Which optic nerve synapse is MOST associated with the control of circadian rhythm?

Which optic nerve synapse is MOST associated with the control of circadian rhythm?

• A. LGN of the thalamus
• B. Pretectal nucleus
• C. Suprachiasmatic nucleus of hypothalamus
• D. Optic chiasm

Answer: (C)

Light input informing the brain’s clock is the key idea here. The suprachiasmatic nucleus in the hypothalamus is the master circadian pacemaker, and it gets direct retinal signals through the retinohypothalamic pathway from intrinsically photosensitive retinal ganglion cells that contain melanopsin. This retinal-to-SCN connection lets the clock read day–night changes and set daily rhythms, including coordinating melatonin release by the pineal gland and shaping sleep–wake cycles. The other structures play different roles: the LGN relays visual information to the cortex for conscious vision, the pretectal nucleus mediates the pupil light reflex, and the optic chiasm is a crossing point for some retinal fibers rather than a center for circadian timing.