M1i / 25B14 / 17A01 / 14B04: Anatomy of the parasympathetic nervous system

25B14: Exam Report

Outline the physiology of the parasympathetic nervous system using the following headings:

  1. the anatomical origins and target organ(s) (45% of marks).
    Responses of the organ(s) are NOT required.
  2. nerve fiber classification and characteristics (20% of marks)
  3. receptor types, locations and neurotransmitters (35% of marks

61% of candidates passed this question.

This question was best answered using the structure provided and ensuring the level of detail
linked to the % of marks to be gained. It could be answered with the following simple information if
presented with appropriate linking.

  1. Craniosacral outflow.
      • CN III – eye,
      • CN VII – parotid gland
      • CN IX -submandibular glands,
      • CN X – cardiac, pulmonary and abdominal organs;
    • Sacral Outflow
      • S2-4 = pelvic and sex organs.
  2. Preganglionic nerve fibres are long type B, partially myelinated and synapse on a ganglion
    close to the target organ. Post synaptic nerves are short type C unmyelinated.
  3. Preganglionic NT is ACh onto N2 ACh receptor which is a ligand gated ion channel allowing
    for Na and K movement. Post ganglionic NT is ACh onto Muscarinic receptors which are G protein coupled.
    • M1, M4 and M5 are in the CNS (Gq, Gi, Gq respectively)
    • M2 – heart and lung (Gi – inhibition of AC and decreased cAMP)
    • M3 stomach, gut and other organs (Gq – IP3/PLC).

17A01: Exam Report

Outline the anatomy and physiology of the parasympathetic nervous system.

32% of candidates passed this question.

An efficient way to answer this question was to describe the anatomy and physiology of both cranial and sacral sections together. High scoring answers included an outline of the relevant nerves, the various ganglia, neurotransmitters and physiological effects. Some candidates described the cellular basis of Nicotinic, Muscarinic and M1-M5 receptors which didn’t attract marks.

14B04: Exam Report

Outline the anatomy and physiology of the parasympathetic nervous system.

0% of candidates passed this question.

Generally there was a lack of detailed knowledge, incorrect facts and at times confusion between the sympathetic and  parasympathetic nervous system functions. A lack of anatomical detail was common (the origin of preganglionic cell bodies was not described clearly, and parasympathetic ganglia were not often named and located). It was expected an answer would mention the central role of Acetylcholine as a neurotransmitter at preganglionic and post ganglionic neurons in the parasympathetic system. Target organs were identified correctly but the exact action was not specified e.g. pupillary constriction vs. dilatation, GI sphincter/bladder – contraction vs. relaxation. Detail concerning receptor physiology was not required.

This is a question covering a core topic that no candidate passed. An overview of the arrangement and function of the autonomic nervous system is provided in several core physiology texts, including Ganong and Guyton.

M1i / 25B14 / 17A01 / 14B04: Anatomy & physiology of the parasympathetic nervous system

  • Parasympathetic NS = division of ANS responsible for restorative function
  • The ANS is the efferent pathway controlling the action of involuntary organs & tissues in order to maintain homeostasis
Nervous System

Parasympathetic Anatomy

Preganglionic Neurons

  • Myelinated B fibres
  • Long preganglionic fibres
  • Passes uninterrupted to ganglia near target organ
  • Craniosacral outflow
  • Preganglionic NT = ACh → acts on nicotinic receptors at ganglia

Postganglionic

  • Unmyelinated C fibres
  • Short due to location of ganglion near target organ
  • Postganglionic NT = ACh → acts on muscarinic receptors

Cranial Nerves

Preganglionic fibre origin

Ganglion

Postganglion Fibre Target

III

→ from oculomotor nucleus

→ ciliary ganglion

→ Eye

  • Ciliary m.
  • Iris sphincter
  • Pupil constriction

VII

→ Superior salivary nucleus

→ submaxillary ganglion

→ Submaxillary & sublingual

  • Salivary glands
  • ↑saliva secretion

IX

→ Inferior salivary nucleus

→ otic ganglion

→ Parotid gland

  • ↑saliva secretion

X

Vagus is the major component of parasympathetic outflow

→ Accounts for 75% of parasympathetic fibres

→ Dorsal nucleus of vagus (X) in medulla

→ Ganglia of visceral plexuses

→ Cardiac plexus, SA node, AV node, conducting system

  • ↓chronotopy, chronotropy, conductivity

→ Pulmonary plexus

  • Bronchoconstriction

→ Gastric plexus

  • Stomach, liver, spleen
  • ↑GI motility & secretions
  • Sphincter relaxation
  • ↑peristalsis

Sacral nerves

S2, 3, 4 of spinal cord

→ Hypogastric plexus

→ Descending colon, rectum, bladder, uterus

  • Rectal contraction
  • Anus relaxation
  • Uterine relaxation
  • Contraction of detrusor m. in bladder wall

CHARACTERISTIC

PARASYMPATHETIC

Origin

Fibre length

Long preganglionic, short postganglionic

Preganglionic fibre

Myelinated B fibre

Ganglia location

Close to effector cells

Preganglionic NT

ACh

Ganglia receptors

Nicotinic

Postganglionic fibres

Unmyelinated C fibres

Postganglionic NT

ACh

Postganglionic receptor

Muscarinic

Function

Conserves & stores E.

ORGAN

ACTION

RECEPTOR

Heart

– inotropy

– chronotropy

– chronotropy

M2

Arteries

Dilatation

NB: not much effort on art/vein as for sympathetic NS

Lung

Bronchial constriction

↑bronchial gland secretion

M3

GI

↑motility

Sphincter relaxation

M3

M3

Liver

Glycogen synthesis

β2 + α

Kidney

Renin release

β1

Bladder

Detrusor contraction (voiding)

M3

Eye

Pupil constriction

↑lacrimal gland secretion

M3

M3

Submandibular & parotid glands

↑salivary secretion

M3