Vagus nerve

1. Main Motor Nucleus; The nucleus ambiguus

The motor component of the vagus nerve arises from the nucleus ambiguus, located deep in the reticular formation of the medulla oblongata. It receives bilateral afferent inputs from the corticonuclear tracts of the cerebral hemispheres. The efferent fibers from this nucleus supply the muscles of the pharynx, the intrinsic muscles of the larynx, and the palatine muscles (except the tensor veli palatini, which is innervated by the mandibular division of the trigeminal nerve).

2. Parasympathetic Nucleus: Dorsal motor nucleus of vagus

The parasympathetic fibers originate from the dorsal motor nucleus of the vagus. This nucleus lies beneath the floor of the fourth ventricle, posterolateral to the hypoglossal nucleus. It receives afferent input from the hypothalamus via descending autonomic pathways and the glossopharyngeal nerve (for the carotid sinus reflex). The efferent preganglionic parasympathetic fibers innervate the involuntary smooth muscle of the bronchi, heart, esophagus, stomach, small intestine, and part of the large intestine (up to the distal third of the transverse colon).

3. Sensory Nuclei: the nucleus of tractus solitarius, and the spinal nucleus of trigeminal nerve

Sensory input is primarily processed in the nucleus of the tractus solitarius, located in the lower medulla. Information about taste sensations from the epiglottis and pharynx travels to this nucleus via peripheral axons with cell bodies in the inferior vagal ganglion.

Somatic sensory input from the external ear, external auditory canal, and certain regions of the dura is also processed by the vagus nerve and relayed to the spinal nucleus of the trigeminal nerve.

1. Intracranial course

The vagus nerve exits the cranial cavity through the jugular foramen in the posterior cranial fossa. At the jugular foramen, the nerve contains two sensory ganglia: a-The superior ganglion (jugular ganglion), related to somatic sensation. b-The inferior ganglion (nodose ganglion), associated with visceral sensation and taste.

2. In the Neck

The vagus nerve descends within the carotid sheath, lying between the internal jugular vein and the common carotid artery.

Important branches in the neck include:

Auricular branch

Provides sensory innervation to the external ear.

Pharyngeal and palatine branches

Supply motor fibers to the pharyngeal and palatine muscles (except stylopharyngeus and tensor veli palatini).

Superior laryngeal nerve

Divides into internal and external branches, responsible for sensation above the vocal cords and motor innervation to the cricothyroid muscle.

3. In the Thorax

The right and left vagus nerves:

Right vagus nerve

The right vagus crosses anteriorly over the subclavian artery and gives rise to the right recurrent laryngeal nerve, which loops under the subclavian artery to ascend back toward the larynx.

Left vagus nerve

The left vagus crosses the aortic arch, where it gives off the left recurrent laryngeal nerve, which loops under the arch and ascends in a similar fashion.

Both recurrent laryngeal nerves supply the intrinsic muscles of the larynx (except the cricothyroid).

Cardiac and pulmonary plexuses

The vagus nerve contributes parasympathetic fibers to the cardiac and pulmonary plexuses. In the cardiac plexus, it provides input that helps regulate heart rate by decreasing it through parasympathetic stimulation. In the pulmonary plexuses, the vagus nerve contributes fibers that facilitate bronchoconstriction and secretion of mucus.

4. In the Abdomen

Esophageal plexus

Both vagus nerves contribute to the esophageal plexus as they descend into the thorax.

Anterior and posterior vagal trunks

After passing through the esophageal hiatus of the diaphragm, the nerves reorganize into anterior vagal trunk (primarily from the left vagus) and posterior vagal trunk (primarily from the right vagus).

- These trunks supply parasympathetic innervation to abdominal viscera, including the stomach, liver, pancreas, kidneys, and intestines (up to the distal third of the transverse colon), via the celiac, renal, and superior mesenteric plexuses.

Somatic Motor Functions

Innervates muscles of the pharynx and larynx (except stylopharyngeus and tensor veli palatini), crucial for swallowing and phonation.

Parasympathetic Functions

Regulates the autonomic functions of the heart (slows heart rate), lungs (bronchoconstriction), and digestive tract (enhances peristalsis and secretions).

Visceral Sensory Functions

Detects changes in thoracic and abdominal viscera, such as blood pressure and distension.

Special Sensory Functions

Provides taste sensations from the epiglottis and pharynx.

Somatic Sensory Functions

Provides sensation to portions of the external ear, external auditory canal, and oropharynx.