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DIGITAL TEXT ANJANA S NAIR B.Ed. NATURAL SCIENCE St THOMAS TRAINING COLLEGE THIRUVANANTHAPURAM 1|Page
STRUCTURE OF HUMAN EYE
Which part of your body lets you read the back of a cereal box, check out a rainbow, and see a softball heading your way? Which part lets you cry when you're sad and makes tears to protect itself? Which part has muscles that adjust to let you focus on things that are close up or far away? If you guessed the eye, you're right
Did you know? Our eyes are sophisticated cameras; they take many pictures in seconds and the brain processes them as messages. Through our eyes, we see the colourful world around us
How many of your family members use spectacles? Let us study how the human eye uses light and enables us to see objects. Eyes are the most valuable organ of the human body, which process the images of objects . in the vicinity. The eyes interpret size, shape, colour and distance of the objects and give a 3D picture of the objects visible.
Human eye is a highly specialized sense organ consisting of an almost spherical eye ball accommodated in the orbit of the skull. The movement of the eye ball is controlled by a set of six extrinsic muscles. The exposed part of the eye ball is protected by movable upper and lower eyelids. The eyelids possess hairs known as eye lashes. The eyeball is kept moist by the secretion of the lacrimal or tear glands. The excess of secretion is passed out by nasolacrimal duct that opens in to the nasal chamber.
The eyeball is composed of three layers. The outermost layer of the eye ball is called the sclera. It is made of an opaque dense fibrous connective tissue,
Which forms the protective layer of the eye. The projected transparent anterior part of the sclera is cornea which refracts light rays to focus on the retina. The conjunctiva layer which covers and protects the front part of sclera except to the cornea. The middle layer of the eye ball is called the choroid. It contains a large number of blood vessels. The part of the choroid seen behind the cornea is iris. Presence of the pigment melanin gives the iris a dark colour. The aperture seen at the centre of the iris is called pupil. The size of this aperture increases and decreases depending on the intensity of light. Just behind the iris is the transparent crystalline lens. Elastic transparent convex lens, connected to ciliary muscles by thread like suspensory ligaments. Ciliary muscles are the circular muscles seen around the lens. The contraction and relaxation of these muscles alter the curvature of the lens. The innermost layer of the eye ball is the light sensitive retina. The retina has a complex structure with two layers- an outer pigment layer and an inner nervous layer. The pigment layer consists of the pigmented epithelium. The nervous layer consists of the light sensitive cells called rods and cones and the other interconnecting neurons arranged in several layers. The receptor cells in the retina are called the rods and cones because of their morphology. Rod cells are more in number than cone cells. Rod cells contain the visual pigment called rhodopsin. This pigments is formed from a protein named opsin and retinal
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which is a derivative of vitamin a. Since they are activated even in dim light, we are able to see objects in dim light. These cells cannot detect primary colours of light- red, green and blue. This diversity is due to the difference in amino acids in the opsin molecule. So, cone cells provide us with colour vision. The rods and cones are not uniformly distributed in the retina.
Near the geometric center of the retina is a slight notch called fovea. This region consist only of cones and therefore when lights falls on this region maximum visual sensation occur. The area where the nerve fibres converge to form the optic nerve has no rods or cones. The images that fall in this small notch like area cannot be perceived. Therefore this area is called blind spot. Aqueous humour is the water like fluid filled in the aqueous chamber between the lens and the cornea. It is formed from blood, and is reabsorbed into blood. Provides oxygen and nourishment to the tissues of the eye. Vitreous humour is the jelly like substance seen in the vitreous chamber between the retina and the lens. Helps in maintaining the shape of the eye.
Formation of image The curvature of the cornea and the lens help light rays which reflect from an object to get focused on the retina. We can see nearby and distant objects clearly. This is because the focal length of the lens can be adjusted according to the distance of the object.
When looking at the object closer to the eye, the ciliary muscles contract and suspensory ligaments slack this increases the curvature of the eye lens and thereby the focal length of the eye lens decreases. This enables to see a near objects clearly.
When looking at the distant object, the ciliary muscles relax and suspensory ligament tense this decreases the curvature of the eye lens and thereby increase the focal length of the lens. This enables us to see the distant object clearly.
The ability of the eye to adjust the focal length of the lens by changing its curvature in accordance to the distance of the object from the eye and form the image on the retina is called the power of accommodation of the eye.
Organization of nervous layer of retina
The retina is the part of the eye that receives the light and converts it into chemical energy. The chemical energy activates nerves that conduct the signals out of the eye into the higher regions of the brain. The retina is a complex nervous structure, being, in essence, an outgrowth of the forebrain. In general, there are four main layers:
(1) Next to the choroid is the pigment epithelium, already mentioned. (2) Above the epithelium is the layer of rods and cones, the light-sensitive cells. The changes induced in the rods and cones by light are transmitted to (3) a layer of neurons (nerve cells) called the bipolar cells. These bipolar cells connect with (4) the innermost layer of neurons, the ganglion cells; and the transmitted messages are carried out of the eye along their projections, or axons, which constitute the optic nerve fibres.
Horizontal cells and amacrine cells, two types of retinal interneurons, modulate or control the information flow from photoreceptors to bipolar cells in the outer plexiform layer and from bipolar cells to retinal ganglion cells in the inner plexiform layer, respectively.
Pathway of light from cornea to photoreceptor cells
Light passes through the front of the eye cornea to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina).In order to reach the photoreceptors rod and cone cells in retina, incoming light must first pass through all the other layers of cells in the retina (see the bottom figure). The first of these is the ganglion cell layer, composed of the bodies of ganglion cells. Next comes the inner plexiform layer, a network of axons and dendrites from ganglion cells, bipolar cells, and amacrine cells. After that comes the inner nuclear layer, composed of the bodies of bipolar, horizontal, and amacrine cells. Next come the outer plexiform layer, composed of the nerve endings of bipolar cells, horizontal cells, and photoreceptor cells, and then the outer nuclear layer, which contains the bodies of the photoreceptor cells. Last comes the outer segment layer, containing the light-sensitive pigments, containing the light-sensitive pigments of photoreceptors cell are located.
Pathway of signals from photoreceptors cells (after receiving light) to Brain
The arrangement of the retinal cells in an orderly manner gives rise to the outer nuclear layer, containing the nuclei of the rods and cones. The plexiform layers are regions in which the neurons make interconnectios.
Thus, the outer plexiform layer contains the rod and cone projections terminating as the rod spherule and cone pedicle; these make connections with the dendritic processes of the bipolar cells, so that changes produced by light in the rods and cones are transmitted by way of these connections to the bipolar cells. (The dendritic process of a nerve cell is the projection that receives nerve impulses to the cell; the axon is the projection that carries impulses from the cell). The inner nuclear layer, containing the nuclei and main cell bodies of the bipolar cells, amacrine and horizontal cells. In the inner plexiform layer are the axons of the bipolar cells and the dendritic processes of the ganglion and amacrine cells. The association is such as to allow signals in the bipolar cells to be transmitted to the ganglion cell layer, the signals then passing out along the axons of the ganglion cells as optic nerve then signal passes to visual cortex of the brain. The visual cortex turns the signals into images (for example, our vision). Some of the eye problems 1. Myopia Otherwise known as short-sightedness, is a common eye condition that causes objects in the distance to appear blurred while close objects are often seen clearly. When someone with short-sightedness tries to look at distant objects, the rays of light are focused in front of the retina, rather than directly onto it, causing the appearance of those objects to become blurred. . This defect can be corrected by using a concave lens.
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2. Hypermetropia Sometimes called hyperopia, is the term used to define being longsighted. If you are hypermetropic, the image of a nearby object is formed behind the retina. This means that light is focused too far back in the eye, causing things which are close up to appear blurred. This defect can be corrected by using a convex lens of appropriate focal length.
3. Presbyopia It is an age-related refractive condition where the patient experiences a loss of focus for objects close by, as the lens inside the eye becomes less flexible with time, and needs reading glasses. It occurs due to hardening of the lens of the eye. Such people often require bi-focal lenses. In the bi-focal lens, the upper portion of the bifocal lens is a concave lens, used for distant vision. The lower part of the bifocal lens is a convex lens, used for reading purposes.
4. Astigmatism It is caused when the cornea or lens develops into an irregular shape. The cornea should have a regular curve, a bit like the shape of a football, but for people with astigmatism, the cornea’s curve is more like a rugby ball. This means the cornea can’t focus light rays properly and can cause blurry vision. This defect can be corrected by using eyeglasses with cylindrical lenses oriented to compensate for the irregularities in the cornea.
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5. Night Blindness The retinal, a part of the visual pigment, is derived from Vitamin A. The deficiency of Vitamin A results in the low production of retinal. This in turn prevents the resynthesis of rhodopsin. In this condition, objects cannot be seen clearly in dim light. 6. Xerophthalmia If there is a prolonged deficiency of Vitamin A, the conjunctiva and cornea become dry and opaque. This causes xerophthalmia and leads ultimately to blindness. 7. Colour Blindness The retina contains cone cells which can detect red, green and blue colours. Some persons cannot distinguish green and red colours due to the defect of cone cells. This condition is called colour blindness. 8. Glaucoma Aqueous humor is the fluid which nourishes the tissues in the eyes. If the reabsorption of aqueous humour does not occur, it causes an increase in the pressure inside the eyes. This causes damage to the retina and the photoreceptor cells and ultimately leads to blindness. It can be rectified by laser surgery.
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10. Cataract It is a condition in which the lens of the eyes become opaque resulting in blindness. This can be rectified by replacing the lens with an artificial one, through surgery. 11. Conjunctivitis This is an infection of the conjunctiva. The causative organisms may be bacteria, virus etc. This disease is transmitted through contact and can be prevented by maintaining personal hygiene. 12. Diabetic Retinopathy Diabetic Retinopathy is a disease of the retina which occurs due to diabetes. It may lead to poor vision and subsequently to complete blindness. The blood vessels in the eye become weak and blood will leak out in the centre of the eye and causes blurry vision. 13. Age related Macular Degeneration It is a physical disturbance in the centre of the retina which is known as macula. It is of two types: Dry macular degeneration and Wet macular degeneration. Causes of age related macular degeneration may be due to age, smoking, diet, exposure to sunlight, high blood pressure.
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Check Your Progress I. Fill in the blanks 1. The ______________ is the transparent front part of the eye. 2. The ______________ is a gel-like substance that helps to keep the eyeball in its proper shape. 3. The ______________is also known as the white of the eye. 4. The ___________ is the extent of the observable world. 5. ___________ vision is one in which each eye is used separately. II. Select the right answer from the following: While viewing nearby objects. (a) ciliary muscles relax (b) curvature of lens decreases (c) ciliary muscles contract (d) focal length increases
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