Retinal anatomy and physiology

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If one were to observe a normal human retina through an ophthalmoscope, one would see the gross morphological features illustrated in Figure 11.2. On the right side of the figure is the head of the optic nerve, also called the optic disk. A number of blood vessels originate from approximately the center of the optic disk and spread to cover much of the inner retina (the side of the retina closest to the lens). These vessels are supplied by the central artery and vein of the retina, which pass through the optic nerve. The outer retina (the side of the retina farthest from the lens and not visible in the figure) is perfused by the blood vessels running behind the retina in the choroid. The blood vessels on the inner retina and in the choroid form the retinal and choroidal circulations, respectively. To the left of the optic disk, one observes a small (~1.5-mm diameter) darker region of the retina devoid of major blood vessels. This region is called the fovea and it subserves the approximately 6 degrees of central vision, equivalent to a 10-cm-diameter circular region at a distance of 1 m. The fovea and the closely surrounding region are called the macula lutea or, more commonly, the macula. As the macular region is not readily seen in the figure, a dashed line indicates the border of the macula. This region, having a diameter of approximately 5 mm, subserves the approximately 20 degrees of the central visual field. This is equivalent to a 35-cm-diameter circular region at a distance of 1 m, or approximately the entire screen on a standard 15-inch monitor observed at 1 m. As the optics of the eye are relatively linear, the magnification factor (millimeters of neural tissue per degree of visual space) is approximately 0.25 throughout the retina.

Figure 11.2 Normal human retina as viewed through an ophthalmoscope, highlighting the regional specialization of the retina. The optic nerve head, also known as the optic disk, is where the axons of the retinal ganglion cells are collected together into the optic nerve as these axons course towards their subcortical targets. This region is devoid of photoreceptors. Additionally, the blood vessel of the retinal circulation originates from the optic nerve head. The fovea, the region of darker pigmentation, subserves the central 6 degrees of visual space and is the region of greatest visual acuity. The photoreceptors in this region are primarily cones. The macula lutea, outlined with a dashed line, subserves the central 20 degrees of visual space. Both rod and cone photoreceptors are found outside the fovea, with rod photoreceptors becoming more prevalent the farther one is from the fovea. The two principal sources of untreatable blindness, age-related macular degeneration and retinitis pigmentosa, are characterized by loss of photoreceptors in the macular region and outside the macular region, respectively. (Adapted from Webvision. With permission.)

Figure 11.2 Normal human retina as viewed through an ophthalmoscope, highlighting the regional specialization of the retina. The optic nerve head, also known as the optic disk, is where the axons of the retinal ganglion cells are collected together into the optic nerve as these axons course towards their subcortical targets. This region is devoid of photoreceptors. Additionally, the blood vessel of the retinal circulation originates from the optic nerve head. The fovea, the region of darker pigmentation, subserves the central 6 degrees of visual space and is the region of greatest visual acuity. The photoreceptors in this region are primarily cones. The macula lutea, outlined with a dashed line, subserves the central 20 degrees of visual space. Both rod and cone photoreceptors are found outside the fovea, with rod photoreceptors becoming more prevalent the farther one is from the fovea. The two principal sources of untreatable blindness, age-related macular degeneration and retinitis pigmentosa, are characterized by loss of photoreceptors in the macular region and outside the macular region, respectively. (Adapted from Webvision. With permission.)

The retina has a laminar neural organization, a highly simplified view of which is presented in Figure 11.3. This organization consists of three nuclear layers (layers of neuronal cell bodies) separated by two plexiform layers (layers of synaptic connections). An inner and outer limiting membrane as well as an outer epithelial layer surround the neural retina. As the photore-ceptors are located in the outermost nuclear layer in humans, the description logically starts with the outermost layer. The retinal pigmented epithelium (RPE) is not properly part of the neural retina but bears discussion due to its association with pathologies of the retina. This layer provides critical nutrients to the photoreceptors as well as phagocytosis of the exhausted components of the photoreceptors that are shed on a daily basis. The outer nuclear layer (ONL) consists of the photoreceptor neurons that convert light's electromagnetic energy into electrochemical energy. In humans, there are two morphological classes of photoreceptor neurons, rods and cones. Cone pho-

Retinal Anatomy

Figure 11.3 Schematic diagram of the organization of the retina, highlighting the principal cells of the retina and the nomenclature of the neural retina. Light entering the eye passes through the entire neural retina before being transduced into electrical signals by the rod and cone photoreceptors of the outer neural layer. These signals are processed in a columnar fashion by the bipolar cells of the inner neural layer and the retinal ganglion cells of the ganglion cell layer as well as being processed horizontally by the horizontal and amacrine cells of the inner nuclear layer. The axons of the retinal ganglion cells are collected together into the nerve fiber layer, which lies between the ganglion cells and the inner surface of the retina, or the epiretinal surface. (Adapted from Webvision. With permission.)

Figure 11.3 Schematic diagram of the organization of the retina, highlighting the principal cells of the retina and the nomenclature of the neural retina. Light entering the eye passes through the entire neural retina before being transduced into electrical signals by the rod and cone photoreceptors of the outer neural layer. These signals are processed in a columnar fashion by the bipolar cells of the inner neural layer and the retinal ganglion cells of the ganglion cell layer as well as being processed horizontally by the horizontal and amacrine cells of the inner nuclear layer. The axons of the retinal ganglion cells are collected together into the nerve fiber layer, which lies between the ganglion cells and the inner surface of the retina, or the epiretinal surface. (Adapted from Webvision. With permission.)

toreceptors are functional in typical daytime viewing conditions and mediate color vision. Humans have three subclasses of cone receptors (S, M, and L) that are most sensitive to blue, green, and red colors, respectively. Rod receptors become functional only at low light levels and, hence, are associated with night vision. As all rod receptors are most sensitive to a single color (blue-green) in humans, at night we can only distinguish relative intensity. Both rod and cone photoreceptors utilize graded voltage potentials to signal a change in stimulus intensity. That is, a change in luminance results in a change in the membrane potential, not an action potential. The densities of rod and cone photoreceptors vary with distance from the fovea (or with retinal eccentricity). Within the fovea, the photoreceptors are exclusively cones. For the first 10-degree radius about the fovea, covering approximately the macular region, the density of cone photoreceptors drops off rapidly to approximately 1/20th the density at the center of the fovea. An inverse relationship occurs with the density of rod photoreceptors, starting with a nil density at the fovea that grows rapidly for the first 20-degree radius about the fovea. The outer plexiform layer (OPL) separates the ONL and the inner nuclear layer (INL) and is the region where the photoreceptors of the ONL form synapses with the two classes of neurons of the INL (bipolar and horizontal neurons). Like photoreceptors, both bipolar and horizontal neurons utilize graded voltage potentials. The third class of neurons of the INL, amacrine cells, fire action potentials. The inner plexiform layer (IPL) separates the INL and the ganglion cell layer (GCL) and is the region where the bipolar and amacrine neurons of INL form synapses with GCL neurons. The neurons of the GCL consist almost exclusively of action potential firing retinal ganglion cells (RGC). The axons of the RGC form the nerve fiber layer (NFL) as they cross the inner surface of the retina, heading towards the optic disk. These fibers are unmyelinated until they reach the optic disk. The inner limiting membrane (ILM) separates the retina from the vitreous humor.

Visual information is processed from the outer to inner layers in a columnar functional organization. That is, the activity of a ganglion cell is modulated by the photoreceptor and bipolar neurons along a column from the inner to outer retina. This way the visuotopic map is preserved across the layers of the retina. Additionally, the horizontal and amacrine neurons integrate information across the neighboring regions of visual space. The degree of convergence of information across visual space depends on the retinal eccentricity of the ganglion cell. At the fovea, there is a one-to-two relationship between photoreceptors and ganglion cells; hence, the fovea is the region of greatest visual acuity. As one moves to the periphery, a large number of photoreceptors drive a single ganglion cell.

The degree of convergence is reflected in the receptive field properties of the RGC. At the fovea, the receptive field size is on the order of a few minutes of arc. At the outer edge of the macula, the receptive field size is as large as 3 to 5 degrees. The majority of the ganglion cells are optimally driven by a center-surround stimulus, or small circular of light at one intensity (or color) surrounded by an annulus of light at another intensity (or color). Cells of the M pathway, representing only 8% of the RGC, are best driven by a luminance difference between the center and surround. Two possible luminance differences are possible: a well-lit circle surrounded by a dark annulus (ON ganglion cell) or a dark circle surrounded by a well-lit annulus (OFF ganglion cell). Cells of the P pathway, representing 80% of the RGC, are best driven by a color difference in the center and surround. The two types of optimal differences are green vs. red and blue vs. yellow. The remaining 12% of the retinal ganglion cells do not fall into the M or P classification and have uncharacterized function.

The receptive field characteristics of bipolar cells are very similar to those of the RGC. In response to the reduction of neurotransmitter release by photoreceptors with increased illumination, bipolar cells either depolarize or hyperpolarize; hence, the two classes of bipolar cells are ON center and OFF center. The opposing surround is thought to result from lateral inhibition by horizontal cells. The similarity between bipolar cell and RGC receptive field properties is thought to be the result of RGC receiving exclusively excitatory input from a small number of bipolar cells.

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