ABSTRACT
A major target for the postganglionic fibers of the autonomic nervous system is smooth muscle, so called because it lacks the striations seen in skeletal and cardiac muscle under the light microscope. Smooth muscle is located in the hollow viscera (e.g. gut, urinary bladder, uterus), in major airways, blood vessels (except capillaries), ducts of exocrine glands and eyes. The neurotransmitters and hormones have either excitatory or inhibitory effects on the muscle, generating junction potentials that are analogous to synaptic potentials. Tonic activity like this is typical of smooth muscle found in arteries and sphincters and is maintained by low frequency firing of autonomic axons which result in depolarization of the smooth muscle, but not action potentials. Axons of postganglionic autonomic neurons have, spaced at intervals along their lengths, numerous swellings called varicosities, each about 1.5 µm in diameter and containing synaptic vesicles.
