Figure 4.

Putative mechanisms underlying around-the-clock activity with no circadian rhythms in animals. (a) The pacemakers controlling circadian cycles such as locomotor activity generate normal rhythms but are uncoupled from motor centres (labelled ‘M’) controlling output (top arrow). The internal rhythms may also be masked by potent external or internal factors (including ultradian rhythms generating pacemakers) that influence circadian output (lower arrow). (b) Some or all pacemaker cells are in an arrested state (marked by a horizontal line). (c) Some or all pacemaker cells have dampened amplitude that is below the threshold needed to drive an overt rhythm. (d) Oscillations persist in individual pacemaker cells, but the phases of these cells are desynchronized (depicted by sine waves with different colours), so that the overall signal is dampened. Alternatively, distinct multicellular pacemakers could interact in ways that promote activity around the clock, for example, if they drive activity at different times of day when they are in anti-phase (depicted by the dashed and solid lines).