48 - Visual Transduction

Abstract

This chapter provides an overview of vertebrate visual transduction. It is mentioned that the process of visual transduction involves a G-protein-mediated second messenger system, which ultimately controls membrane potential and neurotransmitter release. Interesting difference between vertebrate and invertebrate photoreceptors is that the former hyperpolarize in response to light, and the latter depolarize. The chapter discusses photoreceptor cells, physiology of visual transduction, and molecular mechanisms. Photopigment activation and shutoff, cyclic nucleotide cascade are discussed under molecular mechanisms. The process of vertebrate vision begins with photon capture and visual transduction in the rods and cones. The first step in the response to light is the absorption of a photon by a visual pigment molecule. The visual pigment or photopigment consists of a particular protein (an opsin) containing a ubiquitous chromophore, retinal. The photopigment absorption spectra determine the probability that a photon of a particular wavelength will be absorbed. The light-induced decrease in membrane conductance is accomplished by the closure of nonselective cation channels in a specialized region of the photoreceptor called the outer segment. These channels conduct the so-called “dark current” that flows into the outer segment in the dark. The channels are kept open in the dark by cGMP, and they close in the light when the intracellular cGMP concentration falls. The light-induced fall in cGMP occurs when a photoexcited visual pigment molecule activates a G protein, which in turn activates a phosphodiesterase to hydrolyze cGMP. There are elaborate mechanisms for returning the molecular machinery of the photoreceptor to its dark state, and for light and dark adaptation. Although many of the fundamental molecular steps in visual transduction have been determined, numerous mysteries still remain.