Retina and Vision

Two friends were walking in the forest, everything appeared bright and colourful under the morning sunlight. Now, as the evening approached everything looked black and white under the moonlight. Now, light was present at both the times, during the morning time the light source was the Sun and in the night-time the light source was the moon. But colors are visible only in bright light. Well, since the image is formed on the retina so, retina should contain sensory receptors for colors. Now, the same scenery looks different at different times. In the morning it appears bright and colourful but during the nighttime it appears black and white. So are the same sensory receptors responsible for this? No, the same receptors are not responsible. So, there has to be two different receptors for this. Now, let us see the two receptors. The first receptor is known as the cone cells, cone because the top most part of the cell is cone-shaped. Now, the cone cells are activated in bright light and it is responsible for colour vision. Now, the pigment present in the cone cells is called Iodopsin. And Iodopsin helps in colour vision. The second sensory cell is called the Rod cells, rod because this portion of the cell is rod shaped. Now, rod cells are activated in dim light and the pigment present in the rod cells is known as Rhodopsin. Rhodopsin helps in the vision in dim light. Now, different regions of the retina get activated at different times depending on whether it is daytime or if there is bright light or there is dim light. Now, see the light is focusing horizontally opposite to the pupil. So, let us see what is happening. In the retina, the distribution of rod cells and cone cells are not equal. See in this central portion there is mostly cone cells so, they get activated only when there is bright light and it also causes coloured vision, it helps in coloured vision and bright vision. This area is known as the Yellow spot. So, yellow spot is that area on the retina which has the most number of cone cells. Now, if you’ve noticed carefully, you’ll notice that this area has neither cone cells or rod cells, it does not have any sensory receptors. So, this spot is known as the Blind spot, there is no vision or no sensory cells that cause vision in the blind spot. Well, this is because all the axons of the sensory receptors present on the retina bundle here and from here it goes to the occipital lobe of the brain. So, this is the blind spot. Because from here all the axons of the sensory receptors, they bundle together and leave the eye to reach the occipital lobe of the cerebrum. Now, see the image of an object which falls on the retina is inverted but then when the optic nerves takes the visual impulses to the occipital lobe of the cerebrum it becomes erect again and that is why we see an object erect and not inverted. Now see, in a brightly lit room, you’re able to see everything clear and colourful but suddenly there is load sheding and you are unable to see anything for the first few seconds but after that you are able to see a faint outline of the various structures in the room. Now, why does this happen? This is because when there is light in the room the pupil does not remain dilated but when there is less light like in the case of a power cut, the pupil starts dilating with the reduced intensity of light. Now, thus by dilation the area through which light enters the eye, it increases and some more amount of light is now entering into the eye. Also in reduced light cone cells in the retina, they disintegrate and rod cells in the retina, they activate through rhodopsin activation and thus we are able to see in dim light as well. But this rhodopsin activation takes some time and so vision in dim light takes a few seconds to be clear.


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