M1i / 17B07: C+C sympathetic and parasympathetic

17B07: Exam Report

Compare and contrast the sympathetic and parasympathetic nervous systems.

75% of candidates passed this question.

This question was generally well answered A table or diagram lent structure to the answer. More complete answers included details on the function, anatomy, a description of the pre- and post-ganglionic fibres, ganglia, receptors and neurotransmitters involved.

Whilst most commented on ‘fight or flight’ for the SNS and ‘rest and digest’ for the PNS, no candidate observed that the SNS is a diffuse physiological accelerator and that the PNS acts as a local brake. No candidate included the fact that the SNS supplies viscera and skin whilst the PNS only supplies the viscera. Many candidates failed to make reference to the fact that the postganglionic SNS receptor is G protein coupled and the PNS postganglionic receptor is G- coupled on muscarinic receptors but operates an ion channel when nicotinic.

Candidates may have scored higher if they had provided a little more detail in their answers.

M1i / 17B07: Compare and Contrast the sympathetic and parasympathetic nervous systems




Division of the ANS which is activated in “fight-or-flight”

Division of ANS responsible for restorative function


Homeostasis during stress

Conserve & store energy




Pre Ganglionic Fibres

Myelinated slow conducting B fibres

Short preganglionic

Cell bodies in intermediolateral horn

Arise from T1 – L2

Preganglionic fibre leaves spinal cord via ventral root

Passes via white ramus into sympathetic chain

Sympathetic chain = 22 pairs of ganglia

From sympathetic chain:

– Synapse with post-ganglionic cell bodies at level they entered

– Pass up/down to another level of symp. Chain and synapse with that post-ganglionic cell body

– Pass through the symp. Chain & synapse with a collateral ganglion

Myelinated B fibres

Long preganglionic fibres

Passes uninterrupted to ganglia near target organ

Craniosacral outflow


Sacral S2,3,4

Synapse at Ganglia:

III – Ciliary

VI – Submaxillary

IX – Otic

X – Visceral plexi

Sacral – Hypogastric plexus

Pre Ganglionic NT


Ach → acts on nicotinic receptors

Post Ganglionic Fibres

Unmyelinated C fibres

Long postganglionic


-Runs along blood vessels to supply head, neck, thorax & viscera

-Re-enters spinal n’s via grey rami to supply vessels, sweat glands, piloerector m.

-From collateral ganglia synapse close to viscera or in adrenal medulla

NOTE: adrenal medulla is essentially a symp. Ganglion in which postganglionic cells have lost their axons & secetes adrenaline, DA, NA directly into the bloodstream

Unmyelinated C fibres

Short due to location of ganglion near target organ


→ Eye

  • Ciliary m.
  • Iris sphincter
  • Pupil constriction

→ Submaxillary & sublingual

  • Salivary glands
  • ↑saliva secretion

→ Parotid gland

  • ↑saliva secretion

→ 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


→ Descending colon, rectum, bladder, uterus

  • Rectal contraction
  • Anus relaxation
  • Uterine relaxation

Contraction of detrusor m. in bladder wall

Post Ganglionic NT

NA (most), Ach (some)

Ach → acts on muscarinic receptors


Close to CNS

Close to effector cells


Adrenergic or muscarinic

Nicotinic (ganglia receptors)

Muscarinic (postgang receptors)