Swallowing

Phases of Swallowing

Swallowing is a fundamental component of digestion, allowing food to be safely transferred from the oral cavity to the stomach via the oesophagus. The process is divided into three phases:

• Oral phase (voluntary) – propulsion of the food bolus from the mouth into the oropharynx.
• Pharyngeal phase (involuntary) – transfer of the food bolus from the oropharynx into the oesophagus while protecting the airway.
• Oesophageal phase (involuntary) – movement of the food bolus through the oesophagus and into the stomach for further digestion.

Oral Phase

This is the first stage of swallowing and is under voluntary control.

Mastication, which involves the chewing and mixing of food with saliva, leads to the formation of a cohesive food bolus. The bolus is retained within the oral cavity by elevation of the posterior tongue against the soft palate. This prevents premature entry into the oropharynx while the airway remains open.

Following this, the tongue presses against the hard palate and propels the food bolus into the oropharynx. This action briefly inhibits inspiration and stimulates sensory receptors that trigger the swallowing reflex, marking the transition to the pharyngeal phase of swallowing.

The duration of the oral phase varies depending on food texture and conscious control.

Pharyngeal Phase

This is the second stage of swallowing and is under involuntary control.

It begins when the food bolus enters the oropharynx and stimulates pressure receptors in the soft palate and anterior pharynx, activating the swallowing centre in the brainstem. This triggers a series of coordinated actions to protect the airway and facilitate bolus passage:

• Inhibition of respiration
• Elevation of the soft palate – to close the nasopharynx and protect the nasal cavity
• Elevation and anterior displacement of the larynx – to prevent aspiration
• Closure of the glottis and adduction of true vocal cords – to prevent aspiration
• Opening of the upper oesophageal sphincter

The bolus is then propelled from the pharynx into the oesophagus via peristalsis of the pharyngeal constrictor muscles. Gravity plays a minimal role in this process and the speed of bolus movement is primarily determined by bolus viscosity and volume.

Oesophageal Phase

This is the third and final stage of swallowing and is under involuntary control.

The oesophagus is composed of three regions: the upper third contains skeletal (voluntary) muscle, the lower third contains smooth (involuntary) muscle, and the middle third contains a mixture of both.

Once in the oesophagus, the larynx lowers, returning to its normal position. The cricopharyngeus muscle (upper oesophageal sphincter) then contracts to prevent reflux, and normal respiration resumes.

The bolus is subsequently propelled down the oesophagus toward the stomach via peristalsis, coordinated by extrinsic nerves. Each segment of muscle systematically relaxes ahead of the bolus to allow passage, then contracts behind it to move it onward.

Peristalsis is primarily controlled by the autonomic nervous system, and bolus transit occurs at approximately 3-5 cm per second, thus taking around 9 seconds to reach the stomach.

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Fig 1
Diagram showing the three phases of swallowing.

Swallowing Reflex

The swallowing reflex is coordinated by a group of neurones in the medulla oblongata, often referred to as the deglutition or swallowing centre. This centre integrates sensory (afferent) input from the mouth and pharynx and produces a coordinated motor (efferent) output, ensuring swallowing occurs in the correct sequence.

Afferent Arm

When a food bolus touches the oropharynx, sensory receptors are stimulated. The main sensory nerves involved are:

• Trigeminal (CN V) – relays sensory input from the tongue
• Glossopharyngeal nerve (CN IX) – relays sensory input from the oropharynx.
• Vagus nerve (CN X) – relays sensory input from the laryngopharynx.

These signals are relayed to the nucleus tractus solitarius (NTS) in the medulla oblongata.

Efferent Arm

Once the swallowing reflex is initiated, motor signals are transmitted from the medulla oblongata (primarily via the nucleus ambiguus) to the muscles of swallowing through several cranial nerves:

• Trigeminal nerve (CN V3) – innervates the muscles of mastication
• Hypoglossal (CN XII) – allows tongue movements
• Facial nerve (CN VII) – innervates the lips, cheeks and submandibular and sublingual salivary glands
• Glossopharyngeal (CN IX) – innervates a pharyngeal muscle and parotid glands
• Vagus nerve (CN X) – innervates the pharynx, larynx and oesophagus

Clinical Relevance

Dysphagia

Dysphagia is the term for difficulty swallowing. Causes can be broadly classified as problems with motility (tending to affect liquids first) or obstruction (tending to affect solids first).

Motility	Obstruction
Stroke	Tumours
Alzheimer’s disease	Oesophageal strictures
Achalasia	Hiatus hernia

Another common cause of dysphagia is xerostomia (dry mouth).

The presence of dysphagia is typically identified via a barium swallow study. Further tests, such as CT scans, can be used to identify the cause.

Treatment depends on the cause and severity of the patient’s symptoms. Options include swallowing therapy, dietary changes, surgery or, in extreme cases, nasogastric tubes.

Complications of dysphagia include aspiration, dehydration and weight loss.

By Normaler_Schluck-00.jpg (and others): Hellerhoff derivative work: Anka Friedrich [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Fig 2
A normal barium swallow.