Despite its peripheral location, the retina or neural portion of the eye, is actually part of the central nervous system. During development, the retina forms as an outpocketing of the diencephalon, called the optic vesicle, which undergoes invagination to form the optic cup (Figure 11.3; see also Chapter 22). The inner wall of the optic cup gives rise to the retina, while the outer wall gives rise to the pigment epithelium. This epithelium is a melanincontaining structure that reduces backscattering of light that enters the eye; it also plays a critical role in the maintenance of photoreceptors, renewing photopigments and phagocytosing the photoreceptor disks, whose turnover at a high rate is essential to vision.

Figure 11.3

Development of the human eye. (A) The retina develops as an outpocketing from the neural tube, called the optic vesicle. (B) The optic vesicle invaginates to form the optic cup. (C, D) The inner wall of the optic cup becomes the neural retina, while the (more...)

Consistent with its status as a full-fledged part of the central nervous system, the retina comprises complex neural circuitry that converts the graded electrical activity of photoreceptors into action potentials that travel to the brain via axons in the optic nerve. Although it has the same types of functional elements and neurotransmitters found in other parts of the central nervous system, the retina comprises only a few classes of neurons, and these are arranged in a manner that has been less difficult to unravel than the circuits in other areas of the brain. There are five types of neurons in the retina: photoreceptors, bipolar cells, ganglion cells, horizontal cells, and amacrine cells. The cell bodies and processes of these neurons are stacked in five alternating layers, with the cell bodies located in the inner nuclear, outer nuclear, and ganglion cell layers, and the processes and synaptic contacts located in the inner plexiform and outer plexiform layers (Figure 11.4). A direct three-neuron chain—photoreceptor cell to bipolar cell to ganglion cell—is the major route of information flow from photoreceptors to the optic nerve.

Figure 11.4

Structure of the retina. (A) Section of the retina showing overall arrangement of retinal layers. (B) Diagram of the basic circuitry of the retina. A three-neuron chain—photoreceptor, bipolar cell, and ganglion cell—provides the most direct (more...)

There are two types of light-sensitive elements in the retina: rods and cones. Both types of photoreceptors have an outer segment that is composed of membranous disks that contain photopigment and lies adjacent to the pigment epithelial layer, and an inner segment that contains the cell nucleus and gives rise to synaptic terminals that contact bipolar or horizontal cells. Absorption of light by the photopigment in the outer segment of the photoreceptors initiates a cascade of events that changes the membrane potential of the receptor, and therefore the amount of neurotransmitter released by the photoreceptor synapses onto the cells they contact. The synapses between photoreceptor terminals and bipolar cells (and horizontal cells) occur in the outer plexiform layer; more specifically, the cell bodies of photoreceptors make up the outer nuclear layer, whereas the cell bodies of bipolar cells lie in the inner nuclear layer. The short axonal processes of bipolar cells make synaptic contacts in turn on the dendritic processes of ganglion cells in the inner plexiform layer. The much larger axons of the ganglion cells form the optic nerve and carry information about retinal stimulation to the rest of the central nervous system.

The two other types of neurons in the retina, horizontal cells and amacrine cells, have their cell bodies in the inner nuclear layer and are primarily responsible for lateral interactions within the retina. These lateral interactions between receptors, horizontal cells, and bipolar cells in the outer plexiform layer are largely responsible for the visual system's sensitivity to luminance contrast over a wide range of light intensities. The processes of amacrine cells, which extend laterally in the inner plexiform layer, are postsynaptic to bipolar cell terminals and presynaptic to the dendrites of ganglion cells (see Figure 11.4). The processes of horizontal cells ramify in the outer plexiform layer. Several subclasses of amacrine cells that make distinct contributions to visual function. One class of amacrine cells, for example, plays an important role in transforming the persistent responses of bipolar cells to light into the brief transient responses exhibited by some types of ganglion cells. Another type serves as an obligatory step in the pathway that transmits information from rod photoreceptors to retinal ganglion cells. The variety of amacrine cell subtypes illustrates the more general rule that although there are only five basic retinal cell types, there can be considerable diversity within a given cell type. This diversity is the basis for pathways that convey different sorts of information to central targets in a parallel manner.

At first glance, the spatial arrangement of retinal layers seems counterintuitive, since light rays must pass through the non-light-sensitive elements of the retina (and retinal vasculature!) before reaching the outer segments of the photoreceptors, where photons are absorbed (see Figure 11.4). The reason for this curious feature of retinal organization lies in the special relationship that exists between the outer segments of the photoreceptors and the pigment epithelium. The outer segments contain membranous disks that house the light-sensitive photopigment and other proteins involved in the transduction process. These disks are formed near the inner segment of the photoreceptor and move toward the tip of the outer segment, where they are shed. The pigment epithelium plays an essential role in removing the expended receptor disks; this is no small task, since all the disks in the outer segments are replaced every 12 days. In addition, the pigment epithelium contains the biochemical machinery that is required to regenerate photopigment molecules after they have been exposed to light. It is presumably the demands of the photoreceptor disk life cycle and photopigment recycling that explain why rods and cones are found in the outermost rather than the innermost layer of the retina. Disruptions in the normal relationships between pigment epithelium and retinal photoreceptors such as those that occur in retinitis pigmentosa have severe consequences for vision (Box B).

Box B

Retinitis Pigmentosa.