Functional organization of the circadian timing system

Abstract

The circadian timing system establishes daily rhythms in behavior and physiology throughout the body, ensuring that functions like activity, sleep and hormone release are appropriately timed. Research suggests that his temporal synchrony within the body is quite important for health and survival. In mammals, the central circadian pacemaker in the suprachiasmatic nucleus (SCN) drives rhythms in behavior and physiology in large part by stimulating or inhibiting other brain regions responsible for these functions at the appropriate times of day. This timed signal is often indirect, i.e. relayed or possibly processed through a series of neurons in different brain regions before reaching the effector site. The subparaventricular zone (SPZ), a region adjacent to the SCN which is the main recipient of direct neuronal inputs from the SCN, is thought to be a critical relay for SCN signals, since loss of the SPZ results in loss of circadian rhythms in body temperature, activity and sleep/wakefulness. Another important relay site, the dorsomedial hypothalamic nucleus (DMH) gets direct input from both the SCN and SPZ and is critical for normal expression of various circadian rhythms.