Are relaxation oscillators an appropriate model of gastrointestinal electrical activity?

Mathematical models based on relaxation oscillators have heavily influenced the terminology and experimental designs of investigations in gastrointestinal motility for nearly two decades. Relaxation oscillator equations have been used to stimulate the electrical activities of the esophagus, stomach, small intestine, colon, and rectosigmoid region. It has been suggested that many attributes of gastrointestinal electrical activity cannot be adequately explained by classic "core-conductor" or "cable" models of excitation and conduction. This article critically reviews the relaxation oscillator model and provides an explanation for each of the putative inadequacies of core-conductor theory. Furthermore, we question whether relaxation oscillator equations are able to simulate the waveforms of gastrointestinal slow waves, alterations in waveform in response to drugs or electrical stimulation, patterns of slow-wave activity when stimulated at physiological frequencies, prolonged periods of constant resting membrane potential between gastric slow waves and electrotonic spread into inactive regions. We conclude that the relaxation oscillator equations do not fully describe gastrointestinal electrical activity; excitation and propagation can be modeled by a theory that provides for morphological features, ionic conductances, and other elements included in the cable equations.