Canonical Question

ANS Physiology

V5 H3.i Historical V4 K1.i, M1.i 1 appearance

Master answer

Vagus Nerve Anatomy

Nuclei	Dorsal motor nucleus – sends parasympathetic fibers to the GI tract, lungs Nucleus ambiguus – sends efferent motor and parasympathetic fibers to the heart Solitary nucleus – receives special gustatory afferent from the tongue and visceral afferent fibers from organs Spinal trigeminal nucleus – receives general sensory afferent fibers from outer ear, dura of posterior cranial fossa, mucosa of larynx
Course	Exits brain from medulla oblangata – series of rootlets in retro-olivary groove → Travels laterally and exits skull via jugular foramen → Sensory ganglia consist of superior and inferior ganglionic swelling → Joined by cranial root of accessory nerve (CN XI) → Passes down the neck in carotid sheath (between Carotid artery and Internal Jugular Vein) → Enters thorax at base of neck → Left: travels ant to aortic arch, behind Lt main bronchus and into oesophagus → Right: travels behind oesophagus and Rt main bronchus →Both enter abdomen through oesophageal hiatus in diaphragm
Branches	In the jugular fossa: meningeal, auricular branches In the neck: pharyngeal, superior laryngeal, recurrent laryngeal nerves; superior cardiac branches In the thorax: inferior cardiac nerve, anterior bronchial branches, posterior bronchial branches, esophageal branches In the abdomen: gastric, celiac and hepatic branches
Field of innervation	General sensory afferent fibers – sensory information from larynx, auricle, external acoustic meatus, dura mater of the posterior cranial fossa General visceral afferent – information from the aortic body, esophagus, lungs, bronchi, heart, intestines Special afferent – information about taste General visceral efferent – parasympathetic division that simulates smooth muscle and glands of the pharynx, larynx, thoracic and abdominal organs. – Heart via cardiac plexus: SA node: Rt vagus. AV node, ventricles: Lt vagus. – Lungs via pulmonary plexus – Gut (from stomach to proximal to splenic flexure) via gastric plexus Branchial efferent – innervate muscles of mastication and tensor vali palatini
Communications	Trunk and ganglia with CN IX,XI,XII, superior sympathetic ganglion and 1st and 2nd cervical nerves Auricular branch with CN VII Pharyngeal plexus with IX and superior cervical ganglion Cardiac, pulmonary, oesophageal and gastric branches with sympathetic outflow to viscera (Superficial and deep Cardiac plexuses receives branches from cardiac nerves of vagus, recurrent laryngeal nerves and cervical ganglia of sympathetic trunk.)

Physiology

75% of all parasympathetic fibres are in vagus nerve
Pre-ganglionic fibre
- Long fibre
- Release Acetylcholine to stimulate post-ganglionic neuron at nicotinic ACh receptor
- activation is tonic
Ganglia located near or within effector organs
Post-ganglionic fibre
- Short
- Release Acetylcholine to stimulate muscarinic ACh receptor

Effector

Acetylcholine (CCOOCCNH₃)
Formed for Choline and Acetyl CoA
Receptors
- Muscarinic
  - G Protein Coupled
- Nicotinic
  - Ligand gated ion channels
Metabolism
- Acetylcholinesterase

Muscarinic ACh receptor

Known as muscarinic because muscarine also agonises this receptor
Metabotropic, G-protein coupled
Phosphorylate various second messengers
Mediate slow metabolic response via second messenger cascades
5 subtypes: M1-M5
- M1,M3,M5 – excitatory (Gq: IP3/DAG → ↑Ca⁺⁺)
- M2, M4 – Inhibitory (Gi: ↓cAMP)
Receptor at parasympathetic postganglionic terminals
- M1: ANS
- M2: Cardiac
- M3: Lungs

Ganglion / Target organ	Receptor	Signal	Effect
Respiratory: Pulmonary plexus	M3 (Gq)	IP3/DAG ↑Ca⁺⁺	Bronchoconstriction increased mucous production
Cardiovascular: – SA node – AV node	M2 (Gi)	↓cAMP ↑ K⁺(G)	↓ inotropy (Atria > Vent) ↓↓↓ Chronotropy ↓ lusitropy (Atria > Vent) ↓↓↓ dromotropy
stomach to proximal two-thirds of the transverse colon	M1 (Gq)	IP3/DAG ↑ Ca⁺⁺	Increased gastric motility, secretions Relaxation of pyloric sphincter

Exam appearances

Exam	Exact wording	Relationship	Success
2023B Q20	Outline the anatomy (60% marks) and synaptic physiology (40% marks) of the vagus nerve.	historical_member	—