ABSTRACT
As seen in the previous chapter, negative feedback control loops play an important role in enabling many different types of biological functionality, from homeostasis to chemotaxis. When evolutionary pressures cause negative feedback to be supplemented with or replaced by positive feedback, other dynamical behaviours can be produced which have been used by biological systems for a variety of purposes, including the generation of hysteretic switches and oscillations, and the suppression of noise. Indeed, it has recently been argued that intracellular regulatory networks contain far more positive “sign-consistent” feedback and feed-forward loops than negative loops, due to the presence of hubs that are enriched with either negative or positive links, as well as to the non-uniform connectivity distribution of such networks, [1]. In the case studies at the end of this chapter we consider some of the types of biological functionality which may be achieved by positive feedback. First, however, we provide an introduction to some of the tools which are available to analyse these types of complex feedback control systems.
