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Sternohyoid muscle

Image : “The sternohyoid and sternothyroid muscles lie on top of the upper part of the trachea” by Mikael Häggström. License: Public Domain


Larynx: Location and Function

The larynx is an anatomically complex structure composed of important cartilage and muscle that constitute a barrier between the pharynx and the trachea, forming the transition from the upper to lower airways. The epiglottis is a kind of flexible cartilage lid, which closes off the larynx from the esophagus to prevent the aspiration of fluids or foods when swallowing.

Head and Neck Overview

Image: Head and neck overview. By Arcadian, License: Public Domain

The vocal folds, or vocal cords, represent another closing mechanism on the inside of the laryngeal tube. They consist of 2 ‘sails’ of muscular and connective tissue attached to 2 cartilaginous structures surrounding the glottis, the opening between the vocal folds. Complete closure of the glottis allows minimal to no air passage.

The closure of the glottis, coupled with contraction of the diaphragm and the abdominal muscles, raises intra-abdominal pressure, which facilitates laryngeal function. For example, modulation of airflow via glottis is important for phonation, or the utterance of speech. Paralysis of the laryngeal muscles due to a lesion of the vagal nerve or its branches (the superior laryngeal nerve and recurrent laryngeal nerve) results in incorrect or permanent closure of the glottis, which in turn leads to difficulty swallowing, hoarseness, or respiratory distress if lesions are bilateral. Innervation of the larynx is thus a very popular topic in medical exams.

Anatomical Composition of the Larynx

The anatomy of the larynx is complex, and careful examination of the relevant images in the anatomic atlas is key to understanding its anatomical features.

In the straight head posture, the larynx is situated at the level of the 5th and 6th cervical vertebrae, which is slightly higher in women and children. In the longitudinal axis, it is divided into 3 levels based on histological variation (different epithelia of the pharynx and trachea) and anatomical differences. Knowledge of these laryngeal levels is essential for understanding the location of laryngeal carcinomas in clinical practice.

  • The supraglottis, which extends from the entrance of the larynx to the vestibular folds (plicae vestibulares)
  • The glottis, which spans across the vestibular folds to the vocal folds (plicae vocales)
  • The subglottis, which extends from the vocal folds to the lower margin of the cricoid cartilage
Larynx

Image: Anterior and right lateral views of the larynx. By PhilSchatz.com, License: CC BY 4.0

Cartilage of the larynx

The cartilage structure from the top to the bottom is as follows: 

  • The epiglottis, which is elastic cartilage, and closes when swallowing
  • The thyroid cartilage, which is a hyaline cartilage similar to a shield composed of two sheets that meet to form the anterior laryngeal prominence, or the Adam’s apple. In men, it is clearly noticeable through the skin
  • The cricoid cartilage, which is circular hyaline cartilage

These 3 cartilaginous structures are connected to each other via elastic ligaments, to the tongue bone (os hyoideum) cranially, and to the trachea caudally.

Two additional small, paired cartilage structures exist inside the laryngeal frame:

  • The arytenoid cartilage, which is hyaline cartilage that changes the position of the vocal folds
  • The corniculate cartilage, which is elastic cartilage situated at the apex of the respective arytenoid cartilage

Muscles of the Larynx

The classification of the laryngeal muscles is also complex, and referring to the anatomical atlas or watching instructional videos on the internet are helpful in obtaining a better understanding of its structure. 

The infrahyoid, suprahyoid, and the inferior pharyngeal constrictor muscles facilitate the swallowing of food. The small laryngeal muscles are responsible for the fine movements of the vocal folds. 

Extrinsic muscles of the larynx

Cricothyroid muscle Its contraction tilts the cricoid cartilage backward and tightens the vocal cords.

Intrinsic muscles of the larynx

The inner muscles of the larynx originate in the arytenoid cartilage and change the position of the vocal cords via contraction.

Name Function
Posterior cricoarytenoid muscle (posticus) Opening of the glottis (Note: only opener of the glottis)
Lateral cricoarytenoid muscle (lateralis) Also known as the phonation muscle, it consists of two components: the pars intermembranacea and the pars intercartilaginea, which close and open a part of the glottis, respectively.
Transverse cricoarytenoid muscle Closure of the vocal folds
Thyroarytenoid muscle Closure of the vocal folds
Vocal muscle Tension of the vocal folds
Muscles of larynx

Image: Intrinsic muscles of the larynx. By Henry Vandyke Carter, License: Gemeinfrei

Innervation and functional failures of the larynx

Branches of the vagal nerve provide motor and sensory innervation of the larynx. The extrinsic laryngeal muscle is the only innervated by the superior laryngeal nerve (external ramus). The internal ramus supplies the mucosa sensorily above the vocal folds.

The inferior laryngeal nerve (out of the recurrent laryngeal nerve of the vagal nerve) is associated with motor innervation of the intrinsic laryngeal muscles. It also ensures sensory innervation of the laryngeal mucosa below the vocal folds. Injury to the vagal nerve in the brainstem or along its peripheral course results in failure of the laryngeal muscles. Thyroid surgery or local tumors increase the risk of injury.

Histology of the Larynx

Since the larynx represents the transition of the pharynx (stratified non-keratinizing squamous epithelium) to the trachea (ciliated respiratory epithelium), it shows differences in histological tissue composition. The interior of the larynx is divided into 2 pairs of folds: false/vestibular cords (plicae vestibulares) and the aforementioned true/vocal cords (plicae vocales).

Histological cross-sections mostly show the transition of the 2 areas, i.e. a part of the vestibular folds (cranial; stratified non-keratinizing squamous epithelium; sero-mucous glands) and the vocal folds (distal; both stratified non-keratinizing squamous epithelium and ciliated respiratory epithelium; no glands). An individual vocal fold consists of the vocal ligament and the vocal muscle (innermost part of the thyroarytenoid muscle). Between the vocal ligament and the epithelium, there is a loose connective tissue layer called Reinke’s space.

Reinke’s edema is induced by excessive fluid in this space resulting in swelling that impairs phonation and causes hoarseness. Such edema can also be triggered by chronic irritation due to cigarette smoke, which results in the typical smoker’s voice.

Structure of the Trachea and Bronchial Tree

The trachea is approximately 10–12 cm (3.9–4.7 in) in length and is an air-conducting flexible tube. It begins directly below the larynx and runs within the thoracic cage in the middle mediastinum, dorsally to the vessels close to the heart. At the level of the 3rd or 4th thoracic vertebra, the trachea bifurcates into the left and right main bronchi. The trachea is divided into 2 parts: pars cervicalis and pars thoracica

Location Structure
Cranial Larynx
Ventral Thyroid gland
Dorsal Esophagus
Ventral Aortic arch (front), brachiocephalic trunk (right) left common carotid (left)

Bronchial System

Since the bronchi bifurcate repeatedly in order to distribute respiratory air completely into the left and right pulmonary lobes, they are also referred to as the bronchial tree or bronchial system. At the 1st bifurcation (at the level of thoracic vertebrae Th3/Th4), the trachea splits into the left and right main bronchi. The right main bronchus runs steeper than the left one, while the left main bronchus is slightly longer than the right one due to the slightly asymmetric position of the heart. The branching of the bronchial system has real-life implications: aspirated foreign objects are frequently found in the right main bronchus. 

The main bronchi bifurcate up to 23 times after entering the lungs. First, they split into lobar bronchi (2 on the left, and 3 on the right) and later split into segmental bronchi. The smallest units are the bronchioles (bronchioles, terminal bronchioles, respiratory bronchioles), which direct the air into the alveoli, the seat of gas exchange in the lungs.

Histology of the Trachea and Bronchi

The trachea consists of a scaffold of 16–20 semicircular cartilage rings (hyaline cartilage), which are reinforced by collagenous connective tissue. The posterior wall of the trachea is free of cartilage. In this area, the pars membranacea forms a plate out of the smooth muscles (tracheal muscle) and connective tissue, the border to the dorsally running esophagus. This supporting apparatus is histologically referred to as the fibromusculocartilaginous layer (tunica fibro-musculo-cartilaginea). A tunica adventitia composed of loose connective tissue surrounds the trachea.

Trachea

Iimage: The trachea. By OpenStax College, License: CC BY 3.0

The mucosa (tunica mucosa) of the trachea and bronchi is covered by the ciliated respiratory epithelium. The lamina propria contains seromucous tracheal glands. Interspersed, slime-producing goblet cells without kinocilia can be found within the epithelium.

The bronchial wall also consists of 4 layers: mucosa with pseudostratified ciliated respiratory epithelium, bronchial glands and goblet cells; a muscular layer; a support frame of hyaline cartilage; and peribronchial connective tissue. All branches of the bronchial tree that contain cartilage tissue and seromucous glands are bronchi. Bronchioles have neither cartilage nor glands!

Smoking tobacco decreases secretions from the tracheal and bronchial glands, which impairs the elimination of slime via movement of the kinocilia in the ciliated epithelium. Hence, airways are engaged in self-cleaning function.

Anatomy of the Larynx and Trachea in Clinical Presentations

Ventilation with laryngeal masks

The laryngeal mask is a ventilation mask used in anesthesia. It is positioned on the epiglottis and safely encloses the airways. The patient is ventilated using an attached tube. Compared with other procedures, no ventilation tubes are inserted through the glottis into the trachea, thereby decreasing the risk of hoarseness and injury. Thus, the laryngeal mask seals the airways better than a facial mask.

Larynx mask

Image: Laryngeal mask with inflated bead; here, a version with a channel for a gastric tube. By ignis, License: CC BY-SA 3.0

Cricothyrotomy

Cricothyrotomy is the surgical opening of the airways via an incision of the cricothyroid membrane. It is a lifesaving medical emergency procedure frequently used when endotracheal intubation is impossible or contraindicated, and there is an impending loss of the airway.

During cricothyrotomy, the membrane between the cricoid and thyroid cartilages in the larynx, which can be easily palpated at the throat, is opened via an incision or puncture. The endotracheal tube is inserted in order to ensure oxygen supply to the patient. 

Tracheotomy and tracheostoma

In intensive care, tracheotomy is defined as the operational insertion of access to the trachea for patients who depend on long-term ventilation. With a tracheostoma (the tracheal cannula used to create a connection of the trachea to the outside), patients can breathe spontaneously and independently. If needed, however, a ventilation device can be connected.

In addition to long-term ventilation, tracheotomy is indicated in injury to the larynx, a lack of swallowing reflexes, laryngeal or tracheal lesions due to long-term ventilation, certain jaw or ear/nose/throat (ENT) surgeries, or diseases or tumors of the upper airways, which are contraindicated for intubation.

Permanent tracheostoma (plastic tracheostoma) is applied in the case of laryngectomy (laryngeal carcinoma). In this case, the opening is created under the thyroid gland in the jugular fossa.

 laryngeal structures, trachea and invasive procedures

Image: Laryngeal structures, trachea (1–4) and invasive procedures (A–B); (1) thyroid cartilage (2) cricothyroid ligament (3) cricoid cartilage (4) trachea (A) cricothyrotomy (B) tracheostomy. By PhilippN, License: CC BY-SA 3.0

Diseases of the Larynx

See also: An Overview of the Larynx

Inflammation of the larynx (laryngitis)

Laryngeal inflammation can occur after nasal or pharyngeal infection triggered by viruses or bacteria. Also, excessive vocal exertion in a room full of dry air can lead to symptoms of laryngitis: reddened pharyngeal mucosa, burning sore throat, and hoarseness or loss of voice.

In addition to treatment of the underlying cause (e.g., antibiotics for purulent inflammations), patients should speak as little as possible. Abstinence from nicotine exposure, heat treatment (hot throat compresses and warm beverages), and inhalation of chamomile extract are also part of the symptomatic treatment.

Other clinically significant types of inflammation of the lower airways, especially of the larynx, include acute epiglottitis and laryngitis subglottica (croup), which are treated using ENT medicine based on their clinical stage.

Laryngeal cancer (laryngeal carcinoma)

Larynx cancer

Image: Laryngeal cancer. By Paweł Kuźniar, License: CC BY-SA 3.0

Laryngeal carcinoma is a squamous cell carcinoma and the most frequent malignant tumor in ENT medicine. Laryngeal carcinomas are located in the glottis in two-thirds of the cases, in the supraglottis in one-third of the cases, and rarely in the subglottis. An early symptom is a long-lasting hoarseness. Difficulty swallowing and a foreign body sensation (globus sensation) can also occur.

Any event of hoarseness that lasts more than three months should be evaluated for laryngeal carcinoma. The presence of laryngeal carcinomas is closely related to noxious agents such as tobacco smoke, but can also be associated with alcohol. Therapeutically, a partial (in the earlier tumor stages), and in most cases complete, removal of the larynx (laryngectomy), removal of cervical lymph nodes (neck dissection), and radiation are necessary.

The application of tracheostoma is mandatory in patients with the larynx removed (see below). Tracheostoma can be used to create permanent access to the trachea, which is separated from the larynx, through the cervical skin. After this operation, patients no longer have vocal cords and are fitted with avoice prosthesis’ between the trachea and esophagus via laryngeal surgery. However, patients need to relearn speech.

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