of the epiglottis (inferior). In infants, there is no anatomic oropharynx, as the entire tongue resides in the oral cavity, and the soft palate approximates the epiglottis (see Fig. 15.1) [4, 5]. The hypopharynx is the most inferior subdivision of the pharynx and is bounded by the false vocal folds of the larynx (anterior), posterior pharyngeal wall (posterior), the tip of the epiglottis (superior), and the upper esophageal sphincter (inferior). The space between the tongue base and epiglottis is the valleculae, and the spaces posterolateral to the larynx are the pyriform sinuses.

Larynx

See Chap. 7 for a thorough overview of laryngeal structure and function. Structures relevant for swallowing include the true and false vocal folds, the arytenoid cartilages, and the epiglottis (Table 15.1).

Esophagus

The esophagus is a mucous membrane-lined tube that stretches between the upper esophageal sphincter at the junction with the hypopharynx and the lower esophageal sphincter at the junction with the stomach. These sphincters are tonically active to remain closed at rest and sequentially open during swallowing, belching, retching, and vomiting [6]. There are two muscular layers of the esophagus: one with fibers oriented circularly and the other with fibers oriented longitudinally. The rostral third of the esophagus is comprised of striated muscle, with a transition to smooth muscle at the caudal third [7].

Development of Upper Aerodigestive Anatomy

Several aspects of upper aerodigestive anatomy change as an individual develops (Fig. 15.1). The 20 deciduous teeth erupt between 6 and 33 months of age [2, 4, 8]. The sucking pads resorb, resulting in a larger buccal space [4]. As the individual grows, the mandible approximates the size of the maxilla, the tongue takes up a lesser proportion of the oral cavity, the hyoid and larynx descend to a lower position in the neck (~C2–3 to C4–6 for the larynx), and the approximation between the soft palate and epiglottis is lost, resulting in an oropharyngeal space [2, 4, 8–11]. The adenoids grow for the 1st year of life before shrinking around the age of 8, and the angle of the skull base at the nasopharynx becomes more acute [4]. The larynx grows in size and becomes less funnel-shaped, the laryngeal cartilages stiffen, the arytenoid cartilages take up a lesser proportion of the larynx, the angles of the vocal folds approximate parallel to the transverse plane, and the epiglottis stiffens and flattens from an omega shape [4, 10, 11].

Swallowing Physiology

Swallowing can be broken down into four phases: oral preparation, oral transport, pharyngeal, and esophageal [7]. Swallowing is composed of reflexive and voluntary movement patterns, partially controlled by central pattern generators in the brainstem [11, 12]. Swallowing physiology changes as the individual develops, but overall sequencing of biomechanical events remains stable, particularly in the pharyngeal and esophageal phases. An overview of the events that occur in each phase of swallowing is in Table 15.2.

Table 15.2

Events that occur in each phase of swallowing. Swallowing is not a purely sequential process; events within phases overlap in time

Phase	Event
Oral preparation	Food/liquid enters mouth; via sucking in infants
	Cohesive bolus formed and mixed with saliva; mastication if necessary
	Bolus is gathered onto and contained at the center of tongue
	Soft palate contacts posterior oral tongue to prevent bolus spilling into pharynx (except during mastication)
Oral transport	Tongue tip contacts alveolar ridge and strips posteriorly to propel bolus toward pharynx
	Lateral tongue contacts hard palate to prevent bolus spillage into sulci
	Posterior tongue depresses
Pharyngeal	Soft palate raises
	Hyoid moves anterior and superior
	Larynx moves anterior and superior
	Laryngeal vestibule closes
	Epiglottis retroflexes
	Pharyngeal constrictors activate superiorly to inferiorly
	Tongue base retracts to posterior and lateral pharyngeal walls
	Upper esophageal sphincter opens
Esophageal	Peristalsis of esophageal musculature superiorly to inferiorly
	Lower esophageal sphincter opens

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