Corpus: Superior laryngeal nerve

The superior laryngeal nerve is a mixed nerve branch of the vagus nerve, containing both motor and sensory fibers.

The superior laryngeal nerve runs medially to the external carotid artery. At the level of the hyoid bone, it divides into two branches:

• External branch (Ramus externus)
• Internal branch (Ramus internus)

The smaller of the two branches, the external branch, descends caudally beneath the sternothyroid muscle along the larynx. It innervates the cricothyroid muscle, which is crucial for tension and elongation of the vocal cords. Additionally, this branch contributes fibers to the pharyngeal plexus, participating in the innervation of the inferior pharyngeal constrictor muscle.

The larger internal branch travels alongside the superior laryngeal artery, passing through the thyrohyoid membrane to supply sensory innervation to the mucous membrane of the larynx up to the glottis. This branch may form an anastomosis with the terminal branch of the recurrent laryngeal nerve, known as Galen's anastomosis, providing additional sensory supply to the larynx.

The larynx receives sensory innervation from the internal branch of the superior laryngeal nerve above the glottis and from the recurrent laryngeal nerve below the glottis.

The external branch of the superior laryngeal nerve is at risk of injury during thyroidectomy, as it runs closely dorsal to the superior thyroid artery. Damage to the nerve can lead to difficulties in voice modulation due to impaired cricothyroid muscle function. Injuries to the vagus nerve, however, are more commonly associated with procedures such as neck lymph node removal ("neck dissection"), endarterectomy, and cervical spine surgery.

Neuralgia of the superior laryngeal nerve is exceedingly rare, if it occurs at all. Loss of nerve function can result in dysphagia (difficulty swallowing) due to impaired sensation in the pharyngeal area, often leading to aspiration.