Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Sensory systems
Visual sensory system

The peripheral part of the visual sensory system is represented by receptors located in the retina of the eye. However, before studying The Structure of the retina, let us examine the Anatomy of the Eyeball itself.

External appearance of the eye. The eyeball is located in the Orbit of the Skull. In children, it is spherical; in adults, its anteroposterior dimension slightly exceeds its transverse and vertical dimensions, measuring approximately 24 mm. The anterior and posterior poles of the eye are distinguished (Atl. Fig. 142). The line connecting both poles of the eyeball is called its axis. The Optic nerve enters the eyeball slightly medial to its posterior pole.

The eyeball is surrounded by three tunics: the outer fibrous tunic, the middle vascular tunic, and the inner nervous tunic (retina) (Atl. Figs. 142, 148). In the center of the eyeball lies the core, which consists of the lens, the vitreous body, and the aqueous humor—these are the refractive media of the eye. In front of the lens lies the anterior chamber of the eye, which is also filled with fluid.

Tunics of the eyeball. The fibrous tunic (tunica fibrosa bulbi) is the outermost and toughest layer, which helps the eyeball maintain its shape. It consists of two parts. The anterior part, occupying 1/5 of its surface, is formed by the transparent, highly curved cornea, which has light-refracting properties; the posterior part is the white of the eye—the sclera, which resembles the white of a boiled chicken egg in color.

The cornea (cornea) consists mainly of Cytology/practical/45.html">Dense Connective Tissue (the transparent substantia propria of the cornea). Anteriorly, it is covered by stratified squamous non-keratinized epithelium, and posteriorly, from the side of the anterior chamber of the eye, it is lined with a single-layer epithelium—the endothelium. Irritation of the nerve endings that permeate the outer corneal epithelium causes reflex blinking and lacrimation. There are no Blood Vessels in the cornea.

The sclera (sclera) covers the posterior, larger part of the eyeball. It is also formed of dense connective tissue but is opaque due to A large number of Collagen and elastic fibers and a slightly different COMPOSITION OF THE Extracellular matrix. Anteriorly, the sclera transitions into the cornea (Atl. Fig. 143). The boundary between them is a thin, translucent ring—the corneal limbus (border). At the junction of the cornea and sclera runs the venous sinus, through which venous BLOOD AND Lymph drain from the eye. Here, the corneal epithelium transitions into the conjunctiva, which lines the anterior part of the sclera. In the posterior part of the eye, where the optic nerve fibers exit, numerous perforations are formed in the sclera (the cribriform plate). Along its edges, the sclera is thickest and merges with the connective tissue Sheath of the nerve. Thickening of the sclera is also observed anterior to the equator of the eyeball, where the four rectus Muscles of the eye attach to it. Blood vessels pass through the sclera to the choroid and ciliary body.

The vascular tunic (tunica vasculosa bulbi) consists of three parts that differ in Structure and function: the choroid proper, the ciliary body, and the iris.

The choroid proper (chorioidea) is loosely attached to the sclera, with lymphatic spaces located between them. The tunic is thin (up to 0.2 mm) and consists of three layers (laminae). The outermost layer—the suprachoroid lamina—is formed by endothelium and elastic fibers connected to the sclera, with numerous pigment Cells and contacting nerve fibers situated between them. The vascular lamina occupies the middle part of the tunic. It contains large vessels, predominantly Veins, interspersed with connective tissue fibers and pigment cells. In the Deep Layer of the choroid—the choriocapillary lamina—lie large sinusoidal capillaries. Their network is particularly well developed in the area of the macula lutea of the retina (Atl. Fig. 144). The STRUCTURE OF THE capillaries allows blood to pass rapidly from the arterial to the venous bed. At the boundary with the retina lies a semipermeable basement membrane (vitreous membrane, Bruch's membrane), which contains elastic fibers.

At the equator, the choroid is pierced by four veins exiting at equal distances from one another (Atl. Fig. 148). Anteriorly, it transitions into the ciliary body without any sharp boundaries.

The ciliary body (corpus ciliare), which looks like a circular ridge, projects into the eyeball where the sclera transitions into the cornea (Atl. Fig. 143). The posterior margin of the body merges with the choroid proper, while up to 70 ciliary processes extend from its anterior margin. Thin, elastic fibers originate from these processes and attach at their other end to the lens capsule along its equator. These fibers form the suspensory apparatus of the lens, or the ciliary zonule (zonule of Zinn). Within it, between the fibers, remains a space surrounding the lens equator and containing aqueous humor. In the connective tissue stroma of the ciliary body, In addition to blood vessels, there are smooth Muscle fibers—meridional, radial, and circular—which make up the ciliary muscle that provides accommodation.

The iris (iris) is a disc-shaped structure with a central opening—the pupil—and is located behind the transparent cornea. With its outer margin, the iris transitions into the ciliary body, while its inner, free margin borders the pupil. Its connective tissue stroma contains blood vessels, pigment, and smooth muscle. Eye color, ranging from light blue to black, depends on the amount and depth of the pigment. The reddish tint of the eyes of albinos, who completely lack pigment, is caused by blood vessels showing through. The muscle fibers of the iris run in two directions. Radially arranged fibers form the dilator pupillae muscle, while circular fibers around the pupillary margin of the iris form the sphincter pupillae muscle. These muscles allow the iris to act as a Diaphragm that regulates The amount of light entering the eye (Atl. Fig. 149).

The retina (retina) is the innermost tunic of the eyeball. Its outer surface lies adjacent to the choroid, and its inner surface faces the vitreous body. Three parts are distinguished in the retina, of which the posterior, larger part—the optic part—is photosensitive and contains receptor cells. At the level of the posterior border of the ciliary body, it transitions into the ciliary part along an uneven line—the ora serrata. The anterior part of the retina—the iridial part—lines the iris. The latter two parts are insensitive to light.

The optic part of the retina has a complex microscopic structure consisting of 10 layers (Atl. Figs. 145, 146, 147). The outermost layer, adjacent to the choroid, is the pigment epithelium. Directly behind it lies the neuroepithelial layer containing receptor cells. Due to the shape of their outer segments, these cells are called rods and cones. Their peripheral processes, which form the Second layer of the retina, project into the pigment epithelium layer. The number of receptors in the human eye is immense (about 130 million rods and 6–7 million cones). Cones are 'color' receptors and predominate in the central part of the retina; rods provide twilight (scotopic) vision and are located in its peripheral parts. The central processes of the visual receptor cells come into contact with bipolar and horizontal cells, which, in turn, synapse with ganglion cells. The axons of the latter form the optic nerve. There are no blood vessels in the receptor Cell layer; nutrients enter here from the choriocapillary lamina of the choroid.

Thus, in the retina, the receptor cells are located in the outermost layer. The light flux passes through the vitreous body and reaches the deep layers of the retina. To reach the rods and cones, light must travel through the entire thickness of the retina to the pigment layer.

A quantitative Assessment of the cellular composition of the retina showed that the number of cells in its various layers is unequal. It decreases in The sequence of receptor cells — bipolar cells — ganglion cells. This indicates that afferent impulses from several photoreceptor cells converge on a single bipolar cell, and those from several bipolar cells converge on a single ganglion cell. Along with this, horizontal cells are present in the bipolar cell layer, forming synaptic contacts with receptor and bipolar cells, while amacrine cells, which contact bipolar and ganglion cells, are found in the ganglion cell layer.

All the described retinal cells, except for the pigment cells, develop from the wall of the optic vesicle, i.e., they are analogous to Brain Neurons. In addition to them, glial cells, known as radial (Müller) cells, also develop in the retina. These are long, narrow cells whose nuclei are located approximately at the level of the nuclei of bipolar cells. Radial glial cells contact the rods and cones and have A large accumulation of filamentous substance in this part of the retina. Previously, this was considered a membrane and was called the outer limiting membrane. Microvilli on the apical part of the glial cells penetrate between the receptor cells.

According to light Microscopy studies, 10 layers (zones) have been identified in the retina (Atl. Fig. 145).

Layer 1 is formed by pigment epithelial cells.

Layer 2 consists of the photosensitive processes of rods and cones.

Layer 3 is the outer limiting membrane, formed by the processes of glial cells (see p. 244).

Layer 4 is the outer nuclear layer, formed by the nucleated PARTS OF THE receptor cells.

Layer 5 is the outer plexiform layer, formed by the axons of receptor cells and the processes of bipolar and horizontal cells, which form synaptic contacts with one another.

Layer 6 is the inner nuclear layer, consisting of the nucleated parts of bipolar, horizontal, and glial cells.

Layer 7 is the inner plexiform layer, formed by the axons of bipolar cells and the processes of ganglion cells.

Layer 8 is the ganglion cell layer, formed by their cell bodies. Along its outer margin are located amacrine cells and the Blood vessels of the retina.

Layer 9 is the nerve fiber layer, consisting of ganglion cell axons that reach the inner part of the retina, turn at a right angle, and run parallel to its inner surface toward the optic disc. These fibers lack a myelin sheath and Schwann cells, which contributes to the transparency of this layer. Blood vessels and glial cells are also located here.

Layer 10 is the inner limiting membrane, formed by the processes of glial cells and their basement membrane.

Two regions stand out in the posterior part of the retina: the optic disc and the macula lutea. The optic disc is the exit point of the optic nerve from the eyeball; here, the retina contains no photoreceptors. At the disc, the supplying artery enters the retina and the vein exits. Both vessels run inside the optic nerve. The macula lutea is located almost exactly at the posterior pole of the eye; it is the most light-sensitive area of the retina due to its high concentration of cones. The center of the macula dips into the fovea centralis. The line connecting the center of the anterior pole of the eye with the fovea centralis is called the optic axis of the eye. For optimal vision, the eye aligns so that the object being viewed and the fovea centralis lie on the same axis.

Optic nerve fibers become myelinated only after passing through the lamina cribrosa. Consequently, the diameter of the nerve increases.

The refractive media of the eyeball. The lens (lens) is a dense, biconvex, lentil-shaped body (Atl. Fig. 142). Its rounded margin is called the equator. The lens is devoid of Vessels and nerves, completely transparent, and enclosed in a structureless transparent capsule. The posterior surface of the lens is embedded in the vitreous body behind it, while the anterior surface lies against the iris. The lens is suspended by the ciliary zonule. When the muscle fibers of the ciliary body contract, the tension on the zonule relaxes, and the lens, freed from the restricting pressure of its capsule, becomes more convex. This increases its refractive power. The change in the curvature of the lens allows the eye to focus clearly on objects at various distances, a process known as accommodation (Atl. Fig. 150).

The lens is the most powerful refractive medium of the eye (refractive index of 1.43). With age, it becomes denser and flatter, and accommodation weakens.

The vitreous body (corpus vitreum) fills the entire space in the eye between the retina posteriorly and the lens anteriorly. It is closely applied to the retina, helping to keep its pigment and outer layers in place, and assists in stabilizing the lens. The vitreous body consists of a transparent, gelatinous extracellular matrix and is avascular. Its refractive index is 1.33.

Aqueous humor is secreted by the blood Vessels of the ciliary processes and the iris. It fills the ocular cavities: the anterior chamber of the eye, located between the cornea and the iris, and the posterior chamber, between the iris and the lens with its zonule. These two chambers communicate through the pupil, and the aqueous humor bathes the iris, part of the ciliary body, and the lens. Aqueous humor has very low refractive power. Its outflow occurs through the scleral venous sinus.

Accessory apparatus of the eye. The Accessory structures of The Organ of vision include the eyelids, the lacrimal gland, the muscles of the eyeball, the orbital fat body, and the fascia (Atl. Fig. 151)).

The upper and lower eyelids, bounding the palpebral fissure and completing the orbit anteriorly, provide mobile protection for the eyeball. The structural basis of the eyelids is the tarsal plate of dense Fibrous connective tissue, which is permeated by uniquely modified Sebaceous Glands. The latter open onto the free margin of the eyelids and secrete a fatty, whitish substance. Superficial to the plate lies the palpebral part of the orbicularis oculi, a muscle of facial expression. Near the free margin of the eyelids are the follicles of the eyelashes. The inner surface of the eyelids is lined with a membrane called the conjunctiva, which reflects onto the eyeball, covering its exposed surface. The conjunctiva bounds the conjunctival sac, which contains lacrimal fluid that bathes the exposed surface of THE EYE AND possesses bactericidal properties.

At the medial angle of the eye, a space called the lacrimal lake is formed between the margins of the eyelids; at its bottom lies a small elevation, the lacrimal caruncle. On the margin of both eyelids in this area, There is a small opening called the lacrimal punctum, which marks the beginning of the lacrimal canaliculus. Lateral to the lacrimal caruncle, the conjunctiva forms a vertical fold, which is a vestige of the nictitating membrane found in lower vertebrates.

The lacrimal gland is located in the superolateral part of the orbit, in the lacrimal fossa of the Frontal bone. The gland's excretory ducts (10 to 12 in number) open into the lateral part of the conjunctival sac. Lacrimal fluid prevents the cornea from drying out and washes away dust particles. From the conjunctival sac, the lacrimal fluid partially evaporates and partially drains through the lacrimal canaliculi. Beginning at the lacrimal puncta at the medial angle of the eye, the canaliculi run beneath the Skin of the eyelids to the lacrimal sac, located on the medial wall of the orbit, and empty into it. The lacrimal sac, narrowing inferiorly, becomes the nasolacrimal duct, which lies in the nasolacrimal canal and opens into the inferior nasal meatus.

The eyeball is moved by six extraocular muscles: four rectus muscles and two oblique muscles.

The rectus muscles of the eye originate from the common tendinous ring surrounding the optic canal of the skull. These muscles insert into the eyeball anterior to its equator on four sides—lateral, medial, superior, and inferior—after which they are named. Due to their position, these muscles rotate the eyeball around its vertical and frontal axes. However, only the lateral and medial recti turn the eye directly outward and inward; the superior and inferior recti move it not only upward and downward, but also slightly inward.

The superior oblique muscle also originates from the aforementioned tendinous ring, runs toward the medial angle of the orbit, and passes through the trochlea of the frontal bone. The muscle then changes direction, approaching the superolateral aspect of the eyeball behind the equator at an acute angle, where it inserts. Upon contraction, the muscle rotates the eyeball so that the pupil is directed downward and outward.

The inferior oblique muscle originates from the orbital surface of the Maxilla, runs across the orbit, passing beneath the eyeball, and inserts into its lateral surface behind the equator. This muscle directs the pupil upward and outward, rotating the eye, like the superior oblique, around the sagittal axis.

The orbital fat body fills the space between the walls of the orbit and the eyeball with its muscles. It forms a soft and elastic cushion for the eyeball.

The fascia separates the orbital fat body from the eyeball; a slit-like space remains between them, which ensures the mobility of the eyeball.

The conducting and central Pathways of the visual sensory system. The conducting pathway begins in the retina (Fig. 3.62; Atl. Fig. 150). The axons of its ganglion cells form the optic nerves, which enter the cranial cavity through the optic canals and form the optic chiasm. In lower vertebrates (amphibians, reptiles), all optic nerve fibers decussate, so the Movements of the left and right eyes are independent, their fields of vision are separate, and vision is monocular. In monkeys and humans, about half of the optic nerve fibers decussate. This ensures coordinated movements of the eyeballs and binocular vision. Beyond the chiasm, each nerve is referred to as the optic tract. Each optic tract contains fibers from the lateral half of the ipsilateral retina and the medial half of the contralateral retina. The tract winds around the cerebral peduncle and divides into two roots. One of these terminates in the superior colliculus. Its fibers project to the lower effector nuclei of the Brainstem (such as those of the oculomotor and other nerves) as well as to the motor neurons of the Spinal Cord (tectospinal tract). This mediates reflex responses to visual stimuli (e.g., involuntary HEAD and Eye Movements). Neurons of the superior colliculus are involved in the perception of moving objects. There is an orderly PROJECTION OF THE retina (retinotopy) On the surface of the colliculus. Motor neurons responsible for eye movement in a specific direction lie in the deep layers of the Gray matter. Both types of neurons (superficial sensory and deep motor) are interconnected. Centrally located neurons receive projections from the auditory system and somatosensory inputs from the upper half of the body (head, upper limbs).

The other ROOT projects to the pulvinar of the thalamus and the lateral geniculate body. In primates, the neurons of the lateral geniculate body are grouped into 6 layers (Fig. 3.63). Each layer receives axons from the retina of only one eye. Furthermore, each region of the retina projects onto a specific group of neurons. The largest number of neurons receive information from the central retina, including the macula lutea. Thus, a topographic Organization of projections from different retinal zones is evident here. Neurons of the lateral geniculate body also exhibit color sensitivity. In the pulvinar and the lateral geniculate body, visual impulses are relayed to the next neuron, whose fibers travel within the optic radiation to the occipital cortex of the cerebral hemispheres.

The central projection field of the visual system is area 17.

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Fig. 3.63. Lateral geniculate body:

A — histological section; B — diagram (after Szentágothai, 1973); I — medial side; II — ventral side; III — dorsal side; 1—6 — neuronal layers

From it, association fibers project to areas 18 and 19, which contain the secondary and tertiary visual projection areas, respectively. A portion of the optic radiation fibers also projects to these areas. Area 17 is also connected to areas 21 and 7, as well as to the superior colliculus, the pretectal area, the pulvinar of the thalamus, and the lateral geniculate body, while area 19 is connected to areas 17, 18, 21, and 7. Efferent fibers from area 18 project to the pretectal area and the pulvinar of the thalamus. Fibers of varying thickness originating from different subcortical structures terminate in different cortical layers. A clear topographic organization is also observed here; the central part of the retina has a more extensive projection. Electrophysiologically, it has been established that 84% of visual cortex neurons respond to simultaneous stimulation of both retinas, meaning they are responsible for binocular vision (Fig. 3.64).

Neurons of the visual cortex differ not only in their ability to respond to monocular or binocular stimulation, but also in their response to object motion, size, etc.; cortical neurons that form vertical connections with one another are organized into columns. Adjacent columns responding to stimulation of one eye or the other were termed ocular dominance columns (Fig. 3.64). Such columns have been demonstrated morphologically: the injection of labeled Amino Acids into one eye revealed intricately intersecting bands on horizontal (tangential) cortical sections (Fig. 3.64, B, C). The dark bands in this photograph correspond to the localization of the labeled amino acid (3H-Proline) injected into one eye. The amino acid was transported from the retina to cortical neurons via axoplasmic transport along the processes of visual pathway neurons. The laminar ORGANIZATION OF THE lateral geniculate body was demonstrated in a similar manner (Fig. 3.63).

Fig. 3.64. Organization of the visual cortex (after Hubel and Wiesel):

A — diagram of the relationship between the layers of the lateral geniculate body and cortical columns (layer IV); B — specimen and C — diagram of the arrangement of ocular dominance columns in layer IV in a section parallel to the cortical surface; 1 — layer IV of the visual cortex; 2 — ocular dominance columns; 3 — complex cells (binocular); 4 — simple cells (monocular); 5 — lateral geniculate body; L — left eye, R — right eye

The inferior temporal cortex (area 21) is involved in Visual Perception. It is associated with the differentiation of objects by shape, their categorization, and establishing the equivalence of objects projected onto different areas of the retina. The activity of neurons in this area is influenced by the amygdala and hippocampus. Area 7 is involved in the organization of spatial vision.

For a comprehensive analysis of an object (its size, distance from the eyes, etc.), sensations from retinal stimulation are supplemented by sensations from The stimulation of proprioceptors in the accommodative muscles of the ciliary body and the muscles that constrict and dilate the pupil (Atlas Fig. 149, 150).