Photoreceptor Anatomy

Figure 5.1 illustrates the basic components of vertebrate photoreceptors. The outer segment is a specialized cilium containing large numbers of disc-like expansions of the cell membrane. The outer segment is connected by the ciliary stalk to the inner segment, a structure that acts as a light pipe, guiding incident photons to the light-sensitive pigments. The inner segment also contains large numbers of mitochondria that house part of the biochemical machinery required to meet the metabolic needs of the photoreceptor. An axon-like process of variable length connects the cell body to the synaptic terminal, where contact is made with the next elements of the visual pathway. Vertebrate retinas generally possess two types of photoreceptors, called rods and cones, after the shapes of their outer segments. The receptor schematized in Figure 5.1 is a rod. As the name implies, the outer segments of cones taper toward the tip.

The electron micrograph of a rhesus monkey's cone in Figure 5.2 shows the ciliary stalk and part of the outer segment with its stack of discs, including discs newly forming near the ciliary stalk (arrow). The photopigments, which absorb light and initiate the visual process, form a major structural element in these membranous discs. Also visible in the figure are the mitochondria packing the inner segment. In cones, the disc membranes are continuous over the surface of the outer segment and present one face to the extracellular space. Rods differ in that the newly formed discs pinch off from the plasma membrane and are stacked like poker chips inside the more or less cylindrical outer segment.