Most regions of the gastrointestinal tract generate spontaneous electrical and mechanical activity in the absence of stimulation. When electrical recordings are made from slow muscle cells lying in the gastrointestinal tract, a regular discharge of long lasting waves of depolarization, slow waves, is detected. It has recently become apparent that slow waves are generated by a specialized population of smooth muscle cells, known as interstitial cells of Cajal (ICC). ICC can be subdivided into at least two separate groups. In most regions of the gastrointestinal tract, one group of ICC form a network that generates pacemaker potentials, so producing rhythmical membrane potential changes in the adjacent muscle layers. The second group of ICC are distributed amongst the smooth muscle cells and are tightly electrically coupled to them. In some regions of the gut, the second group of ICC augment the waves of pacemaker depolarization, so ensuring that voltage-dependent calcium channels in the smooth muscles are activated during each slow wave cycle. In addition, the second group of ICC are densely innervated by inhibitory and excitatory nerve terminals. Thus intrinsic nerve terminals, rather than communicating directly with smooth muscle cells, selectively innervate ICC and release transmitters directly onto them. The signals that are generated in the ICC, by the neurally released transmitters, then alter the activity of surrounding smooth muscle cells.