Circadian Effects
Organisms in every kingdom of life—bacteria, plants, fungi, and animals—show genetically-based 24-hour rhythms. Although all of these clocks appear to be based on a similar type of genetic feedback loop, the specific genes involved are thought to have evolved independently in each kingdom. Within the animal kingdom, however, a related set of genes are used by a wide variety of animals: the circadian genes in fruit flies, for example, are closely related to those in mammals.
Many aspects of mammalian behavior and physiology show circadian rhythmicity, including sleep, physical activity, alertness, hormone levels, body temperature, immune function, and digestive activity. All of these diverse rhythms are controlled by a single tiny brain area, the SCN, and are lost if the SCN is destroyed. In the case of sleep, for example, the total amount is maintained in rats with SCN damage, but the length and timing of sleep episodes become erratic. The importance of entraining organisms, including humans, to exogenous cues such as the light/dark cycle, is reflected by several circadian rhythm sleep disorders, where this process does not function normally.
The SCN also controls "slave oscillators" in the peripheral tissues, which exhibit their own ~24-hour rhythms, but are kept in synchrony by the SCN.
The SCN receives inputs from specialized photosensitive ganglion cells in the retina, via the retinohypothalamic tract. Neurons in the ventrolateral SCN (vlSCN) have the ability for light-induced gene expression. Melanopsin-containing ganglion cells in the retina have a direct connection to the ventrolateral SCN via the retinohypothalamic tract. If light is turned on at night, the vlSCN relays this information throughout the SCN, in a process called entrainment. Neurons in the dorsomedial SCN (dmSCN) are believed to have an endogenous 24-hour rhythm that can persist under constant darkness (in humans averaging about 24 hours 11 min). A GABAergic mechanism couples the ventral and dorsal regions of the SCN.
The SCN sends information to other hypothalamic nuclei and the pineal gland to modulate body temperature and production of hormones such as cortisol and melatonin.
Read more about this topic: Suprachiasmatic Nucleus
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