Sense Organ Eye
It is a sensory organ of vision that resembles purely a camera. It is a globe or sphere of about 2.5 cm. in diameter with a transparent membrane in the front. It is composed of three distinct coats : Sclera, Choroid and Retina.
Cornea: is the transparent horny coat of sclera and forms a prominent spherical surface. It is circular in shape and has a small radius of curvature. It is not supplied with vessels but is well supplied with sensory nerves. Cornea acts as a transparent window and protects the inner structures and also helps in focussing the image on the retina.
Choroid : It is the middle vascular coat enriched with blood vessels. It has pigmented layer forming inside of eye a dark chamber. In the front part of the eye, it forms the 'iris' which has a central opening called pupil. Attached to the choroid layer and suspended by suspensory ligaments is the 'lens' for focussing the light rays on the retina.
Iris : It is a muscular curtain situated in the front of the lens. It helps to protect the retina. It controls the amount of light entering in the human eye. A powerful light can damage the sensitive structures and therefore, iris reflexly contracts in response to bright light. The iris is in continuation with the ciliary body and the choroid. Pigmented cells are freely scattered on the iris. The pigmented layer of the iris determines the colour of the eye, say blue, brown or grey eyed person. In the centre, there is a circular aperture known as 'pupil'. The pupil is not a structure but a hole in the iris. Iris contains both the circular and radiating muscle fibres. Contraction of the circular muscle fibres (sphinctre pupillae) constrict the pupil and contraction of radial muscle (dilator pupillae) dilates the pupil of the eye. Circular muscle is innervated by parasympathetic fibres and radial muscle is innervated by sympathetic fibres. Thus, stimulation of parasympathetic fibres causes constriction of the pupil (miosis) whereas, stimulation of sympathetic fibres causes dilatation of pupil (mydriasis).
Lens : It is a biconvex transparent body made up of a number of layers. It is situated behind the iris and is attached to the ciliary body by means of suspensory ligaments. The lens is a capsular structure that recoils when suspensory ligaments are slackened and flattens when it is taut. Lens being both transparent and elastic, helps in focussing the clear image on the retina for both near and distant objects by changing its covexity.
Lens and iris alongwith suspensory ligaments and ciliary body divide the entire eye ball into two parts: Aqueous chamber and Vitreous chamber. Aqueous chamber is again divided by the iris and pupil into two chambers: anterior chamber and posterior chamber. The aqueous chamber consists of a watery fluid called aqueous humor. This fluid gives an intraocular pressure that can be measured by tonometer. This fluid from anterior chamber is drained out into the blood stream through the channel of Schlem.
The Vitreous chamber is the space between the lens and the retina. It is filled with jelly like albuminous fluid that keeps retina in contact with choroid and sclera.
Retina : It is a very delicate, transparent and the inner most nervous coat of the eye. In the transverse axis of centre of pupil, there is very sensitive part of retina called macula or yellow spot. It contains the photoreceptors, called cones. Rest of the retina contains photoreceptors called rods. Rods give a purpulish tint due to the presence of a coloring matter known as 'rhodopsin'. In the centre of retina there is a depression known as 'Fovea-centralis'. This part is full of cones and surrounding this is macula or yellow spot. The cones contain the pigment iodopsin. The point in retina from where the optic nerve leaves the eye ball is called blind spot. This spot is about 3.5 mm medial to macula. Histologically, retina is composed of nerve cells, rods and cones arranged in various layers. Outer most layer is pigment layer containing the pigment melanin. This is followed by layer of rods and cones. Inner to rods and cones, there is a layer of nuclei of rods and cones, and a layer of bipolar neurons. They make synapses with horizontal cells and nuclei of rods and cones. Lastly, there is an internal layer of cells (ganglion cells) the axons of which form optic nerve fibres.
Rods and cones are the photoreceptor cells of retina. Each retina contains approximately 6 million cones and 120 millions rods.
The rods are meant for twilight and night vision, i.e.dim vision. They are absent in the fovea but are found scattered in the remaining part of the retina and are concentrated on the periphery. Cones concentrate in the centre of the retina. The spot is called fovea centralis through which one has critical vision. They visualise the objects in their natural colour and concentrate in the centre.
Rods contain the light-sensitive pigment called rhodopsin whereas, cones contain the photochemical called iodopsin. Rhodopsin is made up of protein scotopsin and the carotenoid pigment retinal (derived from retinal or vit. A).lodopsin is made up of the protein photopsin. Structurally rods are elongated cylindrical structures about 40-60u long and 2u in dianmeter whereas, cones are pyramid shaped 28-84u long and 2.5u in diameter. Each cone is connected with its own nerve cell whereas, 10-14 rods are connected to one nerve cell.
Cones are also concerned with color vision. There are three different types of photochemicals present in different cones, thus making these cones selectively sensitive to colors of blue, green and red. The color vision is thus fusion of red, blue and green. Cones are for the critical vision and rods view objects in dim vision.
The field of vision is variable in person to person and not equal on all sides. It can be measured by an instrument called perimeter. The process of measuring the field of vision is called perimetry. Retina can be viewed by an instrument called ophthalmoscope. Fundus can be seen by fundoscopy which aids in the diagnosis of various conditions.
Functions of Retina : Functions of retina are (1) to impart vision and colour vision, (2) conduction of light reflex, (3) conduction of accommodation reflex and (4) to help in maintenance of body equilibrium. Protective Mechanism of eye : It consists of orbit, eyelids and lachrymal apparatus. The orbits are bony cavities situated in the upper part of the face, one on each side of the nose. They are conical in shape. They hold the two eyeballs in position and protect them.
The eyelids are two opaque movable musculo-cutaneous curtains in the front of the eyeball. The inner surfaces of the lids are lined by a mucus membrane known as 'conjunctiva'.
The lachrymal gland secretes a saline-like fluid in front of the eye which keeps the eyes clean and moist. During emotions this saline-like fluid overflows in the form of tears.
PHYSIOLOGY OF VISION
Before light can reach to the rods and cones of the retina, it must pass through the cornea, aqueous humor, pupil, lens, and vitreous humor. The first step, in vision is retinal image formation and activation of photoreceptors. The resulting nerve impulses are then passed to the visual areas of the cerebral cortex.
Retinal Image Formation : This requires four basic processes (1) refraction of light rays, (2) accommodation of the lens. (3) constriction of pupil and (4) convergence of eyes. Light rays entering the eye from the air are refracted at the anterior surface of the cornea, posterior surface of the cornea, anterior surface of the lens and posterior surface of the lens. The degree of refraction that takes place at each surface is very precise and such that rays fall at the fovea centralis.
The lens of the eye is biconvex. Furthermore, it has the unique ability to change the focussing power of the eye by becoming moderately curved at one moment and greatly curved the next. This change in curvature of lens known as accommodation. In far vision, the ciliary muscle is relaxed and the lens is flattened. In near vision, the ciliary muscle contracts pulling the ciliary process and choroid forward towards the lens. This causes shortening, thickening and bulging of the lens and thereby increasing the curvatures. Constriction of pupil occurs in response to light reflex involving autonomic nervous system and it is purely the function of smooth muscles of iris i.e. constrictor or circular muscles of iris.
Human eyes are such that they focus on only one set of the object (single binocular vision). This type of vision is possible due to the phenomenon called convergence. Convergence refers to the medial movement of two eye-balls so that they are directed towards the object being viewed. Convergence is the function of the voluntary muscles attached to the outside of the eye-ball called the extrinsic eye muscles. These are superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique.
Stimulation of Photoreceptors : After the formation of image on the Retina, it is converted into nerve impulses. The steps involved in the generation of nerve potentials are activation of rhodopsin and/or iodopsin of rods and cones respectively. Hyperpolarization occurs as a result of activation of these pigments in response to light. These impulses are then passed through optic nerve to the thalamus. Here the fibres synapse with other neurons whose axons pass to the visual areas of the cerebral Cortex located in the occipital lobe.