Pharynx

The pharynx is a tubular structure that connects the nasal and oral cavities to the esophagus and larynx. It plays multiple crucial roles in both the digestive and respiratory systems, as well as in speech production^[1].

The pharynx begins at the base of the skull and extends down to the level of the sixth cervical vertebra, approximately.

In relation to other structures, it starts posteriorly to the nasal cavities (nasopharynx) and oral cavity (oropharynx). The nasopharynx is continuous with the nasal cavity through the choanae (posterior nasal apertures), and the oropharynx is continuous with the oral cavity through the oropharyngeal isthmus (which is bordered superiorly by the soft palate and inferiorly by the epiglottis).

The pharynx ends at the esophagus inferiorly, around the level of the sixth cervical vertebra where the laryngopharynx (the most inferior part of the pharynx) narrows to become continuous with the esophagus.

Anatomically, the pharynx can be divided into three sections:

1. Nasopharynx: This is the uppermost section, located behind the nasal cavity. It serves as a passageway for air between the nose and the throat. It also contains the opening for the Eustachian tubes, which connect the middle ear to the pharynx and help equalize ear pressure^[7].

2. Oropharynx: This part is located behind the oral cavity, extending from the soft palate to the level of the hyoid bone. Both food and air pass through the oropharynx. It contains the tonsils, which play a role in the immune system^[8].

3. Laryngopharynx (or hypopharynx): This is the lowest part of the pharynx, located behind the larynx, and extends from the hyoid bone to the esophagus and larynx. The laryngopharynx serves as a passageway for food and air, but has a mechanism that prevents food from entering the larynx and trachea when swallowing.

The walls of the pharynx consist of three layers: the innermost layer is mucosa, the middle layer is fibrous and muscular (consisting of both longitudinal and circular muscles), and the outermost layer is composed of loose connective tissue.

The muscles of the pharynx are split into two groups - the constrictors and the longitudinal muscles.

Constrictor Muscles: Composed of the superior, middle, and inferior pharyngeal constrictors, these muscles work in sequence to propel food from the oropharynx to the esophagus.

Longitudinal Muscles: Composed of the Stylopharyngeus (innervated by the glossopharyngeal nerve), Salpingopharyngeus, and Palatopharyngeus muscles, these elevate the pharynx and larynx during swallowing and speech.

The pharynx is innervated by several cranial nerves via the pharyngeal plexus. The primary nerves involved are the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X), along with some contributions from the sympathetic trunk.

1. Glossopharyngeal Nerve (CN IX): This nerve provides sensory innervation to the oropharynx (the middle part of the pharynx behind the mouth). It also provides innervation to the stylopharyngeus muscle, which is involved in swallowing.

2. Vagus Nerve (CN X): The motor innervation to all muscles of the pharynx (except the stylopharyngeus) is provided by the vagus nerve, specifically its pharyngeal branch. This includes the superior, middle, and inferior constrictor muscles, which play a key role in swallowing by pushing food down into the esophagus, as well as the longitudinal muscles (salpingopharyngeus and palatopharyngeus), which help to elevate the pharynx and larynx during swallowing and speech. The vagus nerve also provides some sensory innervation to the laryngopharynx (the part of the pharynx that is continuous with the larynx and esophagus).

3. Sympathetic Trunk: Provides the autonomic (sympathetic) supply to the pharynx.

Overall, the coordinated effort of these nerves allows the pharynx to perform its crucial roles in breathing, swallowing, and speech.

The pharynx performs multiple functions, from breathing and swallowing to speech production, immune function and regulation of middle ear pressure.

The pharynx acts as a conduit for both food and air. It connects the nose and mouth to the trachea (for air) and the esophagus (for food and liquids).

In swallowing, the pharynx has a critical role. The process can be divided into oral, pharyngeal, and esophageal stages^[5].

During the pharyngeal stage, the soft palate elevates to close off the nasopharynx, preventing food from entering the nasal cavity. The tongue pushes the food bolus into the oropharynx. Then, the constrictor muscles contract in sequence to propel the bolus through the oropharynx and laryngopharynx into the esophagus. At the same time, the larynx elevates, and the epiglottis closes to prevent aspiration into the lungs.

This coordinated process is involuntary and involves a complex neural network known as the swallowing center, located in the medulla oblongata of the brainstem. The muscles' contraction and relaxation, coordinated by the swallowing center, ensure safe and efficient passage of food from the oral cavity to the stomach.

In speech, the pharynx acts as a resonating chamber for voice production. Changes in the shape and size of the pharynx, controlled by the pharyngeal muscles, modify the sound produced by the vocal cords. This influences voice quality, including aspects of pitch and timbre.This is especially important for the production of certain speech sounds, known as pharyngeal consonants, although these are relatively rare in the world's languages^[7].

The pharynx houses lymphoid tissue (tonsils and adenoids) that serves as a first line of defense against pathogens that enter the body through the nose or mouth^[8]

1. Tonsils: These are clusters of lymphoid tissue and are part of the body's immune system. They are located in the nasopharynx (adenoids or pharyngeal tonsils), oropharynx (palatine and lingual tonsils), and laryngopharynx (tubal tonsils). The tonsils play a role in immunity by "sampling" bacteria and viruses that enter through the mouth or nose and activating an immune response if needed.

2. Mucosa-associated lymphoid tissue (MALT): This is a diffuse system of small concentrations of lymphoid tissue found in various submucosal membrane sites of the body, such as the pharynx, and acts as a first line of defense against inhaled or ingested foreign pathogens.

These tissues contain immune cells, including B and T lymphocytes, that can respond to pathogens entering through the respiratory or digestive tracts. These immune cells can trigger both local and systemic immune responses to help fight off infections.

The pharynx's role in immunity is especially crucial in children, whose immune systems are still developing. This is why children often have enlarged tonsils and adenoids, as these tissues are actively involved in fighting off various infections. Over time, with repeated exposure to pathogens, the immune system matures and becomes more efficient, and the size of these lymphoid tissues usually decreases.

The nasopharynx contains the openings of the Eustachian tubes, which help equalize pressure between the middle ear and the external environment. This is crucial for proper hearing^[1].