Suprachiasmatic nucleus

The suprachiasmatic nucleus is a paired group of neurons in the hypothalamus that coordinates mammalian circadian rhythms. Light information reaching it from the retina helps synchronize its timing with the day–night cycle.

Retinohypothalamic tract: A neural pathway carrying light-related signals from retinal ganglion cells to the suprachiasmatic nucleus. It delivers retinal information that resets the nucleus in response to light.
Hypothalamus: A brain region that regulates homeostasis, endocrine activity, autonomic functions, and motivated behaviors. The suprachiasmatic nucleus is a small hypothalamic structure above the optic chiasm.
Sleep–wake cycle: The recurring pattern of sleep and wakefulness across the day and night. The nucleus helps align sleep and wake timing with the environmental light cycle.
Robert Moore: A neuroscientist known for research identifying the suprachiasmatic nucleus as a central circadian pacemaker. His lesion studies helped establish the nucleus’s role in circadian rhythms.
Peripheral circadian clock: A circadian oscillator in a tissue outside the brain’s central pacemaker system. Peripheral clocks keep time locally, while the nucleus helps coordinate their timing.
Melanopsin: A light-sensitive protein in intrinsically photosensitive retinal ganglion cells. It enables retinal cells to signal environmental light levels to the nucleus.
Optic chiasm: The brain structure where many optic nerve fibers cross to the opposite side. The nucleus lies just above this landmark, which gives it its name.
Melatonin: A hormone produced mainly by the pineal gland that signals biological night in many vertebrates. Nuclear timing controls the daily pattern of signals that regulate pineal melatonin secretion.
Frederick Stephan: A researcher whose experiments helped demonstrate the suprachiasmatic nucleus’s control of circadian rhythms. His work showed that damaging the nucleus disrupts daily behavioral rhythms.
Circadian rhythm in plants: An approximately 24-hour biological rhythm generated by plant molecular clocks and synchronized by environmental cues. Plants have circadian systems but no mammalian suprachiasmatic nucleus.

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