What is the order of the two main cells in the feedback phototransduction process? (i.e. NOT going toward the optic nerve)

• A. Ganglion cells to photoreceptors
• B. Photoreceptors to ganglion cells
• C. Bipolar cells to ganglion cells
• D. Photoreceptors to bipolar cells

Answer: (A)

In the retina, the immediate feedback that modulates phototransduction happens between photoreceptors and horizontal cells. The photoreceptors are the cells that initially respond to light, and horizontal cells monitor activity across neighboring photoreceptors. They provide inhibitory feedback to the photoreceptors, helping to adjust sensitivity and sharpen contrast. This loop stays within the outer retina and is not directed toward the optic nerve, which is why it isn’t part of the forward path that goes to bipolar and then ganglion cells.

So the correct sequence is photoreceptors first, followed by horizontal cells, with horizontal cells feeding back to the photoreceptors to modulate their response.