Anatomy and Physiology of Swallowing

This article or area is currently under construction and may only be partially complete. Please come back soon to see the finished work! (24/05/2024)

Original Editor - User Name

Top Contributors - Ewa Jaraczewska and Jess Bell  

Introduction

Deglutition is the process of swallowing which defines the movement of liquids or solids from the mouth to the stomach via the pharynx and esophagus. This process accommodates the changes in development as well as changes related to pathology associated with dysphagia. The elements of swallowing such as the suck, swallow, and breathing sequence evolve in the early stages of development and end with a conscious action of swallowing. Multiple organ systems are involved in this process and include musculoskeletal system, neuromuscular system and respiratory system. Normal swallowing requires a coordinated effort of over thirty muscles , the central nervous system, and five cranial nerves. Additionally the nose, the nassal cavity, the oral cavity, and the pharynx are key anatomy structures involved in this process. This article explores anatomy and physiology pertain to the process of normal swallowing.

Developmental Anatomy and Swallowing

Anatomical differences in the swallowing structures are present from infant, through older child, to adulthood. Alongside the anatomical changes , the swallowing adaptation occurs.

Infant:

• jaw and oral cavity are small
• tongue takes up most of the capacity of the mouth
• no teeth
• sucking is possible by using the tongue, sucking pads, and sulci
• sucking pads provide stability
• lack of distinct oropharynx
• the size of the larynx is 1/3 of an adult larynx and is located higher in the laryngeal cavity
• the base of the tongue is close to the larynx
• vocal cords consists of 1/2 of cartilage
• the epiglottis and soft palate touch when at rest

Older child:

• tongue lies at a lower position (floor of the mouth) because the mouth is bigger and the tongue sits behind the teeth
• the mandible extends down and forward
• the oral cavity expands
• the hyoid and larynx are positioned further down
• buccinators (cheek muscles) generate sucking mechanism
• the tongue pushes the food laterally to allow chewing with the teeth
• the sucking pads degenerate
• the pharynx lengthens vertically
• the nasopharynx becomes a 90-degree angle
• the epiglottis becomes wide and flattened
• the base of the tongue and the larynx separate by the age 4
• true vocal cords consist of no more than 1⁄3 of cartilage
• larynx starts to lower by the age of 2

Note: the changes listed above occur at the compromise of aspiration. These changes make the patient prone to aspiration but they are important for speech.

Adult Anatomy and Swallowing

Muscles

Table 1 lists major muscles involved in the swallowing and their function.

Table 1. Muscles of the swallowing

	Muscles	Function
Tongue muscles	Intrinsic tongue muscles: Hyoglossus Geniohyoid Genioglossus Styloglossus	Hyoglossus: depresses and retracts the tongue Geniohyoid: pulls the hyoid bone up and forward during swallowing for effective bolus flow and pharyngeal clearing Genioglossus: maintains upper airway patency by tongue protrusion and stiffening the tongue prevents posterior tongue displacement and upper airway closure Styloglossus: retracts and elevates the tongue
Masticatory muscles	Mylohyoid tensor veli palatini Masseter Temporalis Medial and lateral pterygoid muscles	tense the soft palate assist the levator veli palatini in elevating the palate to occlude and prevent entry of food into the nasopharynx during swallowing.
Muscles of the neck	Anterior belly of digastrics (ABD) Posterior belly of digastrics (PBD)	ABD: stabilizes the hyoid during swallowing PBD: protects the airway while eating
Soft palate and pharynx	Levator veli palatine (LVP) Palatopharyngeous (PPh) Salpingopharyngeous (SPh) Intrinsic laryngeal muscles (IL)	LVP: elevates the soft palate PPh: tenses the soft palate, pulls the pharyngeal walls superiorly, anteriorly, and medially during swallowing, effectively closes off the nasopharynx from the oropharynx. SPh: raises the pharynx and larynx during deglutition (swallowing), laterally draws the pharyngeal walls up IL: open and close the vocal folds, lengthen and shorten the vocal folds
Upper esophageal sphincter	The extrinsic muscles:Anterior group: the suprahyoid (geniohyoid, mylohyoid, stylohyoid, hyoglossus and anterior belly of digastric) the infrahyoid muscles (thyrohyoid, sternohyoid, sternothyroid and omohyoid). The extrinsic muscles: Posterior group: the stylopharyngeus, palatopharyngeus, and pterygopharyngeus. The intrinsic muscles: cricopharyngeus muscle (CPM) inferior constrictor the cranial aspect of the circular esopoesophaguscle	The extrinsic muscles:Anterior group: The suprahyoid muscles move the hyoid bone anterosuperiorly with contraction The infrahyoid muscles pull the larynx up and anteriorly with the hyoid bone. stabilize the larynx during deglutition Posterior group: shorten the pharynx, pulling the UES upwards while concomitantly widening it The intrinsic muscles: prevent the swallowing of air during respiration and phonation and the regurgitation of oesophagal contents into the airway

Cranial nerves

Cranial nerves modulate swallowing execution.Table 2 summerises the function and the clinical relevance of the six cranial nerves involved in the swallowing.

Table 2. Cranial nerves involved in swallowing

Cranial nerves	Function	Clinical Relevance
The trigeminal nerve (TN)CN V	Controls somatosensation of the face and the anterior two-thirds of the tongue Provides motor innervation of the following muscles: the mylohyoid muscle and the anterior belly of the digastric muscle, the masticatory muscles, and the tensor veli palatini muscle.	Impairment of the TN can lead to: Problems in the preparatory oral and oral phase of swallowing due to poor mastication and poor stabilization of the mouth floor. A decreased hyolaryngeal excursion during the pharyngeal phase of swallowing due to mylohyoid and anterior belly digastric muscle impairment.
The facial nerve (FN)CN VII	conveys taste sensation in the anterior two-thirds of the tongue controls the motor movement of the orbicularis oris and buccinator muscle, muscles involved in the closure and prevention of oral residue provides motor innervation of the stylohyoid muscle and the posterior belly of the digastric muscle, which retract the hyoid bone and assist in glossopalatal closure innervates the submandibular and sublingual salivary glands	Impairment of the FN can result in: decreased taste perception poor bolus formation during the preparatory oral phase anterior bolus spilling post swallow oral residue dry mouth
The glossopharyngeal nerve (GN)CN IX	controls somatosensation of the posterior one-third of the tongue, the mucosa of the soft palate and the upper pharyngeal tract provides autonomic innervation of the parotid gland motor innervation of the stylopharyngeus muscle, which assists in the opening of the upper esophageal sphincter	The GN dysfunction can result in: impaired pharyngeal bolus transport impaired upper esophageal sphincter opening, resulting in postswallow pharyngeal pooling
The vagal nerve (VN) CN X	provides motor to all striated muscles of the larynx and pharynx, except the stylopharyngeus muscle and the tensor veli palatini muscle the pharyngeal branches of the VN innervate the levator veli palatini, salpingopharyngeus, palatopharyngeus, palatoglossus, and the uvular muscle. the external superior laryngeal nerve (SLN) supplies the motor innervation of the cricothyroid muscle. the recurrent laryngeal nerve (RLN) is responsible for the motor innervation of all intrinsic laryngeal muscles except for the cricothyroid muscle. the RLN branches and the internal branch of the SLN (ISLN) provide mucosal sensory innervation of the pharynx, larynx, and proximal trachea.	Impairment of the VN can cause: poor velopharyngeal seal and nasal reflux, weak pharyngeal contraction reduced vocal fold adduction resulting in dysphonia and poor cough effectiveness impaired UES opening—post swallow pharyngeal pooling silent aspiration
The hypoglossal nerve (HN) CN XII	innervates all intrinsic and extrinsic tongue muscles, except for the palatoglossus muscle exclusive motor function controlling all movements of the tongue	Impairment of the HN can cause: dysarthria problems with oral control of the bolus, problems with bolus propulsion due to poor lingual pressure premature posterior spill of the bolus to the pharynx post swallow oral residue
The accessory nerve CN XI	motor innervation of the striated portions of the pharynx, larynx and oesophagus in association with the vagus nerve presence of parasympathetic fibres (general visceral efferent) accompanying the vagus nerve fibres prevent pressure from returning to the oral cavity elevates the palate and blocks the possible pressure escape from the oropharynx to the rhinopharynx	Impairment of the accessory nerve can cause: reduced vocal folds adduction

Anatomy structures

"Feeding and breathing share the same anatomy. "

The nose and nasal cavity: Breathing through the nose occurs during eating solids, since the mouth is engaged in processing the food and the lips are sealed to prevent the food from escaping anteriorly. Nasal air pressure oscillates with masticatory jaw movement and becomes, relative to atmospheric pressure, positive during jaw closing and negative during jaw opening.

The oral cavity: tongue movement corresponds with cyclic jaw movement when the food is in the mouth. The tongue and the cheek reposition food laterally (tongue) and medially (cheek).

The pharynx: a route for breathing, mastication and swallowing. The pharynx is dilated to maintain airway for breathing, and is constricted to provide space for bolus aggregation before the pharyngeal swallow.

The larynx and the vocal folds: the posterior aspect of the larynx forms the anterior wall of the upper esophageal sphincter (UES).

The upper oesophageal sphincter (UES): a kidney bean shaped space encompassed anteriorly by the larynx, posterolaterally by the pharyngoesophageal muscles, superiorly by the pharynx and inferiorly by the esophagus.

Neural Coordination of Swallowing

Swallowing requires a coordinated contraction of muscles in the mouth, pharynx, upper esophageal sphincter, and upper esophagus via central control. Swallowing centres activate the voluntary motor centres and inhibit the respiratory centres - this prevents food from entering the trachea. There is also activation of the:

• Reflex centres
• Nuclei of the cranial nerves that are involved in the movement of the tongue, larynx and pharynx

See the swallowing flow chart for details:

Physiology of Swallowing

Drinking and Swallowing of Liquid

Oral Preparatory Phase

• The ONLY voluntary phase of swallowing
• Formation of a liquid bolus in the mouth
• Holding the bolus in the anterior part of the mouth, known as the anterior floor of the mouth or on the surface of the tongue against the hard palate
• The tongue and the soft palate seal the oral cavity posteriorly to prevent the leakage of the liquid into the oropharynx

Resources

• bulleted list
• x

or

1. numbered list
2. x