The Sympathetic Nervous System

The sympathetic nervous system prepares the body to take action during times of threat, stress or excitement, including activating the fight or flight response.

It is one of the major divisions of the autonomic nervous system which controls involuntary functions by regulating the activity of smooth muscle in visceral organs, cardio muscle in the heart, and certain glands such as sweat glands.

When the sympathetic nervous system is activated, preganglionic neurons in the thoracic and lumbar spinal cord send signals to the peripheral nervous system. Some send their signals to a chain of ganglia, clusters of neuronal cell bodies that runs down the side of the spinal column.

From here, postganglionic neurons transmit the signal to target tissues causing responses such as increased heart rate and constriction of blood vessels bringing more blood to muscles.

Other preganglionic neurons send signals to postganglionic neurons in ganglia close to their target organs where they have effects such as inhibiting digestion in the stomach and excretion from the bladder, processes that are low priority in times of stress. 

A third group of preganglionic neurons travels directly to the adrenal gland where they stimulate the production of hormones such as epinephrine, that mediate the stress response throughout the body.