Mechanism of Deglutition and the Reflex Arc
The act of deglutition is a complex process consisting of a sequence of physiological reactions. The reflex arc is triggered by the activation of receptors located on the posterior pharyngeal wall, the base of the tongue, and the soft palate. When a food bolus—formed by the coordinated contractions of the muscles of the tongue and cheeks—mechanically stimulates these areas, afferent signals travel via the glossopharyngeal nerve (n. glossopharyngeus) to the central nervous system.
The swallowing center is located in the medulla oblongata. Its key physiological feature is its close interaction with the adjacent respiratory center. They have a reciprocal (mutually inhibitory) relationship; thus, respiration briefly pauses during a swallow (deglutition apnea).
The efferent limb of the reflex arc travels via fibers of several cranial nerves: the trigeminal, hypoglossal, glossopharyngeal, and vagus nerves. Their impulses produce a precise motor response designed primarily to protect the airway. Contraction of specialized muscles elevates the soft palate, isolating the nasopharynx, while simultaneous elevation of the larynx tightly closes the laryngeal inlet.
Three Phases of the Deglutition Cycle
Physiologists traditionally divide the swallowing process into three phases, which differ significantly in speed and level of voluntary control:
- Oral phase. This is the only fully voluntary phase of deglutition. During this phase, the muscles form a food bolus and intentionally push it toward the root of the tongue.
- Pharyngeal phase. Occurs rapidly, briefly, and completely involuntarily. Together, the oral and pharyngeal phases average only about 1 second. Protective reflexes of the respiratory system are activated during this stage.
- Esophageal phase. Characterized as slow, prolonged, and completely involuntary. The transit time of the bolus through the esophagus depends strongly on its density and physical state. Swallowing liquid takes only 1–2 seconds, whereas solid food travels slowly over 8–9 seconds.
Anatomic and Physiological Features of the Esophagus
The muscular layer of the esophagus exhibits a specific histological structure that changes along its length. Its upper third consists predominantly of striated skeletal muscle fibers, whereas the remaining wall is composed of classic smooth muscle.
A key structure for protecting the gastrointestinal tract is the lower esophageal sphincter (LES). In the resting physiological state, it remains tonically closed. This barrier fulfills an essential function—it prevents the reflux (backflow) of acidic gastric juice from the stomach onto the vulnerable esophageal mucosa.
During a swallow, this sphincter reflexively opens. Its relaxation mechanism is triggered by vagal nerve (n. vagus) excitation and mediated by specialized enteric neurons synthesizing vasoactive intestinal peptide (VIP) and nitric oxide (NO).
Regulation of Esophageal Motility and Peristalsis
Esophageal motor activity is regulated by efferent fibers of the vagus and sympathetic nerves, as well as the intrinsic intramural (enteric) nervous system.
Esophageal muscular contractions take the form of a wave originating in the upper third of the organ and sweeping toward the stomach. Physiologically, this process is strictly coordinated and divided into two components:
- Receptive relaxation wave: A zone of decreased muscular tone. It always travels slightly ahead of the food bolus, preparing the pathway.
- Peristaltic wave: A zone of active muscular contraction directly following the relaxation zone, propelling the food forward.
Thanks to this coordinated mechanism, deglutition and the propulsion of food are entirely independent of gravity. A person can successfully swallow food and liquids while lying horizontally, hanging upside down, or even in microgravity.