Which cells in the retina encounter light first?

Which cells in the retina encounter light first?

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

Answer: (D)

Light is first detected by the photoreceptors—the rods and cones. These cells contain photopigments (rhodopsin in rods, opsins in cones) that undergo chemical changes when photons are absorbed, starting the visual signal. The outer segments of photoreceptors are where this absorption occurs, even though light must pass through several inner retinal layers on its way there. Those inner layers are largely transparent to light and mainly serve processing roles, not light detection. After phototransduction, signals move to bipolar cells, then to ganglion cells, which send the information to the brain. Horizontal and amacrine cells modulate the signals laterally. Because ganglion cells do not respond directly to light but rather relay processed signals from photoreceptors, the initial encounter with light that produces a response occurs in the photoreceptors.

Light is first detected by the photoreceptors—the rods and cones. These cells contain photopigments (rhodopsin in rods, opsins in cones) that undergo chemical changes when photons are absorbed, starting the visual signal. The outer segments of photoreceptors are where this absorption occurs, even though light must pass through several inner retinal layers on its way there. Those inner layers are largely transparent to light and mainly serve processing roles, not light detection. After phototransduction, signals move to bipolar cells, then to ganglion cells, which send the information to the brain. Horizontal and amacrine cells modulate the signals laterally. Because ganglion cells do not respond directly to light but rather relay processed signals from photoreceptors, the initial encounter with light that produces a response occurs in the photoreceptors.