Question 7

Compare and contrast the sympathetic and parasympathetic nervous systems. 

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College Answer

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 Gcoupled 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.  

Discussion

 

Sympathetic 

Parasympathetic 

 

Overview of function

Broadly responsible for what are often summarised as “fight or flight” states i.e. those which involve elevated activity or attention. SNS activation will tend to result in increased heart rate and blood pressure, upregulation of catabolic processes resulting in increased availability of metabolic substrates, and heightened cognitive alertness. Truly a nervous system. 

Often described as the “rest and digest” side of the autonomic coin. Physiological effects of activation vary considerably between effector organs and tissues, and include: regulation of smooth muscle motility, particularly in the GIT; increased secretions including salivation and lacrimation, pupillary constriction, and increased pancreatic exocrine activity.  

 

CNS origins

Intermediolateral nucleus of the spinal cord at the T1-L3 levels

Cranial nerve nuclei in the brainstem and the  intermediolateral cell columns at the S2–S4  level of the sacral spinal cord

 

Efferent fibre origins

Thoracolumbar (T1 to L2)

Cranial nerves and S2-S4

 

Preganglionic fibres 

Unmyelinated B fibres

Unmyelinated B fibres

 

Preganglionic neurotransmitter

Acetylcholine

Acetylcholine

Preganglionic receptors

Nicotinic (ion channel)

Nicotinic (ion channel)

 

Ganglia

Chains of ganglia close to spinal cord

More distal, close to effector organ  

Postganglionic fibres 

Unmyelinated C fibres

Unmyelinated C fibres

 

Ratio of preganglionic to postganglionic fibres

1:20

1:3

 

Postganglionic neurotransmitter

Noradrenaline

Acetylcholine

 

Postganglionic receptors

Noradrenaline receptors (G-protein coupled)

Muscarinic receptors
(G-protein coupled)

 

Directionality

Widespread, directionless; activity often involves the discharge of the entire system

Activity usually localised to specific effector organ

References

Furness, John B. "The organisation of the autonomic nervous system: peripheral connections." Autonomic Neuroscience: Basic and Clinical 130.1 (2006): 1-5.

McCorry, Laurie Kelly. "Physiology of the autonomic nervous system." American journal of pharmaceutical education 71.4 (2007).

Powley, Terry L. "Central control of autonomic functions: organization of the autonomic nervous system." Fundamental neuroscience. Academic Press, 2013. 729-747.

Jänig, Wilfrid. The integrative action of the autonomic nervous system: neurobiology of homeostasis. Cambridge University Press, 2022.