visual system (sensory system) part 1

7/27/2019 Visual System (Sensory System) Part 1

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Visual System (sensory System) Part 1

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 terminatein 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 EyeThe 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 thenreabsorbed 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


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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.


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FIGURE 16-1 Structure of the eye and retina. (A) Different components of the eye. (B) Different layers of the

human retina.
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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 i ts shape becomes thin and flat, which is suited for distant vision.

visual system (sensory system) part 1

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