Joints and Innervations

INNERVATIONS

According to Saran (2011), the superior and inferior laryngeal nerves are the principal nerves that supply innervation to the vocal folds. They are branches of the vagus nerve that provide innervations to the vocal folds. The superior laryngeal nerve divides into two branches: the external laryngeal nerve and the internal laryngeal nerve, which are located about at the level of the larger horn of the hyoid bone. The internal laryngeal nerve communicates with the superior laryngeal artery as it passes through the thyrohyoid membrane. The internal branch of the superior laryngeal nerve transmits it to the mucosa above the vocal folds, and it may be accessible rather simply for anesthesia by cutting through the medial wall of the piriform fossa, which is located beneath the medial wall of the piriform fossa. The cricothyroid muscle receives motor innervation from the external laryngeal nerve, which originates in the larynx. Because of this, it is generally found in close proximity to the superior thyroid artery and superior pole of the thyroid gland.

The inferior laryngeal nerve, also known as the recurrent laryngeal nerve, is responsible for all sensory innervation to structures below the vocal folds. The recurrent laryngeal nerve is so named because of its unusual route, which takes it below the arch of the aorta on the left side and underneath the subclavian artery on the right side. After passing beneath each vessel on the corresponding side, the nerve travels superiorly with the inferior thyroid artery until terminating. It finally travels deep into the inferior constrictor muscles and reaches the larynx posterior to the cricothyroid articulation, where it becomes trapped (Saran, 2011).

JOINTS

According to Grujicic (2021), there are two sets of paired joints in the larynx: the cricoarytenoid joints and the cricothyroid joints. The motions of the vocal cords are modulated by the synovial joints, which come together to produce this.

Each joint is given a name based on its location.

The cricoarytenoid joints are the first pair of joints to be discovered. As the arytenoid cartilage bases articulate with the top surface of the cricoid cartilage, the articulation is completed. The motions of this joint are associated with the abduction and adduction of the vocal cords. Hence, with the opening and shutting of the laryngeal fissure, respectively (rima glottidis).

The cricothyroid joints are the second set of joints.

These are the articulations that occur between the cricoid and thyroid cartilages. When these joints move, the length and tension of the vocal cords are altered, resulting in modulation of the human voice (Grujicic, 2021).

CRICOARYTENOID JOINTS

The cricoarytenoid joint is a synovial articulation between the cricoid and arytenoid cartilages of the larynx that is formed by a paired articulation. In order to construct this joint, two articular facets are required: the superior arytenoid and the inferior cricoid facets. The proximal facet of the arytenoids is positioned on the undersurface of their bodies. Because of this, it is placed in such a way that the short axis of the facet is parallel to the long axis of the cricoid.

The arytenoids make contact with the facets of the cricoid cartilage, which are positioned on the top border of the lamina of the cricoid cartilage. Also elliptical in form, it is slanted inferoanterolaterally and convex on both axes, like the other facet. It is roughly 50 degrees in angle when the long axes of both faces meet posteriorly towards the midline, generating an angle between them.

The articular surfaces are surrounded by a thin and loose joint capsule reinforced by a joint ligament, which protects them from injury. The (posterior) cricoarytenoid ligament is the most important ligament in this joint because it restricts the range of motion. Even though some publications refer to it as "posterior," it is mainly located on the medial surface of the joint. The anterior process (vocal process) of the arytenoid cartilage, which is projected anteriorly, is linked to the vocal ligament by a ligamentous attachment.

Rotation and gliding are the two primary motions that take place in this joint.

The cricoarytenoid muscles turn the arytenoid cartilages in the transverse plane in the longitudinal axis of the cricoid facet when the joint is in a rotational position. The vocal processes of the arytenoid cartilages rotate outwards (dorsomediocranially) or inwards as a result of this rotation (ventrolaterocaudally). The lateral and medial swinging of the vocal processes is associated with the rotation of the arytenoid cartilages, which governs the opening and narrowing of the recess of the rima glottidis. In addition, the arytenoid cartilage on the cricoid cartilage is restricted in its forward mobility by the cricoarytenoid ligaments. There is around a 30° range of motion in total for the rotation.

When the arytenoid cartilages glide down the long axis of the cricoid facet, it is known as cricoid facet glide. The arytenoids will either migrate closer or farther away from one another in this situation. Because of the inferior-anterolateral tilt of the articular surface of the cricoid cartilage, when the arytenoids glide away from each other, they do so in an inferior direction as well as a superior one. When they glide in the opposite direction of each other, the opposite happens. As a result, the vocal cords are either shrinking or growing in length when making vocal modifications.

CRICOTHYROID JOINT

According to Grujicic (2021), the cricothyroid joint joins the cricoid and thyroid cartilages. The inferior horn of the thyroid cartilage and the laterally positioned thyroid articular surface on the cricoid cartilage create this joint.

A fibrous capsule rich in elastin fibers surrounds the joint, reinforced posteriorly by a fibrous band. The posterior, lateral, and anterior cricothyroid ligaments surround this joint. The posterior cricothyroid ligament runs from the articular facet on the thyroid cartilage to the superior cricoid lamina. The lateral cricothyroid ligament runs from the inferior horn of the thyroid cartilage to the posterior portion of the cricoid lamina. The anterior cricothyroid ligament extends from the inferior horn of thyroid cartilage to the lateral side of the cricoid arch above its inferior border.

This joint's main motion is rotation. This rotation is due to the cricoid cartilage rotating on the inferior horns of the thyroid cartilage. This action is best understood as a “tilting” motion. The cricothyroid muscle contracts, tugging on the anterior cricoid cartilage, causing it to tilt upwards. This movement reduces the anterior gap between the inferior thyroid cartilage border and the cricoid arch (Grujicic, 2021).

REFERENCES:

Grujicic, R. (2021). Cricoarytenoid and cricothyroid joints. Kenhub. https://www.kenhub.com/en/library/anatomy/cricoarytenoid-and-cricothyroid-joints

Saran, M., Georgakopoulos, B., & Bordoni, B. (2021b). Anatomy, Head and Neck, Larynx Vocal Cords. PubMed; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK535342/