Photoreceptor signals and vision. Proctor lecture

Abstract

In recent years, there has been rapid progress in understanding the properties and mechanism of generation of the light-evoked electrical signals of vertebrate rods and cones. The graded hyperpolarization that carries information over the length of the cell is generated by closure of cation-selective aqueous pores in the surface membrane of the outer segment. These pores are controlled cooperatively by cyclic GMP, which acts continuously in darkness to keep the pores open. Photoisomerization of rhodopsin or cone pigment produces the rapid amplified activation of phosphodiesterase, which lowers the concentration of cGMP, thereby lowering the conductance of the surface membrane. Calcium ions, once thought to relay excitation to the light-sensitive channels, do not play this role. Instead, they appear to participate in a feedback control mechanism that regulates the nucleotide cascade. Although some general features of the transduction mechanism are now understood, a number of important questions remain. How is the nucleotide cascade shut off? Where does Ca act? What is the structure of the light-sensitive channel? How are stereotyped single photon responses produced? Primate photoreceptors are no longer off limits to single cell electrophysiology. Analysis of the response properties and dark noise of primate rods gives a physiological basis for several fundamental features of human rod vision: single photon detection, poor temporal resolution, the "dark light," rod saturation, scotopic spectral sensitivity, and, perhaps, after-image signals. Primate cones show less sensitive but faster responses shaped by a resonance which may figure in the flicker sensitivity of human cone vision. The spectral sensitivity of the three types of primate cones has been determined over the entire visible region. These sensitivities satisfactorily predict human color matching. The spectral sensitivity curves indicate that the pigment in a given cone is very pure, and that individual cones of a given type normally contain pigments with very similar or identical spectral properties.