Suprachiasmatic nucleus
Nucleus suprachiasmaticus
Also known as: SCN, circadian clock, master clock
A pair of tiny nuclei (≈10 000 neurons each) just above the optic chiasm that act as the body's master circadian clock, entrained by light and driving melatonin and cortisol rhythms.
Overview
Each SCN neuron contains a molecular clock: the transcription factors CLOCK and BMAL1 drive expression of PER and CRY, whose proteins accumulate, enter the nucleus and inhibit CLOCK/BMAL1, giving a self-sustained cycle of ≈24.2 h. Coupling between neurons produces a robust population rhythm that is reset each day mainly by light.
Light reaches the SCN through the retinohypothalamic tract, formed by intrinsically photosensitive retinal ganglion cells containing melanopsin (most sensitive to blue light, ≈480 nm). SCN output to the paraventricular nucleus controls the pineal gland (melatonin at night), the HPA axis (cortisol peak on waking), body temperature and sleep–wake timing, and synchronises peripheral clocks in liver, gut and other tissues.
- Location
Anterior hypothalamus, immediately dorsal to the optic chiasm on either side of the third ventricle.
Function
- Generates the ≈24 h circadian rhythm.
- Entrains the rhythm to the light–dark cycle.
- Drives nocturnal melatonin secretion and the diurnal cortisol rhythm.
Clinical significance
Circadian rhythm disorders
Jet lag, shift-work disorder and delayed sleep phase syndrome reflect misalignment between the SCN and environmental time; timed bright light and melatonin are used to shift the clock. Totally blind people without melanopsin input may have free-running (non-24-hour) rhythms.
Test yourself
Light suppresses melatonin secretion through which pathway?
- Optic nerve → lateral geniculate body → visual cortex → pineal
- Retina → retinohypothalamic tract → suprachiasmatic nucleus → paraventricular nucleus → spinal cord → superior cervical ganglion → pineal
- Retina → superior colliculus → posterior commissure → pineal
- Retina → oculomotor nucleus → ciliary ganglion → pineal
Answer: Retina → retinohypothalamic tract → suprachiasmatic nucleus → paraventricular nucleus → spinal cord → superior cervical ganglion → pineal
Melanopsin-containing retinal ganglion cells signal the SCN, which via the PVN and upper thoracic preganglionic neurons controls postganglionic sympathetic fibres from the superior cervical ganglion. Norepinephrine on pinealocyte β1 receptors (in darkness) drives AANAT and melatonin synthesis.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.