Smooth muscles occur in many parts of the body, usually in sheets enveloping hollow organs or tubes which are either occasionally or always filled. The ultimate function of smooth muscles is to move the fluid or slurry contents forward to some destination. Thus, they can receive and produce signals which then lead to contraction or relaxation. Smooth muscles have been studied since at least the founding of physiology, and have been indispensable assay systems for studying autonomic neural functions, actions of hormones and drugs, and mechanisms of second-messenger systems. They are named for the absence of striations by the early microscopists, and hence are also known as plain muscles. Plain as they might appear, there is nothing bland about them. They are ancient structures in which ionic channels, contractile machinery, and regulatory processes were laid down before the emergence of fast-twitch muscles. In malfunctioning, they afflict more sufferers than all dysfunctions of striated muscles put together. Possibly, in one form or another, ailing smooth muscles consume the largest share of health-care costs.

Yet, we do not know them very well. Their dispersion in relatively small amounts, the abundance of connective and other nonmuscle tissues amongst them, the diversity in their properties, and the small sizes of individual myocytes are all qualities which discourage and deter concerted studies. Perhaps for these reasons, they remain like strangers, recognized sometimes by incompatible transplanted information from other systems.

However, over the past decades, substantial advances have been made in our knowledge of smooth muscles – in such areas as their fine structures, the contractile systems and their regulations, ionic currents of single smooth myocytes, and neurotransmission mechanisms.