EYE : Our Visual Sensory Organ

Eyes are the organs of vision. They detect light and convert it into electro-chemical impulses in neurons. The simplest photo-receptor cells in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain.

LOCATION: Our paired eyes are located in sockets of the skull called orbit.

A BRIEF ACCOUNT OF STRUCTURE AND FUNCTIONS OF THE HUMAN EYE

STRUCTURE: Spherical structure. Eye ball is composed of three layers.

PARTS OF AN EYE :

1. SCLERA: External layer is composed of dense connective tissue called sclera.

2. CORNEA: Anterior portion of sclera 

3. CHOROID: The middle layer which contains many blood vessels so looks bluish in colour and it is thin over the posterior 2/3 of the eye ball.

4. CILIARY BODY:  Choroid becomes thick in the anterior part to form the ciliary body.

5. IRIS: Pigmented and opaque structure of ciliary body and the visible coloured portion of the eye.

6. LENS: Transparent crystalline lens of the eye ball which is held by ligaments attached to the ciliary body.

7. PUPIL: The aperture surrounded by the iris in front of the lens.

* Diameter of the pupil is regulated by the muscle fibres of iris.

8. RETINA: Inner layer which contains 3 layers of the cells-from inside to outside-

ganglion cells, bipolar cells & photoreceptor cells.

9. PHOTORECEPTOR CELLS: Contains the light sensitive proteins called photopigments.

(i) RODS: Twilight (scotopic) vision. Rods contain a purplish-red protein called the rhodopsin or visual purple, which contains derivative of Vitamin A.

(a)  Red, (b)Green, (c)Blue

* Sensation of white light is produced when these cones are stimulated equally.

10. OPTIC NERVES:  It leaves the eye & the retinal blood vessels enter it at a point medial to & slightly above the posterior pole of the eye ball.

11. BLIND SPOT: The region where photo receptor cell are absent.

12. FOVEA: The central pit in the yellowish pigmented spot called macula lutea, at the posterior pole of the eye lateral to the blind spot.

The fovea is a thinned-out portion of the retina where only the cones are densely packed. The visual acuity (resolution) is the greatest.

13. AQUEOUS CHAMBER: The space between the cornea & the lens which contains a thin watery fluid called aqueous humor.

14. VITREOUS CHAMBER: The space between the lens & the retina which is filled with a transparent gel called vitreous humor.

MECHANISM OF VISION

 Light rays in visible wavelength focused on the retina through the cornea & lens generate potentials in rods and cones. The photosensitive compounds in the human eyes are composed of opsin and retinal. Light induces dissociation of the retinal from the opsin resulting in changes in the structure of the opsin. This causes membrane permeability changes. Thus, potential differences are generated in the photoreceptor cells which produce a signal that generates action potential in the ganglion cells through the bipolar cells. These action potentials are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analyzed and the image formed on the retina is recognized based on earlier memory and experience.

DISORDERS OF EYE

MYOPIA

HYPERMETROPIA

CATARACT