There are two types of photoreceptor neurons: rods and cones. Both receptor cell types contain pigment molecules that undergo dissociation in response to light,and it is this photochemical reaction that eventually results in the production of action potentials in the optic nerve. Rods provide black-and-white vision under conditions of low light intensities, whereas cones provide sharp color vision when light intensities are greater
The retina consists of a single-cell-thick pigmented epithelium, photoreceptor neurons called rods and cones, and layers of other neurons. The neural layers of the retina are actually a forward extension of the brain. In this sense, the optic nerve can be considered a tract, and indeed the myelin sheaths of its fibers are derived from oligodendrocytes (like other CNS axons) rather than from Schwann cells.
Since the retina is an extension of the brain, the neural layers face outward, toward the incoming light. Light, therefore, must pass through several neural layers before striking the pho-toreceptors (fig. 10.35). The photoreceptors then synapse with other neurons, so that nerve impulses are conducted outward in the retina.
The outer layers of neurons that contribute axons to the optic nerve are called ganglion cells. These neurons receive synaptic input from bipolar cells, which in turn receive input from rods and cones. In addition to the flow of information from photoreceptors to bipolar cells to ganglion cells, neurons called horizontal cells synapse with several photoreceptors (and possibly also with bipolar cells), and neurons called amacrine cells synapse with several ganglion cells.
Each rod and cone consists of an inner and an outer segment (fig. 10.36). Each outer segment contains hundreds of flattened membranous sacs, or discs, with the photopigment molecules required for vision. The photoreceptor cells continuously add new discs at the base of the outer segment as the tip regions are removed by the cells of the retinal pigment epithelium (see fig. 10.35) through a process of phagocytosis.
Direction of light
■ Figure 10.35 Layers of the retina. Since the retina is inverted, light must pass through various layers of nerve cells before reaching the photoreceptors (rods and cones).
The photoreceptors are in close proximity to the retinal pigment epithelium. These pigment epithelial cells are needed to remove the old tip segments of the photoreceptors; if that function is not performed properly, vision can be impaired (see the discussion of macular degeneration, p. 274). The pigment cells also perform other functions important in vision. For example, the melanin pigment contained in these cells absorbs light that might otherwise reflect back to the photoreceptors and reduce the clarity of vision.
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