Human Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) is a bilateral region of the brain, located in the hypothalamus, that is responsible for controlling endogenous circadian rhythms. The neuronal and hormonal activities it generates regulate many different body functions over a 24-hour period.
The SCN contains several cell types and several different peptides (including vasopressin and vasoactive intestinal peptide) and neurotransmitters, and interacts with many other regions of the brain.
The SCN is situated in the anterior hypothalamus immediately dorsal to the optic chiasm (CHO) bilateral to (on either side of) the third ventricle.

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Circadian effects
The SCN receives inputs from specialized photoreceptive retinal ganglion cells, via the retinohypothalamic tract.
Destruction of the SCN leads to a complete loss of circadian rhythm. Rats with damage to the SCN have no circadian rhythms, i.e., they sleep the same total amount, but polyphasically for random lengths at a time.
The SCN also controls 'slave oscillators' in the peripheral tissues, which exhibit their own ~24 hour rhythms, but are crucially synchronized by the SCN.
The importance of entraining our bodies to an exogenous cue, such as daylight, is reflected by several circadian rhythm sleep disorders, where this process does not function normally.
Neurons in the ventrolateral SCN (vlSCN) have the ability for light-induced gene expression. 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 make an endogenous 24-hour rhythm that can persist under constant darkness (in humans averaging about 24h 11min). Melanopsin-containing ganglion cells in the retina have a direct connection to the SCN via the retinohypothalamic tract.
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.
Other signals from the retina
  • The SCN is one of four nuclei that receive nerve signals directly from the retina.
  • The other three are the lateral geniculate nucleus (LGN), the superior colliculus, and the pretectum:
  • The LGN passes information about color, contrast, shape, and movement on to the visual cortex and itself signals to the SCN.
  • The superior colliculus controls the movement and orientation of the eyeball.
  • The pretectum controls the size of the pupil.
Gene expression
The circadian rhythm in the SCN is generated by a gene expression cycle in individual SCN neurons. This cycle has been well conserved through evolution, and is essentially similar in cells from many widely different organisms that show circadian rhythms.
Neurons in the SCN fire action potentials in a 24-hour rhythm. At mid-day, the firing rate reaches a maximum, and, during the night, it falls again. How the gene expression cycle (so-called the core clock) connects to the neural firing remains unknown.
Many SCN neurons are sensitive to light stimulation via the retina, and sustainedly firing action potentials during a light pulse (~30 seconds) in rodents. The photic response is likely linked to effects of light on circadian rhythms. In addition, focal application of melatonin can decrease firing activity of these neurons, suggesting that melatonin receptors present in the SCN mediate phase-shifting effects through the SCN.

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