Physiology of Night Vision

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Physiology of Night vision The vertebrate eyes have Photosensitive cells called Rods and Cones . Rods are elongated cells mainly confined in the periphery of the retina. These are meant for Dim vision in low light and for peripheral visions. Rods, are extremely light sensitive and their sensitivity is about 500 times greater than the sensitivity of cones. Only one photon is required to stimulate a rod to send a signal to the brain. Nocturnal mammals have rods with unique properties that make enhanced night vision possible. The nuclear pattern of their rods changes shortly after birth to become inverted. Inverted rods have heterochromatin in the center of their nuclei and euchromatin and other transcription factors along the border. The outer nuclear layer in nocturnal mammals is thick due to the presence of millions of rods present to process the lower light intensities of a few photons. Light is passed to each nucleus individually. Cones on the other had are pointed cells confined in the central part of the retina. These are meant for Central vision , Bright vision and Colour vision . Rods have photosensitive pigment called Rhodopsin and cones have Iodopsin. Role of Rhodopsin The molecules of Rhodopsin in the rods undergo a change in shape as light is absorbed by them. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. When exposed to light, it immediately bleaches , and it takes about 30 minutes to regenerate fully. Most of the adaptation occurs within the first five or ten minutes in the dark. Rhodopsin is less sensitive to the longer red wavelengths of light. So many people use red light to preserve night vision. Role of Tapetum Many animals have a tissue layer called the Tapetum lucidum in the back of the eye that reflects light back through the retina. This increases the amount of light entering into the retina. This is found in many nocturnal animals and some deep sea animals. This causes the phenomenon of eye shine in these animals. Tapetum lucidum is absent in human eye. .

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