Orchestrating time: arrangements of the brain circadian clock.

Daily oscillations in physiology and behavior are regulated by a brain clock located in the suprachiasmatic nucleus (SCN). Individual cells within this nucleus contain an autonomous molecular clock. Recent discoveries that make use of new molecular and genetic data and tools highlight the conclusion that the SCN is a heterogeneous network of functionally and phenotypically differentiated cells. Neurons within SCN subregions serve distinctly separate functions in regulating the overall activity of the circadian clock: some cells within the SCN rhythmically express "clock" genes, whereas others exhibit induced expression of these genes after the organism has been exposed to a light pulse. The coordinated interaction of these functionally distinct cells is integral to the coherent functioning of the brain clock.