The Autonomic Nervous System is the part of the nervous system concerned with the innervation of involuntary structures such as the heart, smooth muscles, and glands throughout the body. It is distributed throughout the central and peripheral nervous system.

The autonomic system may be divided into two parts, the sympathetic and the parasympathetic, and in both parts there are afferent and efferent nerve fibers. The activities of the sympathetic part of the autonomic system prepare the body for an emergency. It accelerates the heart rate, causes constriction of the peripheral blood vessels, and raises the blood pressure. The sympathetic part of the autonomic system brings about a redistribution of the blood so that it leaves the areas of the skin and intestines and becomes available to the brain, heart, and skeletal muscles. At the same time, it inhibits peristalsis of the intestinal tract and closes the sphincters. The activities of the parasympathetic part of the autonomic system aim at conserving and restoring energy. They slow the heart rate, increase peristalsis of the intestines and glandular activity, and open the sphincters.

The gray matter of the spinal cord, from the first thoracic segment to the second lumbar segment, possesses a lateral horn, or column, in which are located the cell bodies of the sympathetic connector neurons. The myelinated axons of these cells leave the spinal cord in the anterior nerve roots and then pass via the white rami communicates to the paravertebral ganglia of the sympathetic trunk. The connector cell fibers are called the preganglionic as they pass to a peripheral ganglion. Once the preganglionic fibers reach the ganglia in the sympathetic trunk, they pass to the following destinations- they may terminate in the ganglion they have entered by synapsing with the excitor cell in the ganglion. A synapse is the site where two neurons come into close proximity but not into anatomical continuity. The gap between the two neurons is bridged by a neurotransmitter substance, acetylcholine. The axons of the excitor neurons leave the ganglion and are non-myelinated. These postganglionic nerve fibers now pass to the thoracic spinal nerves as gray rami communicate and are distributed in the branches of the spinal nerves to supply the smooth muscles in the walls of blood vessels, the sweat glands, and the arrector pili muscles of the skin.

Another destination, the nerve fibers entering the ganglia of the sympathetic trunk high up in the thorax, may travel up in the sympathetic trunk to the ganglia in the cervical region, where they synapse with excitor cells. The postganglionic nerve fibers leave the sympathetic trunk as gray rami communicate and most of them join the cervical spinal nerves. Many of the preganglionic fibers entering the lower thoracic and upper two lumbar and sacral regions travel down to the ganglia in the lower lumbar and sacral regions, where they synapse with excitor cells. The postganglionic fibers leave the sympathetic trunk as gray rami communicate that join the lumbar, sacral, and coccygeal spinal nerves.

Another destination of the preganglionic fibers is that they pass through the ganglia on the thoracic part of the sympathetic trunk without synapsing. These myelinated fibers form the splanchnic nerves of which there are three types- the greater splanchnic nerve, the lesser splanchnic nerve, and the lowest splanchnic nerve.