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. Western science typically looks at the SNS as an automatic regulation system, that is, one that operates without the intervention of conscious thought. 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.
The sympathetic nerves run parallel to the spinal cord on both sides of the vertebral column. Sympathetic nerves branch off from these main nerve chains and smaller branches travel into the spinal cord and beyond into major organs, glands, and other groupings of nerves (sometimes called ganglia).
Messages travel through the SNS in a bidirectional flow. Efferent messages 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, activate goose bumps, start sweating and raise blood pressure.
Cells of the SNS originate toward the middle of the spinal column, intermediolateral cell column , near the thoracic vertebrae, numbers 1 thru 12 (T1-T12). This is where the parts of the nerves that have not yet run through a ganglion, preganglionic cell bodies, are located.
Axons (the largest "tentacle" of a nerve cell) leave the spinal cord and synapse (connect, with a space in between) onto the sympathetic chain ganglion; some 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 synapse 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.