Deglutition: The Tongue's Essential Role in Swallowing

What is the role of tongue in deglutition?

The tongue is a highly versatile muscular organ that plays an indispensable and multifaceted role in deglutition (swallowing), facilitating the intricate process from food manipulation and bolus formation to its precise propulsion through the oral cavity and pharynx.

Introduction to Deglutition

Deglutition, commonly known as swallowing, is a complex neuro-muscular process that transports food, liquid, and saliva from the mouth to the stomach. Far from a simple act, it involves the precise coordination of over 30 muscles and multiple cranial nerves across three distinct phases: the oral (preparatory and voluntary), pharyngeal (involuntary reflex), and esophageal (involuntary) phases. At the core of this intricate ballet of muscle contractions and sensory feedback lies the tongue, an organ whose anatomical structure and biomechanical capabilities are perfectly adapted for its pivotal role in ensuring efficient nutrient intake and airway protection.

Anatomy of the Tongue Relevant to Swallowing

The tongue is primarily composed of skeletal muscle, divided into two main groups:

• Intrinsic Muscles: These four pairs of muscles (superior longitudinal, inferior longitudinal, transverse, and vertical) originate and insert entirely within the tongue. They are responsible for altering the tongue's shape – lengthening, shortening, curling, flattening, and narrowing – which is crucial for precise food manipulation.
• Extrinsic Muscles: These four pairs of muscles (genioglossus, hyoglossus, styloglossus, and palatoglossus) originate outside the tongue and insert into it. They are responsible for larger movements, anchoring the tongue, and altering its position within the oral cavity.
  • Genioglossus: Protrudes and depresses the tongue, forming the main bulk of the tongue and crucial for pushing food posteriorly.
  • Hyoglossus: Depresses and retracts the tongue, contributing to bolus control.
  • Styloglossus: Retracts and elevates the tongue, assisting in bolus movement.
  • Palatoglossus: Elevates the posterior tongue and depresses the soft palate, forming the anterior faucial pillars and contributing to the seal between the oral cavity and oropharynx.

The motor innervation for most tongue muscles is provided by the Hypoglossal Nerve (CN XII), highlighting its critical role in tongue movement and, consequently, swallowing. Sensory innervation, crucial for detecting food texture and position, comes from the Trigeminal (CN V), Glossopharyngeal (CN IX), and Vagus (CN X) nerves.

Phases of Deglutition and the Tongue's Involvement

The tongue's role evolves significantly through the different phases of swallowing:

• Oral Preparatory Phase (Voluntary):

  • During this initial phase, the tongue works in concert with the teeth and cheeks to manipulate food, breaking it down and mixing it with saliva to form a cohesive ball called a bolus.
  • The intrinsic muscles precisely shape the tongue to contain the bolus and move it around the oral cavity.
  • The extrinsic muscles position the tongue to facilitate chewing and ensure the bolus remains on the chewing surfaces.
  • The tongue's ability to create a seal with the soft palate (via the palatoglossus muscle) prevents premature spillage of the bolus into the pharynx.

• Oral (Buccal) Phase (Voluntary):

  • Once the bolus is prepared, the tongue takes center stage for its propulsion. The tongue tip elevates to seal against the hard palate or alveolar ridge, followed by a wave-like posterior movement of the tongue dorsum.
  • The genioglossus muscle is particularly active, pushing the bolus backward and downward.
  • This action propels the bolus towards the faucial pillars, triggering the swallow reflex. The precise pressure exerted by the tongue is critical for efficient and safe bolus transfer.

• Pharyngeal Phase (Involuntary Reflex):

  • Upon initiation of the swallow reflex, the tongue's role shifts to a rapid, forceful action that clears the oral cavity and drives the bolus through the pharynx.
  • The base of the tongue retracts forcefully towards the posterior pharyngeal wall, generating significant pressure to push the bolus downward. This retraction also contributes to the inversion of the epiglottis, which covers the laryngeal inlet, protecting the airway from aspiration.
  • Simultaneously, extrinsic tongue muscles, particularly the genioglossus and hyoglossus, contribute to the anterior and superior elevation of the hyoid bone, which in turn pulls the larynx upward and forward, further safeguarding the airway.

• Esophageal Phase (Involuntary):

  • Once the bolus enters the esophagus, the tongue's direct role in propulsion diminishes. Peristaltic waves in the esophageal muscles take over to transport the bolus to the stomach. However, the integrity of the preceding oral and pharyngeal phases, heavily reliant on tongue function, is crucial for successful entry into the esophagus.

Biomechanical Actions of the Tongue During Swallowing

The complex movements of the tongue during deglutition can be broken down into several key biomechanical actions:

• Bolus Containment and Manipulation: Through the coordinated action of intrinsic and extrinsic muscles, the tongue shapes itself to cradle the bolus, preventing spillage and facilitating efficient mastication.
• Anterior-Posterior Propulsion: A critical "stripping" action occurs as the tongue creates a pressure wave, pressing the bolus against the hard palate and progressively moving it from the front to the back of the oral cavity. This action provides the initial force for the swallow.
• Base of Tongue Retraction: The powerful retraction of the tongue base against the pharyngeal wall is essential for generating the necessary propulsive force to move the bolus through the pharynx and into the esophagus. This action also contributes significantly to airway protection by helping to close the laryngeal vestibule.
• Hyolaryngeal Excursion: The extrinsic tongue muscles, particularly the genioglossus and hyoglossus, are instrumental in elevating and moving the hyoid bone and larynx anteriorly. This upward and forward movement widens the pharynx, shortens the distance the bolus must travel, and most importantly, pulls the larynx out of the path of the bolus, preventing aspiration.

Clinical Significance and Dysphagia

Given its central role, any impairment in tongue structure, strength, coordination, or sensation can lead to dysphagia, or difficulty swallowing. Conditions affecting the tongue's musculature (e.g., stroke, neurological disorders like ALS or Parkinson's disease, head and neck cancer treatment, muscular dystrophies) can compromise bolus formation, propulsion, and airway protection. This can result in:

• Oral residue: Food left in the mouth due to inefficient bolus clearance.
• Delayed swallow initiation: Inability to trigger the pharyngeal reflex.
• Aspiration: Food or liquid entering the airway, leading to pneumonia.

Rehabilitation for dysphagia often involves exercises targeting tongue strength, range of motion, and coordination to improve swallowing function and safety.

Conclusion

The tongue is far more than just an organ of taste; it is a meticulously engineered muscular hydrostat that is paramount to the process of deglutition. Its ability to undergo precise shape changes and execute powerful, coordinated movements allows for the efficient preparation, propulsion, and safe passage of food and liquid. Understanding the intricate biomechanics and anatomical contributions of the tongue is fundamental for appreciating the complexity of swallowing and for addressing disorders that compromise this vital physiological function.