K1i / 23B20: Outline the anatomy (60% marks) and synaptic physiology (40% marks) of the Vagus Nerve

23B20: Exam Report

Outline the anatomy (60% marks) and synaptic physiology (40% marks) of the Vagus Nerve

25% of candidates passed this question.

The vagus nerve anatomy was best broken down into a description of the fibers it carries (visceral, parasympathetic and somatic sensory fibres) and then origin and course from the parasympathetic, sensory and motor nuclei in the medulla as the tenth cranial nerve to its branches; the pharyngeal, cardiac, pulmonary and laryngeal branches.

Pre and post-ganglionic physiology involved a detailed description of the Muscarinic Ach receptor and events.

This would also include the 5 subtypes of the muscarinic receptor, with the locations and downstream effects of the M1-M3 locations being the most important to note.

K1i / 23B20: Outline the anatomy (60% marks) and synaptic physiology (40% marks) of the Vagus Nerve

Anatomy

Origin

Within medulla oblongata of brainstem: 4 vagal nuclei

Course

Exits from medulla oblongata -> the nerves emerge by a series of rootlets in retroolivary groove between the olive and the inferior cerebellar peduncle -> travels laterally exiting skull through jugular foramen
Passes down the neck between carotid artery and internal jugular vein within carotid sheath -> enters thorax at base of neck
Left vagus nerve travels anterior to aortic arch, behind primary left bronchus -> through esophageal hiatus of diaphragm into abdomen
Right vagus nerve travels behind esophagus and primary right bronchus -> through esophageal hiatus of diaphragm into abdomen

Nuclei

Dorsal Nucleus: parasympathetic fibres to intestines
Nucleus ambiguus: efferent motor and parasympathetic fibers to the heart
Solitary nucleus: receives special gustatory afferent from tongue and visceral afferent fibers from organs
Spinal trigeminal nucleus: receives general sensory afferent fibers

Branches

Branches in jugular fossa
- Meningeal branch
- Auricular branch
Branches in neck
- Pharyngeal nerve
- Superior laryngeal nerve: internal and external laryngeal branches
- Recurrent laryngeal nerve: innervates all other intrinsic laryngeal muscles except cricothyroid muscle
  - Right: travels under subclavian artery
  - Left: loops around aortic arch distal to ligamentum arteriosus
- Superior cardiac branches
Branches in the thorax
- Inferior cardiac nerve
- Anterior & posterior bronchial branches
- Esophageal branches
Branches in the abdomen
- Gastric branches
- Celiac branches
- Hepatic branches

Fibers

General afferent (sensory) fibers
Special sensory fibers
Visceral afferent (sensory) fibers
Branchial efferent (motor) fibers
Visceral efferent (motor) fibers

Presynaptic physiology

Presynaptic terminals release acetylcholine, postsynaptic membrane carries N2 nicotinic receptors

Postsynaptic physiology

- Muscarinic receptors

M1

Post-synaptic

Gq

CNS: ↑cognitive function, ↑seizure activity
Gastric parietal cell → gastric acid secretion

M2

Post-synaptic

Gi

Heart (SA node & AV node) → negative chronotropy
Bronchial SM → bronchoconstriction
Pupils: miosis (contraction of pupillary sphincter), contraction of ciliary muscle → far vision
CNS
GIT: stimulate gastric secretion, relax gastric/intestinal sphincters, contraction of gallbladder, ↑motility of stomach/intestine

M3

Post-synaptic

Gq

CNS
Glandular tissue (lacrimal, salivary, GI, GU, respiratory & sweat glands) → ↑secretions
Bronchial SM → bronchoconstriction
Vascular SM → vasodilation (via NO release)
GI & GU SM contraction of walls; relaxation of sphincters; stimulate gastric/pancreatic secretion
Uterine contraction
Penile erection
Iris & ciliary muscle contraction
Secretion of sweat glands