SYMPATHETIC NERVOUS SYSTEM

The Sympathetic Nervous System ( SNS ) is a branch of theautonomic nervous system along with the enteric nervous system andparasympathetic nervous system. Sympathetic nervous system involves spinalnerves T1 to L2 or L3.

Like other parts of the nervous system, the sympatheticnervous system operates through a series of interconnected neurons. Sympatheticneurons are frequently considered part of the peripheral nervous system ( PNS), although there are many that lie within the central nervous system ( CNS ).Sympathetic neurons of the spinal cord ( part of the CNS ) communicate withperipheral sympathetic neurons via a series of sympathetic ganglia.

Within the ganglia, spinal cord sympathetic neurons joinperipheral sympathetic neurons through chemical synapses. Spinal cordsympathetic neurons are therefore called presynaptic ( preganglionic ) neurons,while peripheral sympathetic neurons are called postsynaptic ( postganglionic )neurons.

At synapses within the sympathetic ganglia, preganglionicsympathetic neurons release acetylcholine, a chemical messenger that binds andactivates nicotinic acetylcholine receptors on postganglionic neurons. Inresponse to this stimulus, postganglionic neurons principally release noradrenaline( norepinephrine ). 

Prolonged activation can elicit the release of adrenalinefrom the adrenal medulla. Once released, noradrenaline and adrenaline bindadrenergic receptors on peripheral tissues. Binding to adrenergic receptorscauses the effects seen during the fight-or-flight response. These includepupil dilation, increased sweating, increased heart rate, occasional vomiting,and increased blood pressure.

The sympathetic nervous system is responsible for up- anddown-regulating many homeostatic mechanisms in living organisms. Fibers fromthe SNS innervate tissues in almost every organ system, providing at least someregulatory function to things as diverse as pupil diameter, gut motility, andurinary output.

The sympathetic nervous system is perhaps best known formediating the neuronal and hormonal stress response commonly known as thefight-or-flight response. This response is also known as sympatho-adrenalresponse of the body, as the preganglionic sympathetic fibers that end in theadrenal medulla ( but also all other sympathetic fibers ) secreteacetylcholine, which activates the secretion of adrenaline ( epinephrine ) andto a lesser extent noradrenaline ( norepinephrine ) from it. Therefore, thisresponse that acts primarily on the cardiovascular system is mediated directlyvia impulses transmitted through the sympathetic nervous system and indirectlyvia catecholamines secreted from the adrenal medulla.

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