#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:

• parasympathetic
  • visceral motor fibres, producing 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

#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

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

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

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 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

#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

#References

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