In order for the gastrointestinal (GI) tract to carry out its function of digesting and absorbing nutrients, it uses a carefully coordinated series of muscular contractions. These act to propel food along the tract and to aid digestion by churning food and maximizing exposure of nutrients to enzymes and the absorptive epithelial barrier.

There are two major layers of smooth muscle in the GI tract that allow this to happen: an inner, circular layer and an outer, longitudinal layer. Both lie in the muscularis layer of the gut wall, which is superficial to the mucosal and sub-mucosal layers and is surrounded by a connective tissue serosal layer, through which major nerves and blood and lymphatic vessels travel (Figure 1). Some regions of the GI tract have specializations in their muscularis layer, for example, the stomach has an additional oblique smooth muscle layer and the longitudinal layer in the large intestine is organized into ribbon-like taeniae coli.

In addition, different regions of the tract are separated by sphincters composed of smooth muscle, except at the external anal sphincter, where skeletal muscle is present. These sphincters control the movement of substances from one region of the GI tract to another and prevent backflow.

Smooth muscle cells in the GI tract are bundled together and are linked by gap junctions that allow the flow of electrical signals between the cells, allowing them to contract as a functional syncytium. Control of the muscles, and thus gut motility, is complex and is coordinated by the autonomic nervous system, hormones and other mechanisms intrinsic to the GI tract. In this way, the GI tract is able to respond to changes in the luminal environment caused by food intake.