PART 1 Oral cavity, pharynx and esophagus

GI Motility online (2006) doi:10.1038/gimo1
Published 16 May 2006

Physiology of oral, pharyngeal, and esophageal motility

Raj K. Goyal, M.D. and Hiroshi Mashimo, M.D., Ph.D.
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Swallowing is the process by which food is transported from the mouth to the stomach. Functionally, it may be divided into three phases—preparatory, transfer, and transport phases—that follow each other in a sequence. The preparatory phase includes conscious effort to ingest food and reflexes in the oral cavity that help the preparation of the bolus to be swallowed. The transfer phase involves reflex activities in the oral and pharyngeal passages. The transport phase includes transport of the swallowed food bolus through the esophagus into the stomach.

Anatomically, swallowing has been divided into three phases: oral, pharyngeal, and esophageal. The oral phase includes preparatory as well as early transfer phases. The oral preparatory phase includes suckling, chewing, and masticating; the mixing of the food with saliva; and the formation of a bolus of suitable size and consistency. After the bolus is formed, the tongue creates a cup on its dorsal surface that entraps the bolus between it and the palate. The transfer phase begins once the decision to swallow has been made. The tip and sides of the tongue contract against the hard palate to progressively squeeze the entrapped bolus. Simultaneously, the posterior part of the tongue forms a chute that allows the bolus to pass from the isthmus into the oropharynx. If the tongue is weak or paralyzed, the bolus spills over in the oral cavity or into the pharynx. This causes aspiration before the swallow.

As the bolus enters the oropharynx, the soft palate lifts up to close off the nasopharynx from the oropharynx. The posterior pharyngeal wall moves upward, while the posterior part of the tongue moves forward to enlarge the oropharyngeal chamber. Nasal regurgitation of the food occurs if the nasopharynx is not closed off.

After the bolus enters the oropharynx, a ridge-like contraction (Passavant ridge) appears in the uppermost part of the posterior pharyngeal wall. This contraction moves down along with the downward movement of the soft palate. Simultaneously, the relaxed posterior pillars approximate one another, and the posterior tongue contracts against the palate to close off the oral cavity from the oropharynx. Progressive aboral contraction of the posterior pharyngeal wall against the contracting posterior part of the tongue propels the bolus into the pharynx.

During the pharyngeal phase, the tongue seals the oropharynx. The soft palate and proximal pharyngeal wall seal off the nasopharynx. The vocal cords and arytenoids close off the laryngeal opening and the epiglottis swings down to cover the laryngeal vestibule. These actions seal the airway from the pharyngeal cavity. In addition, the hyoid bone and larynx move superiorly and anteriorly, bringing the larynx to a position outside of the path of the bolus. The pharynx also widens and shortens, which is accompanied by an elevation of the upper esophageal sphincter (UES) by several centimeters. These activities move food bolus through the already open UES into the esophagus, and this action is completed by pharyngeal peristaltic contraction.

As the bolus enters the esophagus, the esophagus, including the lower esophageal sphincter (LES), relaxes to receive the bolus. As a result, a large part of a liquid bolus may move into the stomach by gravity alone if the subject is standing. The residual liquid bolus is cleared by the peristaltic contraction wave. Solid bolus usually does not move down by gravity and requires peristaltic contraction for its transport.

A bolus of food travels through the pharynx in less than 1 second; the velocity of the pharyngeal peristalsis is very fast, up to 40 cm/sec. The travel time through the esophagus is 5 to 6 seconds, with a velocity of peristalsis at 3 to 4 cm/sec.

The act of swallowing has voluntary and involuntary components. The preparatory/oral phase is voluntary, whereas the pharyngeal and esophageal phases are mediated by an involuntary reflex called the swallowing reflex. The neuromuscular structure and control of the oral, pharyngeal, and esophageal phases of swallowing are different. The muscles of oral cavity and tongue are voluntary and striated, of the pharynx and cervical esophagus are specialized and striated, and of the thoracic esophagus and LES are smooth. In addition, the sphincteric muscles (UES and LES) are specialized and exhibit tonic contractions. The striated muscles of oral cavity, pharynx, and cervical esophagus are innervated by lower motor neurons that are carried in cranial nerves, including the vagus. Lower motor neurons are all excitatory in nature and excite striated muscles by releasing acetylcholine at motor end plates. Loss of this innervations leads to paralysis of the muscles.

In contrast to these striated muscles, the smooth muscles of thoracic esophagus and LES receive autonomic motor innervations consisting of extrinsic preganglionic fibers that are carried in the vagus nerves and intramural postganglionic neurons that are part of the myenteric plexus. Moreover, the autonomic innervation has both an excitatory pathway and a parallel inhibitory pathway. The excitatory pathway consists of preganglionic and postganglionic neurons, both of which are cholinergic and act by releasing acetylcholine. The inhibitory pathway consists of preganglionic cholinergic and postganglionic nitrergic neurons that exert inhibitory action on smooth muscle by releasing vasoactive intestinal peptide and the gaseous neurotransmitter, nitric oxide.

Under resting conditions (i.e., in between swallows), muscles of the swallowing passages are generally relaxed, except for those of the esophageal sphincters. The UES remains closed passively because of the surrounding structures and tonic contraction of the cricopharyngeus muscle as a result of tonic activity of the nerves innervating it. In contrast, the LES remains closed largely because of the unique property of its muscle.

The swallowing reflex is an elaborate involuntary reflex that involves a swallowing center, or a swallowing pattern generator, in the brainstem. Once activated, the swallowing center neurons send patterned discharges of inhibition and excitation to motor nuclei of the cranial nerves. Pharyngeal and esophageal peristalsis mediated by the swallowing reflex is known as primary peristalsis.

The peristaltic contraction in the striated muscles of the pharynx and thoracic esophagus is the result of sequential inhibition followed by excitation of muscles that are involved in generating peristaltic contraction. The premotor nucleus that mediates swallowing is the central nucleus of the solitary tract that sends fibers to the nucleus ambiguous of the vagus.

The neural mechanism of primary peristaltic contraction in the smooth muscle of the thoracic esophagus is orchestrated by the premotor neurons in solitary tract, which send projections to the caudal and rostral parts of the dorsal motor nucleus of vagus. The caudal part of the dorsal motor nucleus of vagus contains neurons of the inhibitory pathway to the esophagus, whereas the rostral part houses the excitatory pathway neurons to the esophagus. The inhibitory pathway neurons are activated first; this results in inhibition of all ongoing activity in the esophagus and relaxation of the LES. This is followed by sequential activation of neurons to distal areas of the esophagus. The sequence of inhibition followed by excitation can be documented with membrane potential studies as a wave of hyperpolarization followed by depolarization. The peristaltic behavior is the result of a progressive increase in the duration of hyperpolarizations aborally along the esophagus. The gradient of increasing inhibition distally along the esophagus that precedes peristaltic contraction is also called the latency gradient, or deglutitive inhibition.

When a subject is drinking, swallows occur in rapid succession at rates of one swallow every 2 seconds. Under these circumstances, the pharyngeal response follows each swallow, but the esophagus remains inhibited until the last swallow that is followed by peristaltic contraction. The loss of deglutitive inhibition results in nonperistaltic contractions that are sometimes called tertiary contractions.

Swallowing is a highly regulated activity. Normally, it is activated by peripheral receptors located on structures in the posterior part of the oral cavity and oropharynx that are stimulated as the food bolus is pushed into the oropharynx. Afferents in the superior laryngeal nerves are important stimulators of the swallowing reflex. Swallowing can be also activated by stimulation of certain cortical neurons. Often, the cortical and peripheral inputs assist one another in eliciting swallowing.

Primary peristalsis expresses itself differently at different phases of the swallowing process. Esophageal peristalsis that occurs in the thoracic esophagus without the associated pharyngeal contraction is known as secondary peristalsis. Its physiologic role is to clear the esophagus of food residues and refluxed materials by moving them to the stomach. Secondary peristalsis, which is elicited by esophageal distention, is executed entirely by a local intramural reflex. The LES normally relaxes as a part of the peristaltic reflex. Under certain circumstances, the LES may relax without associated peristaltic contraction. This inappropriate LES relaxation is called transient LES relaxation (TLESR), and it may be elicited by gastric vagal afferent stimulation or stimulation of afferents in the superior laryngeal nerve with stimuli that are below the threshold for activating swallowing. Transient LES relaxation may be part of a belch reflex and has been implicated as an important mechanism of gastroesophageal reflux.

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