Pharynx: What is it? Structure, Innervation, Microscopic Anatomy, Pathophysiological Variants and Functions

It is the part of the throat that is behind the mouth and the nasal cavity, above the esophagus and the windpipe.

The pharynx is a part of the digestive system and also the conductive zone of the respiratory system.

The pharynx is the upper expanded part of the alimentary system. It links the oral cavity and nasal cavities in the head with the larynx and esophagus in the neck.

It is a common path for food and air.

The pharynx extends from the base of the skull to the lower border of the cricoid cartilage anteriorly and the lower border of the sixth cervical vertebra (C6) posteriorly.

The pharynx is a fibromuscular tube that is semicircular in cross section and is located directly anterior to the spinal column.

It extends from the base of the skull to the lower edge of the cricoid cartilage.

Structure

Six muscles are predominantly responsible for the voluntary actions of the pharynx:

  • Three pharyngeal constrictor muscles circularly overlapping one on top of the other.
  • Three vertically oriented muscles (stylopharyngeal, salpingopharyngeal and palatopharyngeal).

These muscles are involved in the swallowing process.

The pharynx represents a continuation of the digestive cavity, and provides a route from the oral cavity of the esophagus.

The pharynx communicates with the nasal cavities, the middle ear cavities, and the larynx.

The pharynx is often described from both exterior and interior perspectives.

Depending on its location, there are three sections inside the pharynx: nasopharynx, oropharynx and laryngopharynx.

To better interpret the anatomy of the pharynx, the pharynx is often divided into outer and inner sections.

The external surface is typically described from posterior and lateral views and includes the external superficial muscles that make up the wall of the pharynx, the associated nerves, and the blood supply.

However, the interior of the pharynx is typically described from a sagittal cross section or posterior view as a dissection in which the outer wall of the pharynx is divided in the midline.

The remaining halves are retracted laterally to reveal the internal anatomical sites and muscles of the nasopharynx, oropharynx, and laryngopharynx.

The following sections describe the outer and inner pharynx in detail.

Exterior of the pharynx

From the posterior view, the outer surface of the pharynx lies in front of the cervical vertebrae.

It connects to the occipital bone anterior to the foramen magnum.

The outer surface of the pharynx consists of a voluntary muscle covered externally by a thin oropharyngeal fascia, which is continuous with the outer surface of the buccinator muscle.

The pharyngeal venous plexus and the pharyngeal nerve plexus are located directly in the upper part of the oropharyngeal fascia.

Posterior to the oropharyngeal fascia but anterior to the alar fascia (prevertebral fascia), there is a space free of occupation of connective tissue, known as the retropharyngeal space.

Specifically, the retropharyngeal space extends superiorly to the base of the skull and inferiorly to the infrahyoid region of the neck.

Within the retropharyngeal space, slightly superior to the bifurcation of the carotid artery, there is a large retropharyngeal lymph node along with minor lymph nodes that drain most of the lymphatic vessels of the pharynx.

Continuing with the retropharyngeal space but extending laterally around the pharynx there are the left and right parapharyngeal spaces.

The parapharyngeal spaces are attached superiorly at the base of the skull, but unlike the retropharyngeal space they extend inferiorly only to the submandibular gland sheath in the suprahyoid region.

These spaces are joined medially by the pharynx and laterally by the pterygoid muscles of the infratemporal fossa and the parotid fascia.

The outer wall of the pharynx consists mainly of four muscles: superior pharyngeal constrictor, middle pharyngeal constrictor, inferior pharyngeal constrictor, and stylopharyngeal.

The three pharyngeal constrictor muscles are arranged in a simple manner, as they overlap each other in a vertical arrangement.

Although the three pharyngeal constrictor muscles have different origins anteriorly, all their fibers insert in the midline posteriorly to form the pharyngeal raphe.

The superior pharyngeal constrictor muscle originates from multiple anatomic sites on the lateral aspect before forming the posterior wall of the pharynx.

Specifically, it originates from the posterior border of the pterygomandibular raphe, the pterygoid hamulus of the sphenoid bone, the posterior end of the mylohyoid line of the mandible, and the lateral aspect of the tongue.

Once the fibers reach the posterior aspect of the pharynx, they join the pharyngeal raphe, which attaches to the pharyngeal tubercle in the occipital bone.

Above the superior constrictor muscle is the pharyngobasilar fascia, which serves as an attachment for the pharynx to the base of the skull.

Pharyngobasilar fascia is defined as a distinct submucosal membranous fascia that lies between the muscular and mucosal layers of the pharyngeal wall.

It has attachments to the basilar part of the occipital bone, the petrous part of the temporal bone anterior to the carotid canal, the border of the medial pterygoid plate and, finally, to the pterygomandibular raphe.

The levator veli palatini muscle, the cartilaginous end of the auditory tube (Eustachium), and the ascending palatal artery pierce the pharyngobasilar fascia above the superior border of the superior pharyngeal constrictor muscle.

The median pharyngeal constrictor muscle lies directly below the superior pharyngeal constrictor.

Specifically, it originates from the stylohyoid ligament as well as the greater and lesser horns of the hyoid bones.

As the fibers of the middle pharyngeal constrictor muscle spread backward, some of the fibers overlap with the superior constrictor muscle and the remaining fibers overlap with the inferior pharyngeal constrictor muscle.

The inferior pharyngeal constrictor muscle separates into two separate muscles, the thyropharyngeal and the cricopharyngeus, because they have different origins, attachments, and functions.

As its name implies, the thyropharyngeal originates from the thyroid cartilage and inserts into the pharyngeal raphe.

However, the cricopharyngeus originates from the lateral surface of the cricoid cartilage.

Unlike the other pharyngeal constrictors, the inferior cricopharyngeal fiber bypasses the pharyngeal raphe and inserts into the circular fibers of the esophagus.

Due to its insertion point, the cricopharyngeus muscle serves as an upper esophageal sphincter.

It is in a state of tonic contraction until swallowing begins to prevent regurgitation of gastric and esophageal contents into the pharynx.

Increased tone of the cricopharynx plays a role in the development of false Zenker’s diverticulum, where increased pressure causes extravasation of a mucous bag in a weak area of ​​the pharyngeal wall.

Most of the time, this occurs in Killian’s triangle, which is a small area between the thyropharyngeal and the cricopharyngeal with no musculature.

The stylopharyngeus muscle is the fourth external pharyngeal muscle.

As the name implies, the muscle originates on the medial surface at the base of the styloid process of the temporal bone.

Parts of the fibers fuse between the superior and middle pharyngeal constrictor muscles and the rest attach to the posterior border of the thyroid cartilage.

With an understanding of the major muscles of the outer pharynx, it is easier to delve into some details about the important associated structures, including the nerves and the vasculature.

Superior and posterolaterally from the pharyngobasilar fascia, the jugular foramen serves as an exit site for cranial nerves IX, X, and XI.

Relative to each other, cranial nerve IX exits the jugular foramen anterior to cranial nerve X and cranial nerve XI.

Cranial nerve IX travels posteriorly along the stylopharyngeus muscle and enters the space between the superior and middle pharyngeal constrictor muscle, as does the stylopharyngeus muscle.

Posterior to cranial nerves IX, X, and XI, the superior portion of the internal jugular vein exits the jugular fossa.

Slightly medial to the jugular foramen, the XII cranial nerve exits the hypoglossal canal and travels posterolaterally through the internal jugular vein.

Anteriorly to the jugular foramen is the upper end of the internal carotid artery as it enters the posterior part of the carotid canal.

It should be noted that the sympathetic trunk, including the superior cervical ganglion, is directly behind the internal carotid artery as it travels down the lateral aspect of the pharyngeal constrictor muscles and fuses to form the common carotid artery.

Examining from a lateral perspective, there are spaces between the pharyngeal constrictor muscles.

These gaps serve as entry points for various structures.

As mentioned above, superior to the superior constrictor muscle, the space allows entry of the levator veli palatini muscles and the auditory tube.

Also mentioned above, the stylopharyngeus muscle, cranial nerve IX, and the stylohyoid ligament enter between the superior and middle pharyngeal constrictor muscles.

Continuing along the outer pharyngeal wall, the gap between the middle and lower pharyngeal constrictor muscles allows entry of the internal branch of the superior laryngeal nerve and the superior laryngeal artery.

Finally, the inferior constrictor muscle allows entry to the recurrent laryngeal nerve.

Inside the pharynx

The best way to examine the interior of the pharynx is a sagittal cross section or posteriorly after midline dissection of the outer surface of the pharynx.

Nasal part (nasopharynx)

The nasopharynx is defined anatomically as the region above the soft palate that communicates with the nasal cavity through the choanas.

Specifically, the superior border is defined as the pharyngeal fornix, a mucous membrane that intimately covers the basal parts of the occipital and sphenoid bones.

Laterally and posteriorly, the superior pharyngeal constrictor muscles and the pharyngobasilar fascia define the borders.

The lower border of the nasopharynx is defined as the pharyngeal isthmus, an opening leading to the oropharynx formed by the posterior part of the soft palate and the posterior wall of the pharynx.

The anterior border is essentially non-existent since the nasopharynx communicates anteriorly with the nasal cavity.

The most prominent feature of the nasopharynx is the pharyngeal ostium of the auditory tube, located posterolaterally of the choanas.

The ostium is surrounded by cartilage that produces a horseshoe-shaped elevation known as the torus tubarius.

From the posterior aspect of the torus tubarius arises the salpingopharyngeal fold containing the salpingopharyngeal muscle, which originates in the cartilaginous tubarius vortex and mixes with the palatopharyngeal muscle.

Following the salpingopharyngeal fold, there is a slit-shaped depression called the pharyngeal recess, also known as Rosenmuller’s fossa.

In addition, the salpingopalatine fold arises from the anterior border of the tubal vortex.

Inferior to the bull is another fold called the torus levatorius, formed by the levator veli palatini muscle.

In the posterior, upper part of the nasopharynx and the posterior portion of the auditory tube, reside the lymphoid aggregates known as the pharyngeal tonsil and the tubal tonsil, their frequent extension.

When inflamed, these tonsils are known as adenoids and can cause obstruction of the nasopharynx, resulting in shortness of breath.

Oral part (oropharynx)

The oropharynx is the middle part of the pharynx, directly below the soft palate, which communicates anteriorly with the oral cavity of the isthmus of the fauces, also known as the oropharyngeal isthmus.

Specifically, the oropharyngeal isthmus is linked superiorly by the soft palate, laterally by the palatoglossal arches and inferiorly by the posterior third of the tongue.

The oropharynx is defined anatomically by certain limits as well.

It is joined superiorly by the inferior surface of the soft palate and inferiorly by the root of the posterior tongue and the epiglottis.

Previously, the oropharynx is well delimited from the oral cavity proper by the oropharyngeal isthmus mentioned above.

The posterior root of the tongue exhibits numerous follicles containing lymphatic tissue, which are collectively known as lingual tonsils.

Closely, the lingual surface of the epiglottis curves forward and attaches to the posterior tongue at the midline and lateral margins, forming the median and lateral glossoepiglottic folds.

The depressions created between the median and lateral glossoepiglottic folds are called epiglottic vallecules.

The most prominent feature of the oropharynx are the two folds called the pillars of the jaws, the palatoglossal arch and the palatopharyngeal arch.

The palatoglossal arch contains the palatoglossus muscle and travels anteroinferiorly from the soft palate to the lateral aspect of the tongue.

Posterior to the palatoglossal arch, the palatopharyngeal arch containing the palatopharyngeal muscle moves posteroinferiorly from the soft palate to the lateral part of the pharynx.

Due to the different points of insertion and direction, the arches travel within the oropharynx, creating a triangular space known as the tonsillar fossa, which contains lymphoid tissue known as the palatine tonsil.

The arches are used as anatomical landmarks for the evaluation, manipulation, and surgery of the palatine tonsils.

Laryngeal part (laryngopharynx)

The laryngopharynx is the lower third of the inner pharynx and communicates with the larynx through the inlet of the larynx, also known as aditus.

Specifically, its upper border is the epiglottis and oropharynx and its lower border is the posterior surface of the cricoid cartilage of the larynx.

At the level of the epiglottis, the laryngopharynx is wide but narrows at the level of the cricoid cartilage as it fuses with the esophagus.

At the level of the cricoid cartilage, the laryngopharynx extends to the lateral aspects of the larynx.

In this location, the laryngopharynx is medially surrounded by aryepiglottic folds and laterally delimited by thyroid cartilage, forming a sinus known as the piriformis recess.

Innervation of the pharynx

Most of the sensory innervation of the pharynx is derived from the glossopharyngeal nerve, specifically the pharyngeal and tonsillar branches (cranial nerve IX).

Except for the anterior part of the nasopharynx, which is innervated by a branch of the maxillary nerve (cranial nerve V2) called the pharyngeal nerve.

The pharyngeal nerve is a small sensory nerve that passes through the palatovaginal or pharyngeal canal with its accompanying arterial branch (branch of the pharyngeal artery of the maxillary artery).

It is located between the sphenoid and the palatine bone and provides sensitivity to the parts of the nasopharynx and the auditory tube.

The pharyngeal branch of cranial nerve IX arises before the glossopharyngeal nerve that travels intimately with the stylopharyngeal muscle.

The pharyngeal branch then fuses with the pharyngeal branch of the vagus nerve (cranial nerve X), as well as the cranial part of the spinal accessory nerve, which then advances to the pharyngeal plexus located within the outer fascia of the pharynx.

Although the pharyngeal branch provides most of the sensory innervation, the tonsillar branch of the glossopharyngeal nerve directly supplies the oropharyngeal isthmus as it communicates with the lesser palatine nerve (of cranial nerve V2).

In addition, the lesser palatal branch of the maxillary nerve provides the sensory fibers for the soft palate.

It is worth mentioning that both the pharyngeal branch and the lesser palatal branch arise from the maxillary division of the trigeminal nerve in the pterygopalatine fossa.

As mentioned above, the pharynx consists of 6 main muscles: the superior pharyngeal constrictor, the middle pharyngeal constrictor, the stylopharyngeal inferior pharyngeal constrictor, the salpingopharyngeal, and the palafito pharyngeal.

All derive motor input from the pharyngeal and superior laryngeal branches of the vagus nerve (cranial nerve X) through the pharyngeal plexus, except the stylopharyngeal.

In contrast, the stylopharyngeus muscle derives the motor innervation of the glossopharyngeal nerve (cranial nerve IX) from the fibers of the nucleus ambiguus.

vasculature

The pharynx receives its blood supply from a variety of sources, depending on the anatomical location.

The upper part of the pharynx receives blood from the pharyngeal branch of the ascending pharyngeal artery and from the descending branches of the lesser palatine arteries.

The lower part of the pharynx receives blood supply from the lower thyroid artery and the upper thyroid artery.

The remainder of the pharynx receives blood from the ascending palatal and tonsillar branches of the facial artery, as well as from the maxillary artery.

The main muscles of the pharynx possess their respective blood supplies from the following arteries:

  • The middle and superior pharyngeal constrictors receive their blood supply from the ascending pharyngeal artery (pharyngeal branch) and the facial artery (tonsillar branch).
  • Inferior pharyngeal constrictor receives blood supply from the ascending pharyngeal artery (pharyngeal branch) and the inferior thyroid artery (muscular branches).
  • Palatopharyngeus receives its blood supply from the facial artery (ascending palatal branch), the maxillary artery (descending palatal branch), and the ascending pharyngeal artery (pharyngeal branch).
  • Salpingopharyngeus Like palatopharyngeus receives the blood supply from the facial artery (ascending palatal branch), the maxillary artery (descending palatal branch), the ascending pharyngeal artery (pharyngeal branch).
  • Stylopharyngeus receives its blood supply from the ascending pharyngeal artery (pharyngeal branch).

The pharyngeal veins drain into the pharyngeal plexus, which is located on the posterior wall of the pharynx.

The pharyngeal plexus can drain into the internal jugular vein or the brachiocephalic vein through the inferior thyroid vein.

Microscopic anatomy

The walls of the nasopharynx, oropharynx, and laryngopharynx are made up of four layers.

From the inside (luminal side) to the outside, the layers are the mucosa, the submucosa, the muscular layer, and the fibrous layer.

For the sake of simplicity, the nasopharynx is analyzed in detail on its own.

The oropharynx and laryngopharynx are treated as a group because the microanatomy of the two is nearly identical.

nasopharynx

The mucosal layer consists of the epithelium and lamina propria.

There is significant debate regarding the epithelium of the nasopharynx, whether it is predominantly respiratory epithelium or stratified squamous epithelium and where the anatomical divisions between these two types of epithelium occurred within the nasopharynx.

Most of the literature describes that, as a whole, the nasopharynx consists of 40% respiratory epithelium and 60% stratified squamous epithelium.

The cubic columnar respiratory epithelium is found predominantly directly behind the choanas and on the roof of the posterior wall.

On the other hand, stratified squamous epithelium predominates in the anterior, posterior, and lateral portions of the lower pharyngeal walls.

The remaining portion of the nasopharynx that includes the posterior wall of the middle of the nasopharynx has an alternating pattern of squamous and ciliated columnar epithelium, sometimes called the intermediate epithelium.

This type of epithelium is usually concentrated near the junction of the nasopharynx and oropharynx.

This alternating pattern can also be seen with the pharyngeal tonsil.

The lamina propria contains copious amounts of elastic tissue.

The submucosa contains simple branching tubuloalveolar glands that are typically seromucosal (mixed), producing primarily mucin.

These are predominantly located near the auditory tube. In addition, the submucosa contains lymphatic tissue.

The aggregated lymph node in the posterior superior nasopharyngeal wall forms the pharyngeal tonsil. The muscle layer is made up of skeletal muscle.

Lastly, the fibrosa is a thin layer of fibrous connective tissue.

Oropharynx y Laryngopharynx

The mucosal layer of the oropharynx and laryngopharynx are both lined with stratified squamous epithelium that is typically not keratinized.

However, in autopsies of patients, keratinized squamous stratified biopsies of the oropharynx and laryngopharynx have been identified.

One hypothesis is that these regions may secrete keratin as a defense mechanism after years of irritation and damage to the epithelium.

Similar to the nasopharynx, the oropharynx and laryngopharynx contain lymphoid aggregates in the submucosal and seromucosal glands.

Pathophysiological variants

The inflammation

The most common inflammatory processes that occur in the nasopharynx are tonsillitis and pharyngitis.

Although infectious bacteria are the cause of pharyngitis and tonsillitis, the most common causes are still upper respiratory virus infections.

The viral pathogens most implicated in these infections are echovirus, rhinovirus, adenovirus, and some respiratory syncytial viruses.

A typical presentation of symptoms due to viral causes are: erythema, edema of the mucosa of the nasopharynx and an enlargement of the lymphatic tissue.

When the cause is bacterial, due to beta-hemolytic streptococci and Staphylococcus aureus, the symptoms are: erythema of the mucous membranes of the nasopharynx and an exudative membrane.

Important complications are complications from infections when not properly treated, such as: rheumatic fever and glomerulonephritis resulting from strep throat.

Tumors

Nasopharyngeal angiofibroma

Nasopharyngeal angiofibroma presents as a highly vascular tumor that generally occurs in adolescent males.

It has a tendency to bleed during surgery.

It is seen as a smooth structure in the posterior nasopharynx.

The tumor is benign but very aggressive and causes nasal obstruction and epistaxis.

Performing a biopsy is very dangerous because the tumor is highly vascular.

The tumor can be embolized preoperatively to reduce bleeding.

Nasopharyngeal carcinoma

Nasopharyngeal carcinoma can have three different patterns: carcinoma containing keratinizing squamous cells, squamous but non-keratinizing carcinoma, and undifferentiated carcinomas.

The most common anatomical location for nasopharyngeal carcinoma is the pharyngeal recess.

The development of nasopharyngeal carcinoma may be associated with human papillomavirus infection.

Squamous cell carcinoma

Squamous cell carcinoma is the most common tumor that occurs in the laryngopharynx.

It usually occurs most frequently in the postcricoid region or on the posterior wall, within the piriformis recess.

The consumption of alcohol and tobacco represent the major predisposing factors.

Parkinson’s disease

The pharynx plays a very important role in the swallowing process, and it can cause dysphagia.

Aspiration pneumonia is the most common cause of death in Parkinson’s disease, it is even suggested that the pharyngeal plexus is affected by Parkinsonian pathology.

Parkinson’s disease causes an alteration in nervous sensitivity and alters the swallowing reflex.

Obstructive sleep apnea

Obesity and fat deposition near the pharynx, as well as edema from smoking, cause a decrease in longitudinal traction and wall tension, facilitating collapse and the onset of sleep apnea.

There is also a relaxation function of the pharyngeal musculature itself during sleep, during which even a minimally collapsed airway does not respond to adequate mechanical load.

Features

The pharynx is the widest opposite the hyoid bone, and the narrowest at its lower end which is continuous with the esophagus.

The nasopharynx lies behind the posterior openings of the nasal cavities, above the level of the soft palate. Its function is respiratory.

The oropharynx lies behind the oral cavity, below the level of the soft palate and above the epiglottis.

It has a digestive function.

The laryngopharynx is located behind the larynx and extends from the upper part of the epiglottis to the lower area of ​​the cricoid cartilage, where it becomes a continuation of the esophagus when it narrows.

It is where both food and air pass.

The oropharynx and laryngopharynx are common passages to the digestive and respiratory tracts.