Histology of the retina

The retina is the layer at the back of the eye where light becomes a nerve signal. Under the microscope it is not a smooth sheet: it is a stack of layers, and six cell types make them up. The photoreceptors, which are the cones and the rods. The bipolar cells. The amacrine cells. The ganglion cells. The horizontal cells. And the Müller cells, which are supporting cells.

The route follows a set order. The photoreceptors form the outer nuclear layer and synapse, that is, communicate, with the bipolar cells in the outer plexiform layer. The bipolar cells, in turn, pass the impulse on to the ganglion cells.

The ganglion cell layer is the innermost of them all, the farthest from the photoreceptors, and its axons, the long projections of those neurons, come together and form the optic nerve.

Here is the strange and beautiful part: light detection begins in the deepest cell layer of the retina. It is like a sandwich where the light comes in through the top and is read all the way down to the bottom.