Preganglionic sympathetic nerve fibres have their cell bodies in the intermediolateral horn of spinal segments T1 to L2. Their myelinated axons pass out of the spinal cord by the anterior roots. The anterior root is continuous with a spinal nerve after fusion with the segmental posterior root. The preganglionic sympathetic nerve fibre passes into the spinal nerve and then passes to synapse with the cell bodies of postganglionic sympathetic fibres. The latter exist in either pre- or paravertebral ganglia - see submenu.
A few preganglionic fibres pass directly through the paravertebral ganglia to synapse within the prevertebral ganglia. They form discrete nerves between the ganglia which contain only preganglionic efferent fibres and afferents from assorted viscera. Examples include the splanchnic nerves.
The preganglionic fibres from given spinal levels innervate a certain set of structures - see submenu.
One generalisation about the preganglionic sympathetic neurones is that, compared to preganglionic parasympathetic neurones, they have a relatively short course after leaving the spinal cord before they reach the postganglionic neurone.
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