Physiology of photoreceptor synapses and other ribbon synapses

The first synapses in the visual pathway are at contacts between photoreceptors and second-order retinal bipolar or horizontal cells. Bipolar cells in turn influence the responses of ganglion cells, which project to central visual structures in the brain, either through direct synaptic contacts or indirectly through contacts with amacrine cells. The properties of transmission at synaptic terminals of photoreceptor and bipolar cells thus shape visual information conveyed through the retina and ultimately to the brain. Synaptic transmission at these terminals occurs at special types of synapses called ribbon synapse so named because of the presence of an electron dense ribbon in the presynaptic terminal. This article describes the anatomy, vesicular pools, proteins and calcium regulation at photoreceptor and bipolar cell ribbon synapses, and summarizes our current understanding of the mechanisms of release at ribbon synapses.