Superior Laryngeal Nerve: Structure, Function & Diseases

The superior laryngeal nerve runs in the neck of humans. Its ramus internus consists of sensitive fibers that innervate the upper part of the laryngeal mucosa and some taste receptors. The ramus externus contributes to the motor control of the cricothyroid muscle, which tenses the vocal cords.

What is the superior laryngeal nerve?

The superior laryngeal nerve is the upper laryngeal nerve. It forms a branch of the vagus nerve, which is the tenth cranial nerve and has other branches. These include the ramus meningeus and ramus auricularis in the human head, as well as the cervical branches ramus pharyngeus, ramus laryngeus recurrens and ramus cardiacus. The superior laryngeal nerve is also one of the branches of the neck, after which the vagus nerve extends further to the chest and into the abdomen. For its part, the superior laryngeal nerve has two branches. The ramus internus and ramus externus are each named for their relative locations. The fibers of the superior laryngeal nerve consist of the myelinated processes of neurons and begin at the axon hillock of each cell.

Anatomy and structure

The superior laryngeal nerve receives its fibers from the vagus nerve (tenth cranial nerve) and branches off from it at the inferior nervi vagi ganglion. From there, the superior laryngeal nerve continues to the external carotid artery. At about this point, it divides and gives off two branches: the external branch (ramus externus) and the internal branch (ramus internus). The two differ not only in their course but also in their function. The ramus externus carries predominantly motor fibers, while sensory fibers make up the ramus internus. The internal branch has a larger diameter than the external branch. The internal ramus follows the course of the superior laryngeal artery, which carries oxygenated blood to the larynx. On the side of the blood vessel, the internal branch of the superior laryngeal nerve crosses the quadrangular membrana thyrohyoidea. The nerve fibers of the ramus internus terminate at the mucosa covering the larynx and innervate it to the vocal folds (labia vocalia). Some fibers of the ramus externus extend to the pharyngeal plexus, but most of them extend to the cricothyroid muscle. The laryngeal muscle is also known to medicine as the anticus or externus.

Function and tasks

With its two branches, the superior laryngeal nerve performs sensory and sensory as well as motor functions. The ramus internus innervates the laryngeal mucosa up to the vocal folds (labia vocalis) and from there transmits sensory stimuli to the brain. For this reason, this pathway belongs to the afferent nerves. When swallowing liquids and food, the body needs the information from the superior laryngeal nerve and other sensitive neurons to close the larynx and thus prevent foreign bodies from entering the larynx. A reflex is responsible for this process. In addition, sensory stimulation of the internal ramus plays a role in the cough reflex – for example, if fluid or food enters the airway despite the reflex closure. In addition, the ramus internus of the superior laryngeal nerve supplies some taste receptors and in this way contributes to gustatory perception. The ramus externus embodies a motor nerve pathway. Its efferent fibers transmit commands from the central nervous system to the cricothyroid muscle. Its task is to tighten the vocal cords. To accomplish this, the cricothyroid muscle pulls the cricoid cartilage (cartilago cricoidea) of the larynx downward and backward. When the vocal folds are taut, the voice sounds higher, which is relevant when speaking and singing.

Diseases

Lesions on the superior laryngeal nerve can lead to dysphagia (difficulty swallowing) when the body no longer registers sensitive stimuli from the laryngeal region. The reflex responsible for closing the larynx is often not triggered in these cases, which is why liquid or food pulp can enter the airways (foreign body aspiration). If the foreign body reaches the lungs, it can cause mechanical damage or lead to the development of infection. Failure of the motor ramus externus of the superior laryngeal nerve usually affects the affected person’s voice. This typically sounds broken or hoarse.The frequency spectrum may be restricted and lead to monotonous speech. Damage to the superior laryngeal nerve can be due to various causes. In addition to central nervous diseases, which optionally also affect other nerve tracts, local lesions are possible triggers. A risk of operations in the neck area, for example on the cervical spine, is damage to surrounding tissue. The superior laryngeal nerve is also at risk during thyroidectomy. Such thyroidectomy is indicated in cases of carcinoma of the hormone-producing organ, among others. Thyroid carcinoma embodies a cancer that affects a majority of women and is considered rare. In addition to surgical treatment, radiation therapy may also be considered. However, the specific treatment options depend on the individual case. Surgical treatment of a stenosis, which narrows a blood vessel and thus obstructs blood flow, can also damage the superior laryngeal nerve. Medicine refers to this operation as a peel-out surgery or (end) arteriectomy. It is similar to lymph node excision, also known in the neck as neck dissection, which is also a possible cause of damage to the superior laryngeal nerve. Neuralgic pain is uncommon as a disease of the superior laryngeal nerve.