Abstract. To perform the work necessary to maintain themselves in diverse situations, organismsorchestrate the operations of a host of production mechanisms that individually performspecific physiological and behavioural activities. Effective orchestration depends upon control mechanisms that acquire and integrate information about conditions in the organism and itsenvironment and deploy the results to adjust the operation of production mechanisms. Howare control mechanisms organized? A common assumption is that they need to be organizedin a pyramid, with controllers at a higher level overseeing the activities of numerous sub-ordinate mechanisms, procuring information from them and sending directives to them. In thisarticle, we challenge this view and argue that physiological control is organized heterarchically.To illustrate the distinctive features of heterarchical control, we examine the current under-standing of the regulation of the mammalian gastrointestinal tract. We show that individualproduction mechanisms can be acted on by multiple control mechanisms. However, the activityof a production mechanism is not dictated by these control inputs, which can conflict: one mayrecommend one action, another a different action. It is up to the local production mechanism tointegrate these inputs with its own current state in determining its operation. The integration ofcontrol signals that determines what action is performed is therefore not carried out at a highestlevel by a central decider issuing directives to a host of executors, but it is ultimately executed at thelast step, within the controlled mechanism itself, and this is what we call the ‘last-step integrator.’
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