Which part of a photoreceptor does light encounter first?

• A. Inner segment
• B. Outer segment
• C. Cell body
• D. Synaptic terminal

Answer: (A)

Light entering the retina first encounters the part of the photoreceptor where the photopigments are located. In a photoreceptor, those pigments reside in the outer segment, which consists of stacked membrane discs that capture photons. When light is absorbed by these pigments, it starts the phototransduction cascade that changes the cell’s electrical state and sends signals toward bipolar and then ganglion cells. The inner segment beneath it houses mitochondria and other machinery for energy and protein synthesis, supporting the cell but not absorbing light. The cell body and the synaptic terminal are involved in processing and transmitting the signal to downstream neurons, not in the initial light detection. So the first part of the photoreceptor that light interacts with is the outer segment.