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The act of swallowing is a complex neuromuscular activity that involves the coordination of over 30 nerves and muscles. It is a three-stage process, with the first stage, the oral phase, being entirely voluntary. This is followed by the pharyngeal phase, which is involuntary, and finally, the esophageal phase. The muscles involved in the oral phase include the genioglossus, hyoglossus, styloglossus, and mylohyoid. The pharyngeal phase involves the contraction of the pharyngeal muscles, including the stylopharyngeus, salpingopharyngeus, and palatopharyngeus, and the constriction of the pharynx by the superior, medial, and inferior pharyngeal constrictor muscles. The final stage of deglutition, the esophageal phase, involves rhythmic muscular contractions (peristaltic waves) and pressure within the esophagus to push food downward.
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What You'll Learn
The oral phase
Once the bolus is ready to be swallowed, the superior longitudinal muscle elevates the apex of the tongue to make contact with the hard palate. The bolus is then propelled to the posterior portion of the oral cavity. The tongue plays a crucial role in this phase, receiving supply from CN XII, while the mylohyoid muscle is also involved, receiving innervation via CN V3.
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The pharyngeal phase
The act of swallowing is a complex neuromuscular activity that can be divided into three phases: oral, pharyngeal, and oesophageal. The pharyngeal phase is the first irreversible step in the swallowing mechanism. It is an involuntary process that begins when the bolus of food reaches the palatoglossal arch. This phase involves the coordination of multiple nerves and nerve plexuses, including CN IX, X, and XI, which transmit stimuli to the solitary tract nucleus in the brainstem.
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The esophageal phase
Swallowing is a complex neuromuscular activity that can be broken down into three phases: the oral phase, the pharyngeal phase, and the esophageal phase. The first phase is voluntary, while the latter two are reflex actions. The esophageal phase involves the movement of a bolus of food through the esophagus and into the stomach.
The smooth muscles of the thoracic esophagus and LES receive autonomic motor innervations consisting of extrinsic preganglionic fibers carried in the vagus nerves and intramural postganglionic neurons that are part of the myenteric plexus. The autonomic innervation has both an excitatory and inhibitory pathway. The excitatory pathway consists of preganglionic and postganglionic neurons, which are both cholinergic and act by releasing acetylcholine. The inhibitory pathway, on the other hand, consists of preganglionic cholinergic and postganglionic nitrergic neurons that release a vasoactive intestinal peptide and nitric oxide to exert inhibitory action on the smooth muscles.
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Muscles involved in the oral phase
Swallowing is a complex, multifaceted process involving a variety of muscles and nerves. It is the starting point for the peristaltic transport of food to the stomach. Deglutition or swallowing can be divided into three distinct phases: oral phase, pharyngeal phase, and oesophageal phase. The oral phase is entirely voluntary and is mainly controlled by the medial temporal lobes and limbic system of the cerebral cortex, with contributions from the motor cortex and other cortical areas.
The oral phase of swallowing begins when food or liquid is voluntarily moved to the rear of the oral cavity. The mandible elevates, and the lips adduct to assist in oral containment of the food and liquid. Food is moistened by saliva from the salivary glands and mechanically broken down by the action of the teeth, controlled by the muscles of mastication. This results in a bolus, which is moved from one side of the oral cavity to the other by the tongue. The buccinator helps to contain the food against the occlusal surfaces of the teeth.
The tongue receives its blood supply via the lingual artery, a branch of the external carotid. The genioglossus, hyoglossus, and styloglossus are supplied by CN XII, while the palatoglossus muscle receives supply from the vagus nerve (CN X). The mylohyoid muscle is also involved and receives its innervation via CN V3. Each of these muscles helps the tongue carry out its wide range of potential movements involved in swallowing and are integral components to the oral stage of swallowing.
The bolus is ready for swallowing when it is held together by saliva (largely mucus) and sensed by the lingual nerve of the tongue. The tongue then pushes the bolus to the back of the mouth, and the oral phase ends. The pharyngeal phase then begins when the bolus reaches the palatoglossal arch of the oropharynx.
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Muscles involved in the pharyngeal phase
Swallowing is a complex neuromuscular activity that can be divided into three phases: oral, pharyngeal, and oesophageal. The pharyngeal phase, the first irreversible step in the swallowing mechanism, is an involuntary process that begins when the bolus reaches the palatoglossal arch. During this phase, the tongue blocks the oral cavity, and the nasopharynx is sealed off from the oropharynx and laryngopharynx by the elevation of the soft palate and its uvula. The pharynx then receives the bolus after shortening and widening, and the larynx elevates due to the contraction of suprahyoid muscles and longitudinal pharyngeal muscles.
The longitudinal pharyngeal muscles, including the stylopharyngeus, salpingopharyngeus, and palatopharyngeus, function to condense and expand the pharynx, as well as help elevate the pharynx and larynx during swallowing. The superior, medial, and inferior pharyngeal constrictor muscles constrict the pharynx in a coordinated fashion to propel the bolus toward the upper oesophageal sphincter (UES). These muscles are innervated by the glossopharyngeal nerve (CN X), except for the cricopharyngeus muscle, which is innervated by the recurrent laryngeal nerve.
The pharyngeal phase serves two critical purposes: directing food into the oesophagus and protecting the airway from aspiration. During this phase, the epiglottis closes the trachea, preventing food or liquids from entering the airway. The palatopharyngeal folds on each side of the pharynx are brought together by the superior constrictor muscles, allowing only a small bolus to pass through. The adduction of the vocal cords is achieved by the contraction of the lateral cricoarytenoids and the oblique and transverse arytenoids, all innervated by the recurrent laryngeal nerve of the vagus nerve.
The pharyngeal phase is under the autonomic control of the swallowing centre, located in the lower pons and medulla oblongata of the brainstem. The nucleus ambiguus in the reticular formation is part of the swallowing centre and is responsible for generating general somatic efferent signals. These nerve impulses are transmitted through various cranial nerves to innervate the skeletal muscles of the pharynx and upper oesophagus.
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Frequently asked questions
Swallowing is a complex neuromuscular activity that involves the coordination of 22 different muscle groups, including the oral cavity, pharynx, and esophagus. It is divided into three stages: oral, pharyngeal, and esophageal.
The oral phase is voluntary and involves the tongue, genioglossus, hyoglossus, styloglossus, and mylohyoid muscles. These muscles help the tongue carry out its wide range of movements during swallowing. The masseter, temporalis, and lateral and medial pterygoid muscles are also involved in breaking down food before it is swallowed.
The pharyngeal phase is involuntary and involves the pharyngeal muscles, including the stylopharyngeus, salpingopharyngeus, palatopharyngeus, and superior, medial, and inferior pharyngeal constrictor muscles. The palatoglossus muscle is also involved, receiving supply from the vagus nerve.
