The suprachiasmatic nucleus (SCN) is a tiny pair of nuclei in the front (anterior) part of the hypothalamus, sitting directly above the optic chiasm, one on each side of the third ventricle. It is the master circadian clock of mammals, the brain’s “body clock”. [1][2]
The 3D brain needs JavaScript. The text below still says where the suprachiasmatic nucleus is and what it does.
Opens cut by a side (sagittal) slice through the hypothalamus, which holds the suprachiasmatic nucleus. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
Not a separate shape in this atlas; lit: the part that holds it. Lit: the whole hypothalamus. The suprachiasmatic nucleus is a pair of small nuclei inside the front of the hypothalamus, just above the optic chiasm; it has no shape of its own in the BodyParts3D model, so nothing is drawn at a guessed point. [1][3]
Lit here: the hypothalamus, the part that holds the suprachiasmatic nucleus — see it on its own page.
It sits in the anterior part of the hypothalamus, directly above the optic chiasm (hence “supra-chiasmatic”), on either side of the third ventricle. [1]
The optic chiasm, which it rests on, is where the two optic nerves meet and partly cross, under the hypothalamus and in front of the pituitary stalk. [4]
Size, honestly: it is a few thousand nerve cells, given as about 10,000 neurons in the article on the nucleus itself and about 20,000 in the article on the circadian clock; no source here gives its size in millimetres, and the model cannot measure it, so no size is stated. [1][2]
What it does
It is the central circadian pacemaker: it sets the daily rhythm of sleep and wake and coordinates the subordinate clocks in the cells of the rest of the body. [1][2]
In vertebrates, including humans, the master circadian clock is contained in the suprachiasmatic nucleus, a nerve cluster on both sides of the hypothalamus; it keeps “slave oscillators” in the body’s tissues, which have near-24-hour rhythms of their own, in step. Exactly which signal does that is still not clear. [2]
Daylight resets it every day: light-sensitive retinal ganglion cells containing the pigment melanopsin signal straight to it along the retinohypothalamic tract, so the clock stays matched to the real day. [1]
Left without light cues the body clock still runs, with its own period of close to, but not exactly, 24 hours; that is why light is needed to keep it in step. [2]
Connections
Input: the retinohypothalamic tract from the eyes, reaching mainly its ventrolateral (core) part; its dorsomedial part is called the shell. [1]
Output: mainly to the subparaventricular zone of the hypothalamus next to it, and through the paraventricular nucleus on a long route to the pineal gland, whose melatonin release it times. [1]
Other names
Abbreviated SCN; Latin nucleus suprachiasmaticus, the plural suprachiasmatic nuclei for the pair. It is called the master circadian clock, and in plain speech the body clock. [1][2]
Size
The model has no shape for the suprachiasmatic nucleus itself, so it is not measured here. The hypothalamus that holds it measures about 18 mm front to back, 20 mm top to bottom and 17 mm side to side on this model (the box it fits in, not its volume). [5]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.