Circadian Photoreception in Vertebrates

Different types of photoreceptors in vertebrates may have specialized roles in synchronizing internal circadian clocks to external light signals.

• All vertebrates use specialized photoreceptor cells to perceive light signals that synchronize their internal clocks.
• Mammals primarily utilize photoreceptors in the retina, including rods, cones, and photosensitive retinal ganglion cells containing melanopsin.
• Nonmammalian vertebrates have photoreceptors located in the brain, pineal gland, and retina.
• Evidence suggests that distinct classes of photoreceptors may contribute to various aspects of light entrainment.
• The nonvisual photoreceptive system is also involved in behavioral responses to light, pupil constriction, and reproductive state regulation.

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