Question

Explain the mechanism of generation of light-induced impulse in the retina.

Answer 1

Light induces structural changes in retinal molecules present in the retina. This generates a cascade of biochemical reactions leading to conversion and transmission of the light ray as electrical impulses to the brain where the information is further interpreted.

Complete answer:
In the eye, light enters through the pupil. It is focused by the lens on the retina where it generates an electric impulse. Retina contains the photoreceptor cells which convert light energy to electrical impulses by activating a series of light sensitive cells called rods and cones. They help in the sensation of vision. These impulses are then carried to the brain through the optic nerve. The photopigment (photosensitive compound) present in the retina, rhodopsin, is composed of retinal and opsin.

Light induces structural changes (isomerization) in opsin molecules by dissociating retinal from opsin. Due to such structural changes, the permeability of the cell membrane changes resulting in closing of sodium ion channels of the cell membrane. This further leads to production of an action potential due to potential differences in the photoreceptor cells. Due to the signal produced, an action potential is generated in the ganglion cells through the bipolar cells. These impulses are then transmitted by the optic nerve to the visual area of the cortex in the brain. The neural impulses are then analysed and interpreted by the brain. The image formed on the retina is then recognised.

Note: Cone cells are responsible for colour information and sharpness of the image while rod cells highly sensitive detectors of white light help in night vision. The number of rod cells present in the retina is much higher than that of cone cells.