The larynx serves multiple functions, including control of respiration, airway protection, coordination of swallowing, and phonation. Several nerves in the larynx control these tasks.
Each hemilarynx receives its innervation from ipsilateral branches of the vagus.
Vagus Nerve (Cranial Nerve X)
The vagus nerve emerges from the medulla oblongata where the vagal nerve fibers connected to the following 4 nuclei:
- Spinal nucleus of the trigeminal nerve
- Nucleus of tractus solitarius
- Nucleus ambiguus
- Dorsal vagal motor nucleus
The vagus nerve exits the skull through the jugular foramen. As it emerges from the jugular foramen, the vagus nerve bears the superior and inferior ganglionic swellings that are the sensory ganglia of the nerve. The superior ganglion is smaller and is also called the jugular ganglion, whereas the inferior ganglion is larger and is also called the nodose ganglion.
The vagus descends vertically in the neck in the carotid sheath, between the internal jugular vein (IJV) and the internal carotid artery (ICA) to the root of the neck.
Its further course differs on two sides:
Right side: The nerve passes posterior to the IJV, crosses the first part of subclavian artery, and then enters the thorax.
Left side: The nerve passes between the left common carotid artery and the subclavian artery and posterior to the left brachiocephalic vein to enter thorax.
The vagus supplies the larynx via the following 2 branches:
Superior laryngeal nerve
Recurrent laryngeal nerve
Superior Laryngeal Nerve
The superior laryngeal nerve arises from the inferior ganglion of the vagus. It descends lateral to the pharynx, at first posterior and then medial to the ICA.
At the level of greater horn of hyoid, the superior laryngeal nerve divides into a smaller external laryngeal nerve and a larger internal laryngeal nerve.
Internal branch of the superior laryngeal nerve
The internal branch of superior laryngeal nerve (IBSLN) pierces the thyrohyoid membrane above the entrance of superior laryngeal artery.
The IBSLN divides into an upper branch and a lower branch. The upper branch supplies the mucous membrane of lower part of pharynx, epiglottis, vallecula, and vestibule of the larynx. The lower branch descends in the medial wall of the pyriform fossa beneath the mucous membrane. It supplies the aryepiglottic fold and the mucous membrane of the larynx up to the level of the vocal folds.
The IBSLN provides general sensation, including pain, touch, and temperature for the tissue superior to the vocal folds.
The IBSLN is vulnerable during surgical interventions of the anterior cervical region, including carotid endarterectomy and cervical spine injury, with an anterior or anterolateral approach.  As the nerve passes beneath the mucous membrane of the medial wall of the piriform fossa, it is accessible for injection of local anesthesia, thus providing excellent anesthesia for most of the piriform fossa. 
External branch of superior laryngeal nerve
External branch of superior laryngeal nerve (EBSLN) is the smaller of the two branches of the superior laryngeal nerve. It descends to the region of the superior pole of the thyroid and travels medially along the inferior constrictor muscle. The EBSLN innervates the cricothyroid muscle, which is the only tensor of the vocal cords. The nerve enters the cricothyroid muscle laterally on its deep surface
The EBSLN also contributes innervations to the pharyngeal plexus. The pharyngeal plexus innervates the palate and pharynx and is formed by branches from the external laryngeal nerve, pharyngeal nerves, branches from the cranial nerve IX, and the sympathetic trunk.
In approximately 20% of individuals, the external branch is under the inferior constrictor muscle and cannot be visualized, but it can be stimulated using a nerve probe.  The EBSLN is closely associated with the superior thyroid vascular pedicle at the capsule of the superior pole of the thyroid. Owing to this close anatomical relationship, the nerve is at risk of injury during thyroid surgeries. Several anatomical variations exist between the course of the EBSLN, the superior thyroid artery, and the superior pole of the thyroid. Cernea et al described the level at which the external branch of the SLN crosses behind the superior thyroidal artery. 
Type 1: EBSLN crosses the superior thyroid vessels 1 or more centimeters above a horizontal plane passing through the upper border of superior thyroid pole.
Type 2a: The nerve crosses the vessels less than 1 cm above the plane.
Type 2b: The nerve crosses the vessels below the plane. Cernea type 2b is the most common position.  Cernea type 2b nerves are also the most vulnerable to injury during thyroidectomy.
Recurrent Laryngeal Nerve
The recurrent laryngeal nerve (RLN) is a branch of the vagus.
The course of the recurrent laryngeal nerves on the right and left sides is different:
The right RLN leaves the right vagus nerve as it crosses the right subclavian artery and loops posteriorly under the artery. The right RLN initially traverses at an angle towards the tracheoesophageal groove and then runs parallel to it.
The left RLN originates from the left vagus nerve as it crosses the aortic arch. It then passes posteriorly under the arch and the ligamentum arteriosum. The left RNL travels in a course that is parallel and close to the tracheoesophageal groove. The lower origin and consequently, a longer course of the left RLN makes it more vulnerable to injury than the right RLN.
In the neck, both nerves follow the same course and pass superiorly accompanied by the inferior thyroid artery. As it approaches the thyroid gland, the RLN may pass anterior or posterior to the inferior thyroid artery or between its branches.  The RLN on both sides pass deep to the lower border of the inferior constrictor muscle and enter the larynx posterior to the cricothyroid articulation.
The RLN supplies four intrinsic muscles of the larynx (Lateral cricoarytenoid, posterior cricorytenoid, transverse and oblique interarytenoid and thyroarytenoid) but not the cricothyroid muscle. The interarytenoid muscle, the only unpaired muscle of the larynx, receives innervation from both RLNs. Before entering the larynx, the RLN also sends branches to the inferior constrictor muscle and cricopharyngeus muscle.
The RLN supplies the mucosa of the vocal cord and the subglottis.
Extralaryngeal branching can take place at any point along the course of the nerve but is uncommon inferior to the level of the inferior thyroid artery.  Studies have shown that much variability to the extralaryngeal and intralaryngeal branching exists from person to person, as well as from side to side. [8, 9, 10, 11]
Traditionally, the extralaryngeal branches were described as functionally discrete fibers, separated into the anterior and posterior branches, where the anterior branches solely innervate the adductor muscles (thyroarytenoid, interarytenoid, and lateral cricoarytenoid), whereas the posterior branches innervate the abductor muscles (posterior cricoarytenoid). However, other studies have described no consistent functional pattern of branching of the anterior and posterior laryngeal branches. 
The RLN is at high risk of injury during thyroid surgeries. Although controversy still surrounds whether the identification of the RLN during thyroidectomy will affect the incidence of it is damage or not, most surgeons advocate identification and dissection of the nerve during the procedure to reduce the risk of injury. 
Several surgical landmarks have been proposed to identify the RLN during surgery, including relation of the nerve to inferior thyroid artery, relation to tracheoesophageal nerve, relation to Berry’s ligament, and relation to Zuckerkandl’s tubercle.
Relationship of RLN to inferior thyroid artery
The relationship between the RLN and inferior thyroid artery varies. [13, 14] Steinberg reported that the RLN ascends in the neck between the branches of the inferior thyroid artery in about 6.5% of individuals, posterior to the inferior thyroid artery in 61.5%, and anterior to the inferior thyroid artery in 32.5%.  On the right side, the nerve may be in any of three locations in relation to the artery. On the left side, it is more likely to lie posterior to the artery.
Relationship of RLN to Berry’s ligament
Relationship of RLN and tracheoesophageal groove
The distal end of the RLN was identified along the tracheoesophageal groove. The nerve was identified more consistently at the cricothyroid articulation. Shindo et al describe that most of the right RLNs course between 15-45º when entering the cricothyroid joint, whereas most of the left RLNs course between 0-30º.  This difference is due to the more angled path the right RLN takes when ascending in the neck.
Shindo et al recorded the angle that the RLN forms with the tracheoesophageal groove.  The most common course of the right and left RLN was type II (15°-30°). The next common course on the right side was type III (30°-45°) and on the left was type I (0°-15°).
Relationship of RLN and tubercle of Zuckerkandl
The tubercle of Zuckerkandl represents a thickening where the ultimobranchial body fuses into the median thyroid process and can be enlarged into a nodular process. When enlarged, it is a consistent landmark for the RLN because the nerve almost always courses medial and deep to it.
Anastomosis of RLN
The anastomosis of Galen (also called the ramus anastomoticus or Ansa of Galen) occurs within the framework of the larynx and is an anastomosis between the ipsilateral recurrent laryngeal nerve and the internal branch of the superior laryngeal nerve. Generally, the posterior branch of the RLN contributes to the anastomosis; however, the anterior branch can also contribute to the anastomosis. Traditionally, the anastomosis of Galen has been described to provide purely sensory and autonomic innervation. More recent studies have shown that the anastomosis may also contain motor fibers.
The "human communicating nerve" is an anastomosis between the external branch of the SLN and the distal RLN. Approximately 70% of human larynges have this anastomosis. The human communicating nerve may contain both sensory innervation to the larynx and motor innervation to the thyroarytenoid muscle.
Variants of RLN
Rarely (0.5% to 1% of individuals), in presence of aberrant right subclavian artery, arising from the aorta after the left subclavian artery has given off, the right RLN passes directly from the vagus nerve in the neck towards the larynx and does not recur around subclavian artery. This uncommon anatomic variation of the RLN makes it highly susceptible to surgical injury and is known as right “nonrecurrent” laryngeal nerve.
A left nonrecurrent laryngeal nerve is extremely rare.
The RLN may be in an abnormal location if a large goiter exists, especially if the goiter extends into the substernal or retroesophageal spaces, or when neoplastic changes in the neck have occurred. The nerve can be displaced in any direction and, more commonly, be fixated or splayed to the posterior aspect of the thyroid capsule.