Oral Preparatory Stage

The mandibular branch of the trigeminal nerve (cranial nerve [CN] V) innervates the principal muscles for chewing behaviors. The primary muscles of chewing are the masseter, temporalis, and pterygoid muscles, which attach to the sphenoid wing of the temporal bone. The masseter closes the jaw while the temporalis moves it up, forward, or backward (Table 2-1). The medial pterygoid muscles work bilaterally to elevate the mandible while they shift the jaw to the opposite side unilaterally. The lateral pterygoid muscles work together, pulling down or forward while moving the jaw or chin to the opposite side unilaterally. Both sets of pterygoid muscles cooperate to grind in mastication.

The facial nerve (CN VII) innervates lower facial muscles attached to the maxillae and mandible of the skull. These include the buccinator muscles, which compress the lips and flatten the cheeks in the movement of food across the teeth (Table 2-2). The buccinator fibers blend with those of the orbicularis oris, the sphincter of the lips.

The hypoglossal nerve (CN XII) innervates the tongue, which contains four separate intrinsic muscle masses that have different effects on the shape, contour, and function of the tongue.

Oral/Pharyngeal Stage

The pharyngeal cavity of the neck, which is suspended from the base of the skull and anchored to the top of the sternum, is formed by 26 pairs of striated muscles innervated by six cranial and four cervical nerves. The horseshoe-shaped hyoid bone in the neck serves as a fulcrum that provides a mechanical advantage for pharyngeal musculature associated with swallowing behaviors of the posterior tongue, pharynx, and larynx.

In the nasopharynx, five muscles adjust the position of the velum with respect to the food bolus: the palatoglossal and levator veli palatini muscles (pharyngeal plexus and accessory nerve), which elevate the soft palate and seal the nasopharynx; the tensor veli palatini (mandibular branch of the trigeminal nerve), which tenses the palate and dilates the orifice of the eustachian tube; the palatopharyngeal muscle (pharyngeal plexus and spinal accessory nerve), which depresses the soft palate, approximates the palate or pharyngeal folds, and constricts the pharynx; and the muscularis uvula (spinal accessory nerve), which shortens the soft palate (Table 2-3).

The hypoglossal (CN XII), trigeminal (CN V), and facial (CN VII) nerves innervate the suprahyoid group of muscles. The hypoglossal nerve supplies the geniohyoid, which draws the hyoid bone up and forward, depressing the jaw, and the trigeminal nerve supplies the mylohyoid, which elevates the hyoid bone and tongue and depresses the jaw (Table 2-4). The digastric muscles contain anterior and posterior bellies. The anterior belly is innervated by the mandibular branch of the trigeminal nerve (CN V) and depresses the jaw or raises the hyoid bone, whereas the posterior portion is innervated by the facial nerve (CN VII) and elevates or retracts the hyoid. The facial nerve (CN VII) innervates the stylohyoid muscle, which elevates the hyoid bone during swallowing. In addition, the hyoglossus and the genioglossus serve as laryngeal elevators, as well as extrinsic tongue muscles, and are designed to depress the tongue or help elevate the hyoid bone when the tongue is fixed. The accessory nerve (CN XI), in association with the hypoglossal (CN XII) nerve, innervates the styloglossus, which draws the tongue up and back during swallowing. The glossopharyngeal (CN IX) and accessory (CN XI) nerves also cause the palatoglossus to raise the back of the tongue and lower the velum. The styloglossus and palatoglossus raise the back of the tongue and lower the sides of the soft palate.

The vagus nerve (CN X) and the spinal accessory nerve (CN XI) innervate the muscular pharynx, whose superior, middle, and inferior constrictor muscles constitute its external circular layer and work together to transport a bolus of food toward the esophagus during swallowing. Three other muscles constitute the internal longitudinal layer of the pharynx: the palatopharyngeus, stylopharyngeus, and salpingopharyngeus. The stylopharyngeus (glossopharyngeal nerve) elevates the pharynx, and to some extent the larynx, during swallowing, and the salpingopharyngeus (accessory nerve and pharyngeal plexus) draws the lateral walls of the pharynx up. The palatopharyngeus muscle draws the velum down.

The cricopharyngeal muscle is an important single muscle that lies at the transition level between the pharynx and the esophagus. Functionally, it is separate from both the pharynx and the esophagus and acts as a sphincter, relaxing during passage of the bolus from the pharynx into the esophagus. It is innervated by both pharyngeal branches of the vagus and sympathetic fibers from the middle and inferior cervical ganglia. The key muscles used in the oral and pharyngeal stages of swallowing are shown in Figure 2-4.

Esophageal Stage

The esophagus is a distensible tube, approximately 21 to 27 cm (10 inches) long, connecting the pharynx (at C6) and stomach (at T12). It is separated from the pharynx by the pharyngeal esophageal segment (PES) and from the stomach by the lower esophageal sphincter (LES). Under resting conditions, the esophageal lumen is collapsed, creating a potential space that can easily distend up to 3 cm to accommodate swallowed air, liquids, or solids. The esophagus is lined with a protective, stratified, squamous epithelium that covers an inner layer of circular fibers and an outer layer of longitudinal fibers. At its proximal end (upper fourth) the muscle is striated, whereas the distal two thirds are composed of smooth muscle. The middle third, in the region of the aorta, is a combination of smooth and striated muscle. As it courses through the thorax at the level of the carina, the esophagus runs lateral and posterior to the left ventricle of the heart, creating a natural bend as it courses anteriorly toward the diaphragmatic hiatus. After passing the diaphragmatic hiatus, it connects to the body of the stomach at the level of the LES. The smooth muscle of the LES is arranged in a specialized spiral configuration as it joins the inner oblique muscle zone of the stomach. The relation of the esophagus to the heart and tracheobronchial tree, as well as its path through the diaphragmatic hiatus, is shown in Figure 2-5.

Oral Preparation

Food or liquid in the mouth stimulates taste, temperature, and pressure (touch) receptors. The primary receptors of taste are located on the tongue, on the hard and soft palate, in the pharynx, and in the supralaryngeal region. The receptors are activated by saliva. Saliva is produced by the activation of the submandibular, submaxillary (autonomic aspects of CN VII), and parotid glands (autonomic aspects of CN IX). Activation of these glands is achieved by the actions of the jaw, tongue, and hyoid bone during bolus preparation and by the inherent taste of the bolus. The primary sensory receptors on the dorsum of the tongue responsible for the perception of salt, sweet, sour, and bitter are activated by saliva. In addition to facilitating taste and bolus formation, saliva is important in the maintenance of adequate oral hygiene by controlling microorganisms, in the regulation of the acidity levels in the stomach and esophagus because of it bicarbonate composition, and in the breakdown of carbohydrates. The number of times a person swallows saliva in 1 hour can vary between 18 and 400 and largely depends on the rate of salivary flow.⁶

Sensations of taste are carried by the chorda tympani branch of CN VII on the anterior two thirds of the tongue and through the greater petrosal branch on the hard and soft palate. Taste on the posterior third of the tongue is mediated by CN IX. Sensations of taste are sent to the nucleus tractus solitarii (NTS) in the medulla of the brainstem (see sections on neurologic controls), where they are transmitted to the sensorimotor cortex by the thalamus. Taste receptors in the region of the laryngeal aditus are carried to the NTS by the superior laryngeal branch of CN X. Appreciation of taste depends largely on smell. Smell sensations are carried by direct stimulation of the nasal cavity and by smell elicited by chewing, during which odors travel posteriorly into the nasopharynx. Interpretation of smell is ultimately accomplished through the thalamus to the frontal and temporal cortices by information carried by CN I. Information (memories) relating to smell may be stored in the hippocampus.

The coordinated action of the tongue and jaw moves a bolus laterally onto the molar table for deformation. Further deformation is accomplished by variable contacts of the tongue to the hard palate. Although the tongue may play a large role in containing the bolus in the oral cavity before swallow, evidence indicates that during solid bolus mastication, material is allowed to collect in the vallecular recesses at the tongue base before swallow initiation.⁵ The ultimate role of the tongue is to manipulate, shape, hold, and then transfer the bolus into the oropharynx, signaling the onset of the oral stage of swallow as the swallowing sequence transitions into the pharyngeal stage with the passage of the bolus through the oropharyngeal port. The exact nature of the sensory cues that signal a bolus is ready for swallowing is not completely understood; however, studies have shown that the superior laryngeal nerve branch of the vagus is important in swallow initiation⁷ and in the sensory protective mechanisms of the upper airway.⁸ The mechanics of bolus preparation can be appreciated with videofluoroscopy. The first images are taken as the patient faces the camera and chews a piece of cracker (Video 2-1). The undulating and varied movements of the tongue and jaw are apparent. In the lateral view, the tongue can be seen touching the hard palate as material is pushed toward the tongue base, filling the valleculae before the swallow (Video 2-2).

Oral Stage

Once the bolus is prepared, the tongue tip is elevated to occlude the anterior oral cavity at the alveolar ridge, and the bolus is held against the hard palate. The edges of the tongue dorsum contain the bolus laterally. The tongue tip and dorsum appear to work longer in containment activity than the posterior tongue after the oral stage is initiated; however, the posterior tongue is more responsible for delivering the bolus into the pharynx.⁹ Before—but almost simultaneous with—the first posterior movement of the tongue, respiration ceases (see following section on respiration), followed by arytenoid cartilage approximation precipitating true vocal fold adduction. Retraction of the tongue is primarily accomplished by extrinsic tongue muscles: digastricus (CN V), mylohyoid (CN V), and the geniohyoid (CN XII). The tongue base applies positive pressure to the tail of the bolus by its contact with the velum and posterior pharyngeal wall, which allows the bolus to move rapidly through the pharynx into an open PES. As the tongue propels the bolus posteriorly, the palatopharyngeal folds are pulled medially to form a slit through which the bolus can pass. The levator veli palatinii muscles help elevate the velum to seal the nasopharyngeal opening. The combined action of the tongue’s contact to the velum and posterior pharyngeal wall and sealing the nasopharynx contribute to the maintenance of positive pressure on the bolus as it moves toward zones of negative pressure in the hypopharynx. By the tongue’s connections to the hyoid bone, and the hyoid bone’s connections to the thyroid and cricoid cartilages, the larynx is pulled up and forward, resting under the tongue base that now partially covers the opening to the airway. As the larynx rises, the cartilaginous epiglottis makes its descent over the top of the airway, completing an elaborate system of airway protection that allows the bolus to be directed toward the esophagus rather than into the trachea. The extent of epiglottic descent depends on anterior hyoid displacement, tongue base retraction force, and bolus size.¹⁰ Rapid and complete laryngeal elevation (2 to 3 cm on average) aids in creating negative pressure in the region of the hypopharynx. As the bolus enters the pharynx, it is divided by the vallecular spaces at the level of the tongue base, helping deflect it away from the airway as an additional component of airway protection.