What happens when bright light stimulates ipRGCs and signals the suprachiasmatic nucleus (SCN)?

• A. It promotes alertness via the SCN
• B. It suppresses the immune system
• C. It causes permanent circadian misalignment
• D. It reduces visual acuity

Answer: (A)

Bright light activating ipRGCs provides a real-time cue about the time of day by signaling the brain’s master clock, the suprachiasmatic nucleus. These intrinsically photosensitive retinal ganglion cells (containing melanopsin) send information through the retinohypothalamic tract to the SCN. When the SCN receives this daylight signal, it coordinates the body's rhythms to promote wakefulness and alertness during the day, in part by suppressing melatonin production and aligning peripheral clocks with the day-night cycle. The other options aren’t the direct outcome of this pathway: immune function isn’t immediately dictated by this signal, a single bright-light cue doesn’t permanently misalign the clock, and visual acuity isn’t reduced by this non-image-forming signaling.