Ultrastructural evidence that horizontal cell axon terminals are presynaptic in the human retina.

The organization of the rod spherule and of the horizontal cell axon terminals within the invagination of the rod spherule in the human retina was examined in serial sections by electron microscopy. Twenty-one rod spherules were reconstructed in this study. Axon terminal processes of type I horizontal cells consistently make one or two small punctate synapses onto each rod spherule within the invagination. In addition, these axon terminal processes make distinct synapses upon rod bipolar dendrites outside the spherule before both processes enter the invagination. This is the first positive description of a synapse from a horizontal cell axon terminal process onto a photoreceptor terminal and the first identification of a synapse from a horizontal cell to a rod bipolar cell in the mammalian outer plexiform layer. We speculate that the axon terminal-to-rod synapse is responsible for feedback while the synapse upon the rod bipolar cell is feed-forward and serves to expand the receptive field of the rod bipolar cell beyond its dendritic field. Alternatively, the latter may contribute to a center-surround organization of the rod bipolar's receptive field.