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Swallowing, or deglutition, is a complex neuromuscular activity that involves the coordination of over 20 muscles in the mouth, pharynx, larynx, and oesophagus. These muscles work in harmony to propel food or liquid from the mouth to the stomach. The act of swallowing is initiated voluntarily by muscles in the tongue and mouth, but it is the brain that controls the intricate sequence of muscular contractions that follow, ensuring food travels safely down the oesophagus and into the stomach.
| Characteristics | Values |
|---|---|
| Number of muscles involved | Over 20 |
| Muscle groups involved | Mouth, throat, oesophagus |
| Cranial nerves involved | 12 pairs |
| Muscle control | Voluntary and involuntary |
| Phases | Oral, pharyngeal, oesophageal |
| Brain areas involved | Precentral gyrus, posterior-inferior gyrus, frontal gyrus, cerebral cortex, motor cortex, medulla, brainstem |
| Muscle types | Skeletal, smooth |
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What You'll Learn
The role of the tongue
The tongue is a muscular organ in the mouth that plays a crucial role in the complex process of swallowing, also known as deglutition. This process involves the coordination of over 30 nerves and muscles to transport substances from the mouth to the stomach via the pharynx and oesophagus.
The tongue is essential for manipulating food within the oral cavity and initiating the swallowing process. When chewing, the tongue works in conjunction with the cheeks to position food between the teeth for mastication. The tongue then presses the chewed food against the roof of the mouth, moistening it with saliva to form a bolus. This bolus is then propelled toward the back of the throat by the tongue, marking the beginning of the oral propulsive phase of swallowing.
In the oral propulsive phase, the tongue plays a key role in creating a seal to prevent leakage of food or liquid into the oropharynx. When consuming liquids, the posterior tongue rises to meet the soft palate, ensuring a proper seal. For solids, this seal is not formed, preserving free communication between the oral cavity and oropharynx during mastication. As the swallowing process initiates, the anterior tongue rises to meet the hard palate, while the posterior tongue drops, opening the oropharynx for the passage of the bolus.
The tongue's agility and flexibility are vital for effective swallowing. It can produce a wide range of movements, enabling the propulsion of the bolus toward the back of the throat. The tongue's sensitivity to touch is also important, allowing individuals to detect the texture and consistency of food. This sensitivity acts as a protective mechanism, helping to identify potential hazards such as small pieces of grit or bone fragments.
In summary, the tongue is a key player in the swallowing process, facilitating the manipulation and propulsion of food, creating seals to direct the bolus toward the oesophagus, and providing sensory feedback to ensure safe and efficient deglutition. Its muscular nature and ability to execute precise movements make it an indispensable component of the complex swallowing mechanism.
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The pharyngeal phase
The bolus is moved inferiorly by a peristaltic-like sequential contraction of the superior, middle, and inferior pharyngeal constrictor muscles in a top-to-bottom fashion. A pattern of rapid stimulation and inhibition in the pharyngeal muscles creates a cranial-caudal wave of pressure that directs the food bolus towards the UES. The speed is remarkably fast, occurring at rates of 20 to 40cm/s. Once initiated, the pharyngeal phase behaves in an 'all or none' fashion, much like a reflex, which is why this phase is considered an irreversible step.
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The oesophageal phase
Swallowing is a complex process involving the coordination of many muscle groups and nerves. It is divided into three stages: the oral phase, the pharyngeal phase, and the oesophageal phase. While the oral phase is voluntary, the pharyngeal and oesophageal phases are reflex actions.
During this phase, the bolus moves through the oesophagus, which is composed of striated muscle in the upper third, a mixture of striated and smooth muscle in the middle third, and smooth muscle in the lower third. The exact composition of the oesophageal muscle layers allows for the controlled passage of the bolus into the stomach.
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Involuntary vs voluntary control
Swallowing is a complex behaviour that involves the movement of substances from the mouth (oral cavity) to the stomach via the pharynx and oesophagus. It is a highly coordinated process that involves the action of over 20 to 30 muscles and multiple nerves under voluntary and involuntary control.
The process of swallowing can be divided into three main phases: the oral phase, the pharyngeal phase, and the oesophageal phase. The oral phase is a voluntary process that involves the conscious control of muscles in the mouth to form a food bolus and push it to the back of the throat. This stage is under neuro control by several areas of the cerebral cortex, including the motor cortex. The tongue plays a crucial role in this stage by moistening the food with saliva and pushing the bolus towards the throat.
The pharyngeal phase is the first irreversible step in the swallowing mechanism. It begins when the bolus reaches the palatoglossal arch and involves a rapid phase of muscle contraction to propel the bolus through the upper oesophageal sphincter (UES) and into the oesophagus. This stage is characterised by a wave of coordinated stimuli lasting about a second. It serves two primary purposes: directing food into the oesophagus and protecting the airway from aspiration. The pharyngeal phase is a reflex action that is not under voluntary control.
The oesophageal phase is an autonomous process not under voluntary control. It involves the propagation of the bolus inferiorly by a wave of peristalsis through the oesophagus and into the stomach. This phase occurs much slower than the pharyngeal phase, at a rate of about 3 to 4 cm/s. It ends once the bolus passes through the lower oesophageal sphincter (LES), which is tonically contracted at rest to prevent reflux from the stomach.
While the oral phase of swallowing is typically voluntary, there are instances where swallowing can occur spontaneously without conscious awareness. Spontaneous swallowing is a protective reflex action that occurs due to the accumulation of saliva and/or food remnants in the mouth. It can happen while awake or during sleep and is distinct from voluntary swallowing, which is associated with the desire to eat or drink.
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The role of the brain
The act of swallowing is a complex neuromuscular activity that involves the coordination of over 30 nerves and muscles. This process is controlled by the brain, which communicates with the muscles through several cranial nerves. These cranial nerves emerge from the brainstem, located at the base of the brain, and control functions such as swallowing, smelling, tasting, seeing, moving the face and eyes, and shrugging the shoulders.
The cerebral cortex, a part of the brain, plays a crucial role in the regulation of swallowing. Studies in animals and humans have shown that stimulation of the cerebral cortex can evoke the full swallowing sequence. In anesthetized sheep, for instance, swallowing can be triggered by stimulating a rostral region in the orbitofrontal cortex. In primates, the main cortical areas for eliciting a swallow are the dorsolateral and anterolateral frontal cortex, including the cortical masticatory area.
The cerebral cortex is involved in processing sensory and motor information related to swallowing. Functional magnetic resonance imaging (fMRI) and other advanced imaging techniques have helped researchers understand the neuroanatomy of swallowing and the role of the cerebral cortex. These studies have revealed that swallowing is mediated by multiple distinct cortical and subcortical regions, and lesions to these regions can result in dysphagia (difficulty swallowing).
The medulla, a small area of the brainstem, is critical in carrying out the swallowing reflex. Strokes involving the medulla can cause significant swallowing problems, and individuals may require temporary or permanent feeding tubes to prevent choking and aspiration pneumonia. The medulla receives information from various processing centres in the brain, including the cerebral cortex, and coordinates the efforts of the muscles involved in swallowing.
Swallowing is a complex behaviour that is learned early in development, with the suck-swallow-breath sequence evolving as infants are introduced to new foods. The coordination of swallowing improves with age, and the maturation of more refined swallowing abilities typically completes by the third or fourth year of life.
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Frequently asked questions
The process of swallowing, also known as deglutition, involves the movement of substances from the mouth (oral cavity) to the stomach via the pharynx and oesophagus.
Swallowing involves over 20 muscles of the mouth, throat and oesophagus.
The process of swallowing can be divided into three main phases: oral, pharyngeal, and oesophageal. The oral phase is entirely voluntary, while the pharyngeal and oesophageal phases are reflex actions.
The pharyngeal phase involves the contraction of the pharyngeal muscles, including the superior, middle and inferior pharyngeal constrictor muscles.
The act of swallowing is controlled by a group of nerves called cranial nerves, which emerge from the brainstem. The brain communicates with the muscles involved in swallowing through these cranial nerves.
