The Visual System

Chapter 17 The Visual System

Chapter Outline

It is clear from everyday experience that we are a visually oriented species. Although it is arguable which of our senses is most important, loss of vision certainly has a greater impact on humans than loss of, for example, olfaction or taste. Partly because of its importance (and partly for anatomical and technical reasons discussed later), a great deal of research has been done on the visual system. Currently, we know more about the visual system than about any other sensory system, and it is likely that with further study we will understand in some detail how this portion of the central nervous system (CNS) actually works.

Some lizards, fish, and amphibians have a photosensitive pineal organ that constantly stares up at the sky as an unblinking third eye. In mammals, however, with very few exceptions, all photic information is transduced in the rods and cones of the retina and then conveyed to the brain by way of the axons of the output cells (called ganglion cells) of the retina. These axons, together with the axons of higher-order cells on which they synapse, form a visual pathway that begins anteriorly in the eyes and ends posteriorly in the occipital lobes. Throughout most of this course a precise retinotopic arrangement of fibers is maintained, so that particular small regions of the retina are represented in particular small regions of more central parts of the pathway. Damage at many different locations within this system results in visual deficits, and knowledge of the anatomy involved makes it possible to understand these deficits. This in turn means that the same knowledge can be used to deduce the site of a lesion.

The Eye Has Three Concentric Tissue Layers and a Lens

Eyes and cameras both need to deal with similar sets of issues—maintaining a stable relationship between a focusing apparatus and a photosensitive surface, focusing on near and far objects, regulating the amount of light reaching the photosensitive surface, and recording the pattern of incoming light—so, not surprisingly, they have many analogous components. The retina is an outgrowth of the diencephalon (Fig. 17-1), and one result of this origin is numerous parallels between the eye and the brain and meninges. The eye can be thought of as formed from three roughly spherical, concentric tissue layers, with a lens suspended inside them (Fig. 17-2). Each layer contributes to different structures in different parts of the eye (Table 17-1).

The outermost tissue layer is continuous with the dura mater. Like the dura, it is a feltwork of collagenous connective tissue. Most of this layer forms the sclera, the “white of the eye,” which continues posteriorly as the sheath of the optic nerve. Beginning at a circular transition zone called the limbus, the anterior sixth of this layer is the transparent cornea, which lets light into the eye.

The heavily vascularized middle layer, the uvea* or uveal tract, is similar in some ways to the arachnoid and pia. This is the principal route through which blood vessels and nerves (other than the optic nerve) travel within the wall of the eye. Over most of its extent the uvea is sandwiched between the sclera and the retina as the densely pigmented choroid. Choroidal capillaries supply retinal photoreceptors, and choroidal pigment absorbs stray light (much like the flat black paint job inside a camera does). The uvea continues anteriorly to form the bulk of the ciliary body (containing the ciliary muscle) and the stroma of the iris.

The innermost layer is an outgrowth of the CNS and is itself a double-layered structure, reflecting its origin from the two layers of infolded optic cup (Fig. 17-1). Over most of its extent, this double layer comprises the retina, which lines the choroid. The outer portion of the retina, adjacent to the choroid, is the retinal pigment epithelium, whereas the inner portion, adjacent to the interior of the eye, is the neural retina. Under normal conditions no space exists between the pigment epithelium and the neural retina in adults. However, the mechanical connections between the two are not very strong, and under certain circumstances this potential space opens and retinal detachment results (see Fig. 17-14E). Retinal receptors are metabolically dependent on pigment epithelial cells and on the adjacent choroid-al vasculature, so detached areas stop working. The photosensitive retina ends anteriorly at a serrated border (the ora serrata), but the same two layers continue as the double-layered ciliary epithelium covering the ciliary body and the double layer of pigmented epithelium covering the posterior surface of the iris.


Figure 17-14 Use of optical coherence tomography (OCT) to visualize the retina and intraocular pathology. A, OCT image of the fovea and parafoveal retina. The outlined areas are enlarged in B and C. D, Normal optic disk (OpD), with a thick nerve fiber layer (NFL) converging on it from either side. Scale mark = 250 μm. E, A case of retinal detachment. The space (*) between the retinal pigment epithelium (RPE) and the photoreceptor layer (IS/OS) is apparent. The maintained reflectivity of the photoreceptor layer is a good prognostic sign, indicating probable recovery once the photoreceptors are reattached to the pigment epithelium. Scale mark = 250 μm. F and G, A case of acute angle-closure glaucoma in a 63-year-old woman who presented to the emergency department with a red, painful left eye and reduced vision. OCT revealed that her iris was edematous and bowed forward, presumably because of contact between the lens and the pupillary margin of the iris. The increased pressure in the posterior chamber pushed the iris forward, narrowing the angle (arrow in F) between the iris and cornea and obstructing the outflow of aqueous humor. A laser was used to perforate the peripheral iris, providing a direct route to the anterior chamber for aqueous trapped in the posterior chamber (much like using a shunt to treat hydrocephalus). Her left iris assumed a more normal configuration (G), and her symptoms resolved. GCL, ganglion cell layer; INL, inner nuclear layer; IPL, inner plexiform layer; IS/OS, photoreceptor inner and outer segments; ONL, outer nuclear layer; OPL, outer plexiform layer.

(A, B, D, and E, from Huang D et al: Optical coherence tomography. In Huang D et al, editors: Retinal imaging, Philadelphia, 2006, Mosby Elsevier. C, courtesy Dr. James G Fujimoto, Massachusetts Institute of Technology. F and G, from Kalev-Landoy M et al: Acta Ophthalmol Scand 85:427, 2007.)

Intraocular Pressure Maintains the Shape of the Eye

Cameras have rigid bodies, designed to keep the film in a stable position relative to the lens. In contrast, the shape of the eye (and position of the retina) is maintained in much the same way as inflating a soccer ball maintains its shape. The collagenous sclera and cornea correspond to the wall of the soccer ball, and intraocular fluid pressure replaces air pressure. The pressure is generated by a now familiar process of fluid production, circulation, and reabsorption (Fig. 17-3). The ciliary body functions as a small outpost of choroid plexus, secreting aqueous humor across the ciliary epithelium and into the posterior chamber, the space between the iris and the lens. Pushed along by hydrostatic pressure, the aqueous humor passes through the pupil and into the anterior chamber, filters through the collagenous trabecular meshwork (analogous to arachnoid granulations) at the iridocorneal angle, and enters an endothelium-lined scleral venous sinus (the canal of Schlemm), which communicates directly with the venous drainage of the eye. The production rate (about 2 μL/min) is sufficient to completely replace the aqueous humor about 15 times a day. The space behind the lens, constituting most of the intraocular volume, is filled with gelatinous vitreous (Latin for “glassy”) humor, so the resistance to aqueous outflow afforded by the trabecular meshwork and the wall of the scleral venous sinus causes a pressure of about 15 mm Hg that is transmitted throughout the eye, maintaining its shape.

In much the same way that blocking the circulation or reabsorption of cerebrospinal fluid causes increased intracranial pressure, headache, and neural damage, processes that interfere with the circulation or reabsorption of aqueous humor cause the painful condition of glaucoma and, ultimately, retinal damage (see Fig. 17-14F and G).

The Cornea and Lens Focus Images on the Retina

Focusing an image requires refraction of light across one or more interfaces where there is a change in refractive index. The aqueous and vitreous humors have a refractive index only slightly lower than that of the lens, so the lens accounts for only about a third of the refractive power of the eye; its major role is adjusting the focus of the eye for near and far objects, as described later. Hence for nonaquatic vertebrates like us, most of the refraction occurs at the air-water interface at the front surface of the cornea;* its curved shape, maintained by intraocular pressure, accounts for our ability to see 90 degrees or more to the side (see Fig. 17-32).

One can imagine a variety of strategies for changing the focus of an optical device to accommodate to near objects—moving the photosensitive surface, moving the refractive elements, or changing the shape of the refractive elements. Different animals have adapted each of these strategies. Conventional cameras are adjusted for near or far objects by moving their lenses closer to or farther from the film; similarly, fish and most amphibians have intraocular muscles that move the lens back and forth. Arthropods cannot move or deform lenses that are part of the exoskeleton, but some have muscles that move the retina closer to or farther from the lens. Some animals have muscles attached to the cornea that can change its curvature. Terrestrial vertebrates use intraocular muscles to change the shape of the lens. Our lens is suspended by strands of connective tissue called zonules (Fig. 17-3, inset), attached at one end to the lens and at the other end to the ciliary body. At rest, the tension of this zonular suspension keeps the lens slightly flattened and the eye focused on distant objects. The ciliary muscle has some fibers oriented circumferentially that act as a kind of sphincter; contraction of these fibers pulls the ciliary attachment points of the zonules toward the center of the pupil and relaxes some of the tension in the zonular suspension. Other ciliary muscle fibers are oriented parallel to the surface of the eye; contraction of these pulls the ciliary attachment points partly anteriorly and partly toward the center of the pupil, again relaxing some of the tension in the zonular suspension. Hence, somewhat counterintuitively, contraction of the ciliary muscle allows the lens to fatten as the eye accommodates to near objects: the posterior surface of the lens is embedded in the vitreous humor and does not move much, but the anterior surface bulges out slightly.

The Iris Affects the Brightness and Quality of the Image Focused on the Retina

The range of light intensities over which we have useful vision, from starlight to bright sunlight, is an astonishing 1012 or so—a million million-fold. This is a much greater range of intensities than receptor potentials and frequencies of action potentials can encode directly, so there are mechanisms for adapting visual sensitivity to the ambient illumination. Most of these mechanisms depend on the physiology and wiring of retinal neurons, but in addition, the iris plays a role in regulating the amount of light reaching the retina. The two posterior epithelial layers are densely pigmented, and in brown-eyed individuals the stroma contains substantial additional pigment, so essentially all light reaching the retina must first pass through the pupil, the aperture in the middle of the iris.

The size of the pupil is controlled by two smooth muscles in the iris (Fig. 17-2); both are highly unusual, in that they are derived from the same layers of neural ectoderm that give rise to the retina. The circumferentially arranged pupillary sphincter encircles the pupil at what was, embryologically, the edge of the optic cup.* The pupillary dilator, whose fibers are arranged like spokes radiating from the sphincter, is located at the interface between the pigment epithelial layers and the stroma. The sphincter is the stronger of the two, and reflex connections mediated by the optic and oculomotor nerves constrict the pupil in response to increased levels of illumination (see Fig. 17-39). The pupillary sphincter can contract by about 80%, much more than other muscles and enough to vary the diameter of the pupil from about 8 mm to 1.5 mm. However, this corresponds to only about a thirtyfold change in area, consistent with the idea that retinal mechanisms play the major role in adjusting visual sensitivity.

In addition to decreasing the amount of light reaching the retina, a smaller pupil improves the optical performance of the eye (just as, within limits, a smaller aperture improves the optical performance of a camera lens). This is particularly important when focusing on near objects (see Figs. 17-40 and 17-41).

The Retina Contains Five Major Neuronal Cell Types

One reason so much research has been done on the visual system is the overall anatomical simplicity of the neural retina relative to other parts of the nervous system. Although it contains hundreds of millions of neurons, there are only five basic types involved in the processing of visual information, and their patterns of interconnections are fundamentally the same throughout the retina.

The five cell types have their somata neatly arranged in three layers and make most of their synapses in two additional layers interposed between the layers of cell bodies. In each synaptic layer, one cell type brings visual information in, another type carries information out, and a third type serves as a laterally interconnecting element.

A simplified, schematic illustration of these basic connection patterns is shown in Figure 17-4. Starting peripherally, photoreceptor cells, stimulated by light, project to the first layer of synapses, where they terminate on the aptly named bipolar and horizontal cells. Bipolar cells then project to the next layer of synapses, whereas horizontal cells spread laterally and interconnect receptors, bipolar cells, and other horizontal cells. In the second layer of synapses, bipolar cells terminate on ganglion cells and amacrine cells.* Axons of ganglion cells leave the eye as the optic nerve, whereas processes of amacrine cells spread laterally and interconnect bipolar cells, ganglion cells, and other amacrine cells.

Retinal Neurons and Synapses Are Arranged in Layers

The entire retina is conventionally described as a 10-layered structure, beginning with the pigment epithelium (Fig. 17-5; see also Fig. 17-14); five of these layers are the layers of cell bodies and synapses just mentioned. In naming these layers, the term nuclear refers to cell bodies and the term plexiform to synaptic zones. Inner and outer refer to the number of synapses by which a structure is separated from the brain, so that, for example, photoreceptors are “outer” with respect to bipolar cells. From outside in, the 10 layers of the retina are as follows.

2. Rods and cones are the two different types of vertebrate photoreceptors. Each consists of several regions (Fig. 17-6): an outer segment, an inner segment, a cell body, and a synaptic terminal. (Strictly speaking, rod or cone refers to only the outer segment plus the inner segment of a photoreceptor cell, but these terms are commonly used to refer to entire receptors.)

The outer segment of a rod is relatively long and cylindrical, whereas that of a cone (except in the fovea) is shorter and tapered (Figs. 17-6 and 17-7). Each type of outer segment is filled with hundreds of flattened membranous sacs, or disks. In cones, the interior of most of these disks is continuous with extracellular space, but in rods, almost all the disks have pinched off from the external membrane and are wholly intracellular. The major protein constituent of the outer segment membranes of both rods and cones is the visual pigment, which is called rhodopsin in rods. (There is no universally accepted name for the visual pigments of cones, and they are often referred to simply as cone pigments.) Hence photons traversing the outer segment of a rod or cone must pass through hundreds or thousands of sheets of membrane, each full of visual pigment molecules. As one might expect from this localization of visual pigment, the outer segment is the site of visual transduction; photons absorbed here cause a receptor potential that then spreads to the rest of the cell. The photosensitive portion of the receptor cells, oddly enough, is located in the part of the neural retina farthest removed from incoming light (i.e., the retina is inverted with respect to the path of light through it [Figs. 17-2 and 17-4]). This curious situation is universally true among vertebrates. However, this does not detract substantially from visual sensitivity or acuity, because the retina is thin (Fig. 17-5) and nearly transparent (Fig. 17-8), and because other anatomical modifications (discussed shortly) are found in the retinal area of greatest acuity.

Each outer segment is an elaborately specialized cilium that remains connected to its inner segment by a narrow ciliary stalk. The inner segments contain, among other organelles, a very prominent collection of mitochondria. These mitochondria supply the energy necessary for processes associated with transduction and for the synthesis of visual pigments. These pigments are continually renewed, being synthesized in the inner segment, transported through the ciliary stalk, and incorporated into disk membranes. “Old” disks at the apical ends of the outer segments of rods and cones are then phagocytosed by the pigment epithelium. (Certain types of retinal degeneration are caused by a defect in this renewal-phagocytosis process.)

As discussed later in this chapter, rods mediate low-acuity, monochromatic vision in dim light, whereas cones mediate high-acuity color vision, but require more light to do so.

8. The ganglion cell layer contains the cell bodies of the ganglion cells, whose dendrites ramify in the inner plexiform layer and whose axons leave the eye as the optic nerve. This cell layer is considerably thinner than either the outer or the inner nuclear layer in most retinal locations, reflecting the fact that there are about 5 million cones and about 100 million rods in a human retina, but only about 1 million ganglion cells. Clearly, a good deal of convergence is involved in retinal processing, but the convergence is not uniform across the retina. As discussed shortly, some regions are specialized for high spatial acuity and have little convergence, whereas other regions are specialized for high sensitivity and have a great deal of convergence.

Figure 17-24 Formation of center-surround receptive fields at the level of bipolar cells, using foveal cones and midget bipolar cells as examples. A, Formation of receptive field centers. In the dark, photoreceptors are depolarized and release glutamate onto the superficial synapses made by OFF-center bipolar cells (1) and onto the invaginating processes of ON-center bipolar cells (2) and horizontal cells (3). Because of the nature of the postsynaptic receptor molecules, glutamate hyperpolarizes the processes of ON-center bipolar cells and depolarizes the other two kinds of processes. Hence in the dark, ON-center bipolar cells are relatively hyperpolarized, and horizontal cells and OFF-center bipolar cells are relatively depolarized. Light in the receptive field center hyperpolarizes the cone (4), decreases glutamate release, and reverses all these polarizations: ON-center bipolar cells (as their name implies) depolarize (5), and the ganglion cells to which they project fire more rapidly; OFF-center bipolar cells hyperpolarize (6); and horizontal cells also hyperpolarize (7), but only moderately, because glutamate is still being released onto their other processes. B, Role of horizontal cells in formation of the antagonistic surround. Light delivered to cones surrounding the central cone in A causes a large hyperpolarization of horizontal cells (1) because of diminished glutamate release on many of their processes. This hyperpolarization spreads to the synaptic terminal of the central cone (2), in turn causing increased release of glutamate (as though it just got darker in the center of the receptive field), hyperpolarization of the ON-center bipolar cell (3), and depolarization of the OFF-center bipolar cell (4). The mechanism of this increased glutamate release is still being investigated, but it probably involves the effects of altered pH in the synaptic cleft on voltage-gated Ca2+ channels in the photoreceptor synaptic terminal.

Visual information travels in several parallel streams, just as somatosensory information travels rostrally through the spinal cord and brainstem in multiple parallel pathways. In the case of the visual system, the axons of several anatomically and functionally distinct classes of ganglion cells share the same optic nerve in their course toward the brain. In the primate visual system, approximately 80% of all ganglion cells form a single class of small cells that are particularly responsive to the colors of visual objects and to details of their shapes. Some general aspects of the distinctive connections of this and other ganglion cell classes are mentioned later in this chapter.

The Retina Is Regionally Specialized

Cross sections through the retina do not have the same appearance at all locations. For example, no photoreceptors, interneurons, or ganglion cells are present at the optic disk, where the axons of ganglion cells leave the eye to form the optic nerve (Fig. 17-9). These axons originate near the vitreous, so they must turn posteriorly and traverse the retina before passing through the sclera. Because there are no photoreceptors at the optic disk, we are blind to any object whose image falls on this part of the retina. Although the blind spot can be demonstrated easily (Fig. 17-10), we have no awareness as we walk around a blank spot in visual space. One might think this is because the left eye can see the part of the visual field that falls on the right eye’s blind spot, and vice versa (see Fig. 17-32). This cannot be the explanation, though, because we are unaware of the blind spot even with one eye closed. The real reason is that the nervous system simply “fills it in.” We are actually quite skillful at this, and patients with damage to their visual systems can become blind in surprisingly large areas of their visual fields without being aware of it.

Beginning near the lateral edge of the optic disk is a circular portion of the retina, about 5 mm in diameter, in which many of the cells contain a blue-absorbing pigment. This gives the area a yellowish color (Fig. 17-8) when examined with appropriate illumination and has led to its being called the macula lutea (Latin for “yellow spot”), usually shortened to macula. In the center of the macula is a depression about 1.5 mm in diameter, called the fovea, which is particularly rich in cones. In the central part of the fovea is a pit, only about 350 μm across, that contains only elongated cones (no rods) and is directly in line with the visual axis (Fig. 17-11). The central fovea is specialized for vision of the highest acuity; all the neurons and capillaries that are present elsewhere (and that light would otherwise traverse before reaching the receptors) are collected around the edges of the fovea. Specialized interneurons called midget bipolar cells receive their inputs from individual foveal cones. These bipolars in turn contact individual midget ganglion cells, so that an anatomical basis for highly detailed foveal vision is maintained.*

The fovea is one extreme in a changing rod-cone distribution across the retina (Fig. 17-12). The packing density of cones decreases sharply outside the fovea, whereas that of rods increases, reaching a maximum just outside the macula. From here to the edge of the retina, the cone density remains at a low level, and the rod density slowly declines as well (Fig. 17-13). Given the properties of rods and cones, it follows from these distributions that the fovea is used for high-acuity color vision in reasonably bright light, whereas extrafoveal regions function at lower light levels.

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Jul 31, 2016 | Posted by in NEUROLOGY | Comments Off on The Visual System
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