photoreceptors

Meaning: Photoreceptors are the rods and cones, cells at the back of the retina that transduce light into a neural signal. Rods are very sensitive, work in dim light, and give no color information; they are most numerous in the periphery and absent from the fovea. Cones need brighter light, give color and fine acuity, and are packed in the fovea. Humans have three cone types tuned to short, medium, and long wavelengths (trichromatic theory). In darkness photoreceptors are depolarized and release neurotransmitter steadily; light hyperpolarizes them and reduces release.

On the MCAT: Night vision and peripheral vision point to rods; color and detail in central vision point to cones. Passages may describe dark adaptation, where rods slowly regain sensitivity, or color deficiency from a missing cone type. The blind spot is where the optic nerve leaves the retina, so no photoreceptors are there.

Common confusion: Light does not depolarize photoreceptors the way a stimulus excites most sensory neurons; it hyperpolarizes them. Also, photoreceptors do not send signals to the brain directly: bipolar cells and then ganglion cells, whose axons form the optic nerve, carry the signal onward.