This phase is composed of a series of sequential events for the passage of the food bolus from the pharynx to the esophagus along with protection of the airway and nasopharynx (Fig. 2.2).

The passage of food bolus through the fauces was earlier thought to act as a sensory input to trigger the initiation of the pharyngeal stage [1, 9, 10]. The sensory input for the initiation of this reflex is carried by the vagus and the glossopharyngeal nerves to the swallowing center in the brainstem [10]. Variable bolus position at swallow initiation has been demonstrated by various studies, which have shown that bolus entry into the pharynx may occur before swallow initiation in healthy individuals when drinking liquids [1, 11, 12]. Food may enter the hypopharynx before a swallow, especially in case of food that contains both solid and liquid components [1]. Thus the trigger for swallow initiation may depend on multiple factors [12]. Also while eating solid food, the chewed bolus is aggregated in the oropharynx or valleculae before swallowing.

When the bolus enters the oropharynx, the nasopharynx is closed by elevation and contact of the soft palate with the lateral and posterior pharyngeal walls, thus preventing nasal regurgitation. This velopharyngeal closure, brought about by the levator palatine muscles also provides a surface to propel the bolus in a downward direction. The velopharyngeal closure is shortly followed by reflex closure of the laryngeal inlet. Closure of the true and false vocal cords occurs. The hyoid bone and larynx are pulled upward and forward due to suprahyoid and thyrohyoid muscle contraction, so that the larynx is covered by the tongue base [1, 7, 8, 10]. Backward movement of the epiglottis, probably thought to be due to hyolaryngeal elevation, pharyngeal constriction, bolus movement, and tongue base retraction covers the laryngeal inlet [13]. This results in apnea which may last from 0.5 to 1.5 s [1, 14, 15]. Retraction of the base tongue pushes the bolus against the pharyngeal walls. Sequential contraction of the constrictor muscles of the pharynx from above downward propels the bolus downward. The volume of the pharyngeal cavity reduces due to decrease in vertical pharyngeal length. The constrictor muscles contract involuntarily, but their actions are coordinated via the pharyngeal plexus [10]. The duration of this phase is about 1 s [10].

The esophageal phase begins when the food bolus enters the esophagus. The esophagus is made up of smooth and striated muscle and is innervated by the esophageal plexus of nerves. The upper esophageal sphincter (UES) is in a state of constant contraction [16]. The UES opens up to allow the passage of the food bolus into the esophagus. Impaired UES opening can lead to food retention in the piriform sinuses and hypopharynx, increasing the risk of aspiration following a swallow. Factors responsible for the opening of the UES are [1]:

The lower esophageal sphincter, which is also contracted at rest [16] (which helps in prevention of gastric reflux), relaxes when the food bolus reaches it. The bolus is transported by a peristaltic wave through the lower esophageal sphincter into the stomach [10]. The peristalsis in the thoracic esophagus is “true peristalsis” regulated by the autonomic nervous system [1]. The peristaltic wave consists of an initial wave of relaxation that accommodates the bolus, followed by a wave of contraction that propels it [1]. This is further assisted by gravity when in an upright position [1]. During this phase, the soft palate is lowered by the relaxation of the tensor and levator palatine muscles, the hyoid drops down, the epiglottis goes back to its original position, and the laryngeal vestibule opens [8] (Fig. 2.3).

A normal swallow requires coordination between mastication, respiration, and swallowing [1, 8, 10]. The process of swallowing is partially voluntary and partly involuntary. Centers in the brain are responsible for the voluntary control of swallowing. Bilateral areas in the primary sensory and motor cortex, in the prefrontal areas (anterior to the face region of the precentral gyrus, corresponding to Brodmann area 6), and in the parietal cortex are related to the voluntary initiation of swallowing [8]. The control of swallowing in humans is complex, with some areas being controlled bilaterally, while others are under unilateral control [8].

In addition to the mechanical sealing of the larynx during the pharyngeal phase of swallowing, respiration is also interrupted by the inhibition of the respiratory center in the brainstem [1, 17]. Swallowing of a liquid usually starts at the expiratory phase of respiration, and completion of swallow is followed by resumption of respiration in the expiratory phase. This acts as a protective mechanism, as it helps in clearing out any residues, thus preventing aspiration [1, 14, 15]. The respiratory rhythm is also altered during mastication of solid food. During swallowing of solid food, the apneic phase may be slightly longer [1].

The physiology of swallowing is a complex process involving coordination between various systems. Any abnormality in the swallowing pathway, anatomical or neural, can lead to swallowing dysfunction. Also, central neurological disorders can affect swallowing in the presence of an anatomically normal swallowing pathway. Aspiration pneumonia, malnutrition, etc., are some of the consequences of swallowing disorders. A detailed understanding of the factors controlling the swallowing mechanism with respect to its structure and neural control would help in diagnosing and treating swallowing disorders.