Vagus nerve is a cranial nerve [arises from brain] that contains both motor and sensory fibers. It is the longest cranial nerve and has the widest distribution from neck to thorax to the abdomen where its somatic and visceral afferent fibers are distributed. There are 12 cranial nerves on each side of the skull and vagus nerve is also called Xth cranial nerve.
The name of the nerve is derived from the Latin ‘vagary’ – meaning wandering.
It is also called wandering nerve.
Origin and Course of Vagus Nerve
The vagus nerve originates from the medulla oblongata of the brainstem and exits from it in the groove between the olive and the inferior cerebellar peduncle.
Exit from Skull
It comes outside the skull by entering the middle compartment of jugular foramen, with the glossopharyngeal and accessory nerves (CN IX and XI respectively).
In Jugular Foramen
In jugular foramen, the nerve has upper and lower sensory ganglia of the nerve.
The superior ganglion is called jugular ganglion and is less than 0.5 cm in diameter.
The inferior ganglion or ganglion nodosum is about 1 cm distal to the jugular ganglion. Below this ganglion, the cranial root of the accessory nerve joins the vagus nerve.
In the Neck and Further Course
The vagus nerve passes into the carotid sheath and travels downward medial to the internal jugular vein and common carotid artery.
At the base of the neck, the right and left nerves have different pathways. Along the course, several branches are given out. These branches and their distribution is discussed in next section.
- The right vagus nerve passes anterior to the subclavian artery and posterior to the sternoclavicular joint, entering the thorax. It travels into the fat behind the innominate vessels reaching the right side of the trachea which separates it from the right pleura. It then goes down behind the hilum of the right lung and courses medially toward the esophagus where it joins the left vagus nerve to form esophageal plexus.
- The left vagus nerve passes inferiorly between the left common carotid and left subclavian arteries, posterior to the sternoclavicular joint, entering the thorax. It moves down on the left side of the aortic arch which separates it from the left pleura. Further, it travels behind the phrenic nerve. It courses behind the root of the left lung and then deviates medially and downwards to reach the esophagus where it joins its counterpart to form the esophageal plexus by joining the opposite (right) vagus nerve.
The plexus also receives fibers from the sympathetic trunk.
The anterior and posterior gastric nerves arise from the esophageal plexus. The left vagus nerve contributes mainly to anterior gastric whereas the right gastric contributes to posterior gastric.
However, each gastric nerve has some fibers from the other side’s vagus nerve too.
The gastric nerves are responsible for supply to all the abdominal organs and the gastrointestinal tract ending just before the left colonic or splenic flexure.
Branches of Vagus Nerve
Vagus nerve gives several branches along its course. These are discussed region-wise.
In Jugular Foramen
A meningeal branch is given at the superior ganglion that re-enters the skull through the jugular foramen to supply the posterior fossa dura.
The auricular branch from the superior ganglion enters the mastoid canaliculus in the lateral part of the jugular foramen and supplies the sensation to the posterior part of the external auditory canal and external ear. This nerve communicates with branches of the seventh (facial) and ninth (glossopharyngeal) cranial nerves.
Branches of the Vagus Nerve in the Neck
The pharyngeal branches contain sensory and motor fibers and arise from the inferior ganglion. The motor fibers are the contribution of cranial nerve XI. These branches cross between external and internal carotid arteries to reach the middle constrictor muscle.
They reach the pharyngeal plexus formed by cranial nerve IX and contribution from the sympathetic chain.
Branches from the plexus supply
- The pharyngeal muscles and mucous membrane
- Palate except for the tensor palatini muscle
Vagal fibers from the pharyngeal plexus, fibers from the glossopharyngeal nerve and from the sympathetic chain form the intercarotid plexus at the carotid bifurcation.
These mediate chemoreceptors impulses in the carotid body. The carotid body is a small cluster of chemoreceptors and supporting cells located in the outer layer of the carotid vessel at the carotid bifurcation.
Superior laryngeal nerve
The superior laryngeal nerve arises from the inferior ganglion. It descends against the lateral wall of the pharynx, first along the posterior and then the medial surface of the internal carotid artery.
Superior laryngeal nerve divides into the external and internal laryngeal nerve branches at the tip of the hyoid
The external nerve passes downward with the superior thyroid vessels and to the inferior pharyngeal constrictor muscle. It supplies cricothyroid muscle.
The internal laryngeal nerve enters the larynx by piercing the pierces the thyrohyoid membrane. It supplies most of the mucosa above the glottis. It has three divisions:
- First division supplies mucosa of the laryngeal surface of the epiglottis
- Middle division supplies the mucosa of the vocal folds (both true and false folds) as well as the aryepiglottic fold
- Inferior division supplies
- the arytenoid mucosa
- The anterior wall of the hypopharynx
- The upper esophageal sphincter
- Part of the subglottis (major is by the ipsilateral recurrent nerve)
Recurrent Laryngeal Nerve [Inferior Laryngeal Nerve]
Right recurrent laryngeal nerve branch comes out from the vagus nerve around the right subclavian artery at the root of the neck. It further goes upward in the tracheoesophageal groove to enter the larynx between the cricopharyngeus and the esophagus.
The nerve passes behind the thyroid outer lobe [ posterior, middle part] and enters the larynx underneath the inferior constrictor muscle.
The left recurrent laryngeal nerve has a similar course except that it loops around the aortic arch distal to the ligamentum arteriosus.
The left recurrent laryngeal nerve is longer than the right because of its long course as it loops around the aortic arch.
Thus all the intrinsic muscles of the larynx except cricothyroid are supplied by the recurrent laryngeal nerve. Cricothyroid, as we noted before, is supplied by the superior laryngeal nerve.
All the muscles receive supply from ipsilateral [same side] recurrent laryngeal nerve. However, the interarytenoid receives supply from both the recurrent laryngeal nerves.
The nerve of Galen or ramus communicans is a connecting nerve between the superior and the recurrent laryngeal nerves. It carries motor supply to the tracheal and esophageal mucosa.
Superior cardiac nerve
The superior cardiac nerves originate just distal to the inferior vagal ganglion. Each nerve consists of two or more branches and may receive fibers from the sympathetic ganglia.
These nerves carry general visceral parasympathetic motor fibers to the cardiac plexus.
Vagus nerve branches in the thorax
The inferior cardiac branch arises from the trunk of the vagus as it lies beside the trachea on the right side and from recurrent laryngeal nerve on the left side. These branches end in the deep part of the cardiac plexus.
The anterior bronchial branches
- Are 2-3 in number
- Lie on the anterior surface of the root of the lung
- Join the branches from sympathetic trunk to form the anterior pulmonary plexus
The posterior bronchial branches
- Are larger than the anterior branches
- Lie on the posterior surface of the root of the lung
- Join sympathetic fibers to form the posterior pulmonary plexus
The esophageal branches
- Anterior and posterior branches
- Join together to form the esophageal plexus
- Plexus supplies posterior surface of the pericardium
Vagus nerve branches in the abdomen
- Supply the stomach
- Form anterior and posterior gastric plexus
- Anterior gastric plexus – left vagus fibers
- Posterior gastric plexus – right vagus fibers
- Mainly from the right vagus nerve
- Join the celiac plexus
- Supply the pancreas, spleen, kidneys, adrenals, and intestine
- They come from the left vagus
- Join the hepatic plexus
- Distributed to the liver
Natural Anatomical Variations
- The recurrent nerve may divide into two branches before entering the larynx
- The anterior (or medial) branch – supplies the adductor muscles
- The posterior (or lateral) branch- supplies the abductors.
- Branching usually occurs below the cricoid cartilage at a variable distance
- Rarely recurrent laryngeal nerve may have 4-6 branches.
- These may be esophageal branches or supply the inferior pharyngeal constrictor
- Sometimes, Right subclavian artery is retroesophageal
- Arises from the aorta distal to the ligamentum arteriosus
- In such situations, the right recurrent nerve does not loop around the artery
Types of Fibers of the Vagus Nerve and Their Distribution
Vagus nerve carries different kinds of fibers to affect different physiologic functions. Each type of fiber has a different origin.
Parasympathetic Efferent Fibers
These are also called efferent general visceral fibers or visceral motor fibers.
They arise from the dorsal motor nucleus of the vagus (medulla). They are responsible for the control of involuntary muscles and gland activity.
These fibers are distributed to
- Thoracic and abdominal viscera
- Bronchial tree
- Heart (inhibitory fibers)
- Small intestinal motor fibers
- Secretory fibers to the stomach and pancreas
Somatic motor fibers
(Efferent special visceral or Branchial motor)
These fibers arise from the cells of the nucleus ambiguus (situated deep in medullary reticular formation). Nucleus ambiguus is the motor nucleus for striated muscles of the pharynx, larynx, and palate.
Therefore, these fibers help in swallowing and phonation
Sensory Visceral Fibers
These are of two types -afferent general visceral and special visceral fibers.
Afferent general visceral fibers arise from the cells of the jugular ganglion or superior ganglion of vagus whereas special visceral fibers are from ganglion nodosum of the vagus nerve( inferior ganglia of the vagus).
General visceral fibers are responsible for visceral sensibilities whereas special visceral fibers carry the taste sensations
The somatic sensory fibers
(Afferent General Somatic)
These fibers are few in number and their nucleus is the spinal trigeminal nucleus. They are concerned with skin sensations.
These give auricular branch to the external ear, meatus, and tympanic membrane.
Functions of Vagus Nerve
The vagus nerve supplies motor parasympathetic fibers to all the organs except the adrenal glands, from the neck down to the second segment of the transverse colon.
The vagus also controls the following skeletal muscles
- Cricothyroid muscle
- Levator veli palatini muscle
- Salpingopharyngeus muscle
- Palatoglossus muscle
- Palatopharyngeus muscle
- Superior, middle and inferior pharyngeal constrictors
- Muscles of the larynx (speech).
Thus, through these parasympathetic fibers, the vagus nerve is responsible for
- Maintaining heart rate
- Movements of the gastrointestinal system
- Speech muscles
Afferent fibers innervate the outer ear canal via Alderman nerve or auricular branch of the vagus nerve.
Part of the meninges is supplied by fibers from the vagus nerve.
- Vagus nerve is responsible for gag reflex which is carried by muscles of the pharynx
- Also participates in cough reflex
- In gastroenteritis 5-HT3 receptor-mediated vagus stimulation leads to vomiting
- Vasovagal response- Fainting occurs because of hypotension that follows emotional stress. For example, fainting after seeing blood.
Vagal tone and heart rate
Vagal tone refers to the activity of the vagus nerve. The increased vagal tone which has a parasympathetic effect is often found in athletes and is associated with decreased heart rate.
Clinical Procedures Related to Vagus Nerve
Vagus nerve stimulation by implanting a neurotransmitter in the chest is being used as a treatment by delivering electrical impulses to the vagus nerve.
Depression and epilepsy are two diseases where it is used.
Cutting of the vagus nerve was used as a procedure to treat peptic ulcer disease but is obsolete now.
Its role is being studied in the treatment of weight loss and Parkinson’s disease.