Function of the parts of human eye
- Refracts and focuses light.
- Regulates light entrance.
- Admits light.
- Absorbs extra light.
- Refracts light.
- Refracts light.
- Ciliary body:
- Holds lens in place.
- Contains receptors.
- Allow black and white vision.
- Allow colour vision.
- Optic nerve:
- Transmits impulse.
- Fovea centralis:
- Region of cones in retina.
Working of eye
1.Mechanism of light perception
The human eye has two functional parts- Dioptric or focussing part and Receptor part.
It consits of conjunctiva,cornea,aqueous humour,lens and vitreous humour. These part are transparent and act as lenses.They refract the light rays passing through the eye to bring them to a focus on the retina.Maximum refraction is caused by the cornea,which places the image approximately on the retina.The lens effects fine adjustment and brings the image into a sharp focus.
3. Receptor part
It comprises the retina.The image formed on the retina is inverted and smaller. It converts the energy of specific wave lengths of light into action potential in nerve fibre.
4.Pathway of sensory impulses from eye to brain.
Biochemistry of eye
The receptor cell of eye are called photoreceptor or visual cell. They are two types__
The rod cells contain a purplish pigment called visual purple or rhodopsin. They function in dim light and at night.They produce poorly defined images.Bright light splits rhodopsin into a lipoprotein scotopsin and a carotenoid pigment retinal(retinine)a process called bleaching.The splitting of rhodopsin depolarizes the rod cell and it releases a neurotrasmitter,passing the nerve impulse via bipolar neuron and ganglion cells to the optic nerve. In the dark ,rhodospin is resynthesized from scotopsin and retinal.This process is called 'dark adaptatioin'. It makes the rods functional.It takes sometime for rhodopsin to be reformed. This is why on entering a dark room at daytime or on coming out of a well lighted room at night we feel blind for a while,when we go from darkness into bright light, we feel difficulty in seeing properly for a moment till rhodopsin is bleached and cones become functional.
Cones contain iodopsin which is visual violet and made up of photopsin +retinal. The 3 types of cones are erythrolabe(755 nm sensitive to red).cyanolabe (430nm sensitive to blue) and chlorolabe ( sensitive to green 535 nm). However ,if all the cone,types are simultaneously stimulated by equal amounts of coloured light than sensation for white light is perceived.
Diurnal animals are adapted to see during day light(photopic vision) and can perceive colour.In dark, colours are not perceived.Such animals have more cones in their eye than rods.
Differences between Rod cells and Cone cells
Rods secreted by rod cells. Produce "Rhodopsin" which is visual purple and made up of scotopsin+11 cis retinal. Vitamin-A is needed for the formation of Rhodopsin. Rhodopsin is very sensitive to light. Rods are active in dim light or low intensity light. Rod cell are absent in fovea centralis of retina. Rods are more in number in peripheral region of retina. Rods are more in nocturnal animals. In owl only rods are present and cones are absent. 120 million rods in human.