When Clocks Go Bad: Neurobehavioural Consequences of Disrupted Circadian Timing
The SCN acts as a “master” clock, sending neuronal and humoral output to a number of regions in the CNS. The SCN can passively drive rhythmicity in these regions, or nuclei may themselves have their own automomous clocks. Within the SCN, every cell is a potentially independent oscillator unit. Extensive intra-SCN signalling is used to synchronise the rhythms of multiple cells. Entrainment is a process whereby environmental stimuli can alter the timing or phase of central rhythms so that they are coincidental with the prevailing daily cycle (far left). Time cues reach the SCN via several pathways involving intermediate brain areas. Some of these relay nuclei may contain their own oscillators and/or have rhythms in their function (blue and orange arrows and oscillators). Others may not have rhythmic function (green arrow). Extra-SCN oscillators may directly utilise time cues (blue and orange oscillators) or may rely solely on the SCN to synchronise/entrain rhythms (lilac arrows to grey oscillators). These oscillators may also provide feedback which impacts on the operation of the SCN (grey return arrows). In addition, extra-SCN oscillators may drive/influence rhythmicity in other brain areas (smaller grey oscillator) and communicate with each other independently from the SCN (reciprocal grey arrows). Within the CNS, some neurotransmitters/neuromodulators involved in signalling are indicated. The overall output of this complex system (large red arrow) creates the daily rhythmicity seen in a range of neurological functions and behaviours (far right). There is a wider communication with clock/oscillator units throughout the entire body. However, these so-called “peripheral” oscillators and their actions are not shown here.