Recurrent laryngeal nerve
||The lead section of this article may need to be rewritten. (June 2012)|
|Nerve: Recurrent laryngeal nerve|
|POSTERIOR VIEW: The tracheobronchial lymph glands (I. and E. Recurrent nerves visible at top.)|
|Course and distribution of the glossopharyngeal, vagus, and accessory nerves.|
|Latin||nervus laryngeus recurrens|
|Gray's||subject #205 912|
The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve (tenth cranial nerve) that supplies motor function and sensation to the larynx (voice box). It travels within the endoneurium. It is the nerve of the 6th pharyngeal arch.
Other than the cricothyroid muscle which is innervated by the superior laryngeal nerve, motor control of all the intrinsic muscles of the larynx, the thyroarytenoid, the posterior and lateral cricoarytenoid, and arytenoid muscles depends upon the recurrent laryngeal nerve. Additionally, it transmits sensory information from the mucous membranes of the larynx from the lower surface of the vocal fold, downwards.:847–9
The nerves are named "recurrent" (from the Latin re-, "back", and currere, "to run") because they are oriented in the opposite direction to the vagus nerves from which they branch. Neurons which innervate the laryngeal muscles in the neck do so via a circuitous route, descending into the thorax and looping around a large artery before rising up along the trachea and esophagus before returning to the neck.
The vagus nerves exit the skull and run alongside the carotid arteries through the neck into the chest. In the common anatomic configuration, the RLN branches off the vagus at the vessel where the carotid originates, the aortic arch on the left and the subclavian artery on the right. The recurrent branches pass anteriorly to (in front of) the vessel, then wrap under and behind the vessel. The left RLN is longer than the right, because it crosses under the arch of the aorta at the ligamentum arteriosum. The recurrent nerves typically ascend along the groove at the junction of the trachea and esophagus. As the recurrent nerve hooks around the subclavian artery or aorta, it gives off several cardiac filaments to the deep part of the cardiac plexus. As it ascends in the neck it gives off branches, more numerous on the left than on the right side, to the mucous membrane and muscular coat of the esophagus; branches to the mucous membrane and muscular fibers of the trachea; and some pharyngeal filaments to the superior pharyngeal constrictor muscle.
The nerve splits into anterior and posterior rami before supplying muscles in the voice box – it supplies all laryngeal muscles except for the cricothyroid, which is innervated by the external branch of the superior laryngeal nerve.
The recurrent laryngeal nerve enters the pharynx, along with the inferior laryngeal artery and inferior laryngeal vein, below the inferior constrictor muscle to innervate the Intrinsic Muscles of the larynx responsible for controlling the movements of the vocal folds.
During human (and all vertebrate) development, a series of pharyngeal arch pairs form, projecting ventrally (towards the front of the face and neck). Each arch develops its own artery, nerve which controls a distinct muscle group, and skeletal tissue. Starting from the head, they are numbered 1, 2, 3, 4, and 6 to correspond with the six arches of the most closely related fish. If arch 5 forms at all, it is a transiently existing rudiment. Arches 4 and 6 produce the laryngeal cartilages. The nerve of Arch 6 becomes the recurrent laryngeal nerve. The nerve of Arch 4 gives rise to the superior laryngeal branch of the vagus nerve which controls several muscles involved in speech and swallowing; its arteries, which project between the nerves of the fourth and sixth arches, become left-sided arch of the aorta and the right subclavian artery. On the right side, the artery of Arch 6 is obliterated while, on the left side, the artery persists as the ductus arteriosus; circulatory changes immediately following birth cause the vessel to close down leaving a remnant, the ligamentum arteriosum. During growth, these arteries descend into their ultimate positions in the chest, creating the elongated recurrent paths.
In roughly 1 out of every 100 to 200 people, the right inferior laryngeal nerve is nonrecurrent, branching off the vagus nerve around the level of the cricoid cartilage. Typically, such a configuration is accompanied by variation in the arrangement of the major arteries in the chest; most commonly, the right subclavian artery arises from the left side of the aorta and crosses behind the esophagus. A left nonrecurrent inferior laryngeal nerve is even more uncommon, requiring the aortic arch be on the right side (mirror image anatomy—situs inversus) accompanied by an arterial variant which prevents the nerve from being drawn into the chest by the left subclavian.
Injury to the recurrent laryngeal nerves can result in a weakened voice (hoarseness) or loss of voice (aphonia) and cause problems in the respiratory tract. Because the actions of the muscles during vocalization is complex and there are different mechanisms of injury, the effects vary. If the nerve on one side is injured, there are several possible effects:
- The voice may be weakened and sound breathy because one vocal cord is not acting.
- Beyond vocalization, the larynx acts to close of the respiratory tract during swallowing. If this function is impaired, food and liquids may enter the respiartory tract (aspiration).
- If the vocal cord is paralyzed in the adducted (closed) position, the restricted air flow may cause difficulty in breathing during physical activity.
Bilateral nerve damage can result in breathing difficulties and aphonia, the inability to speak.
The nerve receives close attention from surgeons since during neck surgery, especially thyroid and parathyroid surgery, the nerve is at risk for injury. Nerve damage can be assessed by laryngoscopy, during which a stroboscopic light confirms the absence of movement in the affected side of the vocal cords. The right recurrent laryngeal nerve is more susceptible to damage during thyroid surgery because it is close to the bifurcation of the right inferior thyroid artery, variably passing in front of, behind, or between the branches.:820–1 Similar problems may follow non-surgical trauma to the neck.
The RLN may be compressed by tumors. Studies have shown that 2–18% of lung cancer patients develop hoarseness because of RLN compression, usually left-sided. This is associated with worse outcomes, and when found as a presenting symptom, often indicates inoperable tumors. The nerve may be severed intentionally during lung cancer surgery in order to fully remove a tumor.
In Ortner's syndrome or cardiovocal syndrome, a rare cause of left RLN palsy, expansion of structures within the heart or major blood vessels impinges upon the nerve, causing symptoms of unilateral nerve injury.
Horses are subject to a condition which causes injury, equine recurrent laryngeal neuropathy, a disease of the axons. The cause is not known, although a genetic predisposition is suspected. The length of the nerve is a factor since it is more common in larger horses and left side is affected almost exclusively. As the nerve cells die, there is a progressive paralysis of the larynx, causing the airway to collapse. The common presentation is a sound, ranging from a musical whistle to a harsh roar, accompanied by worsening performance. The condition is incurable, but surgery can keep the airway open. Experiments with nerve grafts have been tried.
Galen demonstrated the nerve course and the clinical syndrome of recurrent laryngeal nerve paralysis in animals when both nerves were severed, and described the same effect in two human infants who had undergone surgery for goiter. In 1838, five years before he would introduce the concept of homology to biology, the famed anatomist Richard Owen reported upon the dissection of three giraffes, including a description of the full course of the left recurrent laryngeal nerve.
Evidence of evolution
The extreme detour of this nerve (about 15 feet in the case of giraffes) is cited as evidence of evolution. The nerve's route would have been direct in the fish-like ancestors of modern tetrapods, traveling from the brain, past the heart, to the gills (as it does in modern fish). Over the course of evolution, as the neck extended and the heart became lower in the body, the laryngeal nerve was caught on the wrong side of the heart. Natural selection gradually lengthened the nerve by tiny increments to accommodate, resulting in the circuitous route now observed.
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- SUNY Figs 21:04-01
- cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (X)
- Example of Vocal Cord Paralysis
- Dissection of a giraffe displaying the laryngeal nerve