#The parasympathetic nervous system

#Define the parasympathetic nervous system

Part of the autonomic nervous system that controls various unconscious functions, mostly via control of visceral organs. May be considered a physiological brake.

#Outline the anatomy of the vagus nerve

#Functional pathways

The vagus nerve (cranial nerve X) is a long, important nerve that has many fibre types which serve various parasympathetic and somatic functions:

• visceral motor fibres
  • produce secretory and motor effects in the heart, vasculature, bronchi, and GI tract
• visceral sensory fibres - aortic baroreceptors and chemoreceptors - dura - heart, respiratory tract and GI tract - epiglottis (taste)
• somatic sensory fibres, supplying the external auditor meatus
• somatic motor fibres, supplying the skeletal muscles of larynx/pharynx/upper oesophagus

#Origin

The vagus nerve originates from 4 nuclei in the medulla oblongata

1. Dorsal vagal nucleus - carres mixed visceral sensory/motor fibres
2. Nucleus ambiguus (NA) - carries motor fibres to the pharynx/larynx (known as ‘branchial’ motor neurones)
3. Nucleus tractus solitarius (NTS) - carries sensory fibres including taste
4. Spinal trigeminal nucleus - a minor contribution to the vagus nerve, carrying somatic sensory fibres

By Montoya, S., Portanova, A. & Bhatt, A.A, licensed under CC-BY 4.0

#Course

Head

Exits the cranium via the jugular foramen

There are two sensory ganglia at this level:

1. superior ganglion of the vagus nerve (aka jugular ganglion)
   • gives off the meningeal branch → dura
2. inferior ganglion of the vagus nerve (aka nodose ganglion)
   • gives off the auricular branch → external auditory meatus

Cranial ganglia of the vagus nerve.

Neck

Descends in carotid sheath between internal jugular vein and carotid artery, lateral to trachea.

Branches

• pharyngeal branch → pharyngeal plexus
• superior laryngeal nerve (SLN) → larynx (motor and sensory)
• cardiac branches → superficial and deep cardiac plexuses

Superior and recurrent laryngeal nerves.

By Jkwchui, licensed under CC-BY-SA 3.0

Chest

Enters thorax anterior to both subclavian arteries.

Branches

• cardiac branches → superficial and deep cardiac plexuses
• recurrent laryngeal nerves (RLN) → neck → larynx - L) RLN passes under the aortic arch
• R) RLN passes under the R) suvbclavian artery
• oesophageal plexus → oesophagus
• pulmonary branches - anterior pulmonary branches → anterior pumonary plexus
• posterior pulmonary branches → posterior pulmonary plexus

Branches and plexuses of the vagus nerve.

Abdomen

Enters abdomen through oesophageal hiatus in diaphragm at the level of T10 - L) vagus → anterior vagal trunk - R) vagus → posterior vagal trunk

Branches - gastric branches → stomach - coeliac branches → coeliac plexus and superior mesenteric plexus → renal/adrenal/GI - hepatic branches → hepatic plexus

#Outline the non-vagal components of the parasympathetic nervous system

#Cranial

CN III (oculomotor nerve)

• the parasympathetic nucleus of CN III is known as the Edinger-Westphal nucleus
• it supplies the ciliary ganglion → pupil constriction and accomodation

CN VII (facial nerve)

• supplies two ganglia:
  • pterygopalatine ganglion → lacrimal gland → lacrimation
  • submandibular ganglion → submandibular/submental salivary glands → salivation

CN IX (glossopharyngeal nerve)

• supplies the otic ganglion → parotid gland → salivation
• also receives signals from the carotid baroreceptors and chemoreceptors

#Sacral

Pelvic splanchnic nerves originate from S2-4. Often described as ‘the mechanism for emptying'.

• gastrointestinal effects
  • anal sphincter relaxation
  • stimulates distal colon (from splenic flexure) and rectum peristalsis
• genitourinary effects
  • detrusor contraction
  • bladder sphincter relaxation
  • erection

Innervation of the parasympathetic nervous system

By Blausen Medical, licensed under CC-BY 3.0

#Outline the nerve fibre types and synaptic physiology

#Nerve fibres

	Preganglionic	Postganglionic
Length	Long	Short
Fibre type	Myelinated B-fibres	Unmyelinated C-fibres
Synapse location	Autonomic ganglia close to the target organ	On target organ
Neurotransmitter released	Releases acetylcholine (ACh) which activates type-2 nicotinic (N2) receptors on postganglionic nerves	Releases ACh which activates muscarinic receptors on target organs

#Receptors and neurotransmitters

• N2 receptors
• Muscarinic receptors

Pre- and post-ganglionic nerve and neurotransmitter types in parasympathetic, sympathetic, and somatic neurones.

By Daniel Walsh and Alan Sved, licensed under CC-BY-SA 4.0

#References

Ellis, Feldman, Harrop-Griffiths, Lawson. Anatomy for anaesthetists, 8th edition. Blackwell Publishing. 2004.