Chemical and electrical synapses between photoreceptors in the retina of the turtle, Chelydra serpentina.

Electron microscopic observation of the distal retina of the turtle, Chelydra serpentina, revealed that photoreceptors contact each other by means of a variety of junctions. The synaptic terminals of the primary and accessory members of a double cone invariably make punctate contact with each other distal to the basal surfaces. This type of contact was only rarely seen between the synaptic terminals of single photoreceptors. Photoreceptor telodendria which emanate from the basal surface of the synaptic terminal and ramify laterally for up to 40 microns in the outer plexiform layer give off branchlets which contact the terminals of both rods and cones. These contacts resemble superficial basal junctions and can be of the "wide-gap" or "narrow-gap" type. In cones the branchlets of the telodendria sometimes make a "distal junction" with one of the lateral elements of a synaptic dyad. Gap junctions were not found between the fins radiating from the myoids of adjacent receptors but were seen to occur en passant between photoreceptor telodendria in the outer plexiform layer. These junctions, though small, are probably of sufficient size to account for the rod-rod coupling observed physiologically in this retina. A prominent organelle resembling a synaptic lamella was occasionally seen in processes making contact with photoreceptors. These processes were identified as bipolar cell dendrites.