Biology THE EYE

### KEY TOPICS

star Parts of an Eye
star Types of photoreceptors
star Mechanism of Vision

### PARTS OF AN EYE

● color{violet}("Our paired eyes") are located in sockets of the color{violet}("skull") called color{brown}("orbits.")

● The color{violet}("adult human eye ball") is nearly a color{brown}("spherical") structure.

● The color{violet}("wall of the eye ball") is composed of three layers.

● The color{violet}("external layer") is composed of a color{violet}("dense connective tissue") and is called the color{brown}("sclera. ")

● The color{violet}("anterior portion") of this layer is called the color{brown}("cornea. ")

● The middle layer, color{brown}("choroid,") contains many color{violet}("blood vessels") and looks color{violet}("bluish in colour. ")

● The color{violet}("choroid layer") is thin over the posterior two-thirds of the color{violet}("eye ball,") but it becomes thick in the anterior part to form the color{brown}("ciliary body.")

● The color{violet}("ciliary body") itself continues forward to form a pigmented and opaque structure called color{violet}("the iris") which is the visible coloured portion of the color{violet}("eye.")

● The color{violet}("eye ball") contains a transparent color{brown}("crystalline lens") which is held in place by ligaments attached to the color{violet}("ciliary body.")

● In front of the color{violet}("lens"), the aperture surrounded by the iris is called the color{brown}("pupil.")

● The diameter of the color{violet}("pupil") is regulated by the color{violet}("muscle fibres") of iris.

● The inner layer is the color{brown}("retina") and it contains three layers of cells – from inside to outside – color{violet}("ganglion cells,")

### TYPES OF PHOTORECEPTORS

● There are two types of color{violet}("photoreceptor cells"), namely, color{brown}("rods") and color{brown}("cones.")

● These color{violet}("cells") contain the color{violet}("light-sensitive proteins") called thecolor{brown}(" photopigments.")

● The daylight (color{brown}("photopic")) vision and colour vision are functions of cones and the twilight (color{brown}("scotopic")) vision is the color{violet}("function of the rods.")

● The color{violet}("rods") contain a color{violet}("purplish-red protein") called the color{brown}("rhodopsin or visual purple,") which contains a derivative of color{brown}("Vitamin A.")

● In the color{violet}("human eye,") there are three types of cones which possess their own characteristic color{violet}("photopigments") that respond to red, green and blue lights.

● The sensations of different colours are produced by various combinations of these color{violet}("cones") and their color{violet}("photopigments.")

● When these color{violet}("cones") are stimulated equally, a color{violet}("sensation of white light") is produced.

● The color{brown}("optic nerves") leave the color{violet}("eye") and the color{violet}("retinal blood vessels") enter it at a point medial to and slightly above the color{violet}("posterior pole") of the color{violet}("eye ball.")

● color{violet}("Photoreceptor cells") are not present in that region and hence it is called the color{brown}("blind spot.")

● At the color{violet}("posterior pole") of the color{violet}("eye lateral") to the color{violet}("blind spot,") there is a yellowish pigmented spot called color{violet}("macula lutea") with a central pit called the color{brown}("fovea.")

● The color{violet}("fovea") is a thinned-out portion of the color{violet}("retina") where only the color{violet}("cones") are densely packed.

● It is the point where the color{brown}("visual acuit (resolution)") is the greatest.

● The space between the color{violet}("cornea") and the color{violet}("lens") is called the color{brown}("aqueous chamber") and contains a thin watery fluid called color{violet}("aqueous humor.")

● The space between the color{violet}("lens") and the color{violet}("retina") is called the color{brown}("vitreous chamber") and is filled with a transparent gel called color{violet}("vitreous humor.")

### MECHANISM OF VISION

● The light rays in color{brown}("visible wavelength") focussed on the color{violet}("retina") through the cornea and lens generate color{violet}("potentials (impulses)") in rods and cones.

● As mentioned earlier, the color{brown}("photosensitive compounds (photopigments)") in the human eyes is composed of color{brown}("opsin") (a protein) and color{brown}("retinal") (an aldehyde of vitamin A).

● color{violet}("Light induces") dissociation of the color{brown}("retinal from opsin") resulting in changes in the structure of the color{violet}("opsin.")

● This color{violet}("causes membrane") permeability changes.

● As a result, color{violet}("potential differences") are generated in the color{violet}("photoreceptor cells.")

● This produces a color{violet}("signal") that generates action color{violet}("potentials") in the color{brown}("ganglion") cells through the color{violet}("bipolar cells.")

● These color{violet}("action potentials") (impulses) are transmitted by the color{violet}("optic nerves") to the color{brown}("visual cortex") area of the color{violet}("brain,") where the color{violet}("neural impulses") are analysed and the image formed on the color{violet}("retina") is recognised based on earlier memory and experience.