Organization of the Mammalian Circadian System

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Abstract
The mammalian circadian timing system is a set of related neural structures whose function is to provide a temporal organization for physiological processes and behaviour. The system has three major components, entrainment pathways, pacemakers and output pathways that couple the pacemakers to effector systems that express circadian functioning. The retinohypothalamic tract is a direct retinal projection to the circadian pacemakers, the suprachiasmatic nuclei. The retinohypothalamic tract arises from a discrete set of retinal ganglion cells that receive photic information from a unique population of retinal photoreceptors and it mediates photic entrainment of the suprachiasmatic nuclei. The geniculo-hypothalamic tract arises from neurons of a specialized subdivision of the lateral geniculate complex, the intergeniculate leaflet. The intergeniculate leaflet and geniculohypothalamic tract appear to provide integrated photic and non-photic input to the suprachiasmatic nuclei to modulate pacemaker function. The suprachiasmatic nuclei comprise individual neuronal oscillators coupled into a neural network. The output of the suprachiasmatic nuclei is quite restricted but becomes amplified by a set of downstream components of the system that appear to provide a widespread circadian signal.