The Hypothalamus and Limbic System Quiz

See Major Nuclei and First Rules. Option B is correct because the suprachiasmatic nucleus (SCN), a small paired cluster of neurones at the front of the hypothalamus, lies just above the optic chiasm (the point where the two optic nerves meet and partly cross). It receives light information straight from the retina through the retinohypothalamic tract, a dedicated pathway that starts in specialised light-sensing retinal ganglion cells containing the pigment melanopsin. That direct line is what makes the SCN the master circadian pacemaker. Circadian means "about a day", and a pacemaker is the clock that sets the pace for everything else. The SCN times other hypothalamic and pituitary rhythms, including the night-time rise in melatonin (the hormone made by the pineal gland, a small gland deep in the brain, that signals darkness), so the body anticipates day and night instead of only reacting to them. The usual trap is to mix up the SCN with the pineal gland: the SCN is the clock, and the pineal gland is one of the organs the clock instructs. To remember it: the SCN sits where the eyes' signals cross, so light goes in there and timing comes out.

• Location: the suprachiasmatic nucleus lies just above the optic chiasm at the front of the hypothalamus
• Input: light information reaches it directly from the retina through the retinohypothalamic tract
• Role: it is the master circadian pacemaker and times rhythms such as the night-time rise in melatonin