Q&A: Neuroanatomy of the Vagus Nerve

The vagus nerve (cranial nerve [CN] X) is the longest cranial nerve in the body, containing both motor and sensory functions in both the afferent and efferent regards. The nerve travels widely throughout the body affecting several organ systems and regions of the body, such as the tongue, pharynx, heart, and gastrointestinal system. Because of the wide distribution of the nerve throughout the body, there are several clinical correlations of the vagus nerve.

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In this article:
Origin of the vagus nerve
MCQ: clinical scenario
MCQ: answer
MCQ: explanation

Origin of the vagus nerve

The vagus nerve has its origin in the medulla oblongata and exits the skull via the jugular foramen. There are two ganglia on the vagus nerve (superior and inferior) as it exits the jugular foramen; the spinal accessory nerve (CN XI) joins the vagus nerve just distal to the inferior ganglion.

The origin of cell bodies for the vagus nerve originates from the nucleus ambiguous; the dorsal motor nucleus of X, superior ganglion of X, and the inferior ganglion of X. The nerve fibers from the nucleus ambiguous are efferent, special visceral (ESV) fibers which help to mediate swallowing and phonation. Fibers originating from the dorsal motor nucleus of X are efferent, general visceral (EGV) fibers which provide the involuntary muscle control of organs it innervates (cardiac, pulmonary, esophageal) and innervation to glands throughout the gastrointestinal tract. Superior ganglion of X provides afferent general somatic innervation to the external ear and tympanic membrane. The inferior ganglion of X provides afferent general visceral fibers to the carotid and aortic bodies; the efferent fibers of this nerve travel to the nucleus tractus solitarius; the inferior ganglion also provides taste sensation to the pharynx and relays this information to the nucleus tractus solitarius.

The vagus nerve continues by traveling inferiorly within the carotid sheath where it is located posterior and lateral to the internal and common carotid arteries, and medial to the internal jugular vein.

MCQ: clinical scenario

Which receptor is innervated by nerves that combine with the vagus nerve?

a) Carotid sinus receptors
b) Aortic arch receptors
c) Central chemoreceptors
d) Pulmonary artery receptors
e) Atrial receptors

MCQ: answer

The correct answer is B

MCQ: explanation

The aortic arch baroreceptors are innervated by the aortic nerve, which then combines with the vagus nerve (X cranial nerve) traveling to the brainstem (see figure below); bilateral vagotomy, therefore, denervates the aortic arch baroreceptors.

CARDIOVASCULAR PHYSIOLOGY
The most important arterial baroreceptors are located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch (see figure below).

Arterial baroreceptors. Credit: Cardiovascular Physiology Concepts

The carotid sinus baroreceptors are innervated by the sinus nerve of Hering, which is a branch of the glossopharyngeal nerve (IX cranial nerve). The glossopharyngeal nerve synapses in the nucleus tractus solitarius (NTS) located in the medulla of the brainstem. The aortic arch baroreceptors are innervated by the aortic nerve, which then combines with the vagus nerve (cranial nerve X) traveling to the NTS. The NTS modulates the activity of sympathetic and parasympathetic (vagal) neurons in the medulla, which in turn regulate the autonomic control of the heart and blood vessels.

Reference(s)
1). Brian J. Kenny; Bruno Bordoni: Neuroanatomy, Cranial Nerve 10 (Vagus Nerve). Treasure Island (FL): StatPearls Publishing; 2020 Jan. Creative Commons Attribution 4.0 International License.
2). Richard E. Klabunde, PhD: Arterial Baroreceptors. Cardiovascular Physiology Concepts. 2012.