Table of Contents
- Larynx: Location and Function
- Anatomical Composition of the Larynx
- Muscles of the Larynx
- Histology of the Larynx
- The Structure of the Trachea and the Bronchial Tree
- Bronchial System
- Histology of Trachea and Bronchi
- Anatomy of Larynx and Trachea in Clinical Presentations
- Diseases of the Larynx
- Popular Exam Questions on the Lower Airways
Larynx: Location and Function
The larynx is an anatomically complex structure. It is helpful to recall the anatomy of the cervical area: The larynx is an anatomical-functional unit composed of important cartilages and muscles that builds a barrier between pharynx and trachea, thus forming the transition from upper to lower airways. The epiglottis, being a kind of flexible cartilage cover, closes off the larynx from the esophagus whereby it prevents the aspiration of fluids or food during swallowing.
However, if this happens anyway, the coughing reflex makes sure that the foreign material is catapulted out of the airways. The vocal folds (plicae vocales) build another closing mechanism on the inside of the laryngeal tube. They consist of two ‘sails’ of muscular and connective tissue that are fixated at two cartilages. The gap between them is called the glottis (rima glottis). If the glottis is completely closed, minimal to no air can pass through it.
Closing the glottis and contracting the diaphragm and the abdominal muscles can raise the intraabdominal pressure, which makes the larynx play a functional role for the abdominal muscular pressure. Modulation of airflow via the glottis is important for phonation. Paralysis of the laryngeal muscles due to a lesion of the vagal nerve or its branches – superior laryngeal nerve and recurrent laryngeal nerve – can result in incorrect or permanent closure of the glottis, which in turn leads to difficulty swallowing, hoarseness, or – when there are lesions on both sides – even respiratory distress. Innervation of the larynx is thus a very popular subject in medical exams.
Anatomical Composition of the Larynx
In order to comprehend the anatomy of the larynx, one should definitely take a close look at the respective images in the anatomical atlas. At straight head posture, the larynx is situated on the level of the fifth and sixth cervical vertebra, slightly higher in women and children. In the longitudinal axis, it is divided into three levels. This division is due to histological differences (different epithelia of pharynx and tracheal area) and differences in anatomical structure. Not only for exams but also for the description of the location of laryngeal carcinomas in clinical practice, knowing these three laryngeal levels is essential.
- Supraglottis: from the entrance to the larynx to the vestibular folds (plicae vestibulares)
- Glottis: from the vestibular folds to the vocal folds (plicae vocales)
- Subglottis: from the vocal folds to the lower margin of the cricoid cartilage
Cartilages of the Larynx
The cartilage frame of the larynx is structured in a complex manner. The following laryngeal cartilages can be found from top to bottom:
- Epiglottis: elastic cartilage, closes at swallowing
- Thyroid cartilage (cartilago thyroidea): hyaline cartilage, looks like a shield, the anterior laryngeal prominence represents the Adam’s apple. In men, it can mostly be seen through the skin.
- Cricoid cartilage (cartilago cricoidea): hyaline cartilage, it is shaped like a signet ring.
Through elastic ligaments, these three cartilages are connected with each other, to the tongue bone (os hyoideum) cranially, and to the trachea caudally.
On the inside of the laryngeal frame, there are two more small, paired cartilages:
- Arytenoid cartilage (cartilago arytaenoidea): hyaline cartilage, changes the position of the vocal folds
- Corniculate cartilage (cartilago corniculata): elastic cartilage (this one really sits on the apex of the respective arytenoid cartilage)
Muscles of the Larynx
The classification of the laryngeal muscles is also hard to understand at first glance. It is worthwhile to look it up in the anatomical atlas or to watch instructional videos on the internet. Once understood, a lot of functions and questions concerning the larynx can be deduced logically, which saves a lot of learning effort.
During swallowing, the larynx is moved as a complete structure by the infrahyoid and suprahyoid muscles and the inferior pharyngeal constrictor muscle. The small laryngeal muscles are responsible for fine movements of the vocal folds. Their individual tasks and innervations are popular subjects in exams.
Extrinsic Muscles of the Larynx
|Cricothyroid muscle||Tension of the vocal folds. Its contraction tilts the cricoid cartilage backwards and tautens the vocal cords.|
Intrinsic Muscles of the Larynx
The inner muscles of the larynx originate from the arytenoid cartilage and change the position of the vocal cords by contracting.
|Posterior cricoarytenoid muscle (posticus)||Opening of the glottis (Note: only opener of the glottis)|
|Lateral cricoarytenoid muscle (lateralis)||Closes (pars intermembranacea) and opens (pars intercartilaginea) a part of the glottis, respectively. Also referred to as phonation muscle.|
|Transverse cricoarytenoid muscle||Closure of the vocal folds|
|Thyroarytenoid muscle||Closure of the vocal folds|
|Vocal muscle||Tension of the vocal folds|
Innervation and Functional Failures of the Larynx
Branches of the vagal nerve ensure the motor and sensory innervation of the larynx. The extrinsic laryngeal muscle is the only one that is innervated by the superior laryngeal nerve (external ramus). The internal ramus supplies the mucosa sensorily above the vocal folds.
The intrinsic laryngeal muscles’ motor innervation is performed by the inferior laryngeal nerve (out of the recurrent laryngeal nerve of the vagal nerve). It also ensures the sensory innervation of the laryngeal mucosa below the vocal folds. If the vagal nerve is injured in the brainstem or in its peripheral course, specific failures of the laryngeal muscles occur. There is a special risk of injury at thyroid surgery or local tumors.
Histology of the Larynx
Since the larynx represents the transition of pharynx (stratified non-keratinizing squamous epithelium) to trachea (ciliated respiratory epithelium), there are differences in histological tissue composition. The interior of the larynx is divided by two pairs of folds: false/vestibular cords (plicae vestibulares) and the already mentioned true/vocal cords (plicae vocales).
Histological cross-sections mostly show the transition of the two 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!). The individual vocal fold (plica vocalis) is made up of the vocal ligament (ligamentum vocale) and the vocal muscle (innermost part of the thyroarytenoid muscle). Between the vocal ligament and the epithelium, there is a loose connective tissue layer, Reinke’s space.
A Reinke’s edema occurs when there is too much fluid in this space. The result is a swelling that impairs phonation and makes the patient hoarse. Such an edema can also be triggered by chronic irritation due to cigarette smoke, which results in the typical smoker’s voice.
The Structure of the Trachea and the Bronchial Tree
The trachea is a roughly 10-12 cm long, air conducting flexible tube. It begins directly under the larynx and runs within the thoracic cage in the middle mediastinum, dorsally of the vessels close to the heart. On the level of the third or fourth thoracic vertebra, the trachea bifurcates into the left and right main bronchus. With respect to its pathway, the trachea is divided into two parts: pars cervicalis and pars thoracica. Relations concerning the location of the trachea in the thorax are important and often subject of exams:
|Ventral||Aortic arch (left!), brachiocephalic trunk (right!)|
Since the bronchi bifurcate further and further in order to spread the respiratory air completely into the left and right pulmonary lobes, it is also referred to as bronchial tree or bronchial system. At the first bifurcation (bifurcation tracheae on the level of Th3/Th4), the trachea splits into the left and right main bronchus. The right main bronchus runs steeper than the left one. Therefore, relevant for exam questions and real life: aspirated foreign objects are more frequently found in the right main bronchus. Due to the position of the heart and the resulting slightly asymmetric position of the heart, the left main bronchus is slightly longer than the right one.
The main bronchi bifurcate up to 23 times after they enter the lung. Firstly, they split into lobar bronchi (two on the left, three on the right), and they split into segmental bronchi. The smallest units are the bronchioles (bronchioles, terminal bronchioles, respiratory bronchioles), which lead the air into the alveoli, the place in the lung where gas exchange happens.
Histology of Trachea and Bronchi
The trachea consists of a support frame 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. At this area, the paries membranacea forms a plate out of 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.
The mucosa (tunica mucosa) of trachea and bronchi is covered by ciliated respiratory epithelium. The lamina propria contains sero-mucous glands, the tracheal glands (glandulae tracheales). Interspersed, slime producing goblet cells without kinocilia can be found within the epithelium.
The bronchial wall also consists of three layers: mucosa with pseudostratified ciliated respiratory epithelium, glands (glandulae bronchiales) 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 sero-mucous glands are bronchi. Bronchioles neither have cartilage nor glands!
Smoking tobacco can decrease production of secretion of the tracheal and the bronchial glands, which impairs the elimination of slime via the movement of the kinocilia of the ciliated epithelium and, hence, the self-cleaning function of the airways.
Anatomy of Larynx and Trachea in Clinical Presentations
Ventilation with Laryngeal Masks
The laryngeal mask, a ventilation mask, is used in anaesthesia. It lies on the epiglottis and safely encloses the airways. The person is ventilated using an attached tube. Compared to other procedures, this has the advantage that no ventilation tubes have to be pushed through the glottis into the trachea (risk of hoarseness and injury) and the airways are even better sealed than with a facial mask.
Cricothyrotomy is the opening of the airways at the height of the larynx. It is a lifesaving medical emergency procedure but is very rarely used in practice and represents a last resort.
In a cricothyrotomy, the membrane between the cricoid and thyroid cartilage of the larynx, which can easily be palpated at the throat, is opened via an incision or puncture. Then, a thracheostomy or a endothracheal tube is inserted in order to ensure oxygen supply for the patient. Knowing this might make a good impression in exams but later on, it will only be useful if you aspire to become an emergency doctor.
Tracheotomy and tracheostoma
In intensive care, the tracheotomy method is applied, i.e., the operational insertion of an access to the trachea for patients who depend on longterm ventilation. With a tracheostoma (the tracheal cannula which creates a connection of the trachea to the outside), patients can breath spontaneously on themselves. If needed however, a ventilation device can be connected.
Beside longterm ventilation, indications for tracheotomy are injuries of the larynx, a lack of swallowing reflexes, laryngeal or tracheal lesions due to long-time ventilation, certain jaw or ENT surgeries, or diseases or tumors of the upper airways which would inhibit intubation.
A permanent tracheostoma (plastic tracheostoma) is indicated in case of a laryngectomy (laryngeal carcinoma). In this case, the opening is located under the thyroid gland in the jugulum.
Diseases of the Larynx
Inflammation of the Larynx (Laryngitis)
An inflammation of the larynx can occur in the course of an infection of the nose and pharynx region with viruses or bacteria. Also, an overload of voice at dry room air can lead to the symptoms of laryngitis: reddened pharyngeal mucosa, burning sore throat, and hoarseness up to the loss of voice.
Beside the treatment of the causes (antibiotics at purulent inflammations), the patients should speak as little as possible. Nicotine abstinence, heat treatment (hot throat compresses and warm beverages), and the inhalation with chamomile extract are also part of the symptomatic treatment.
Other clinically significant inflammations of the lower airways, especially of the larynx, are the acute epiglottitis and the laryngitis subglottica (croup), which is treated in ENT medicine in the clinical stage.
Laryngeal Cancer (Laryngeal Carcinoma)
The 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, and very seldom in the subglottis. An early symptom is long lasting hoarseness. Difficulty swallowing and foreign body sensation (globus sensation) can also occur.
At the event of hoarseness lasting more than three months, a laryngeal carcinoma always has to be ruled out! The formation of laryngeal carcinomas is closely related to noxae like tobacco smoke, but also associated with alcohol. Therapeutically, a partial (at earlier tumor stages), mostly complete removal of the larynx (laryngectomy) with removal of cervical lymph nodes (Neck dissection), and radiation is necessary.
The removal of the larynx makes the application of a tracheostoma obligatory (see below). At this, a permanent access to the trachea, which is separated fro the larynx, is made to the cervical skin. After this operation, patients do not have vocal chords anymore and can be provided with a ‘voice prosthesis’ between trachea and esophagus during this larynx surgery. However, speaking has to be learned again.
Popular Exam Questions on the Lower Airways
Solutions can be found below the references.
1. Which assignment concerning the laryngeal muscles is not correct?
- Closure of the vocal folds – transverse arytenoid muscle
- Opening and closure of the vocal folds – lateral cricoarytenoid muscle
- Tension of the vocal folds – cricothyroid muscle, vocal muscle
- Closure of the vocal folds – posticus muscle
2. Which of the following structures of the larynx is punctured at cricothyrotomy?
- Medial part of the thyroid cartilage.
- Crcothyroid ligament between cricoid cartilage and thyroid cartilage.
- Thyrohyoid membrane between the tongue bone and the thyroid cartilage.
- The punction occurs via the thyroid gland.
- A punction of the larynx is not possible.
3. At which level is the bifurcation of the trachea located?
- On the level of the thoracic aperture.
4. A little boy is brought to the emergency room due to acute dyspnea. Where is an aspirated foreign body most likely be found in imaging modalities?
- Tacheal bifurcation
- Right main bronchus
- Left main bronchus