The principal function of the digestive system is to process food. This processing involves six essential steps: ingestion, propulsion, mechanical and chemical digestion, absorption and defecation. Some of these steps are carried out by a single organ, for example, only the mouth ingests food. However, the majority of these processes require the coordinated activity of several organs.
Since the late 1960s, slow wave propagation in the gastrointestinal tract has been the subject of mathematical models. In 1968 Nelson & Becker suggested that a chain of relaxation oscillators could simulate the electromechanical activity in the small intestine. During the early 1970s, Sarna et al. developed this idea further by using an array of bidirectionally coupled oscillators to simulate different aspects of the gastrointestinal tract activity. For the next two decades, the pioneering work by these authors had a significant influence on the terminology and experimental methods used in investigations of gastrointestinal motility.
More recently, models based on coupled oscillators have been challenged. It has been argued that relaxation oscillators cannot simulate the electrical behaviour of single cells. Electrophysiological models of cardiac cells, such as the ventricular myocyte models of Luo and Rudy (1991, 1994a,b) and Noble et al. (1998), consider ionic mechanisms of membrane activity. Equivalent models for the gastrointestinal tract are called core conduction models.
As part of the Physiome Project, the Digestive System Research Group at the Bioengineering Institute is currently developing a mathematical model which describes the electrical and magnetic activity of the gastrointestinal system. Their research can be divided into three main areas: cellular; forward and inverse modelling.
At the cellular level, they wish to develop mathematical models that describe the electrophysiology of the smooth muscle cells and the interstital cells of Cajal (ICC) within the gastrointestinal system. Initially, this modelling will be of healthy cells, however in the longer term, they plan to incorporate the cellular changes that occur in disease states such as mesenteric ischemia.
In forward modelling, the aim is to determine what is happening on the surface of the body. The group is interested in producing detailed models of the electrical control activity (ECA) in the stomach and small intestine. These detailed models would incorporate the models of cellular electrophysiology. The organ level models are placed within a torso model, and are then used to calculate the electrical fields on the body surface along with the magnetic fields outside the body that are generated by the ECA.
Inverse modelling, in contrast to forward modelling, is where the aim is to determine what is happening inside the body from measurements recorded on, or just outside, the body surface. Non-invasive electrical measurements are taken by placing recording electrodes on the abdomen. Non-invasive magnetic recordings are taken by positioning a SQUID (Superconducting QUantum Interference Device) magnetometer above the abdomen. The aim of this research is to quantify the health of the stomach and intestines based on these recordings.