Which cells are primarily responsible for initiating phototransduction when light is detected?

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

Answer: (D)

The process of phototransduction starts in photoreceptor cells. When light hits the outer segments of rods and cones, the photopigment (opsin bound to a light-sensitive retinal molecule) absorbs a photon. This causes the retinal to change shape (11-cis to all-trans), activating the opsin and triggering a G-protein cascade. Transducin activates phosphodiesterase, which lowers the level of cGMP. With less cGMP, the cGMP-gated ion channels close, and the photoreceptor membrane becomes hyperpolarized. This reduces the release of the neurotransmitter glutamate onto bipolar cells, thereby initiating the signal that will be processed downstream through bipolar and ganglion cells to the brain. Horizontal and amacrine cells contribute to processing and refinement of the signal, but they do not initiate phototransduction—the initial detection and conversion of light into an electrical signal happens in the photoreceptors.