Swallowing Reflex

The swallowing reflex, also known as deglutition, is a complex process that moves food from the oral cavity into the stomach. It involves a coordinated sequence of voluntary and involuntary actions, ensuring efficient food transport while preventing aspiration.

Mechanisms

Swallowing is divided into several phases: oral, pharyngeal, and esophageal.

• Oral Phase (Voluntary)

    ◦ Begins with chewing, which lubricates food by mixing it with saliva and decreases the size of food particles to facilitate swallowing and initial digestion.

    ◦ The food bolus is propelled into the pharynx by the tongue pushing against the hard palate.

    ◦ This phase can be voluntarily initiated, but a stimulus (even a small amount of saliva) is required to trigger the involuntary reflex.

• Pharyngeal Phase (Involuntary Reflex)

    ◦ This is a rapid phase, typically less than 1 second.

    ◦ Upon initiation, the nasopharynx closes to prevent food from entering the nasal cavity, and breathing is temporarily inhibited (deglutition apnea).

    ◦ The glottis closes, and the epiglottis moves downward to seal off the trachea, preventing food from entering the airway. This inhibition of breathing is a reflex phenomenon where sensory nerve endings in the pharynx mucosa are stimulated, sending afferent impulses via the 5th, 9th, and 10th cranial nerves to the deglutition center in the medulla, which then inhibits the respiratory center.

    ◦ A peristaltic contraction of the pharyngeal musculature propels the bolus into the esophagus.

    ◦ Simultaneously, the upper esophageal sphincter (UES) relaxes shortly before the distal pharyngeal muscles contract, allowing bolus passage.

• Esophageal Phase (Involuntary Reflex)

    ◦ This phase is slower, with peristaltic contractions moving down the esophagus at 2 to 6 cm/second, potentially taking 10 seconds for the bolus to reach the lower end.

    ◦ The body of the esophagus undergoes a peristaltic contraction, which begins just below the UES and moves sequentially towards the stomach.

    ◦ There are two types of esophageal peristalsis:

        ▪ Primary peristalsis: Initiated by the act of swallowing.

        ▪ Secondary peristalsis: Initiated by stimulation of receptors within the esophagus (e.g., from distention by residual food).

    ◦ Shortly before the peristaltic contraction reaches the lower esophageal sphincter (LES), both the LES and the orad (proximal) region of the stomach relax. This process is termed receptive relaxation.

    ◦ Receptive relaxation allows the stomach to accommodate large volumes of ingested material with only a minimal rise in intragastric pressure, for example, 1600 cubic centimeters (cc) of air with a pressure rise of no more than 10 mm Hg.

    ◦ After the bolus passes, the LES contracts back to its resting tone.

Essential Features

• Coordinated Involuntary Reflex: Once initiated, swallowing proceeds as a highly coordinated involuntary reflex.

• Sequential Muscle Activation: Pharyngeal and esophageal musculature is activated in a proximal-to-distal manner, leading to peristaltic contractions.

• Sphincter Regulation: The UES and LES act in a coordinated fashion, relaxing to allow bolus passage and then contracting to maintain barrier function between swallows.

• Gastric Accommodation: The orad stomach relaxes simultaneously with the LES, preparing to receive the swallowed bolus without a significant increase in internal pressure (receptive relaxation).

Regulation

The coordination of the swallowing reflex is primarily central in origin, involving both neural and hormonal influences.

• Swallowing Center: An area within the reticular formation of the brainstem acts as the swallowing center. This center integrates afferent impulses and coordinates the activity of various cranial nerve nuclei.

• Neural Pathways:

    ◦ Afferent (Sensory) Impulses: Sensory input from the pharynx is directed to the swallowing center. Information is carried by cranial nerves, including the 5th (trigeminal), 9th (glossopharyngeal), and 10th (vagus) nerves.

    ◦ Efferent (Motor) Impulses:

        ▪ For the pharyngeal musculature and striated muscle of the upper esophagus, efferent impulses are distributed via nerves from the nucleus ambiguus (part of the vagal nuclei). These nerves directly synapse with the striated muscle fibers.

        ▪ For the smooth muscle areas of the esophagus (lower esophagus), the LES, and the orad stomach, activation occurs via the dorsal motor nucleus (part of the vagal nuclei). These visceral motor nerves synapse with ganglion cells of the intrinsic (enteric) nerve plexuses, which then innervate the smooth muscle cells.

• Central vs. Peripheral Control:

    ◦ Pharyngeal and upper esophageal (striated muscle) peristalsis is primarily regulated by the central nervous system, with sequential impulses from the swallowing center.

    ◦ Lower esophageal (smooth muscle) peristalsis and LES relaxation are coordinated by both central nervous system pathways and intrinsic (enteric) nerves. Peristalsis can still occur after bilateral vagotomy in smooth muscle areas, indicating local coordination by intrinsic nerve plexuses or smooth muscle cells.

• Neurotransmitters:

    ◦ The transient relaxation of the LES and the receptive relaxation of the orad stomach during swallowing are mediated through enteric inhibitory nerves.

    ◦ While the exact neurochemical basis is not fully known, vasoactive intestinal peptide (VIP) and nitric oxide (NO) have been proposed as neurotransmitters responsible for smooth muscle relaxation in these regions. VIP physiologically mediates the relaxation of GI smooth muscle, and its effects are often mediated by NO.

Applied Clinical Aspects

• Injury to Swallowing Center: Damage to the swallowing center can lead to abnormalities in the pharyngeal component of swallowing.

• Achalasia: This is a condition characterized by a failure of the LES to relax properly during swallowing and a loss of peristalsis in the lower esophagus.

• Diffuse Esophageal Spasm: This involves uncoordinated and forceful contractions of the esophageal smooth muscle, which can impair bolus transit.

• Gastroesophageal Reflux Disease (GERD) / Heartburn: Occurs if the tone of the LES is decreased, allowing gastric contents (acid) to reflux back into the esophagus.

    ◦ A useful test for episodes of acid reflux is 24-hour monitoring of intraesophageal pH, where a small pH probe is positioned 5 cm above the LES.

• Vagotomy: Section of the vagus nerves can result in a decrease in gastric emptying of solids and affects contractions of the caudad stomach. In the context of swallowing, cutting the vagus nerve (cervical vagotomy) still allows peristalsis in smooth muscle areas of the esophagus, indicating the role of intrinsic nerves.

• No Receptive Relaxation of Orad Stomach: If manometry reveals normal peristalsis in both upper and lower esophagus but no receptive relaxation of the orad stomach, this suggests a defect in the vagal nerves or intrinsic nerves that mediate this relaxation, specifically those originating from the dorsal motor nucleus of the brainstem.