Vision is one of the most important sensory functions. It serves as the basis for our perception of the outside world. For example, our ability to detect forms, images, colors, and movement of objects is derived from the functions of the visual system. The initial processing of light signals received by the photoreceptors occurs in the retina. The axons emerging from the retina terminate in a relay nucleus located in the dorsal thalamus. The neurons located in the thalamic relay nucleus, in turn, project to the visual cortex where further processing occurs for visual perception. Details of different components of the visual system are described in the following section.
Components of the Eye
The receptor organ for the visual system is the eye (Fig. 16-1A). Three layers of tissue enclose the eye. The outermost layer is called the sclera and consists of a tough white fibrous tissue. An anterior portion of the sclera, the cornea, is transparent and permits light rays to enter the eye.
The middle layer, the choroid, is highly vascularized. It is continuous with the iris and the ciliary body. The iris is the colored portion of the eye that is visible through the cornea. The iris has a central opening, which is called the pupil. The size of the pupil is neurally controlled via the circular and radial muscles of the iris.
The innermost layer of the eye is the retina. The optic nerve exits the retina at a pale circular region called the optic disc or optic nerve head (Fig. 16-1A). Blood vessels supplying the eye enter via the optic disc. Because there are no photoreceptors in the optic disc, it is called the blind spot. Near the lateral edge of the optic disc lies a circular portion that appears yellowish in appropriate illumination because of the presence of a yellow pigment in the cells located in this region. This region of the retina is called the macula lutea (or simply macula). This part of the retina is for central (as opposed to peripheral) vision. At the center of the macula lies a depression called the fovea, which contains primarily cones. The layers of cell bodies and processes that overlie the pho-toreceptors in other regions of the retina are displaced in the fovea. A small region at the center of the fovea, known as the foveola, is also devoid of blood vessels. The fovea, including the foveola, represents the region of retina with highest visual acuity because there is minimum scattering of light rays due to the absence of layers of cells and their processes and blood vessels in this region. The structure and function of photoreceptors (rods and cones) and their distribution in different regions of the retina are described later in this topic.
Different tissue layers enclosing the eye are continuous with certain structures of the eye. For example, sclera is continuous with the cornea, choroid is continuous with the iris and ciliary body, neural retina is continuous with ora serrata, and nonneural retina is continuous with epithelium of the ciliary body. Ora serrata is the serrated margin located just behind the ciliary body and represents the limits of the neural retina (photoreceptors and other cells associated with sensing and processing of light stimulus). Details of the structure of the retina and the structure and function of photoreceptors (rods and cones) and their distribution in different regions of the retina are described later in this topic.
The space between the lens and the cornea, called the anterior chamber, is filled with a watery fluid called aqueous humor. This fluid is produced continuously by the epithelial cells of the ciliary processes that constitute the vascular component of the ciliary body. The ciliary processes are located around the rim of the posterior chamber (the space between the lens and the iris). The aqueous humor flows into the anterior chamber through the pupil and provides nutrients to the lens and cornea. It is then reabsorbed through a specialized collection of cells (trabec-ulae) into the canal of Schlemm (a venous channel) that is located at the junction of iris and cornea (the anterior chamber angle). Under normal circumstances, the production and uptake of aqueous humor is in equilibrium. When this equilibrium is disrupted, there is an accumulation of the aqueous humor in the anterior and posterior chambers, and the pressure in these chambers increases. Because the posterior chamber is in contact with the vitreous body (see next paragraph), an increase in the pressure within the anterior and posterior chambers is exerted within the entire eyeball. The increase in intraocular pressure reduces blood supply to the eye, causing damage to the retina. This eye disease, known as glaucoma, is a major cause of blindness. There are two major types of glaucoma: open-angle and closed-angle (also called narrow-angle or angle-closure) glaucoma. In open-angle glaucoma, the removal of aqueous humor is decreased due to reduced permeability through the trabeculae into the canal of Schlemm. In closed-angle glaucoma, the anterior chamber angle is narrowed by the forward movement of the iris, thus obstructing the removal of the aqueous humor. Open-angle glaucoma is a chronic condition and is treated by cholinomimetic drugs (e.g., pilocarpine, applied topically) and diuretics (e.g., dorzolamide, applied topically, or aceta-zolamide, administered orally). The most popular drugs for the treatment of open-angle glaucoma are prostaglandin analogs (e.g., lantanoprost) and beta-adrenergic receptor blockers (e.g., timolol) applied topically. Prostaglandin analogs increase the outflow of the aqueous humor from the anterior chamber, whereas beta-adrenergic receptor blockers decrease the secretion of aqueous humor from the ciliary epithelium. Acute closed-angle glaucoma is associated with a painful increase in intraocular pressure, which must be treated with drugs on an emergency basis or prevented by surgical removal of the iris (iridectomy).
As mentioned earlier, a thick, gelatinous material, called vitreous body or vitreous humor, fills the space between the lens and retina. It contains phagocytes that remove blood and debris in the eye under normal circumstances. In certain situations, such as aging, the debris particles are too large to be removed by the phagocytes in the vitreous humor. These floating debris particles, called floaters, cast shadows on the retina.
FIGURE 16-1 Structure of the eye and retina. (A) Different components of the eye. (B) Different layers of the human retina.
The iris, ciliary body, and choroid constitute the uveal tract. Inflammation of these structures, which usually is secondary to an injury or infection, is called uveitis. Typical treatment consists of administration of atropine to relieve ciliary muscle spasm, which is the cause of pain in this condition. Topical application of steroids is usually effective in relieving inflammation.
Three pairs of extraocular muscles that move the eyeball within the bony orbit are attached to the sclera. The extraocular muscles are not visible normally because of the presence of conjunctiva, a membrane that folds back from the eyelids and attaches to the sclera.
Light rays pass through the cornea, lens, and anterior and posterior chambers and reach the photoreceptors (rods and cones) located in the retina. Focusing of images on the photoreceptors depends on refraction (bending) of light rays as they pass through the cornea and the lens. The change in refractive power of the lens is called accommodation. Radially arranged connective tissue bands hold the lens in place; these bands are called zonule fibers and are attached to the ciliary muscle. The ciliary muscle forms a ring. When it contracts, the zonule fibers relax, the tension on the lens is reduced, and its shape becomes rounder and thicker, which is suited for near vision. Under normal circumstances, the ciliary muscle is relaxed, the zonule fibers are stretched to exert tension on the lens, and its shape becomes thin and flat, which is suited for distant vision.
Different Layers of The Retina
The human retina consists of the following layers (Fig. 16-1B).
The Pigment Epithelium Layer
The pigment epithelium layer is the outermost layer of the retina consisting of pigmented cuboidal cells that contain melanin. The bases of these cuboidal cells are firmly attached to the choroidal layer of the eye located outside the retina. The presence of tight junctions between these cuboidal cells prevents the flow of ions and plasma. The cuboidal cells have microvilli at their apical regions, which interdigitate with photoreceptors. The pigmented epithelium cells provide nutrition (glucose and essential ions) to photoreceptors and other cells associated with them. The black pigment, melanin, absorbs any light that is not captured by the retina and prevents it from reflecting back to the retina, which would otherwise result in the degradation of the image. Thus, the pigment epithelium layer protects the photoreceptors from damaging levels of light.
The pigmented epithelium layer develops from the outer aspect of the optic cup as a component of the choroidal layer. The rest of the retina develops from the inner aspect of the optic cup, which folds inwards and becomes apposed to the pigmented epithelium. A potential space persists between the pigmented epithelium and rest of the retina. This anatomic arrangement renders the contact between the pigmented epithelium layer and the neural retina (photoreceptors and other cells associated with the sensing and processing of light stimulus) mechanically unstable. Therefore, the pigment epithelium layer sometimes detaches from the neural retina. In this condition, known as retinal detachment, the photoreceptors can be damaged because they may not receive the nutrition that is normally provided by the pigment epithelium layer. Retinal detachment is now repaired by laser surgery.
The Layer of Rods and Cones
Rods and cones are photoreceptors. The structure and function of photoreceptors are described later. The light-sensitive portions of these photoreceptors are contained in the layer of rods and cones. In most regions of the retina, the rods outnumber the cones (there are approximately 100 million rods and 5 million cones in the human retina). One exception to this rule is the region of greatest visual acuity, the fovea (a depression in the center of the macula). The fovea contains only cones. High visual acuity at the fovea, especially in the foveola, is attributed to the presence of an extremely high density of cone receptors in this region of the retina. Other anatomical features that contribute to high visual acuity at the fovea are diversion of blood vessels away from the fovea and displacement of layers of cell bodies and their processes around the fovea. These anatomical features allow minimal scattering of light rays before they strike the photoreceptors. Disorders affecting the function of fovea, which, as mentioned earlier, is a restricted region of the retina (1.2—1.4 mm in diameter), cause dramatic loss of vision (other prominent defects in vision are described later in this topic).
The External Limiting Membrane
The photosensitive processes of rods and cones pass through the external limiting membrane in order to be connected with their cell bodies. This region also contains processes of Muller cells (these cells are homologous to the glial cells of the central nervous system [CNS] and are unique to the retina).
The Outer Nuclear Layer
The cell bodies of rods and cones are located in the outer nuclear layer.
The Outer Plexiform Layer
The outer plexiform layer contains the axonal processes of rods and cones, processes of horizontal cells, and den-drites of bipolar cells. This is one of the layers where syn-aptic interaction between photoreceptors and horizontal and bipolar cells takes place.
The Inner Nuclear Layer
The inner nucleus layer contains the cell bodies of ama-crine cells, horizontal cells, and bipolar cells. Amacrine and horizontal cells, or association cells, function as interneurons. Amacrine cells are interposed between the bipolar and ganglion cells and serve as modulators of the activity of ganglion cells. The role of horizontal and bipolar cells in the processing of signals from the photorecep-tors is discussed later.
The Inner Plexiform Layer
The inner plexiform layer contains the axons of bipolar cells, processes of amacrine cells, and dendrites of ganglion cells. This is another layer where synaptic interaction between different retinal cells takes place.
The Layer of Ganglion Cells
The cell bodies of multipolar ganglion cells are located in the layer of ganglion cells. The fovea centralis of the retina has the greatest density of ganglion cells. The final output from the retina after visual stimulation is transmitted to the CNS by the ganglion cells via their axons in the optic nerve. The ganglion cells are the only retinal cells that are capable of firing action potentials.
TABLE 16-1 Comparison of Rods and Cones
|
|
Cones |
Rods |
|
Sensitivity to light stimulus |
Low |
High |
|
Photosensitive pigments |
Less abundant |
More abundant |
|
Response to light stimulus |
Fast |
Slow |
|
Specialized for |
Day vision |
Night vision |
|
Effects of damage |
Loss of cones causes decreased visual acuity (legal blindness) |
Loss of rods causes night-blindness and loss of peripheral vision |
|
Acuity of vision |
Acuity of vision mediated by cones is high |
Acuity of vision mediated by rods is low |
|
Saturation |
Saturate when light is very intense |
Saturate in day light |
|
Role in color vision |
Mediate color vision (3 types of cone cells) |
They are achromatic |
|
Concentration in fovea |
High |
Absent in fovea |
|
Relative numbers |
Less numerous than rods |
More numerous than cones (20:1) |
The Optic Nerve Layer
The optic nerve layer contains the axons of ganglion cells and processes of Muller cells. The axons of the ganglion cells travel towards the posterior pole of the eye and exit at the optic disc through the sclera to form the optic nerve.
The following layers of the retina are concerned with sensing and processing of light stimulus. They constitute the neural retina, which includes the layer of rods and cones, external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, layer of ganglion cells, and optic nerve layer.
