ANATOMY OF THE VISUAL ANATOMY OF THE VISUAL SYSTEMSYSTEM

ANATOMY OF THE VISUAL SYSTEM

The visual system has three parts:• the eye for the reception of visual

information• the visual pathway for the

transmission of visual information • the occipital cortex, the center of

the visual system.

• The eye has three layers and three chambers. Some of the structures are transparent and involved in the transmission of light.

• The three layers are:• outer layer• middle layer• inner layer.

The sclera represents the largest part (5/6) of the outer layer. It is a white, opaque capsule. strong and indistensible. The scelera becomes thin and sieve-like at the level of lamina cribrosa, where the axons of the ganglion cells exit and form the optic nerve. The extraocular muscles insert into the scleral collagen. A large number of channels penetrate the sclera:four anterior for the anterior ciliary arteries• four posterior for the vortex veins• one larger posterior channel, for the optic nerve, central retinal artery and vein.

The sclera consists of three layers:1. Episclera — a dense and vascular connective tissue that separates the conjunctiva and sclera.2. Scleral stroma — composed of bundles of collagen and elastic fibers of varying size, not as uniformly oriented as in the cornea.3. Lamina fusca — the inner scleral layer that blends with the uveal tract.

CorneaThe cornea occupies the anterior part (1/6) of the external layer. This structure is transparent, wet because of the precorneal tear film, avascular, and richly supplied by sensory nerves. The cornea appears elliptical when viewed from the front. In the adult it measures about 12 mm in the horizontal meridian and 11 nun in the vertical meridian. The radius of curvature is about 7.8 mm. The eye’s normal astigmatism is produced by the steeper vertical meridian. The peripheral cornea is thicker (1 mm) than the central cornea. The transition zone between the peripheral cornea and sciera is known as the limbus.The cornea forms a positive meniscus lens of about 45 D in air and constitutes the main refractive element of the eye. It represents the anterior border of the anterior chamber.

Anatomically, the cornea consists of the following five layers:1. Epithelium is made up of nonkeratinized stratified squamous epithelium. This layer regenerates easily.2. Bowman’s layer is an acellular structure consisting of fine collagen fibers. It does not regenerate after injury.3. Corneal stroma makes up about 90 % of corneal thickness. It is composed of collagen fibers that have uniform orientation, cells and ground substance. There are fixed cells (keratocytes, keratoblasts) and many types of leukocytes that migrate.4. Descemet’s membrane is a thin layer of collagen fibrils, more resistant than Bowman’s layer. This structure is able to regenerate.5. Endothelium is a single layer of cells with a high metabolic rate. Its major role is to control dehydration of the cornea.

The transparency of the cornea is due to:• integrity of the epithelium• uniform orientation of cornea! fibers• normal pump activity of the endothelium• absence of blood vessels.

Middle LayerThe middle layer - uvea - has three parts:• iris• ciliary body• choroid.

Iris•The iris is the most anterior extension of the uveal tract. Its distinctive color is referred as being the “color of the eye”. The mobility of the iris allows the pupil to change size and to react to light.•The iris has the shape of a diaphragm that divides the anterior part of the eye into two chambers: the anterior chamber and posterior chamber.•The anterior chamber is bordered anteriorly by the corneal endothelium and posteriorly by the iris and the anterior lens capsule.•The posterior chamber is bordered anteriorly by the iris and the ciliary processes and posteriorly by the lens.•Near the pupillary margin, the iris is more pigmented. The peripheral area, called the root of the iris, merges with the ciliary body.

The iris consists of:1. Endothelium - has the same structure as the cornea! endothelium.2. Stroma - is composed of connective tissue, vessels, nerves, pigmented and non-pigmented cells. There are two smooth muscles:• the dilator of the pupil, with radial fibers and sympathetic innervation• the sphincter of the pupil, with circular fibers and parasympathetic innervation.

Miosis is constriction of the pupil and is produced by contraction of the sphincter. Mydriasis is dilation of the pupil and is produced by contraction of the dilator.3. Epithelium - is densely pigmented and continuous with the neurosensory part of the retina.

Ciliary BodyThe ciliary body is the middle part of the uveal tract. It is 5 — 7 mm wide, appears triangular in shape and has apex, base and two surfaces:the apex is directed posteriorly toward the choroid• the base is directed anteriorly toward the root of the iris• the external surface underlies the sciera• the internal surface overlies the vitreous.The ciliary body has two major functions: aqueous humor formation and accommodation of the lens. It may also play a role in the uveo-scleral outflow of aqueous humor.The ciliary body is composed of two parts:• pars plana• pars plicata

The pars plana is a flat area extending from the ora serrata to the ciliary processes. The ora serrata marks the boundary between the retina and pars plana.The pars plicata is the anterior part of the ciliary body. It has two structures:• ciliary processes — 70 radial vascular folds that produce the aqueous humor• ciliary muscle — a smooth muscle with longitudinal, radial and circular fibers. Its function is accommodation.The zonula is the link between the lens and the ciliary muscle.

Aqueous Humor• Both anterior and posterior chambers

contain a clear aqueous humor fluid secreted into the posterior chamber by the ciliary epitelium

• It passes in front of the lens , trough the pupil into the anterior chamber and returns to the venous circulation trough the Canal of Schlemm, situated in the angle of anterior chamber

ChoroidThe choroid is the posterior part of the uveal tract. It is a thin layer, pigmented and richly vascularized. The ora serrata borders the choroid anteriorly; the posterior border is the optic nerve. This is the major nutritional reservoir of the eye, nourishing the outer retina and the posterior part of the sclera. Because of its pigmentation, it makes a sort of “dark chamber” around the receptors of light

The choroid has three layers:1. Large vessel layer2. Medium vessel layer3. Choriocapillaris (capillary lamina of the choroid).Bruch’s membrane separates the choriocapillaris and the retinal pigment epithelium. In fact these three structures form a complex unit that influences the activity of the outer retinal layers.

RetinaThe retina is the innermost layer of the eye. It is a thin, transparent structure where there are light receptors and the first elements of the visual pathway. Its color appears orange red because of the combination of the pigmentation of the choroid vessels and the pigment of the retinal receptor cells.The retina has two distinct areas:• pars optica — retina with visual function• pars coeca — retina without visual function that covers the iris and the ciliary body. The ora serrata marks the boundary between these two areas.The

The most important landmarks of the retina are the papilla and the macula lutea.The papilla (optic nerve disc) has an oval shape, it is pale compared to the rest of the retina. The vertical diameter is usually larger than the horizontal; both are about 1.5 - 1.8 mm. There is a depression in the middle, the physiologic cup, located slightly temporally and representing less than 0.3 of the entire disc diameter. The papilla is located nasally. At this level the optic nerve and the central retinal vein exit from the eye and the central retinal artery enters the eye. Retina is divided into four quadrants: superior - nasal and temporal; inferior — nasal and temporal. The size and location of retinal changes are expressed in relation to the optic nerve disc.The macula is the area of sharpest vision. It is oval, has a diameter of 2 - 3 mm and is placed at the end of the anteroposterior axis. In the center of the macula there is a depression called fovea centralis, which has a typical light reflex during ophthalmoscopy. The specific pigment and the agglomeration of cone cells give its yellow color.

Histologically, retina consists of:• outer, pigmented epithelial layer• inner, neurosensory retina.The neurosensory retina is composed of neuronal, gun! and vascular elements. The photoreceptors are highly specialized cells called rods and cones, according to the shape of their outer segment.

The rods are the photoreceptors of vision in reduced illumination (scotopic vision). There are about 130 million rods throughout the retina, except in the fovea, where there are only cones. In the periphery of the retina there are only rods. The outer segment of a rod consists of multiple laminated discs, which look like a stack of coins. The specific visual pigment of rods is called rhodopsin. The pigment absorbs light and that causes a chain of chemical reactions. As a result rhodopsin is divided into scotopsin and trans-retinal, which is a vitamin A compound. The metabolism of vitamin A is a function of the pigment epithelium. Vitamin A deficiency or some chorio-retinal diseases influence dark adaptation.The cones are the photoreceptors for daylight vision and color perception. The number of cones is less than the number of rods. In the fovea there are only cones: toward the periphery of the retina their number decreases. Each fundamental color has a specific visual pigment, for red, green and blue. The outer segments of cones contain only one type of visual pigment per cell.

VesselsThe inner and outer retina have a different vascular supply. This is not a double one each sector has to be intact in order to maintain the retina in a functional state.The complex Bruch’s membrane-choriocapillaris nourishes the outer retina (pigment cell epithelium).The inner portion of the retina (neurosensory retina) is perfused by branches of the central retinal artery. In some individuals, a cilioretinal artery contributes to the macular circulation.The central retinal artery is a branch of the ophthalmic artery. At the level of the optic disc it divides into a superior and an inferior branch. Each branch has a temporal and a nasal branch, which divide further giving small ramifications that extend up to the ora serrata. The capillaries of the retina do not anastomose, like capillaries in the heart and brain. If one vessel is occluded, that territory will suffer from ischemia.Retinal veins follow a similar course. They drain the blood to the central retinal vein, which leaves the eye at the level of the optic disc and terminates in the superior ophthalmic vein.

OPTICALLY CLEAR MEDIAThe transparent structures of the eye, which are involved in the transmission and refraction of light, are:1. Cornea2. Aqueous humor3. Lens4. Vitreous body.

The cornea has the shape of a meniscus-convergent lens and a power of +45 diopters. This is the most important component of ocular refraction.The aqueous humor has a composition similar to that of cerebrospinal fluid. It is produced by the ciliary body and has circulates within the eye. From the posterior chamber it passes through the pupil to the anterior chamber. At this level there is a permanent motion of the fluid induced by heat. The aqueous outflow toward the episcleral venous plexus is the function of some complex structures placed at the level of the irido-corneal angle. Aqueous humor contains all the nutrients for the avascular lens, cornea, and trabecular meshwork. It also removes all their waste products. In conditions such as trauma and inflammation, the protein content of aqueous humor may increase, a reaction that is very important as a mechanism in uveitis, secondary glaucoma and complicated cataracts.

The lens is a biconvex, convergent lens and has a power of +20 - 22 diopters. It lies behind the iris and in front of the vitreous body. It has some degree of elasticity and is perfectly transparent under normal conditions.The anterior part of the lens is just behind the pupil and its relation to the iris depends on mydriasis or miosis. The lens limits the anterior chamber and the posterior chamber of the eye. The position of the lens in the eye is maintained by the zonula of Zinn, which extends from the equator of the lens to the ciliary muscle. The equator is the largest diameter of the lens. It has an anterior and a posterior pole placed, as the center of the lens, on the anteroposterior axis.The lens consists of:a anterior capsule: convex, thicker than the posterior capsule• anterior and posterior cortex• nucleus• posterior capsule: concave, a very thin membrane.The nucleus of the lens has a central part, the embryonic nucleus, surrounded by the fetal nucleus and the adult nucleus. The fibers of the lens are in continuous transformation; those that start to form near the equator push the older ones toward the center.The lens has the following structure:• anterior and posterior capsule• anterior and posterior epithelium• lens fibers or cells• ground substance.

Accommodation is the process that focuses the image of a near object on the retina. During accommodation the ciliary muscle contracts, the zonula is relaxed and mainly the anterior part of the lens becomes more convex.

A cataract results from lens reaction to many kinds of injuries. Opacification of the lens is due to an increase in calcium levels, reduced potassium levels, changes in the composition of proteins, or oxidative damage.

The vitreous body occupies the major volume of the globe, behind the lens and in front of retina. There is a ligament between the posterior capsule of the lens and the vitreous, a connection that is stronger in young patients. Aging makes this link weaker and permits intracapsular extraction of the lens.The vitreous is a gel composed of a collagen framework interspersed with hyaluronic acid molecules. The components of the vitreous are water (99 %), proteins, polysaccharides, lipids, and minerals. Like the other optically clear media, the vitreous body has no blood vessels. All the chemical changes for repairing an injury need more time than for other tissues.

AQUEOUS HUMOR OUTFLOW SYSTEM

The drainage of the aqueous humor is the most important function of the structures located deep at the level of limbus. Other drainage routes, accounting for less than 10 % of total flow, are uveoscleral drainage and diffusion into the iris.The most important structures involved in this complex process are:• trabecular meshwork, consisting of perforated connective tissue sheets, arranged in a laminar pattern that result in holes of different sizes. The meshwork represents the most important obstacle to aqueous outflow.• scleral venous sinus (Schlemm’s canal) is a large venous channel similar to a lymphatic vessel.• collector channels begin at the level of the scieral venous sinus. They drain into the episderal venous plexus, formed by aqueous vessels, which are sometimes visible on biomicroscopy under the conjunctiva.