Zum Mechanismus der biologischen 24-Stunden-Periodik

Summary

The biological 24-hour-periodicity is based upon an endogenous (self-sustained) oscillation which is synchronized with the earth's rotation by periodically changing factors of the environment, primarily by the alternation of light and dark. These external „Zeitgebers” affect the phase of the endogenous oscillation. Theoretically, there are four different simple types of phase-control; all complicated types are combinations of these four types. In model experiments the behaviour of an oscillation in each of the four cases of phase-control is clearly demonstrated.

The comparison of model experiments and biological experiments suggests that in organisms a specific combination-type of phase-control occurs. In this combination-type, a change in frequency is always positively correlated with a change in average level of the oscillation. Both parameters of the oscillation increase in light-active organisms and decrease in dark-active organisms with increasing light-intensity (“circadian rule”). In organisms both parameters are coupled by means of non-linear elements.

The differential equation describing the 24-hour-periodcity is characterized by certain non-linearities. One of these makes the oscillation self-sustained and simultaneously couples the frequency of the oscillation to the average level, in the sense postulated by the circadian rule. The magnitude of the non-linearity is such that the resulting oscillation is intermediated between a harmonic and a relaxation type of oscillation, but has more characteristics of a harmonic oscillation. A second non-linearity which also couples frequency and level positively concerns the energy of recoil.

All general properties of the biological 24-hour-periodicity can be reproduced by the described oscillator model. Some special properties (e.g. “pattern”) are more easily understood by the assumption of two coupled oscillators; the second oscillator, following the same general laws described above, is controlled by the first one. The oscillator hypothesis can be applied to biological periodicities with other frequencies; in general, the higher the frequency of a system the more the oscillation tends towards a relaxation type of oscillation.