Anatomy and Physiology of Swallowing

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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:The anterior group: the suprahyoid (geniohyoid, mylohyoid, stylohyoid, hyoglossus and anterior belly of digastric) the infrahyoid muscles (thyrohyoid, sternohyoid, sternothyroid and omohyoid). The extrinsic muscles: The posterior group: the stylopharyngeus, palatopharyngeus, and pterygopharyngeus. The intrinsic muscles: cricopharyngeus muscle (CPM) inferior constrictor the cranial aspect of the circular esopoesophagus muscle	The extrinsic muscles:The 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 The 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 vagus 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 oesophageal 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 oesophagus.

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

The act of swallowing occurs during four phases: (1) oral preparatory phase, (2) oral propulsive phase, (3) pharyngeal phase, and (4) oesophageal phase. When drinking or swallowing liquid or eating solids, the initial phases of swallowing are different. The following two phases characterise the initiation of the swallowing of liquid and drinking:

1. The oral preparatory phase
2. The oral propulsive phase

When swallowing solids, the following two phases characterise the initial process of swallowing and they can overlap in time:

1. The oral preparatory phase which consists of:
   • stage 1 transport phase
   • food processing phase
2. The propulsive phase which includes:
   • stage 2 transport phase
   • bolus aggregation phase

Oral Preparatory Phase

Swallowing of Liquid

The oral preparatory phase starts as soon as you take a sip. It is a voluntary phase of swallowingand include the following steps:

1. Formation of a liquid bolus in the mouth
2. 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
3. The tongue and the soft palate seal the oral cavity posteriorly to prevent the leakage of the liquid into the oropharynx

Swallowing of Solids

The mastication and swallowing completed with food being pushed to the oropharynx for bolus formation is called the Process Model of Feeding. It includes two stages of transport and one stage of food processing.

1. Transport (stage 1)
   • the bolus is taken to the post-canine dentition and is rotated
   • the chewing and biting of the food happens in the occlusal surfaces of the post-canine dentition
2. Food Processing

• mastication and chewing to reduce the food particle size
• saliva softens the food, improves the consistency of the food, and makes the food very feasible for swallowing
• cyclical jaw movement, involving the tongue, cheek, soft palate, the hyoid bone
• the posterior cavity is open as compared to sealed posterior cavity when swallowing of liquid
• the cyclical movement of the jaw and soft palate allows the pumping of the air into the nasal cavity through the pharynx to deliver the aroma of the food to the chemoreceptors of the nose
• when the bolus of food is ready to be swallowed, it is placed on the surface of the tongue and is propelled backward to the oropharynx
• chewing continues to occur when the food remains in the oral cavity. The stages of chewing are as follow:
  • early jaw opening: lips are sealed, the tongue and the jaw moves forward and downward
  • late jaw opening: lips are open, the tongue has curled to prevent from getting bitten, the tongue moves mediolaterally and anteroposteriorly, and the hyoid bone acts as a stabiliser for the jaw and the tongue.

Clinical Relevance: Aging correlates with an increase number of chewing cycles and activity of the jaw adductor muscles during mastication. Despite the increase in mastication time, the bolus size at the time of swallow onset is larger in the elderly than in the young.

Oral Propulsive Phase

Swallowing of Liquid

The oral propulsive phase immediately follows the oral preparatory phase.

1. The tip of the tongue rises to touch the hard palate
2. As the tip of the tongue rises, the oral cavity opens
3. The tongue-to-palate contact area gradually widens in an anterior-posterior direction
4. The bolus is pushed backward along the pharynx

To prevent the aspiration of liquid, "the bolus is commonly held in the oral cavity until just before the initiation of the pharyngeal swallow." With solids, the food is propelled to the oropharynx, accumulated and bolus is formed prior to swallowing.

Swallowing of Solids

Transport (stage 2)

• the chewing continues to occur till the food is in the mouth
• food is transported to the oropharynx

Bolus aggregation

• bolus aggregation takes place until swallow onset

Clinical relevance: "The initial consistency of food affects the duration of oropharyngeal bolus aggregation before the swallow and the number of chewing cycles. Oropharyngeal bolus aggregation time is longer, and the number of chewing cycles greater, with hard than soft foods."

Pharyngeal Phase

The food is propelled from the pharynx to upper oesophageal sphincter to the oesophagus. During the pharyngeal phase the following activities must take place:

1. The passage of the food
   • tongue elevates and pushes the bolus against the wall of the pharynx
   • tongue elevation blocks the oral cavity
   • the pharyngeal constrictor muscles contract creating a peristaltic wave
   • the bolus is pushed in the downward direction.
2. The airway protection.

The epiglottis, arytenoids, and vocal folds are the three gatekeepers for aspiration prevention.

• The soft palate elevates and become in contact with the lateral and posterior walls of the pharynx. This activity closes up the nasopharynx
• The elevation of the soft palate prevents regurgitation of the food
• The vocal cords close and seal the epiglottis
• The transverse arytenoid muscles tilt forward to contract the epiglottic base and seal the epiglottis
• The suprahyoid muscle and the thyrohyoid muscle contract and pull the hyoid bone up
• Since the larynx is attached to the hyoid bone, it moves up and forward following muscle contraction
• As a result, the backward tilting of the epiglottis seals the laryngeal vestibule

All the above activities must occur before the upper oesophageal sphincter opens because once it is open, the bolus is going to pass down to the oesophagus.

In the next step of the swallowing process, the upper oesophageal sphincter (UES) opens, and the bolus is transported forward. For the UES to open, the following events must occur actively:

• relaxation of the cricopharyngeus muscle
• the bolus creates a pressure which helps in opening of the UES

Oesophageal Phase

The oesophagus is a tube-like structure. It extends from the upper oesophageal sphincter to the lower oesophageal sphincter. The upper one-third of UES is called a cervical oesophagus, and is made up of striated muscles. The thoracic oesophagus located in the lower two-thirds of the UES is made up of smooth muscles.

The oesophageal phase include the following activities:

• food passes from the upper oesophageal sphincter to the thoracic oesophagus
• a peristaltic wave develops which transport the bolus
• the peristalsis movement transport the bolus through the lower two-thirds of the UES

Swallowing and Respiration

In adults, the swallowing and breathing are tightly coordinated and swallowing usually starts during the expiratory phase of breathing. During swallowing , breathing briefly ceases due to physical closure of the airway by elevation of soft palate and tilting of epiglottis and a neural suppression of respiration in brainstem. This pause continues for 0.5-1.5 seconds during swallowing, and the respiration resumes with continued expiration after swallowing. This sequence of exhale-swallow-exhale prevents the inhalation of remaining food in pharynx.Other patterns of swallowing and respiration include (1) “inhale – swallow – exhale”, (2) “exhale-swallow-inhale” and (3)“inhale-swallow-inhale” and they rarely occur in healthy adults.

Note: "When performing sequential swallows while drinking from a cup, respiration can resume with inspiration." During mastication, the respiratory cycle duration decreases and than increases with swallowing following the sequence of exhale-swallow-exhale.

Resources

• Hao N, Sasa A, Kulvanich S, Nakajima Y, Nagoya K, Magara J, Tsujimura T, Inoue M. Coordination of Respiration, Swallowing, and Chewing in Healthy Young Adults. Front Physiol. 2021 Jul 13;12:696071.
• Matsuo K, Palmer JB. Coordination of Mastication, Swallowing and Breathing. Jpn Dent Sci Rev. 2009 May 1;45(1):31-40.
• Feeding and Swallowing Development in Children