How opposing interactions between internal clock neurons help adjust to light-dark cycles and cause lasting changes

Attractive coupling from the core to shell regions of the suprachiasmatic nucleus (SCN) accounts for observed phase relationships under light-dark cycles.

• The SCN is the primary circadian pacemaker in mammals, receiving light signals and regulating rhythms in peripheral tissues.
• The free-running period (FRP) of animals in constant darkness is influenced by the preceding light-dark cycle, a phenomenon termed after-effect.
• Phase oscillator models suggest that attractive and repulsive couplings affect the phase differences between SCN oscillators.
• Repulsive coupling from the shell to the core enhances the SCN's ability to synchronize with light-dark cycles.
• After transitioning to constant darkness, the FRP gradually reverts to the intrinsic SCN period, demonstrating the after-effect.

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