Entrainment of the suprachiasmatic nucleus network by a light-dark cycle

TL;DR

This paper models how the brain's circadian clock synchronizes with light-dark cycles, revealing a sharp transition in neuron behavior driven by bifurcations, influenced by signal strength and cell coupling.

Contribution

It introduces a simple realistic model that explains the sharp entrainment transition in the SCN network via bifurcation analysis.

Findings

Entrainment transition is explained by a supercritical Hopf-like bifurcation.

The period and strength of light signals critically affect synchronization.

The model predicts conditions for the disappearance of endogenous oscillations.

Abstract

The synchronization of biological activity with the alternation of day and night (circadian rhythm) is performed in the brain by a group of neurons, constituting the suprachiasmatic nucleus (SCN). The SCN is divided into two subgroups of oscillating cells: the ventro-lateral (VL) neurons, which are exposed to light (photic signal) and the dorso-medial (DM) neurons which are coupled to the VL cells. When the coupling between these neurons is strong enough, the system synchronizes with the photic period. Upon increasing the cell coupling, the entrainment of the DM cells has been recently shown to occur via a very sharp (jumping) transition when the period of the photic input is larger than the intrinsic period of the cells. Here, we characterize this transition with a simple realistic model. We show that two bifurcations possibly lead to the disappearance of the endogenous mode. Using a mean field model, we show that the jumping transition results from a supercritical Hopf-like bifurcation. This finding implies that both the period and strength of the stimulating photic signal, and the relative fraction of cells in the VL and DM compartments are crucial in determining the synchronization of the system.