Eye

RETINA

The retina has several distinct layers. Rods and cones form synaptic connections with bipolar and horizontal cells. Bipolar cells are relay neurons that transmit visual signals from the inner to the outer plexiform layer of the retina; horizontal cells are interneurons activated by rods and cones and send their axons laterally to act on neighboring bipolar cells. As a result of the actions of horizontal cells, bipolar cells have concentric receptive fields; that is, their membrane potentials are shifted in one direction by light reaching the center of their receptive field, and in the opposite direction by light reaching the surrounding area. Neither bipolar nor horizontal cells generate action potentials; all information is transferred by changes in membrane potential, which spread passively through the cell bodies and axons.

The processes of bipolar cells that reach the outer plexiform layer form synapses with ganglion cells and amacrine cells. Ganglion cells are output neurons whose axons comprise the optic nerves and optic tracts; amacrine cells are interneurons. Unlike other retinal neurons, both amacrine and ganglion cells generate action potentials.

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