Oscillations

Positive feedback is at the heart of many electronic oscillators, and this is also true of biochemical systems [4], the paradigm being a product which activates its own synthesis in a cooperative fashion [4]: the product slowly builds up while the synthesising enzyme is active at a low level, activates the enzyme when it reaches a sufficient concentration, which results in substrate depletion and thus a reduced synthesis rate. Glycolytic oscillations, as well as many cases of Ca$^{2+}$ oscillations, often involve positive feedback [4] (interestingly, positive feedback can in some situations disfavour glycolytic oscillations [42], as can enhanced negative feedback due to proteic confinement and non-Michaelian kinetics [43]).

Certain types of oscillators use "negative resistance" components, often provided by positive feedback circuits [44]. Similarly, it has been known for a long time that a positive feedback circuit enhances outer hair cell movements, opposing viscous forces, and is necessary for high frequency selectivity [45] (see [46] for a recent review); it has in fact been shown that the hair bundles of these cells have negative stiffness in a certain range of displacements [47], making realistic a model based on the principle of negative-resistance oscillators. In this case, positive feedback circuits could thus have the role of freeing the system from viscous forces (as in a regenerative amplifier), as first proposed in [48], bringing it closer to an "ideal" behaviour. This parallels the role of negative feedback circuits discussed in the next section.