Functional Neuroanatomy of the Retina

Prologue In the eight years since Dr. Paul Witkovsky reviewed retinal anatomy and function for this series, a sea change has occurred. We have progressively uncovered detailed molecular architectures for many structures and processes. We have concretely linked circuitry and retinal disease. There is now an overwhelming amount of literature to be screened. A PubMed search on the topic of “retina AND neuron” over 20012008 retrieves > 10,000 papers. If you started today and read all day, every day without fail, it would take two-to-three years to catch up to 2008, and then you would still be three years behind an exponentially growing field. After severe winnowing the reference list has grown to over 250 papers, yet it is certain that I have neglected key publications. I apologize to those authors in advance. What justifies this expansion? First, we have now assembled a nearly complete catalogue of cells in the retina (perhaps 90%) and have learned a tremendous amount about neuronal phenotypes and connections. We simultaneously know more and, paradoxically, understand less about primate color coding mechanisms. New retinal cell types have been discovered and refined models of synaptic signaling have emerged. Our understanding of the molecular mechanisms of synaptic function, of neurotransmitter receptor molecular biology, of modulatory mechanisms, and gap junctions has exploded. Further, we now have strong evidence of postnatal and diseaseinduced neuroplasticity in the mature retina. Some problems persist. We still do not know how horizontal cells (HCs) work; how red/green color coding happens; why we need so many kinds of bipolar cells (BCs) and amacrine cells (ACs); how the retina develops (though great advances have been made); what retinal efferents do; nor the exact details of any ganglion cell (GC) micronetwork. The study of retinal structure remains a dynamic, challenging enterprise.