Retinal Anatomy

Primate Retinal Basics

Among mammals, primates (monkeys, apes, and humans) are the sole group that possesses trichromatic color vision, that is, they possess three different types of receptors, called cones, which are responsible for daylight vision and code for color. They have peak sensitivities in the short (S, blue), middle (M, green) and long (L, red) regions of the spectrum. A further group of receptors, the rods, are responsible for vision in dim light, when we are colorblind. The visual systems of primates are very similar to ours, no matter whether anatomical, physiological, or behavioral criteria are used.

The retina lies at the back of the eye with the receptors farthest from the lens. The structure of the eye is sketched in Figure 29(a) of the color insert, with an enlargement of a small retinal circuit. After light has passed through the lens, it passes through the neural layers of the retina (which are virtually transparent) to be absorbed by the receptors. These are laid out in a regular array across the back of the eyeball, with the greatest density in the center of the visual field, which is called the fovea. The neural layers consist of bipolar cells that contact the cones and rods, and activate the ganglion cells, which send their axons and signals to the brain.

Figure 29 Retinal Anatomy—The Eye and Retina

Note: (a) Sketch of the eye. The retina lies at the back of the eye. The fovea is the region of central vision. The basic retinal circuit (inset) consists of photoreceptors (anchored to the pigment epithelium) that give their output to the bipolar cells that give their output to the ganglion cells, which send their output to the brain. (b) The retina shows a regular structure with different lamina; light must pass through them to reach the receptors. (c) A wiring diagram of the different channels in primate retina. Different channels mediate achromatic, red-green, and blue-yellow signals. Each has the same pattern—cone to bipolar cell to ganglion cells, but there are differences in detail. See the Retinal Anatomy entry for additional information. (pp. 868–871).

A cross section of the retina (color insert, Figure 29b) shows a layered structure with the receptors visible at the right as oblong, pencil-like elements pointing toward the incoming light. With the simple dye used to stain this section, it is impossible to disentangle the jumble of neuronal elements in the different layers. But with more sophisticated techniques, the layers and their neurons have been revealed and it is now possible to draw a wiring diagram of the retina, as seen in color insert, Figure 29(c). This looks complicated, but all pathways are variants of the same scheme—receptor to bipolar cell to ganglion cell.

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