Spread of Excitation and Background Adaptation in the Rod Outer Segment

This chapter discusses the spread of excitation and background adaptation in a rod outer segment (ROS). There is evidence that both excitation and background adaptation in a ROS spread from the site of photoisomerization. The chapter illustrates that both excitation and desensitization in the ROS are confined to near the site of photon absorption. Subsequently, the inside of the outer segment is known to be tightly packed with membranous disks arranged in a stack, each disk spanning the entire cross-sectional area of the outer segment. A substance diffusing across the outer segment is relatively unobstructed, but longitudinal diffusion can occur only around the edges of the disks. Free diffusion coefficients for the substances mediating both excitation and desensitization are therefore about l0⁻⁵ cm² sec⁻¹, similar to that for small ions or molecules in aqueous solution.