Surgical anatomy and physiology of swallowing

Deglutition is a complex process, which transports ingested food and liquid from the mouth to the stomach. Swallowing requires the coordination of 3 anatomically and functionally separate upper aerodigestive tract structures, which comprise the following swallowing apparatus: the oral cavity, the pharynx, and the larynx.^{,  ,  ,}  These structures act as a hydrodynamic pump with valves that allows food and liquid to be transferred into the stomach without entering the respiratory tract.^,  The act of swallowing is divided into the following 3 phases: the oral phase, pharyngeal phase, and esophageal phase, each of which corresponds to the location of the food bolus in the swallowing apparatus. The initial stage of deglutition, the oral phase, is voluntary and triggers the subsequent involuntary pharyngeal and esophageal phases.^{,  ,}  Dysphagia is a symptom caused by a multitude of diverse diseases that can affect each phase of deglutition, and cause a serious deterioration in the quality of life of affected patients. To surgically treat dysphagia, a thorough understanding of the anatomy and physiology of deglutition is required to both identify the affected stage(s) of swallowing and the appropriate intervention needed to treat them.

The oral phase of swallowing occurs in the oral region that consists of the lips, cheeks, teeth, gums, oral cavity, hard and soft palate, and the palatine tonsils. Food processing and bolus formation occurs in the oral cavity proper that is the space between the upper and lower dental arches. It is limited laterally and anteriorly by the dental arches, superiorly by the palate and inferiorly by the tongue. Posteriorly, the oral cavity proper transitions into the oropharynx that is the superior portion of the pharynx (Figure 1, Figure 2). The lips surround the mouth and form a sphincter using the orbicularis oris muscle that controls entrance and exit from the oral cavity. They are covered externally by specialized skin, internally by a mucous membrane, and contain the superior and inferior labial muscles, vessels, and nerves. The cheeks form the lateral walls of the oral cavity and are continuous with the lips with the same skin and internal mucous membrane. Internally, the cheeks contain the buccinator muscles and the buccal fat pads, which lie superficial to the buccinators. The hard palate composes the anterior two-third of the palate and forms a bony concave roof covered with a mucous membrane. The soft palate is a mobile fibromuscular fold, which comprises the posterior one-third of the palate and separates the nasal cavity from the nasopharynx. There is a subtle color change from pink of the hard palate to yellow-red of the soft palate.^,  The soft palate is suspended anteriorly from the posterior edge of the hard palate by the palatine aponeurosis, which is thick anteriorly and thins posteriorly as it combines with the muscular section. Posterior and inferiorly, the soft palate has a curved free margin from which the uvula hangs. Laterally, the soft palate is continuous with the palatoglossal and the palatopharyngeal folds, which join it to the tongue and the pharynx respectively. The space between the oral cavity and the pharynx is termed the fauces, and is bound superiorly by the soft palate, laterally by the palatoglossus and the palatopharyngeal arches, and inferiorly by the root of the tongue. The soft palate contains 5 muscles which emanate from the base of the skull and descend to the palate.^,  The levator palatini muscle that moves the soft palate upwards and backwards during swallowing, extends inferiorly, medially, and anteriorly from both the petrous part of the temporal bone and the cartilage pharyngotympanic tube, and inserts on the mid partition of the palatine aponeurosis. The tensor veli palatini muscle, responsible for tensing the soft palate and opening the pharyngotympanic tube during swallowing, extends from the scaphoid fossa of the medial pterygoid plate, spine of sphenoid bone, and cartilage of pharyngotympanic tube.^,  It extends first inferiorly, and then turns at a 90° angle medially over the pterygoid hamulus to disperse horizontally in the palatine aponeurosis. This course allows the tensor veli palatine tendon to form a pulley around the pterygoid hamulus to pull horizontally on the aponeurosis.^{,  ,}  The paired palotopharyngeal muscles constitute the arch of the soft palate, and elevate the pharynx and larynx during the pharyngeal phase of swallowing as well as depress the tensed palate at the end of the oral phase.^,  They originate from the hard palate and palatine aponeurosis and extend inferiorly and posteriorly to form part of the lateral wall of the pharynx.^,  The palatoglossus muscle extends inferiorly from the palatine aponeurosis to the lateral side of the tongue and draws the soft palate and tongue together by elevating the posterior aspect of the tongue.^,  The musculus uvulae shortens the uvula and pulls it superiorly during swallowing. The muscles of the soft palate are innervated by the pharyngeal branch of the vagus nerve with the exception of the tensor veli palatini, which receives its innervation from the mandibular division of the trigeminal nerve.^,  Both the hard and soft palate receive their blood supply via the greater and lesser palatine branches of the maxillary artery. The tongue is a mobile muscular structure that sits partly in the oral cavity and partly in the oropharynx.^,  It is covered in a mucous membrane and has 8 pairs of muscles that are grouped into intrinsic and extrinsic sets. The 4 intrinsic muscles control the shape of the tongue and consist of the vertical, transverse, superior longitudinal, and inferior longitudinal muscles. The 4 extrinsic muscles control the position of the tongue and are composed of the genioglossus, hyoglossus, palatoglossus, and styloglossus muscles.^{,  ,}  The extrinsic muscles originate outside of the tongue and insert on the tongue. All the muscles of the tongue are innervated by the hypoglossal nerve with the exception of the extrinsic palatoglossus muscles, which receive their innervation from the pharyngeal plexus. The tongue is divided into 2 areas of sensation. General sensory information in the anterior two-third of the tongue is carried by the trigeminal nerve, whereas the posterior one-third is served by the lingual-tonsillar branch of the glossopharyngeal nerve. The arteries of the tongue are derived from the lingual artery that branches from the external carotid artery.

The oral phase of deglutition occurs in the oral cavity and consists of 2 stages, the preparatory stage and propulsive stage.^,  The preparatory stage begins as food or liquid is placed into the mouth and results in the formation of a bolus. Bolus formation occurs through the coordination of lip, buccal, mandibular, and tongue movements, as well as the closure of the upper esophageal sphincter to maintain the food or liquid in the oral cavity until the person is ready to initiate swallowing. Liquids require no preparation for bolus formation, whereas solid food requires processing to reduce the size of particles through mastication and softening by saliva.^,  This process is under voluntary control via 3 cranial nerves. The trigeminal nerve coordinates mastication and relays sensory information, the facial nerve coordinates lip and buccal movement to assist in food positioning and retention, and the hypoglossal nerve controls tongue movement. Once a suitable bolus for swallowing is formed, it is positioned in a groove formed by the tongue against the hard palate, and the propulsive stage of the oral phase begins.^,  Via a wave-like elevation of the tongue that starts from the tip and moves posteriorly, the bolus is propelled posteriorly in a squeeze-like action through the fauces into the opening of the pharynx^{,  ,  ,}  (Figure 3A).

The pharynx extends from the cranial base to the inferior border of the cricoid cartilage anteriorly and the inferior border of the C6 vertebra posteriorly. It is divided into the following 3 parts: the nasopharynx, posterior to the nose and superior to the soft palate; the oropharynx, from the soft palate to the hyoid bone; and the laryngopharynx, posterior to the larynx (Figure 1, Figure 2).^,  The wall of the pharynx is a muscular wall composed of longitudinal muscles internally surrounded by an external circular layer of muscles, giving it a tubular shape. The external muscles consist of the superior, middle, and inferior constrictor muscles, and are responsible for constricting the walls of the pharynx during swallowing (Figure 4). They contract involuntarily in a sequential manner from the superior-to-inferior end of the pharynx to propel the bolus through the pharynx and into the esophagus. The superior pharyngeal constrictor has complex attachments, and originates from the pterygoid hamulus, pterygomandibular raphe, posterior end of mylohyoid line of mandible, and the transverse musculature of tongue. These muscle extensions join and extend posteriorly to insert on the pharyngeal tubercle on the basilar part of the occipital bone. The middle pharyngeal constrictor muscle originates from greater and lesser horns of the hyoid bone and the stylohyoid ligament which runs from the styloid process in the skull base to the minor processes of the hyoid bone.^,  The middle constrictor then inserts in the posterior midline to the pharyngeal raphe. The inferior pharyngeal constrictor is the thickest of the pharyngeal constrictor muscles and consists of 2 parts, the thyropharyngeus and the cricopharyngeus (Figure 5). The thyropharyngeus part originates from the oblique line of the thyroid cartilage, the lateral aspect of the cricoid cartilage, and the thyroid ligament. It extends posteriorly surrounding the pharynx, and inserts into the median pharyngeal raphe. The upper fibers ascend obliquely to overlap the middle constrictor.^,  The cricopharyngeus part of the inferior constrictor originates from the side of the cricoid cartilage between the origin of the cricothryroid and the articular facet of the inferior horn of the thyroid cartilage. Its fibers pass horizontally and posteriorly encircling the pharyngo-esophageal junction to insert at the same site on the opposite side of the cricoid cartilage, and are continuous with the inner circular fibers of the esophagus. Posteriorly, between the lower fibers of the thyropharyngeus and the upper fibers of the cricopharyngeus, there is a small triangular space known as Killian dehiscence. This space is anatomically normal but lacks the support that is present in the rest of the inferior constrictor. This leads Killian dehiscence to be prone to the formation of a diverticulum during abnormal uncoordinated pharyngeal peristalsis, in which the peristaltic waves reach the cricopharyngeus before it has relaxed. The superior pharyngeal constrictor is innervated by the pharyngeal branch of the vagus and pharyngeal nerves, whereas the middle and inferior constrictors are innervated by the pharyngeal branch of the vagus nerve and pharyngeal plexus, as well as branches of the external and recurrent laryngeal nerves. All the pharyngeal constrictor muscles receive blood supply from branches of the ascending pharyngeal artery. The superior pharyngeal constrictor muscle receives additional blood from the tonsillar branch of the facial artery.

The internal longitudinal muscles of the pharynx consist of the palatopharyngeus, stylopharyngeus, and salpingopharyngeus (Figure 4). These muscles shorten and widen the pharynx during swallowing by elevating the pharynx and larynx. The palatopharyngeus is the largest of the longitudinal muscles. It originates from the hard palate, palatine aponeurosis, and pterygoid process and extends inferiorly to insert on the posterior border of the thyroid cartilage, as well as within the pharyngeal constrictor musculature and esophagus.^,  The stylopharyngeus muscle extends from the styloid process of the temporal bone inferiorly and anteriorly in a gap between the superior and middle pharyngeal constrictors. It extends inferiorly to insert on the posterior and superior borders of the thyroid cartilage with the palatopharyngeus as well as to the edges of the epiglottis. The salpingopharyngeus originates from the cartilaginous part of the pharyngotynpanic tube and extends downward to merge with the palatopharyngeus. The palatopharyngeus and salpingopharyngeus muscles receive their innervation from the pharyngeal branch of the vagus nerve and the pharyngeal plexus, whereas the stylopharyngeus muscle is innervated by the glossopharyngeal nerve.

During swallowing, the larynx must be closed to prevent aspiration of ingested material into the airway. As stated before, this is accomplished by elevation of the larynx to underneath the base of the tongue, closure of the vocal folds, and closure of the epiglottis over the laryngeal vestibule. Laryngeal elevation is accomplished by the 4 suprahyoid muscles, which elevate the following hyoid bone: the digastric, mylohyoid, stylohyoid, and geniohyoid muscles. The anterior belly of the digastric muscle originates from the digastric fossa of the mandible, and the posterior belly originates from the mastoid notch of the temporal bone. As these muscle bellies descend toward the hyoid bone, they are joined by an intermediate tendon, which is then held to the greater horn of the hyoid by a fibrous sling that allows the tendon to slide anteriorly and posteriorly. The anterior belly of the digastric muscle is innervated by the mylohyoid branch of the trigeminal nerve and the posterior belly is innervated by the digastric branch of the facial nerve. The mylohyoid muscle originates from the mylohyoid line of the mandible and extends inferiorly to insert on the mylohyoid raphe and body of the hyoid. The mylohyoid is innervated by the nerve to the mylohyoid, a branch of the inferior alveolar nerve. The geniohyoid muscle originates from the inferior mental spine of the temporal bone, and the stylohyoid muscle originates from the styloid process of the temporal bone. Both of these muscles extend inferiorly and insert on the body of the hyoid. The geniohyoid is innervated by the hypoglossal nerve, and the stylohyoid is innervated by the stylohyoid branch of the facial nerve. As the suprahyoid muscles elevate the hyoid bone and the larynx, the thyrohyoid muscle, an infrahyoid muscle, approximates the hyoid bone and thyroid cartilage. The thyrohyoid originates from the oblique line of the thyroid cartilage and inserts on the inferior border of the body as well as the greater horn of the hyoid. It is innervated by the hypoglossal nerve.

Closure of the glottis occurs in an inferior-to-superior peristaltic fashion in which the true vocal folds close first, followed by the false folds and then the aryepiglottic folds.^,  This important during swallowing as it allows for clearance of any bolus material that may have entered the airway during swallowing. Constriction of the lateral cricoarytenoid muscles, transverse and oblique arytenoid muscles, and aryepiglottic muscles close the aryepiglottic folds in a sphincteric action, which when relaxed forms a triangular opening to the larynx (Figure 2). Additionally, contraction of these muscles pulls the arytenoid cartilages toward the epiglottis.

The epiglottis is a flexible, heart-shaped cartilaginous structure covered in a mucous membrane that sits posterior to the root of the tongue and hyoid bone and anterior to the laryngeal inlet (Figure 1). Its superior portion is broad and freely mobile with no attachments. The inferior portion tapers and attaches to the angle formed by the thyroid laminae via the thyroepiglottic ligament. The epiglottis is fixed by fibroelastic ligaments to the hyoid bone, thyroid cartilage, and quadrangular membrane. During laryngeal elevation and hyoid approximation to the thyroid, the suspended epiglottis passively shifts from a vertical-to-horizontal orientation over the laryngeal vestibule.^{,  ,}  Once horizontal, compression from the advancing bolus, peristaltic contraction of the pharyngeal constrictors, and contraction of the thyroepiglottic muscle complete the closure of the epiglottis over the laryngeal inlet.^, 

The pharyngeal phase of swallowing is initiated voluntarily as the bolus crosses the anterior tonsillar pillars by sensory information relayed via the glossopharyngeal and vagus nerves.^,  Once initiated, the remainder of the phase is involuntary and completed in approximately 1 second. As the tongue pushes the bolus into the pharynx, the soft palate shifts superiorly to seal the nasopharynx from the oropharynx (Figure 3B). The palatopharyngeal folds simultaneously move toward the midline to form a funnel that guides the bolus through the upper pharynx. The hypoid bone and larynx are then raised anterosuperiorly away from the cervical spine, which shortens and enlarges the pharynx while creating a negative pressure in the hypopharynx. Bolus propagation through the pharynx is accomplished by the following 4 mechanisms: tongue driving pressure, contraction of the pharyngeal constrictor muscles, negative hypopharyngeal pressure, and gravity. As the bolus passes through the lower pharynx, the airways are protected by elevation of the larynx to underneath the base of the tongue, closure of the vocal folds, and closure of the epiglottis over the laryngeal vestibule. The pharyngeal phase concludes as the bolus passes from the lower pharynx into the esophagus through the upper esophageal sphincter (Figure 3C).

The esophageal phase of swallowing takes place entirely within the esophagus that is a muscular tube approximately 25 cm (10 in) in length with an average diameter of 2 cm. In the resting state, the upper two-third of the esophagus is collapsed while the lower one-third is rounded. As it moves inferiorly toward the stomach, there are 3 constrictions created by impressions of adjacent structures that can be used as landmarks, namely, the cervical constriction (upper esophageal sphincter), the thoracic constriction, and the diaphragmatic constriction. The cervical constriction is located at the beginning of the esophagus and is created by the cricopharyngeus aspect of the inferior pharyngeal constrictor. The upper esophageal sphincter measures 4-6 cm in length and is consists of the cricopharyngeus muscle, the lower fibers of the thyropharyngeus muscle, and the upper muscle fibers of the esophagus. The thoracic constriction is a compound constriction created by the crossing of the esophagus by the aortic arch and the left mainstem bronchus. The diaphragmatic constriction is located where the esophagus passes through the esophageal hiatus of the diaphragm. After passing through the esophageal hiatus, the abdominal esophagus is only 1.25 cm in length and widens as it terminates by entering the stomach at the cardiac orifice. This esophagogastric junction lies to the left of the T11 vertebra, and is designated the Z-line, a jagged line where the esophageal mucosa transitions into gastric mucosa.

Structurally, the esophagus consists of the following 3 layers: the mucosa, submucosa, and muscularis. The musculature of the esophagus consists of 2 layers, an inner circular layer and external longitudinal layer. The internal circular musculature is thinner superiorly and thickens inferiorly as it approaches the stomach. In the external layer of muscle, the superior third consists of striated muscle only, the middle third contains both striated and smooth muscle, and the lower third consists of only smooth muscle. The esophagus is innervated by the esophageal plexus, formed by the anterior and posterior gastric branches of the vagus trunk and the greater splanchnic and periarterial plexus branches of the thoracic sympathetic trunk. The arterial blood supply of cervical esophagus is supplied via branches of the right and left superior and inferior thyroid arteries. The thoracic esophagus is supplied by branches of the tracheobronchicial arteries, and the intra-abdominal esophagus is supplied via branches from the left gastric artery.

The esophageal phase is involuntary and begins when the advancing bolus crosses the upper esophageal sphincter and enters the esophagus. The upper esophageal sphincter relaxes at the triggering of the pharyngeal phase, and remains open until the bolus passes the relaxed cricopharyngeus portion of the inferior pharyngeal constrictor (Figure 2, Figure 5). Therefore, cricopharyngeus muscle relaxation is crucial for normal swallowing, and failure to do such in the proper extent or time may generate discoordination from the remainder of the swallowing mechanisms, hence, leading to dysphagia. Once the bolus passes through the upper esophageal sphincter, it is propelled through the esophagus toward the stomach via peristaltic waves that generate a positive pressure (Figure 3D). Secondary peristaltic waves arise in response to local distension, allowing for the clearance of any bolus retained after the primary peristaltic wave.^,  Peristaltic waves in the superior two-third of the esophagus progress more rapidly than the inferior one-third. This is owing to the fact that the superior aspect of the esophagus contains striated muscle, whereas the inferior one-third is nonstriated. At the bottom of the esophagus, there is a segment approximately 3 cm in length that extends above and below the diaphragm with a raised pressure 8 mm Hg greater than intragastric pressure on average. This segment forms the lower esophageal sphincter, and is normally under tonic contraction owing to intrinsic and extrinsic forces. The intrinsic force is created by the internal circular muscle fibers of the esophagus, and the extrinsic force is created by diaphragmatic pressure. After a bolus passes into the esophagus, these forces relax, and the lower esophageal sphincter opens just before the peristaltic wave carrying the bolus reaches it.

Dysphagia is a debilitating symptom, which may significantly decrease the quality of life in affected patients. Owing to the complexity of swallowing, a thorough understanding of the anatomy and physiology is essential for successful diagnosis treatment of dysphagia.

The author report no proprietary or commercial interest in any product mentioned or concept discussed in this article.