Deglutition reflex

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Deglutition Reflex (Swallowing)

Swallowing (deglutition) is a complex coordinated act that propels a food bolus from the mouth to the stomach. It involves both voluntary and involuntary components, and requires precise neuromuscular coordination to protect the airway.

Swallowing mechanism showing swallowing center in medulla, cranial nerves, pharynx, epiglottis, vocal cords, and esophageal peristalsis

Stages of Swallowing

1. Voluntary (Oral) Stage

Food is chewed and formed into a bolus on the tongue.
The tongue presses upward and backward against the hard palate, squeezing the bolus into the pharynx.
Once the bolus enters the posterior mouth and pharynx, swallowing becomes entirely automatic and cannot normally be stopped.

2. Pharyngeal Stage (Involuntary)

This is the reflex stage proper. The bolus stimulates swallowing receptor areas (epithelial mechanoreceptors) located in a ring around the pharyngeal opening, with the greatest sensitivity on the tonsillar pillars. The following events occur in rapid, automatic sequence (completed in less than 2 seconds):

Step	Event	Purpose
1	Soft palate pulled upward	Closes posterior nares - prevents nasal regurgitation
2	Palatopharyngeal folds pulled medially	Form a sagittal slit - prevents incompletely masticated food from passing
3	Vocal cords strongly approximated; larynx pulled upward and anteriorly	Epiglottis swings backward over laryngeal opening - protects airway
4	Upper esophageal sphincter (pharyngoesophageal sphincter) relaxes	Opens esophageal entrance
5	Pharyngeal wall contracts in a peristaltic wave (superior → inferior)	Propels bolus into upper esophagus

Key protective note: The tight approximation of the vocal cords is the most essential airway protection mechanism. The epiglottis provides a secondary guard. Destruction of the muscles approximating the vocal cords greatly increases aspiration risk.

Inhibition of Respiration

The pharyngeal stage temporarily interrupts respiration - typically for less than 6 seconds. The swallowing center actively inhibits the respiratory center of the medulla during this time.

3. Esophageal Stage (Involuntary)

The esophagus conducts food from the pharynx to the stomach via two types of peristalsis:

Primary peristalsis: A direct continuation of the pharyngeal peristaltic wave. Travels the length of the esophagus in 8-10 seconds.
Secondary peristalsis: Initiated by distension of the esophagus by any retained food. Continues until all food is cleared into the stomach - controlled by the intrinsic myenteric nervous system and partly by extrinsic reflexes via the vagus.

The lower esophageal sphincter (gastroesophageal sphincter) relaxes ahead of the peristaltic wave and remains relaxed until the peristaltic wave has driven food into the stomach, after which it closes tightly to prevent reflux.

Neural Control - The Deglutition (Swallowing) Center

Component	Detail
Afferent (sensory) pathway	Trigeminal (CN V) and glossopharyngeal (CN IX) nerves carry impulses from pharyngeal receptors to the tractus solitarius in the medulla
Swallowing center	Located in the reticular substance of the medulla and lower pons - coordinates the entire automatic sequence
Efferent (motor) pathway	CN V (trigeminal), CN IX (glossopharyngeal), CN X (vagus), CN XII (hypoglossal), and some superior cervical nerves

The sequence and timing of the swallowing reflex is stereotyped - it remains constant from one swallow to the next.

Summary Flow

Voluntary tongue movement → Bolus enters posterior pharynx
        ↓
Stimulation of tonsillar pillar receptors
        ↓
Afferents via CN V & IX → Tractus solitarius → Swallowing center (medulla/pons)
        ↓
Efferents via CN V, IX, X, XII
        ↓
Soft palate up | Larynx up + epiglottis folds | UES relaxes | Pharyngeal peristalsis
        ↓
Bolus enters esophagus → Primary/Secondary peristalsis → LES relaxes → Stomach

Clinical Relevance

Dysphagia can arise at any stage - oral, pharyngeal (most dangerous due to aspiration risk), or esophageal.
Damage to CN IX, X, or XII (e.g., stroke, bulbar palsy) disrupts the reflex arc.
A depressed or absent gag reflex correlates with pharyngeal stage dysfunction.
Aspiration pneumonia results from failure of the laryngeal/epiglottic protective mechanisms.
Achalasia is failure of the LES to relax (esophageal stage dysfunction).

Source: Guyton and Hall Textbook of Medical Physiology, p. 788-789

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