The last-step integrator thesis: Heterarchical control of the mammalian gastrointestinal tract

To perform the work necessary to maintain themselves in diverse situations, organisms orchestrate the operations of a host of production mechanisms that individually perform specific physiological and behavioural activities. Effective orchestration depends upon control mechanisms that acquire and integrate information about conditions in the organism and its environment and deploy the results to adjust the operation of production mechanisms. How are control mechanisms organized? A common assumption is that they need to be organized in a pyramid, with controllers at a higher level overseeing the activities of numerous subordinate mechanisms, procuring information from them and sending directives to them. In this article, we challenge this view and argue that physiological control is organized heterarchically. To illustrate the distinctive features of heterarchical control, we examine the current understanding of the regulation of the mammalian gastrointestinal tract. We show that individual production mechanisms can be acted on by multiple control mechanisms. However, the activity of a production mechanism is not dictated by these control inputs, which can conflict: one may recommend one action, another a different action. It is up to the local production mechanism to integrate these inputs with its own current state in determining its operation. The integration of control signals that determines what action is performed is therefore not carried out at a highest level by a central decider issuing directives to a host of executors, but it is ultimately executed at the last step, within the controlled mechanism itself, and this is what we call the 'last-step integrator.'