Human Larynx: Innervation

According to Stavroulaki and Birchall (2001), transplantation is being viewed as a means of increasing both the quality of life (QOL) and quantity of life (pg. 257). The notion of returning phonation and respiratory function via transplantation has fascinated laryngologists since the 1950’s. Strome performed the first true human laryngeal transplantation in 1998 and the patient continues to do well. However, according to Stravroulaki and Birchall (2001) re-innervation of the transplanted larynx is crucial for a successful functional outcome

The human larynx receives innervation from the vagus nerve via the superior laryngeal nerve (SLN) and recurrent laryngeal nerve (RLN). The SLN travels from the back of the larynx to the front initially under and then medial to the internal carotid artery (pg. 258). The SLN divides two branches: the large internal branch (InSLN) and the small external branch (ExSLN).

The InSLN descends above the superior laryngeal artery on the thyroid membrane. It then pierces through a hiatus at the upper edge of the inferior pharyngeal constrictor (IPC) muscle (pg. 258). Within the thyroid membrane, it divides into multiple branches descending along the aryepiglottic fold. The most superior branches supply the mucosa of the valleculae and the epiglottis (sensory fibers).

The ExSLN passes to the back of the larynx (posterior) to the sternothyroid muscle, which is deeper to the superior thyroid artery. The ExSLN descends within the inferior pharyngeal constrictor muscle sending branches to the pharyngeal plexus. Just under the lower edge of the thyroid cartilage or level of the “oblique line”, it curves to the front and middle of the larynx and reaches the cricothyroid (CT) muscle. Prior to entering the CT muscle the nerve splits and supplies both the oblique and rectus bellies of the CT muscle (motor fibers).

The recurrent laryngeal nerve (RLN) also derives from the vagus nerve. On the right side of the larynx the RLN winds around the subclavian artery, running closer to the trachea as it travels upward towards the tracheo-esophageal groove. On the left it passes lateral to the ligamentum arteriorsum behind the aortic arch and enters the trachea-esophageal groove in the chest. Branches on the left are distributed to the esophagus, trachea, and IPC muscles as they ascent towards the larynx. In the majority of cases, the RLN divides outside the larynx into an anterior motor branch and medial sensory branch. The anterior RLN carries adductor and abductor fibers which supply the PCA and Interarytenoid (IA) muscles of the larynx.

The posterior cricoarytenoid (PCA) muscle is frequently innervated by two or three compartmental nerve branches that arise of the main trunk of the AnRLN. In some cases, the nerve is innervated by a single nerve branch, which splits as soon as it enters the muscle. The inferior and superior branches innervate bellies of the PCA muscle.

         Reinnervation of the transplanted larynx is important for a successful outcome. A major step towards achieving the goal of successful innervation has been the use of appropriate animal models (dogs, cats, and rats). According to Stavroulaki and Birchall (2001), these animals have been widely used as preclinical models and a thorough understanding of the larynx structure and function. 

Resource:

Stavroulaki, P., Birchall, M.(2001). Comparative study of the laryngeal innervations in humans

and animals employed in laryngeal transplantation research, The Journal of Laryngology & Otology, 115, 257-266