The autonomic nervous system (ANS) is a division of the peripheral nervous system that supplies smooth muscle and glands. It acts to regulate numerous body processes, such as blood pressure and respiratory rate, acting as the effector component of homeostasis.

The autonomic nervous system has two sub-divisions:

  • The sympathetic nervous system
  • The parasympathetic nervous system

 

The sympathetic nervous system

The sympathetic nervous system (SNS) functions to regulate the body’s ‘fight or flight’ responses. The SNS originates from the thoracolumbar segments of the spinal cord and uses short preganglionic neurons and long post ganglionic neurons.

The preganglionic neurons use acetylcholine as a neurotransmitter, and the postganglionic neurons use noradrenaline as a neurotransmitter. An exception to this general pattern of innervation is that of the postganglionic neurons supplying the sweat glands and chromaffin cells of the adrenal medulla, which both use acetylcholine as a neurotransmitter.

The action of the SNS are most apparent when the body is faced with stressful situations. It acts to mobilise energy stores, allowing us to cope with the stress and increase our chances of survival. It is, however, constantly active at a basic level as part of the maintenance of homeostasis.

 

The parasympathetic nervous system

The parasympathetic nervous system (PNS) responsible for the body’s ‘rest and digest’ and ‘feed and breed’ activities that occur when the body is at rest. The PNS originates from the craniosacral segments of the spinal cord.

This PNS consists of long preganglionic neurons and short post ganglionic neurons. The preganglionic and postganglionic neurons both use acetylcholine as a neurotransmitter.

 

Overview of the comparative features of the SNS and PNS

FeatureSNSPNS
Response‘Fight or flight’‘Rest and digest’ and; ‘feed and breed’
Spinal cord distributionThoracolumbar segmentsCraniosacral segments
Preganglionic neuronsShortLong
Postganglionic neuronsLongShort
Preganglionic neurotransmitterAcetylcholineAcetylcholine
Postganglionic neurotransmitterNoradrenalineAcetylcholine

 

Overview of the connections of the SNS and PNS

Overview of the connections of the ANS, image sourced from Wikipedia
Courtesy of Thomas Splettstoesser CC BY-SA 3.0

 

 


Thank you to the joint editorial team of www.frcemexamprep.co.uk for this ‘Exam Tips’ post.

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