The sympathetic nervous system (SNS) is part of the autonomic nervous system (ANS), which also includes the parasympathetic nervous system (PNS).
The sympathetic nervous system activates what is often termed the fight or flight response.
Like other parts of the nervous system, the sympathetic nervous system operates through a series of interconnected neurons.
Sympathetic neurons 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 (which is part of the CNS) communicate with peripheral sympathetic neurons via a series of sympathetic ganglia.
Within the ganglia, spinal cord sympathetic neurons join peripheral sympathetic neurons through chemical synapses.
Spinal cord sympathetic neurons are therefore called presynaptic (or preganglionic) neurons, while peripheral sympathetic neurons are called postsynaptic (or postganglionic) neurons.
At synapses within the sympathetic ganglia, preganglionic sympathetic neurons release acetylcholine, a chemical messenger that binds and activates nicotinic acetylcholine receptors on postganglionic neurons.
In response to this stimulus, postganglionic neurons principally release noradrenaline (norepinephrine).
Prolonged activation can elicit the release of adrenaline from the adrenal medulla.
Once released, noradrenaline and adrenaline bind adrenergic receptors on peripheral tissues.
Binding to adrenergic receptors causes the effects seen during the fight-or-flight response.
These include pupil dilation, increased sweating, increased heart rate, and increased blood pressure.
Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal cord and are thought to extend to the second or third lumbar segments.
Because its cells begin in the thoracic and lumbar regions of the spinal cord, the CNS is said to have a thoracolumbar outflow.
Axons of these nerves leave the spinal cord in the ventral branches (rami) of the spinal nerves, and then separate out as 'white rami' (so called from the shiny white sheaths of myelin around each axon) which connect to two chain ganglia extending alongside the vertebral column on the left and right.
These elongated ganglia are also known as paravertebral ganglia or sympathetic trunks.
In these hubs, connections (synapses) are made which then distribute the nerves to major organs, glands, and other parts of the body.