Overview
The throat (see the image below) is part of both the digestive and respiratory systems and is responsible for coordinating the functions of breathing, swallowing, and phonation while protecting the airway from aspiration. [1] From superior to inferior, the throat is subdivided into three sections: oropharynx, laryngopharynx, and larynx. Together, they function to sense and propel a food bolus from the mouth to the esophagus in a coordinated fashion while protecting the airway. [1]
Gross Anatomy
The pharynx is bounded by the base of the skull superiorly; the cricoid cartilage inferiorly; and the nasal cavities, the oropharyngeal inlet, and the base of the tongue anteriorly.
Oropharynx
The boundaries of the oropharynx are the lower edge of the soft palate superiorly and the hyoid bone inferiorly. The anterior border is formed by the oropharyngeal inlet and the base of the tongue, and the posterior border is formed by the superior and middle pharyngeal constrictor muscles and their overlying mucosa.
The oropharynx communicates with the oral cavity via the oropharyngeal inlet, through which it receives a food bolus. The oropharyngeal inlet is made of the lateral palatoglossal folds, just anterior to the palatine tonsils. The folds are themselves made of the palatoglossus muscle, which originates on the palate itself and its overlying mucosa.
Inferiorly, the posterior one third of the tongue, or the base of the tongue, continues the anterior border of the oropharynx. The vallecula, which is the space between the base of the tongue and the epiglottis, forms the inferior border of the oropharynx. This is typically at the level of the hyoid bone.
Within the lateral walls of the oropharynx are the paired palatine tonsils, sitting in a fossa separated anteriorly by the palatoglossal folds and posteriorly by the palatopharyngeal folds. The tonsils are masses of lymphoid tissue that are involved in the local immune response to oral pathogens.
The muscles that form the posterior wall of the oropharynx are the overlapping superior and middle pharyngeal constrictors and their overlying mucous membrane. The glossopharyngeal nerve and the stylopharyngeus muscle enter the pharynx at the border between the superior and middle constrictors.
Laryngopharynx
The laryngopharynx, also known as the hypopharynx, constitutes the most inferior portion of the pharynx. [1] The borders of the laryngopharynx are the hyoid bone superiorly and the upper esophageal sphincter (UES), or cricopharyngeus muscle, inferiorly.
The anterior boundary of the laryngopharynx consists largely of the laryngeal inlet, which includes the epiglottis and the paired aryepiglottic folds and arytenoid cartilages. The posterior surface of the arytenoid cartilages and the posterior plate of the cricoid cartilage complete the anteroinferior border of the laryngopharynx. Lateral to the arytenoid cartilages, the laryngopharynx consists of the paired piriform sinuses, which are bounded laterally by the thyroid cartilage.
The posterior pharyngeal wall consists of the middle and inferior pharyngeal constrictor muscles and their overlying mucous membranes. Below this, at the level of the cricoid cartilage, the circumferential cricopharyngeus muscle forms the UES. This muscle tonically contracts during rest and relaxes during swallowing in order to allow a food bolus to pass into the esophagus.
The laryngopharynx is divided into three subsites: [1, 2]
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Piriform sinuses - These are paired recesses located on either side of the laryngeal inlet, bounded laterally by thyroid cartilage and medially by aryepiglottic folds.
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Posterior cricoid region - This area extends from the arytenoid cartilages to the inferior border of the cricoid cartilage and is composed of posterior cricoarytenoid muscle.
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Posterior pharyngeal wall - This region joins the lateral limits of the piriform sinus and is composed of inferior constrictor muscle. The esophagus and UES forms the inferior boundary of the laryngopharynx.
Larynx
The larynx extends from the epiglottis to the cricoid cartilage and includes the vocal folds, which can rapidly adduct when the laryngeal mucosa is stimulated, to prevent aspiration of food and other contents into the trachea. In addition to its role in airway protection, the larynx contains a number of cartilages and muscles that serve in voice production. It can be vertically subdivided into: [3]
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Supraglottis (from the epiglottis to the vocal folds) - It includes key structures such as the epiglottis, aryepiglottic folds, false vocal cords (vestibular folds), and the laryngeal ventricles.
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Glottis (at the level of the vocal folds) - This region includes the vocal cords and the space (rima glottidis) between them. The glottis plays a central role in sound production and airflow regulation during breathing and speaking.
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Subglottis (from the vocal folds to the level of the cricoid cartilage) - It is the region below the glottis, extending to the inferior border of the cricoid cartilage.
Musculature
The muscular layer of the pharynx consists of an inner longitudinal layer and an outer circular layer (see the images below). The inner layer includes the paired stylopharyngeus, salpingopharyngeus, and palatopharyngeus muscles. The outer layer includes the overlapping superior, middle, and inferior constrictor muscles.
The stylopharyngeus originates from the styloid process and inserts into the posterior border of the thyroid cartilage. It is unique as it's the only pharyngeal muscle innervated by the glossopharyngeal nerve (cranial nerve IX). Its primary function is to elevate the pharynx and larynx during swallowing. [2, 4]
The salpingopharyngeus originates from the cartilage posterior to the opening of the Eustachian tube and inserts into the fibers of the palatopharyngeus muscle. Innervated by the vagus nerve (cranial nerve X), it assists in elevating the pharynx and larynx during swallowing and contributes to the opening of the Eustachian tube to equalize middle ear pressure. [2, 4]
The palatopharyngeus muscle forms the posterior pillar of the palatine tonsil, and it originates from the posterior border of the hard palate and inserts on the posterior border of the thyroid cartilage, along with the stylopharyngeus. It is innervated by the pharyngeal plexus and functions in raising the throat and closing the oropharyngeal aperture.
The pharyngeal constrictors function in a coordinated manner during swallowing to propel a food bolus through the throat and into the esophagus. Each of the pharyngeal constrictors circumscribes the posterior wall of the pharynx and inserts on the median raphe, which is a midline continuation of the buccopharyngeal fascia (see Microscopic Anatomy).
The superior pharyngeal constrictor originates from the pterygoid hamulus, the pterygomandibular raphe, the posterior end of the mylohyoid line of the mandible, and the side of the tongue. It inserts into the pharyngeal raphe and the pharyngeal tubercle of the occipital bone. Innervated by the pharyngeal branch of the vagus nerve through the pharyngeal plexus, it constricts the upper part of the pharynx to facilitate swallowing. [2, 4]
The middle pharyngeal constrictor originates from the greater and lesser horns of the hyoid bone and the stylohyoid ligament and inserts into the pharyngeal raphe. Also innervated by the vagus nerve, it constricts the middle portion of the pharynx to facilitate the downward movement of the bolus. [2, 4]
The inferior constrictor originates from the oblique line of the thyroid cartilage and the cricoid cartilage and inserts into the pharyngeal raphe. Its innervation is primarily via the vagus nerve, with contributions from the recurrent laryngeal nerve and the external branch of the superior laryngeal nerve. This muscle constricts the lower part of the pharynx, playing a key role in the final stage of pharyngeal swallowing. [2, 4]
At the superior end of the superior constrictor lies the Passavant's ridge, which is a visible constriction during the elevation of the pharynx while swallowing. It is created by the interdigitation of palatopharyngeal fibers with the superior pharyngeal constrictor.
Vasculature
The arterial supply to the pharynx comes from four branches of the external carotid artery. The primary contribution is from the ascending pharyngeal artery, which comes off the external carotid just superior to the carotid bifurcation and passes posterior to the carotid sheath, giving off branches to the pharynx and tonsils. Along its path, it gives off pharyngeal branches that supply the pharyngeal constrictors, soft palate, palatine tonsil, and pharyngotympanic tube. It also forms anastomoses with branches of the ascending palatine artery (from the facial artery) and the ascending cervical artery (from the vertebral artery). [5]
The palatine branch of the facial artery enters the pharynx just superior to the superior pharyngeal constrictor. The facial artery also gives off ascending palatine and tonsillar branches, which help supply the superior pharyngeal constrictor and palate. The maxillary artery gives off the greater palatine artery and a pterygoid branch, and the dorsal lingual artery coming off the lingual artery adds a minor contribution.
Blood drains from the pharynx via an internal submucosal plexus and an external pharyngeal plexus contained in the outermost buccopharyngeal fascia. The plexus drains into the internal jugular vein and, occasionally, the anterior facial vein. Extensive communication exists between the veins that drain the throat and those that drain the tongue, esophagus, and larynx.
Lymphatic drainage
The pharynx drains into the deep cervical nodes along the carotid sheath. The hypopharynx may also drain into the paratracheal nodes.
The lymphatic vessels of the larynx drain into the deep cervical nodes, pretracheal nodes, and prelaryngeal nodes. The lymphatic drainage of the larynx is divided based on its anatomical regions. Structures located above the vocal folds drain into the superior deep cervical lymph nodes, while those below the vocal folds initially drain into pretracheal and paratracheal nodes before reaching the inferior deep cervical nodes. [2]
Nerve supply
The pharyngeal plexus of nerves provides the efferent and afferent nerve supply of the pharynx and is formed by branches of the glossopharyngeal nerve (cranial nerve IX), the vagus nerve (cranial nerve X), and sympathetic fibers from the cervical chain. Other than the stylopharyngeus, which is innervated by the glossopharyngeal nerve, all of the muscles of the pharynx are innervated by the vagus nerve.
All of the intrinsic muscles of the larynx are innervated by the recurrent laryngeal nerve, a branch of the vagus nerve, except for the cricothyroid muscle, which receives its innervation from the external branch of the superior laryngeal nerve, also a branch of the vagus nerve.
The pharyngeal plexus receives branches of the glossopharyngeal and vagus nerves to supply sensory innervation to the pharynx as well. The posterior one third of the tongue, in the oropharynx, receives both its taste and somatic sensation from the glossopharyngeal nerve. The cricopharyngeus, a subdivision of the inferior pharyngeal constrictor, forms the upper esophageal sphincter (UES) and receives its parasympathetic input for relaxation from the vagus nerve and its sympathetic input for contraction from postganglionic fibers from the superior cervical ganglion.
Microscopic Anatomy
The pharyngeal wall layers, from inside to outside, are as follows:
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Mucous membrane
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Pharyngobasilar fascia
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Muscle layer
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Buccopharyngeal fascia
The mucous membranes of the oropharynx and laryngopharynx consist of nonkeratinizing stratified squamous epithelium, which provides protection against mechanical stress from food passage. [1] Deep to this is an elastic lamina propria; this layer separates the mucous membrane from the fibrous pharyngobasilar fascia. This lamina propria contains a dense network of elastic fibers, blood vessels, and lymphatics, facilitating nutrient exchange and immune responses. [1]
The pharyngobasilar fascia is a submucosal layer that provides structural support to the pharyngeal wall. It is thickest at its superior attachment to the skull base, where it anchors to the basilar part of the occipital bone and the medial pterygoid plates of the sphenoid bone. As it descends, the fascia becomes thinner, blending with the outer circular muscle layer of the pharynx. This fascia acts as a scaffold for muscle attachment and contributes to maintaining the integrity of the pharyngeal lumen during swallowing. [6]
The muscles of the pharynx include an inner longitudinal layer and an outer circular layer (see Gross Anatomy). The outermost buccopharyngeal fascia is a thin, fibrous layer containing the pharyngeal plexus of nerves and veins. Its attachments are the prevertebral fascia posteriorly and the styloid process and the carotid sheath laterally.
Pathophysiologic Variants
A diverticulum in the pharynx is an outpouching of tissue caused by the herniation of mucosa or submucosa and muscle through the pharyngeal wall. The most likely location for a diverticulum to develop is an area just along the posterior pharyngeal wall where there is a zone of sparse muscle fibers (known as Killian's triangle), just superior to the cricopharyngeus portion of the inferior pharyngeal constrictor muscle.
A Zenker's diverticulum forms from increased pressure within the lumen of the pharynx, usually as a result of uncoordinated peristalsis and cricopharyngeal hyperactivity. It most commonly occurs during the seventh or eighth decades of life and is far more common in males than females. Its most common presenting symptom is dysphagia, but it may also manifest with regurgitation (reappearance of ingested food in the mouth), weight loss, aspiration (food regurgitated into airway), cough, and halitosis (food trapped is digested by microorganisms). The standard treatment for a Zenker’s diverticulum is a cricopharyngeal myotomy. The condition is usually painless. [7]
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Mouth and throat, sagittal section.
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Throat, external lateral view.
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Muscles of the pharynx, partially opened posterior view.
