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 six 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. 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. Cranial nerves involved in swallowing

Cranial nerves	Function
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.
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
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

Anatomy structures

the nose

the nassal cavity

the oral cavity

larynx and vocal folds

• The posterior aspect of the larynx, including the arytenoid and cricoid cartilages also forms the anterior wall of the upper esophageal sphincter (UES).
• The inferior constrictor muscles attach to the thyroid laminae, while the CPM attaches to the posterolateral aspects of the cricoid cartilage. At rest, these cartilages press against the spine, closing the potential space of the UES. During deglutition, the larynx is elevated off the spine by cervical and pharyngeal muscles decreasing the pressure required by the pharynx to push a food bolus into the UES and to the esophagus beyond.

the pharynx

the esophagus:

• the upper esophageal sphincter (UES) guards the entrance of the esophagus
• the relaxation of the cricopharyngeus muscle (CPM), elevation of the larynx and propulsion of the food bolus by the pharynx coordinate to open the UES and facilitate deglutition
• The UES is a kidney bean shaped potential space, about 4 cm in length, that is bounded anteriorly by the larynx, posterolaterally by the pharyngoesophageal muscles, superiorly by the pharynx and inferiorly by the esophagus
• The muscular control of the UES is primarily extrinsic by the suprahyoid and infrahyoid muscles, while intrinsic control of opening and closing is by the CPM, inferior constrictor muscle and the cranial circular esophageal muscle.

Neural Coordination of Swallowing

Central control of the swallowing process is required for orderly contraction of muscles in the mouth, pharynx, upper esophageal sphincter, and upper esophagus. These regions are striated muscle and need guiding motor input for their contraction. The more distal smooth muscle esophagus, including the lower esophageal sphincter, has a number of intrinsic control mechanisms for orderly contractile function that must interact and cooperate with the central control. Sensory information from the mouth and pharynx, and peripheral sensory–motor control mechanisms in the esophagus integrate and coordinate with the central control. This combination is a remarkable example of the brain–gut axis at work

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