The efferent innervation of tissues other than skeletal muscle is by way of the autonomic nervous system. It regulates involuntary action, such as the heart beat, smooth muscle movement or contraction (liver, intestines), and glands. The autonomic nervous system is made up of two neurons in series that connect the central nervous system and the effector cells. The first neuron has its cell body in the central nervous system. The synapse between the two neurons is outside the central nervous system, in a cell cluster called an autonomic ganglion. The nerve fibers passing between the central nervous system and the ganglia are called preganglionic fibers; those passing between the ganglia and the effector cells are postganglionic fibers.
Anatomical and physiological differences within the autonomic nervous system are the basis for its further subdivision into sympathetic and parasympathetic components. The nerve fibers of the sympathetic and parasympathetic components leave the central nervous system at different levels —the sympathetic fibers from the thoracic (chest) and lumbar regions of the spinal cord, and the parasympathetic fibers from the brain and the sacral portion of the spinal cord. Therefore, the sympathetic division is also called the thoracolumbar division, and the parasympathetic is called the craniosacral division. The two divisions also differ in the location of ganglia. Most of the sympathetic ganglia lie close to the spinal cord and form the two chains of ganglia —one on each side of the cord—known as the sympathetic trunks. Other sympathetic ganglia, called collateral ganglia —the celiac, superior mesenteric, and inferior mesenteric ganglia—are in the abdominal cavity, closer to the innervated organ. In contrast, the parasympathetic ganglia lie within or very close to the organs innervated by the postganglionic neurons.
The anatomy of the sympathetic nervous system can be confusing. Preganglionic sympathetic fibers leave the spinal cord only between the first thoracic and third lumbar segments, whereas sympathetic trunks extend the entire length of the cord, from the cervical levels high in the neck down to the sacral levels. The ganglia in the extra lengths of sympathetic trunks receive preganglionic fibers from the thoracolumbar regions because some of the preganglionic fibers, once in the sympathetic trunks, turn to travel upward or downward for several segments before forming synapses with postganglionic neurons (numbers 1 and 4). Other possible paths taken by the sympathetic fibers are numbers 2, 3, and 5.
In both sympathetic and parasympathetic divisions, acetylcholine is the major neurotransmitter released between pre- and postganglionic fibers in autonomic ganglia. In the parasympathetic division, acetylcholine is also the major neurotransmitter between the postganglionic fiber and the effector cell. In the sympathetic division, norepinephrine is usually the major transmitter between the postganglionic fiber and the effector cell.