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
Sympathetic
Parasympathetic
Definition
Division of the ANS which is activated in “fight-or-flight”
Division of ANS responsible for restorative function
Function
Homeostasis during stress
Conserve & store energy
Anatomy
Thoracolumbar
Craniosacral
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
CN III, VII, IX, X
Sacral S2,3,4
Synapse at Ganglia:
III – Ciliary
VI – Submaxillary
IX – Otic
X – Visceral plexi
Sacral – Hypogastric plexus
Pre Ganglionic NT
ACh
Ach → acts on nicotinic receptors
Post Ganglionic Fibres
Unmyelinated C fibres
Long postganglionic
Either:
-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
CN:
→ 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
Sacral:
→ 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
Ganglia
Close to CNS
Close to effector cells
Receptors
Adrenergic or muscarinic
Nicotinic (ganglia receptors)
Muscarinic (postgang receptors)