The Anatomy of the Superior Laryngeal Nerve

Paralysis of this nerve can change your voice

In This Article

The superior laryngeal nerve is involved in speech as well as protecting your airway from food and drink when you swallow. It branches off from the vagus nerve and then divides into two major branches, one sensory (gathering information from your senses) and one motor (involved in movement). Since it provides both kinds of function it is classified as a mixed nerve.

Anatomy

The nerves of your body all originate either from the brain or from the spinal cord. They then travel outward, giving off branches that allow your muscles to move, your fingers to feel, your eyes to see, etc.

Twelve generally symmetrical pairs of cranial nerves emerge from your brain, and the vagus nerve, or tenth cranial nerve (CN X), is one of them. From its point of origin in the brainstem, it runs down through your neck to reach the chest and abdomen. It’s the longest of the cranial nerves and is involved in numerous critical functions, including heartbeat and blood-pressure regulation, sweating, digestion, and carrying sensory information from your internal organs to your brain.

While it travels down through the neck, the vagus nerve gives off several branches, including:

  • Pharyngeal branch, which provides motor function to most of the muscles in the pharynx (area behind the nose and mouth) and soft palate of your mouth
  • Recurrent laryngeal nerve, which exists only on the right side and provides motor function to the majority of the muscles inside the larynx (“voice box”)
  • Superior laryngeal nerve, which provides sensory function to the lower part of the pharynx and the upper portion of the larynx

Structure and Location

Along the vagus nerve are several ganglia, which are like sensory relay stations for nerve signals. In the throat, in front of the jugular vein, is a ganglion called the ganglion nodosum (or nodose ganglion). The superior laryngeal nerve arises from the middle of this ganglion.

The nerve then combines with a branch from the superior cervical ganglion and runs downward alongside the pharynx and behind the internal carotid artery.

The superior laryngeal nerve then divides into two branches:

  • External branch
  • Internal branch

External Branch

The external branch of the superior laryngeal nerve is the smaller of the two branches and is sometimes called the ramus externus. It runs down past the larynx and under the sternothyroid muscle. It then:

  • Connects with the cricothyroid muscle in the larynx
  • Sends branches to the pharyngeal plexus (a network of nerve fibers) and constrictor pharyngis inferior muscle in the pharynx
  • Communicates with the superior cardiac nerve, which is behind the common carotid artery

Internal Branch

The internal branch of the superior laryngeal nerve, which also is called the ramus internus, travels down to and through the thyrohyoid membrane (in the larynx) along with the superior laryngeal artery. Its fibers are then distributed to the larynx’s mucous membrane. Specifically, they connect to the:

  • Epiglottis
  • Base of the tongue
  • Epiglottic glands

Some fibers also run backward to innervate the:

  • Mucous membrane that surrounds the entrance to the larynx
  • Lining of the laryngeal cavity down to the vocal folds

A filament then continues down to join the recurrent laryngeal nerve on the inner surface of the thyroid cartilage.

Anatomical Variations

While nerves have a typical route through the body, the course does vary in some people. It’s important for doctors to know about possible anatomical variations, especially during surgery or procedures such as nerve blocks.

The superior laryngeal nerve’s variations aren’t well understood, but some variations to its course are documented. The course has some known variations in relation to the superior thyroid vessels, which is important during thyroid and parathyroid surgery.

Four different routes have been described:

  1. About 60% of people have the standard distance of at least 1 centimeter (cm) between the external branch and a structure called the superior thyroid pole.
  2. 17% have a distance shorter than 1 cm.
  3. In 20%, the branch crosses below the upper edge of the thyroid.
  4. In 3%, the branch can’t be seen because it runs beneath the fascia or inside the muscle, so its specific course is unknown.

The second and third types pose an increased risk during surgical removal of the thyroid gland. These variations appear to be more common in people of Mexican, Chinese, and Indian descent. Physical height and the size of the thyroid gland are believed to play a role in some of the variations, as well.

When it comes to the superior constrictor muscle, the nerve’s course may be completely outside of it, completely inside of it, or partially inside of it.

In less than half of people, there’s a connection from the exterior branch to the thyroarytenoid muscle that’s called the human communication nerve. Whether it provides function to the muscle isn’t yet known.

Function

As a mixed nerve, the superior laryngeal nerve provides both motor and sensory function to different areas of the throat and mouth that are primarily involved in speech and keeping food and drink out of your airway.

Motor Function

The external branch of the nerve is the motor branch. It connects to and allows for movement of the cricothyroid muscle, which is sometimes referred to as “singer’s muscle.” This muscle tenses and stretches your vocal ligaments, which tilts your thyroid forward and makes it so you can produce forceful sounds with your voice. Additionally, the cricothyroid works with the thyroarytenoid muscle to create high-pitched sounds.

The cricothyroid is unique in a couple of aspects. First, it’s the only laryngeal muscle innervated by the superior laryngeal nerve; the rest are served by the recurrent laryngeal nerve. Second, it’s the only tensor muscle of the larynx that plays a role in making vocal sounds.

Sensory Function

The internal branch of the superior laryngeal nerve is believed to be purely sensory. It’s crucial for protecting your airway when you swallow so food doesn’t “go down the wrong pipe” and obstruct your breathing.

When you swallow, it activates special receptors that are innervated by the internal branch. The receptors communicate with neurons (brain cells) that control swallowing, closure of the larynx, and your breathing rhythm.

Associated Conditions

As with any nerve, the superior laryngeal nerve may be damaged by trauma, abnormal physiological structures, and disease that affect nerves. A major cause of problems with this nerve, especially the external branch, is surgery.

External Branch

The external branch of the superior laryngeal nerve is at high risk of being damaged during thyroidectomy (surgery to remove the thyroid gland). Some estimates of injury rates during this procedure put it as high as 58%. Surgeons typically monitor patients closely during procedures in order to minimize the risk.

The external branch can also be injured during a cricothyrotomy, which is an emergency incision made in the neck to open an alternate airway when a person can’t breathe due to an obstruction.

When this nerve is damaged, you may lose the ability to yell or create high-pitched sounds. The overall pitch of the voice may change, and the voice may tire easily and become monotone.

Interior Branch

The interior branch is more protected and thus less likely to be damaged. It can become irritated easily if you breathe in food or drink, and that generally leads to uncontrolled coughing. If this branch is damaged, you can lose the reflex that causes that cough. This raises your risk of a lung infection that occurs when food enters the lung, which is called aspiration pneumonia. Interior-branch damage also can lead to a loss of sensation from portions of the larynx, which can cause problems with swallowing (dysphagia).

Common Trunk

The common trunk of the superior laryngeal nerve (before it branches into internal and external) can happen during surgery at the base of the skull or due to trauma to the vagus nerve itself. That can lead to any of the symptoms associated with damage to the branches. Other symptoms depend on the location of the trauma.

Rehabilitation

Treatments for damage to the superior laryngeal nerve include voice therapy and several types of surgery. The treatment course depends on factors such as specific symptoms and the extent of the damage.

Voice therapy is the most common option for chronic cases of nerve paralysis in which only the superior laryngeal nerve is involved. The goal of therapy is to build strength in the cricothyroid muscle. The outcome is variable when it comes to improving the voice, and researchers believe this is due to the difficulty of diagnosing paralysis in this nerve as well as differences in symptoms from one person to the next.

Surgical options include:

  • Type 1 thyroplasty (medialization laryngoplasty): The paralyzed vocal fold is moved to the middle of the larynx, where the un-paralyzed can make contact with it and completely close. Closure is necessary for making sound and sealing off the lungs when you swallow.
  • Modified type 4 thyroplasty (cricothyroid approximation): The vocal folds are surgically lengthened in order to raise the pitch of your voice.
  • Reinnervation using nerve-muscle pedicle technique: A nerve cable is grafted to the functional cricothyroid muscle on one side, then attached to the paralyzed cricothyroid muscle on the other side.

Most of the evidence for these treatments is anecdotal, as little or no research has been done on them. Your doctor can help guide you to the appropriate treatments.

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