In the retina, which cells encounter light first?

• A. Photoreceptors
• B. Bipolar cells
• C. Ganglion cells
• D. Horizontal cells

Answer: (C)

Light entering the eye travels through the inner layers of the retina before reaching the cells that actually detect photons. The layers closest to the vitreous humor are the nerve fiber layer and the ganglion cell layer. Because photons must pass through these inner layers first, the very first retinal cells that light encounters are the ganglion cells (and their fibers), even though they do not perform light detection in the way photoreceptors do. The photoreceptors, rods and cones, are located deeper back in the retina and do the actual light transduction once the light has passed through the earlier layers. So the first cellular interface light meets in the retina is the ganglion cell layer, while photoreceptors are the first to respond to light in terms of phototransduction.