Question 20

Outline the anatomy (60% marks) and synaptic physiology (40% marks) of the vagus nerve

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College Answer

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.

Discussion

Anatomy of the vagus nerve:

  • Fibres:
    • Autonomic (parasympathetic)
      • cardiac muscle
      • visceral muscle of thoracic and abdominal viscera
    • Visceral sensory, from:
      • baroreceptors of aortic arch
      • chemoreceptors of aortic bodies
      • taste fibres from epiglottis
      • skin of external acoustic meatus and behind auricle,
      • mucous membrane of pharynx and larynx 
    • Somatic
      • to the skeletal muscle of the pharynx, upper oesophagus, palate and larynx 
  • Path:
    • Two nerves, right and left
    • Emerge from the jugular foramina
    • Forms two ganglia here:
      • superior ganglion
      • Inferior ganglion
    • Then,  runs straight down the neck, contained in the back of the carotid sheath, between carotid artery and jugular vein.
    • Passes in front of the subclavian artery to enter the mediastinum
    • The right vagus travels in contact with the trachea, left vagus runs along the great arteries
      • On the arch of the aorta the left vagus nerve flattens out and gives off its recurrent laryngeal branch
      • The right recurrent laryngeal nerve is given off in the root of the neck and hooks around the right subclavian artery
    • Both pass behind the lung root and contribute to the pulmonary plexuses
    • Both add to the oesophageal plexuses on the surfaces of the lower oesophagus, where they become mixed
  • Branches:
    • meningeal branch that supplies sensation to the dura mater of the posterior fossa
    • auricular sensory branch that supplies a part of the tympanic membrane
    • carotid body branch forms a plexus with the carotid sinus branch of the glossopharyngeal nerve and collects sensation from the glomus
    • pharyngeal branch supplies the muscles of the pharynx muscles of the pharynx (except stylopharyngeus) and the muscles of the soft palate (except tensor palati)
    • superior laryngeal nerve, which supplies the piriform recess and the cricothyroid and the cricopharyngeus part of the inferior constrictor.
    • cervical cardiac branches to the cardiac plexus
    • recurrent laryngeal nerve (supplies the muscles of the larynx)

Presynaptic physiology

  • presynaptic terminals release acetylcholine, postsynaptic membrane carries N2 nicotinic receptors

Post-synaptic physiology

  • Postsynaptic neurotransmission is muscarinic
  • 5 types of receptors
    • M1, M3 and M 5 are Gq protein coupled: the second messenger is IP3, and the result is calcium release.
    • M2 and M4 are Gi protein coupled: they deactivate adenylyl cyclase, and decrease the levels of cAMP
  • Downstream effects of the ... with the locations and downstream effects of the ... M1-M3 locations.. ?
    • M1 receptors are mostly in the CNS and are involved in memory
    • M2 receptors are found in many parasympathetic end organs:
      • Pupil (miosis)
      • Lacrimal glands
      • SA/AV node
      • Stomach (motility and secretion)
      • gallbladder
      • Intestinal sphincters
      • motility
    • M3: same as above, and also:
      • Bladder detrusor muscle contraction
      • Relaxation of the trigone sphincter
      • Erection
      • Generalised secretion of the sweat glands
      • Increased secretion of the pancreatic juice

Obviously there is absolutely no way you could produce this much detail in the exam over the course of ten minutes, but one could fashion this into an actual answer by pruning some of the detail. For example, nobody asked as to where the fibres go to or from, they just wanted a "description of the fibers it carries". A bare minimum, with a total of 181 words (i.e. achievable over ten minutes), could look something like this:

Anatomy of the vagus nerve:

  • Fibres:
    • Autonomic (parasympathetic)
    • Visceral sensory
    • Somatic motor
  • Path:
    • Two nerves, right and left
    • Emerge from the jugular foramina
    • Form two ganglia here (superior and inferior)
    • Travel between the carotid and jugular, into the chest
    • Right along the trachea, left along the greater vessels
    • Both pass behind the lung root
    • Join and merge to form oesophageal plexuses
  • Branches in order of proximal to distal:
    • meningeal branch 
    • auricular sensory branch
    • carotid body branch
    • pharyngeal branch
    • superior laryngeal nerve
    • cervical cardiac branches to the cardiac plexus
    • recurrent laryngeal nerve (supplies the muscles of the larynx)

Presynaptic physiology

  • presynaptic terminals release acetylcholine, postsynaptic membrane carries N2 nicotinic receptors

Post-synaptic physiology

  • Postsynaptic neurotransmission is muscarinic
  • 5 types of receptors
    • M1, M3 and M5 -  Gq protein coupled: increase intracellular calcium
    • M2 and M4 - Gi protein coupled: decrease the levels of cAMP
  • Downstream effects:
    • M1: in the CNS,  involved in memory
    • M2: pupils, lacrimal glands, SA/AV node, stomach (motility and secretions), gallbladder, intestinal sphincters and intestinal motility
    • M3: same as above, and also bladder detrusor, trigone relaxation, erection, sweating, and pancreatic secretion

References

Câmara, Richard, and Christoph J. Griessenauer. "Anatomy of the vagus nerve." Nerves and nerve injuries. Academic Press, 2015. 385-397.

Berthoud, Hans-Rudolf, and Winfried L. Neuhuber. "Functional and chemical anatomy of the afferent vagal system." Autonomic Neuroscience 85.1-3 (2000): 1-17.