The retina is part of the central nervous system (CNS) and has a long-standing history as an approachable part of the brain. With its still complex neural circuit it is responsible for converting light into a pattern of electrical impulses that informs the brain about the visual world. The retina has a common architecture across non- and mammalian species with six classes of neurons: including two types of light sensitive cells: cones (daytime color vision) and rods (low light sensors). Photoreceptor signals are processed through three types of interneurons: horizontal cells, bipolar cells and amacrine cells. The cell bodies of these neurons, along with the retinas’ major support cell type, Müller glia, are located in the inner nuclear layer. In the outer plexiform layer the synaptic terminals of rods and cones connect with horizontal cells and bipolar cells. These two cell types modify the incoming signals and then relay them to the dendrites of the amacrine and ganglion cells via synapses in the inner plexiform layer. The amacrine cells further process the incoming signals (e.g. motion detection), while the ganglion cells relay the visual information to the higher brain areas via their axons in the optic nerve.
FIGURE: (left) Cross section of the human eye. (middle) Cross section through a retina with all cell nuclei labeled in blue. The retinal ganglion cell layer is at the top. (right) Schematized distribution of retinal cell types according to the cross section in the middle. Retinal pigment epithelial (RPS) cells are part of the brain-blood barrier and support photoreceptors (rods (R) and cones (C)). Interneurons, bipolar cells (BP), amacrines (AC) and horizontals (HC) receive and modulate signals from the photoreceptors which are relayed to the higher brain center through the axons of the retinal ganglion cells (RGC).
