The superior laryngeal nerve is a branch of the vagus nerve (cranial nerve X) that provides both sensory and motor innervation to structures in the neck, specifically related to the larynx.
Structure
- Branching: The superior laryngeal nerve branches off from the vagus nerve in the neck. It further divides into two main branches: the internal laryngeal nerve and the external laryngeal nerve.
- Fiber Types: It contains both sensory (afferent) and motor (efferent) fibers. The internal laryngeal nerve is primarily sensory, whereas the external laryngeal nerve is primarily motor.
- Course: The superior laryngeal nerve travels alongside the superior laryngeal artery, a branch of the superior thyroid artery, in its course towards the larynx.
Location
- Origin: The nerve originates from the vagus nerve in the cervical region, usually at the level of the nodose ganglion.
- Neck: After branching off from the vagus nerve, it descends in the neck, often passing medial to the carotid artery.
- Division: The nerve usually divides into its internal and external branches at or near the level of the greater cornu of the hyoid bone.
- Internal Laryngeal Nerve: This branch pierces the thyrohyoid membrane to enter the larynx and provides sensory innervation to the laryngeal mucosa above the level of the vocal cords.
- External Laryngeal Nerve: This branch travels more superficially and innervates the cricothyroid muscle, a muscle that adjusts tension and length of the vocal cords during speech and singing.
- Relation to Structures: The superior laryngeal nerve is closely related to various anatomical structures like the carotid artery, thyroid gland, and hyoid bone in its course.
Functions
The superior laryngeal nerve has both sensory and motor functions related to the larynx and surrounding structures.
Sensory Functions
- Laryngeal Mucosa: The internal laryngeal nerve, a branch of the superior laryngeal nerve, provides sensory innervation to the mucosa lining the larynx above the vocal cords. This sensation is crucial for initiating the cough reflex to expel foreign substances, as well as for monitoring airflow and pressure during vocalization.
- Pain and Temperature: The internal laryngeal nerve also transmits information about pain and temperature from the area it innervates.
Motor Functions
- Cricothyroid Muscle: The external laryngeal nerve, another branch of the superior laryngeal nerve, supplies motor innervation to the cricothyroid muscle. This muscle plays an essential role in pitch modulation during speech and singing by adjusting the tension and length of the vocal cords.
- Inferior Pharyngeal Constrictor: Some fibers of the superior laryngeal nerve may also supply the inferior pharyngeal constrictor muscle, which assists in swallowing.
Integrated Functions
- Swallowing: The sensory input from the superior laryngeal nerve contributes to the complex neural circuitry that controls swallowing, helping to ensure that food enters the esophagus rather than the airway.
- Voice Modulation: The motor function of the external laryngeal nerve is crucial for the fine control of pitch in the human voice. Professionals who rely heavily on voice control, such as singers and public speakers, depend on the proper functioning of this nerve.
- Cough Reflex: Sensory fibers of the superior laryngeal nerve contribute to the initiation of the cough reflex, helping to protect the airway from aspirated material or irritants.
- Breathing: The sensory function plays a role in monitoring airflow during breathing, sending information back to the central nervous system for processing.
Clinical significance
The superior laryngeal nerve holds considerable clinical importance due to its sensory and motor functions related to the larynx and speech.
Voice and Speech
Voice Disorders: Injury or dysfunction of the superior laryngeal nerve, particularly the external laryngeal branch, can lead to voice issues such as hoarseness, pitch changes, and fatigue while speaking or singing. This is particularly significant for individuals whose professions rely on voice quality, such as singers, actors, and public speakers.
Vocal Cord Paralysis: Damage to the nerve can result in paralysis or paresis of the cricothyroid muscle, affecting vocal cord tension and leading to vocal issues.
Swallowing and Airway
Swallowing Disorders: Impairment of the sensory function of the internal laryngeal nerve can compromise the swallow reflex, potentially leading to aspiration or dysphagia (difficulty swallowing).
Airway Protection: The nerve’s sensory function contributes to the cough reflex, which protects the airway from aspiration of foreign material. Dysfunction can compromise this protective mechanism.
Surgical Considerations
Thyroid Surgery: The superior laryngeal nerve is at risk during thyroid surgeries. Surgeons must be aware of its anatomy to avoid injuring it, which could lead to postoperative voice changes and swallowing issues.
Laryngeal Surgery: Procedures involving the larynx, such as tumor removal, also pose a risk to the superior laryngeal nerve. Preoperative mapping and intraoperative monitoring of the nerve may be undertaken to minimize risks.
Diagnostic Importance
Laryngoscopy: The superior laryngeal nerve may be evaluated during laryngoscopy for conditions affecting the voice or airway.
Nerve Conduction Studies: Electrophysiological studies can be performed to assess the nerve’s integrity in cases of unexplained voice or swallowing issues.
