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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
DISORDERS OF EYE
MYOPIA
HYPERMETROPIA
CATARACT
