Introduction to Feedback Loops

Feedback loops are processes that connect output signals back to their inputs. The history of biological feedback goes back at least 130 years to observations by scientists that organs and other living systems can satisfy their own needs by self-regulation. The ability to self-regulate is based on the use of feedback loops that can adjust to changes (stimuli or forcing factors) to return to stable conditions and maintain homeostasis. Mammalian species use signalling proteins and second messengers to build hundreds of cell-specific signalling systems. Many of the signalling components have multiple upstream regulators and downstream targets, creating a web of connectivity within and between signalling pathways. Without both positive and negative feedback systems, living organisms would be unable to control and maintain an array of important processes including thermoregulation (if body temperature changes, mechanisms are induced to restore normal levels) and glycaemic regulation (eg hormonal regulation including insulin which lowers blood glucose when concentrations are higher than normal; glucagon which raises blood glucose when concentrations are lower than normal).