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Chapter 15: The Visual System

INTRODUCTION

The visual system conveys more information to the brain than any other afferent system. This information is processed within the brain so as to form a set of maps of the visual world. Humans are “Visual animals” and relatively large proportion of human brain tissue is devoted to vision. The visual system includes the eye and retina, the optic nerves, and the visual pathways within the brain, where multiple visual centers process information about different aspects (shape and form, color, motion) of visual stimuli.

THE EYE

The functions (and clinical correlations) of the cranial nerves (III, IV, VI) involved in moving the eyes have been discussed in Chapter 8, along with the gaze centers and pupillary reflexes. The vestibulo-ocular reflex is briefly explained in Chapter 17. This chapter discusses the form, function, and lesions of the optic system from the retina to the cerebrum.

Anatomy and Physiology

The optical components of the eye are the cornea, the pupillary opening of the iris, the lens, and the retina (Fig 15–1). Light passes through the first four components, the anterior chamber, and the vitreous to reach the retina; the point of fixation (direction of gaze) normally lines up with the fovea. The retina (which develops as a portion of the brain itself, and is considered by some neuroscientists to be a specialized part of the brain, located within the eye) transforms light into electrical impulses (Fig 15–2).

FIGURE 15–1

Horizontal section of the left eye; representation of the visual field at the level of the retina. The focus of the point of fixation is the fovea, the physiologic blind spot on the optic disk, the temporal (lateral) half of the visual field on the nasal side of the retina, and the nasal (medial) half of the visual field on the temporal side of the retina. (Reproduced with permission from Simon RP, Aminoff MJ, Greenberg DA: Clinical Neurology, 4th ed. New York, NY: Appleton & Lange; 1999)

An illustration shows structures in a horizontal section of the left eye and visual field at the level of the retina. Illustration shows the following structures: Optic nerve; Optic disk; Fovea; Nasal retina; and Temporal retina. Point of fixation is a straight line which extends through the midpoint of cornea and lens, and reaches the retina. Nasal visual field is an oblique line transecting the point of fixation. It extends from the temporal retina. Temporal visual field is an oblique line transecting the point of fixation, and perpendicular to nasal visual field. It extends from the nasal retina. Physiologic blind spot is a narrow zone extending from the fovea and transecting the point of fixation. The closest line is at 13 degrees, while farthest line in physiologic blind spot is at 18 degrees.
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FIGURE 15–2

Section of the retina of a monkey. Light enters from the bottom and traverses the following layers: internal limiting membrane (ILM), ganglion cell layer (G), internal plexiform layer (IP), internal nuclear layer (IN) (bipolar neurons), external plexiform layer (EP), external nuclear layer (EN) (nuclei of rods and cones), external limiting membrane (ELM), inner segments of rods (IS) (narrow lines) and cones (triangular dark structures), outer segments of rods and cones (OS), retinal pigment epithelium (RP), and choroid (C). ×655.

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The retina, organized into 10 layers, contains two types of photoreceptors (rods and cones) and four types of neurons (bipolar cells, ganglion cells, horizontal cells, and amacrine cells) (Figs ...

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