What is a biological oscillator?

Biological oscillators are amenable to qualitative analysis even before they have been described exhaustively in quantitative terms. Qualitative analysis can identify the elements essential for generating the oscillations and can enhance our understanding of underlying oscillator mechanisms. Two essential elements of a biological oscillator are 1) an inhibitory feedback loop, which includes one or more oscillating variables, and 2) a source of delay in this feedback loop, which allows an oscillating variable to overshoot a steady-state value before the feedback inhibition is fully effective. The analysis of the patterns of interactions and delays observed in biological oscillators is simplified by the translation of variables, interactions, and delays into schematic representations. To illustrate how such translations can be implemented, three biological oscillators are described schematically: 1) the glycolytic oscillator, 2) the bursting of the molluscan neuron, R15, and 3) the oscillations underlying smooth muscle contractions.