Structure and Function of the Autonomic Nervous System

The autonomic nervous system (ANS), part of the peripheral nervous system (PNS), is responsible primarily for maintaining homeostasis, or stability in the body. The sympathetic and parasympathetic nervous systems are divisions of the autonomic nervous system. The ANS uses two nerve fibers that meet at a ganglion (a mass of tissue that contains several cell bodies) in order to reach a target organ. The preganglionic fiber is a nerve fiber that is covered with myelin (the mass of lipid-rich material that wraps around and insulates the axon of a nerve cell) and travels from the central nervous system (CNS) to a ganglion. The postganglionic fiber not only is unmyelinated, or not covered with myelin, but it also travels from the ganglia to effector organs.

The sympathetic nervous system increases the activities of the body to be more alert and excited. This is performed while inhibiting digestion. Sympathetic nerves arise from the lumbar and sacral spinal nerves. The adrenal medulla of the adrenal gland, which is a gland found right above the kidneys, secretes hormones into the bloodstream that travel throughout the body and have the same effect as sympathetic stimulation. Preganglionic neurons of the sympathetic nervous system commonly release the neurotransmitter acetylcholine, which binds to nicotinic cholinergic receptors on postganglionic neurons. Acetylcholine is able to bind to this receptor as well as a muscarinic receptor, both of which signal the body to elicit different responses after neurotransmitter-receptor binding.

The parasympathetic nervous system is active when the body is resting and calm. It also plays a role in the stimulation of digestive activity. Parasympathetic nerves arise from the brain and sacral region of the spinal cord. In the parasympathetic pathway, acetylcholine is released from both preganglionic and postganglionic neurons. This neurotransmitter binds to nicotinic cholinergic receptors on the postganglionic neuron and to muscarinic cholinergic receptors of the effector cells that are part of the somatic nervous system. The digestive system has its own nervous system called the enteric nervous system, which is part of the ANS, a division of the PNS, that controls local blood flow, movement, and mucus secretions. The enteric nervous system can respond according to its nutrient contents and acts using reflexes to promote motility of the walls of the gastrointestinal tract. The enteric nervous system is regulated by both divisions of the sympathetic and parasympathetic nervous systems. Parasympathetic nerves can stimulate gastrointestinal function, while stimulation by the sympathetic nervous system can inhibit enteric activity. Although it communicates with the CNS, the enteric nervous system can function independently of the brain and spinal cord. This explains why the enteric nervous system is sometimes called the body's "second brain."