Anatomy, Physiology, and Pathology of the Organs of Hearing, Vision, and Speech - Shvetsov A.G. 2006

Anatomy, Physiology, and Pathology of the Visual Analyzer
Anatomy and Physiology of the Visual Analyzer

The function of Vision is carried out by a complex system of various interconnected structures forming the visual analyzer, which consists of three divisions:

§ peripheral — receptors of the retina;

§ conducting — optic nerves that transmit excitation to the Brain;

§ central — subcortical and Brainstem centers (lateral geniculate bodies, pulvinar of the thalamus, superior colliculi of the Midbrain tectum), as well as the visual cortex in the occipital lobe of the cerebral hemispheres.

The Water/140.html">Anatomical Structure of the sensory visual system, essentially its peripheral division, is the eye—a paired, nearly spherical organ with a diameter of 24 mm and a weight of 6—8 g, located in the orbits of the Skull (Fig. 1). The eye is anchored here by four rectus and two oblique Muscles that control its movements. The shape of the eye is maintained by the hydrostatic pressure (25 mm Hg) of the aqueous humor and the vitreous body.

The human eye perceives light waves of only a specific wavelength—approximately from 380 to 770 nm. The eye's sensitivity to light varies: it increases in the dark and decreases in the light. The ability of the eye to adapt to the perception of light of varying brightness is called visual adaptation. Impairment of dark adaptation is expressed in a reduced ability to orient in space under insufficient illumination, up to the loss of mobility. This condition is called nyctalopia ("night blindness"). Nyctalopia can occur due to Vitamin A deficiency, infectious diseases, poor Nutrition, etc. Light adaptation is the adjustment of The Organ of vision to high levels of illumination, which occurs quite rapidly (50—60 sec). Thus, if a person enters a brightly lit room from the dark, temporary blindness occurs, which quickly passes. People with impaired light adaptation see better in twilight than in bright light. Light rays from the objects being viewed pass through The Optical System of the eye (cornea, lens, and vitreous body) and focus on its inner lining (retina), which is the actual visual receptor because light-sensitive Cells—photoreceptors (cones and rods)—are concentrated here.

Light perception is the most delicate function of The Organ of vision. Thanks to it, a person has The ability to distinguish light by brightness and intensity, and can see not only during the day but also at twilight. The retina consists of 10 layers, but the 2nd, 6th, and 9th layers are involved in light perception (Fig. 2).

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Fig. 1. Schematic diagram of the human Eyeball

The human retina contains approximately 5-6 million cones and 120 million rods (Fig. 3). Cones are responsible for color, daytime vision, while rods are responsible for light perception in twilight (colorless) conditions. The sensitivity of the rods depends on the concentration of rhodopsin (visual purple) within them and the neural elements of the visual analyzer.

I — pigment epithelium; II — layer of rods and cones; III — outer nuclear layer; IV — outer plexiform layer; V — horizontal Cell layer; VI — bipolar cell layer (inner nuclear); VII — amacrine cell layer; VIII — inner plexiform layer; IX — ganglion cell layer; X — Optic nerve fiber layer

Fig. 2. Diagram of The structure of the human retina

The most important and highly delicate area of the retina is the macula lutea ("yellow spot") with the fovea centralis, where the bulk of the cones is concentrated. Moving toward the periphery, the density of cones decreases, while the density of rods increases. Cones, which possess high resolving power, primarily provide daytime Color Vision and are involved in the precise perception of the shape, color, and details of an object. The macula, especially its fovea centralis, is the site of the sharpest, so-called central vision.

A — rod: 1 — outer segment; 2 — inner segment; 3 — fiber; 4 — Nucleus; 5 — synaptic terminal. B — cone: 1 — outer segment; 2 — inner segment; 3 — nucleus; 4 — fiber; 5 — pedicle

Fig. 3. Structure of a retinal rod and cone

The ability of the eye's optical system to form a sharp image on the retina is called visual acuity, which is based on the resolving power of the eye, i.e., its ability to perceive two separate points at a minimum distance between them. If rays coming from two adjacent points stimulate the same or two neighboring cones, both points are perceived as a single larger one. For them to be seen separately, there must be at least one unstimulated cone between the stimulated ones. Consequently, the maximum possible visual acuity depends on the diameter of the cones in the fovea centralis of the macula. Visual acuity varies slightly depending on the intensity of illumination. Under the same illumination, visual acuity can vary significantly. With fatigue, visual acuity decreases.

With distance from the macula, the number of cones decreases, while the number of rods increases; at the periphery of the retina, only rods are present. Rods, which have low resolving power but, at the same time, very high light sensitivity, facilitate the perception of objects in twilight or at night ("scotopic vision").

The areas of the retina surrounding the macula provide peripheral, or lateral, vision, in which the shape of an object is perceived less clearly. Therefore, while central vision enables the examination of fine details and the identification of objects, peripheral vision is an extremely important function that expands the capacity for free spatial orientation. It is determined by the field of vision, which is captured by a fixed eye at any one time. Without peripheral vision, a person is practically blind, unable to move around without assistance. With a normal field of vision, a person is able, within certain limits, to view objects and phenomena holistically, simultaneously, in their mutual connections and relationships, and to encompass distantly located objects with their gaze. The field of vision in children is somewhat smaller than in adults, which is one of the reasons for the increased frequency of traffic accidents involving children. Significant concentric narrowing of the field of vision occurs in retinitis pigmentosa and glaucoma (so-called "tunnel vision"). Changes in the field of vision associated with its partial loss in the center or at the periphery of the retina (scotomas) also occur. The presence of small scotomas in the field of vision leads to the appearance of shadows, spots, circles, ovals, and arcs, complicating the perception of objects and making reading and writing difficult. The latter becomes impossible with extensive bilateral scotomas.

The Optical System of the eye. In addition to the receptor apparatus located in the retina, the eye includes an optical system which, by focusing light rays, ensures the creation of a sharp image on the retina of objects located both near and far from the eye. This ability of the eye is called accommodation.

The optical System of the eye consists of the cornea, the lens, and the vitreous body, but the accommodative function of the eye depends mainly on the cornea and the lens.

From an object located more than six meters away, practically parallel rays of light enter the eye, whereas rays coming from closer objects diverge noticeably. In both cases, for the light to focus on the retina, it must be refracted (i.e., its path bent), and for near objects, the refraction must be stronger. A normal eye is capable of precisely focusing light from objects located at a distance from 25 cm to infinity. Refraction of light occurs when it passes from one medium to another with a different refractive index, specifically at the air-cornea interface and at The surface of the lens.

The cornea—the anterior part of the sclera of the eye—is a spherically shaped, avascular, highly sensitive, transparent, optically homogeneous membrane with a smooth, mirror-like, shiny surface. The shape of the cornea cannot change, so refraction here depends only on the angle of incidence of light on the cornea, which, in turn, depends on the distance of the object. The strongest refraction of light occurs in the cornea, while the function of the lens consists of the final "fine-focusing."

The lens is a transparent, elastic structure shaped like a biconvex lens. The lens is covered by a vitreous, structureless, transparent, very dense, and highly refractive capsule, along the entire edge of which thin but highly elastic fibers (zonules of Zinn) extend to the ciliary Muscle of the ciliary body. They are tightly tensioned and keep the lens in a stretched (flattened) state, but when viewing near objects, the tension of the zonules of Zinn decreases, the tension of the capsule relaxes, and the lens, due to its elasticity, becomes more convex. Its refractive power increases, resulting in accommodation of the eye for near vision. When looking into the distance, the increased tension of the zonules of Zinn leads to the opposite effect: the lens becomes flatter, and its refractive power becomes minimal.

The lens of young people consists predominantly of soluble Proteins, but after the age of 20, the protein COMPOSITION OF THE lens gradually changes: the proportion of its insoluble fractions increases while the soluble ones decrease. As a result, a dense nucleus forms within the lens, which enlarges even further with age, causing the lens to lose almost all of its elasticity. The permeability of the lens capsule gradually declines, which impairs its nutrient supply and leads to clouding (senile cataract), with all the subsequent consequences for light transmission and the accommodative function of the eye.

The cavity of the eye behind the lens is filled with a transparent, amorphous, jelly-like substance called the vitreous body, which occupies the space between the retina and the lens. The vitreous body contains up to 98% water and a negligible amount of protein and salts. It has no Blood Vessels or nerves, but it gives shape and turgidity to the eyeball and serves as an important component of the eye's optical system; in pathological conditions, it becomes cloudy. All three structures refract light rays in such a way that a reduced and inverted image of visible objects is formed on the retina, but this does not interfere with their correct perception, as the key lies not in the spatial position of the image on the retina, but in its interpretation by the brain.

The refractive power of the eye at rest, which ensures the focusing of the image on the retina, is called refraction. Refraction can be:

1. Proportionate (normal) — emmetropia.

2. Disproportionate:

§ farsightedness (hypermetropia) — is a consequence of a short longitudinal axis of the eye. It can be associated either with an irregular shape of the eye (shortened eyeball) or with an abnormal curvature of the lens. In these cases, the image is focused behind the retina. To shift the image onto the retina, a farsighted person must increase their refractive power by increasing the curvature of the lens. Glasses with biconvex lenses are required;

§ nearsightedness (myopia) — in this case, parallel rays coming from distant objects intersect in front of the retina, without reaching it. This is due to an excessively long longitudinal axis of the eye, or a greater than normal refractive power of the eye (lens). To see clearly into the distance, a nearsighted person must use biconcave lenses, which reduce the refractive power of the lens and thereby shift the image back onto the retina.

§ astigmatism — is caused by pathological changes in the cornea, which loses its sphericity in certain areas; as a result, different Regions of the cornea have different refractive powers, and optical lenses with a single degree of curvature cannot provide the necessary focusing of the image on the retina.

Color vision. In the visible part of the spectrum, the human eye absorbs light of all wavelengths, perceiving them as seven colors (represented by the mnemonic "Richard — red, Of — orange, York — yellow, Gave — green, Battle — blue, In — indigo, Vain — violet"), each of which corresponds to a specific region of the solar spectrum. The ability of the human eye to distinguish A large number of color shades (up to several thousand) is achieved due to the presence of Three types of cones in the retina—"red", "green", and "blue"—which contain different pigments and, according to electrophysiological studies, absorb light of different wavelengths. Color vision is explained by the trichromatic theory, according to which the perception of various colors and shades is determined by the degree of stimulation of each cone type by light reflected from an object. Thus, for example, equal stimulation of all cones produces the sensation of white. The color mixing effect underlies color television, photography, and painting.

The extreme periphery of the retina perceives only white; approaching the center is accompanied by the sensation of blue, then yellow, then red, while green is perceived predominantly by the macula lutea. Primary color differentiation occurs in the retina, but the final perceived color is determined by the integrative Functions of the brain.

An important condition for normal vision is the interaction of both eyes, i.e., the ability to see with both eyes simultaneously while perceiving the observed object as a single whole. This visual ability is called binocular vision. It allows for a three-dimensional image of objects and the determination of their relative distance from the observer. Stereoscopic vision, i.e., the perception of an object's shape, begins to develop at 5 months, and by 9 months, a child acquires the capacity for stereoscopic spatial perception, distinguishing depth and the distance of objects. However, the complete development of binocular vision is not finalized until 7–15 years of age.

Finally, an important characteristic of human vision is its stereoscopic nature. Two separate flat images received by the right and left eyes 'fuse' into one in the cortical visual center, forming a stereoscopic image.

The Mechanism of Visual Perception. Light hitting the photoreceptors triggers a conformational change in the visual pigments they contain: the visual pigment of the rods, rhodopsin, decomposes into retinal (a derivative of vitamin A) and the protein opsin. Retinal, subsequently converted into vitamin A, is used to regulate the permeability of The Cell membranes of retinal pigment cells; however, to ensure night vision, the reverse resynthesis of vitamin A and opsin back into rhodopsin is required. If vitamin A is deficient, night blindness ('nyctalopia') develops. Instead of rhodopsin, cones contain iodopsin, which differs slightly in structure from rhodopsin and does not require vitamin A to perform its visual function.

During the conformational change of visual pigments, nerve impulses are generated and transmitted to the subsequent retinal Neurons (bipolar and ganglion cells) and then into the optic nerve, which originates from the ganglion cells. The area of the retina where the optic nerve exits lacks both cones and rods and is therefore incapable of perceiving light. It is called the 'blind spot'.

Exiting the Orbit through the cribriform plate of the sclera and the optic canal, the fibers of the optic nerve (the conducting pathway of the visual analyzer) HEAD toward the brain (Fig. 4).

Fig. 4. Visual analyzer

After entering the cranial cavity, the optic NERVES OF THE right and left eyes form a partial decussation (optic chiasm) at the Base of the brain, in the region of the sella turcica; only the fibers originating from the nasal (medial) halves of the retina decussate, while the fibers from the temporal (lateral) halves of the retina do not. Beyond the chiasm, they form the optic tracts.

Thus, the right optic tract contains fibers from the temporal half of the retina of the right eye and the nasal half of the left eye, while the left optic tract contains, conversely, the uncrossed fibers of the temporal half of the left eye and the crossed fibers of the nasal half of the right eye (Fig. 5).

Within the optic tracts, the nerve fibers reach the subcortical visual centers in the lateral geniculate bodies, the superior colliculi of the corpora quadrigemina, the thalamus, and the Hypothalamus. This marks the end of the peripheral part of the visual analyzer.

The central part of the visual analyzer begins at the axons of the subcortical visual centers, where visual impulses are relayed to the Pathways of the brain, through which they reach the Cerebral Cortex in the occipital lobe. The cortical visual centers comprise Brodmann areas 17, 18, and 19 of the cerebral cortex (Fig. 5).

Fig. 5. Cortical representation of the visual analyzer (Brodmann areas 17-19)

a — lateral surface of the occipital lobe of the cerebral hemispheres; b — Cytology/practical/54.html">Longitudinal section of the occipital lobe of the cerebral hemispheres

In this context, the central core of the cortical end of the visual analyzer—the organ of higher analysis and synthesis of visual stimuli that forms the visual image—is Brodmann area 17, while areas 18 and 19 are associative. Damage to cortical area 17 can result in cortical blindness, whereas lesions in areas 18 and 19 impair spatial orientation.

Oculomotor mechanisms of vision. Normal eye function requires mobility and the capacity for fine adjustments essential for any precision optical instrument. To obtain a clear image of the viewed object on the retina, the object must lie on the visual axis of the eye, which passes through the center of the lens and the macula lutea of the retina.

Proper alignment of the visual axes is achieved through:

§ body movements and head rotation — coarse adjustment;

§ extraocular muscle movements — fine adjustment;

§ lens accommodation — ultra-fine adjustment, regulated by the CNS and facilitated by the ciliary muscle of the eye;

§ convergence — The process of aligning the visual axes until they intersect on the object being viewed, i.e., at the point of fixation. This is driven by the contraction of the rectus Muscles of the eye. Convergence disorders lead to binocular vision anomalies, primarily associated with The Development of strabismus or nystagmus.