The primary functions of the gastrointestinal (GI) tract are the digestion of ingested food, the absorption of water, nutrients, electrolytes and vitamins, and the excretion of indigestible and waste products. The GI tract should not be thought of as a single organ, but a series of organs each with specialised functions. Each section of the GI tract has characteristic motor and secretory properties to accomplish a particular role in the overall function of the gut.

Gastrointestinal motility

Apart from the proximal part of the oesophagus, the GI tract has a remarkably uniform structure consisting of three layers of smooth muscle. These are arranged as an outer longitudinal layer, a middle circular layer and an inner submucosal layer (muscularis mucosa) (Figure GI1).

The basic contractile unit of the circular and longitudinal muscle layers is the smooth muscle cell. Across each cell membrane a transmembrane potential of between −40 and −70 mV (negative intracellular charge) is maintained by an ATP-dependent Na+K+ATPase pump. In most areas of the GI tract, the transmembrane potential of smooth muscle cells rhythmically depolarises and repolarises, which is called basic electrical rhythm, slow-wave activity or electrical control activity (ECA). Gap junctions between individual cells allow transmembrane ionic movements to be conducted from cell to cell. Slow-wave activity is conducted along lengths of bowel in a synchronised pattern due to this electrical continuity between cells. Segments of intestine with similar electrical activity, therefore, behave as a functional syncytium.