Chapter 9: Vision

OBJECTIVES

After studying this chapter, you should be able to:

• Describe the various parts of the eye and list the functions of each.

• Describe the organization of the retina.

• Explain how light rays in the environment are brought to a focus on the retina and the role of accommodation in this process.

• Define hyperopia, myopia, astigmatism, presbyopia, and strabismus.

• Describe the electrical responses produced by rods and cones, and explain how these responses are produced.

• Describe the electrical responses and function of bipolar, horizontal, amacrine, and ganglion cells.

• Trace the neural pathways that transmit visual information from the rods and cones to the visual cortex.

• Describe the responses of cells in the visual cortex and the functional organization of the dorsal and ventral pathways to the parietal cortex.

• Define and explain dark adaptation and visual acuity.

• Describe the neural pathways involved in color vision.

• Identify the muscles involved in eye movements.

• Name the four types of eye movements and the function of each.

INTRODUCTION

The eyes are complex sense organs that have evolved from primitive light-sensitive spots on the surface of invertebrates. They gather information about the environment; and the brain interprets this information to form an image of what appears within the field of vision. The eye is often compared to a camera, with the cornea acting as the lens, the pupillary diameter functioning like the aperture of the camera, and the retina serving as the film. However, the eye, especially the retina, is far more sophisticated than even the most expensive camera. Within its protective casing, each eye has a layer of photoreceptors that respond to light, a lens system that focuses the light on these receptors, and a system of nerves that conducts impulses from the receptors to the brain. A great deal of work has been done on the neurophysiology of vision; in fact it is said to be the most studied and perhaps the best understood sensory system. The way the components of the visual system operate to set up conscious visual images is the subject of this chapter.

ANATOMY OF THE EYE

The principal structures of the eye are shown in Figure 9–1. The outer protective layer of the eyeball is the sclera or the “white of the eye” through which no light can pass. It is modified anteriorly to form the transparent cornea, through which light rays enter the eye. The lateral margin of the cornea is contiguous with the conjunctiva, a clear mucous membrane that covers the sclera. Just inside the sclera is the choroid, which is a vascular layer that provides oxygen and nutrients to the structures in the eye. Lining the posterior two-thirds of the choroid is the retina, the neural tissue containing the photoreceptors.