Coupled oscillators and biological synchronization.

weight at the end of a string can take any of an infinite number of closed paths through phase space, depending on the height from which it is released. Biological systems (and clock pendulums), in contrast, tend to have not only a characteristic period hut also a characteristic amplitude. They trace a particular path through phase space, and if some perturbation jolts them out of their accustomed rhythm they soon return to their former path. If someone startles you, say, by shouting, "Boo!", your heart may start pounding but soon relaxes to its normal behavior. Oscillators that have a standard waveform and amplitude to which they return after small pertUIbations are known as limit-eycle oscillators. They incorporate a dissipative mechanism to damp oscillations that grow too large and a source of energy to pump up those that become too smali.