Sympathetic nervous system
The sympathetic nervous system (SNS) is one half of the autonomic nervous system; the parasympathetic nervous system (PNS) is the other.
The sympathetic nervous system activates what is often termed the "fight or flight response" of the body. Some evolutionary theorists suggest that the sympathetic nervous system operated in early man to maintain human survival (Origins of Consciousness, Robert Ornstein; et al.), as the sympathetic nervous system is responsible for priming the body for action.
Nerves of the SNS connect into major organs, glands, and other nerves. Messages travelling through the SNS can trigger changes in different parts of the body simultaneously. For example, the sympathetic nervous system can: accelerate heart rate, widen bronchial passages, decrease motility (movement) of the large intestine, constrict blood vessels, cause pupil dilation and raise blood pressure.
The sympathetic nervous system originates from the intermediolateral cell column of vertebral levels T1-T12 where the preganglionic cell bodies of the SNS are located. Axons leave the spinal cord and synpase onto a nearby sympathetic chain ganglion; axons leaving the sympathetic chain then synapse on their destination organs. The first synapse (in the sympathetic chain) is mediated by nicotinic receptors physiologically activated by acetylcholine, and the target synpase is mediated by adrenergic receptors physiologically activated by either norepinephrine or epinephrine. The one exception is with sweat glands which receive sympathetic innervation but have muscarinic acetylcholine receptors which are normally characteristic of the PNS.
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