Introduction

The neural retina serves both to transduce the visual scene into biologically relevant electrical signals and to “preprocess” the visual scene before transmission to the brain. The demands of the visual system require rapid and reliable signal transmission through the retinal synapses. Extraction and enhancement of salient features in the visual scene by the retina are motivated, in part, by the limited carrying capacity of the optic nerve exiting from the eye. In humans, only 1 million fibers are available to carry visual information to the brain. This section of the book discusses the anatomical and physiological processes underlying visual processing in the retina. This chapter focuses on the pathways, properties, and underlying mechanisms by which excitatory signals are conducted from rods and cones to bipolar cells to RGCs (retinal ganglion cells). This circuit is commonly referred to as the “vertical” or “radial” pathway. This chapter begins with the anatomical wiring of this vertical pathway, then discusses functional attributes of signal transfer, and finally ends with the cellular and molecular mechanisms that mediate signal transmission. Many of the topics discussed herein are covered in other previously published reviews (Dowling, 1987; Matthews, 1996; Thoreson and Witkovsky, 1999; Toyoda et al., 1999; Wilson, 1994; Wu, 1994).