Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
The Organ of Vision. Visual Analyzer
Optical System of the Eye - Pathway of the Visual Analyzer

The photoreceptive, sensitive component of the visual analyzer (the first link) consists of rods and cones located in the retina. The pathway from the cones and rods to the Cerebral Cortex represents the second link of the visual analyzer. The central (third) link is the visual cortex on the Medial surface of the occipital lobe of the cerebral hemispheres.

The Processing of visual information in the visual analyzer begins directly in the retina. The outer segments of rods and cones appear as stacked membrane discs. These discs are formed by folds of Cell/30.html">The Plasma Membrane and contain molecules of photosensitive pigments: rhodopsin in rods and iodopsin in cones.

Light entering the eye penetrates the deepest layers of the retina, where it stimulates the rod-shaped and cone-shaped neurocytes (rods and cones). The conversion of light energy into nerve impulses occurs As a result of chemical processes within the rods and cones. Under METABOLISM/18.html">The Influence of light, Chemical Reactions take place in the outer segments of the photoreceptor Cells, causing visual pigments (rhodopsin in rods, iodopsin in cones) to break down into simpler chemical substances. These substances act on the rods and cones, inducing excitation in them. After the light stimulus ceases, rhodopsin and iodopsin are resynthesized. Consequently, these chemical reactions generate a receptor potential in the photoreceptor cells, which in turn triggers a Nerve Impulse.

Rod-shaped neurocytes (rods) are unable to distinguish colors; they are used primarily in dim-light and night Vision to recognize objects by their shape and illumination. Cone-shaped neurocytes (cones) function in daylight and are essential for Color Vision. Based on structural and chemical characteristics, some cones perceive blue light, others green, and still others red—meaning specific types of cones respond to specific wavelengths of light.

The nerve impulse originating in the rods and cones is transmitted to bipolar cells located within the retina, and then to ganglion neurocytes. The axons of the ganglion cells converge at the blind spot to form the Optic nerve, which enters the cranial cavity. On the ventral surface of the Brain, the right and left optic nerves form a partial decussation (optic chiasm). At the optic chiasm, not all nerve fibers cross to the opposite side, but only those originating from the medial part of the retina. Thus, beyond the optic chiasm, the optic tract contains nerve fibers from the lateral (temporal) part of the retina of the 'same' eye and the medial (nasal) part of the retina of the contralateral eye. Further on, the nerve fibers proceed to the subcortical visual centers—the lateral geniculate body and the superior colliculi of the Midbrain roof. In these centers, impulses from the retinal ganglion cell fibers are relayed to subsequent Neurons whose projections extend to the cortical visual center—the visual cortex of the occipital lobe (projection area 17), where higher-order analysis of Visual Perception takes place. The partial decussation of the visual pathways ensures binocular vision.

Binocular, black-and-white, and color vision. Vision with both eyes (binocular vision) enables the perception of three-dimensional images of objects, their spatial depth, and distance estimation. When viewing an object, the right eye sees more of its right side, while the left eye sees more of its left side. At the same time, the brain perceives these two images as a single, stereoscopic image. Binocular vision is possible because the image forms on corresponding, matching areas of the retina in both the right and left eyes. Working together and integrating visual information, both eyes provide stereoscopic vision, which yields a more precise understanding of the shape, volume, and depth of objects.

Light adaptation of the eyes. Moving from a dark room to a brightly lit one, or vice versa, requires some time for adjustment—adaptation. Adaptation to bright light (light adaptation) occurs quickly, within 4–6 min. Adaptation to darkness is considerably slower. When moving from a brightly lit environment into the dark, adaptation takes up to 45 min or more, during which the sensitivity of rod neurocytes (rods) increases sharply.

Color vision is provided exclusively by cone neurocytes (cones). The visual centers of the brain also participate in color perception. Color vision deficiency (color blindness) occurs in approximately 8% of men and 0.5% of women. In such cases, the perception of either red, green, or blue is impaired or absent. Total color blindness (achromatopsia) is rare.