Biological Timing: Circadian Oscillations, Cell Division, and Pulsatile Secretion

Abstract

The term “rhythms” in biology refers to a wide range of phenomena that have one feature in common, the process or variable of interest oscillates with time. There are numerous examples of biological processes that oscillate, some in a sustained fashion and some with damping. Common examples of such biological processes are the heart beat, respiration, and the sleep-waking cycle, processes readily monitored without invasive or sophisticated procedures. Heart beat and respiration, as well as flagellar rotation in bacteria, are in the middle range of biological oscillations (arbitrarily rates of ≈ 0.1 to 300 Hz). Sleep-waking and annual cycles (migration, hibernation) are in the low to very low frequency range (10-5 to 10-8 Hz). There are some biological processes that are driven by external stimuli that operate at very high frequencies (> 104 Hz), such as the membrane ion currents due to the resonance of stereocilia on hair cells in the inner ear, and these are probably the highest biological frequencies that cells, as intact structures, can generate. The wide range of such biological oscillations, some 12 orders of magnitude, suggests that very different mechanisms must be at work to cover such a range.