[Mathematical model of two pathways of light perception by the photoreceptor cell]

On the basis of the concept postulating polyfunctional role of rhodopsin in the photoreceptor cell excitation a mathematical model of two pathways of excitation was formulated. One pathway involves the appearance of Ca2+ ions in the cell, which block the sodium channels. The second one results in cGMP splitting which also brings about the blocking of sodium channels. A change in the concentration of acting agents and development of cell excitation under illumination was calculated. The calcium branch is shown to be rapid and direct, but of a low sensitivity. The cGMP branch is a slow one but has a high intensification coefficient. The action time of the calcium branch does not depend on illumination, while the time of cGMP branch decreases with the illumination rise and is in an inverse proportion to the square root of the flash intensity. Under repeated illumination the amplitude of calcium response decreases, and its time remains constant. In cGMP branch the time of the response to the second flash decreases. The amplitude of the second response depends on the intensity of the first flash, it can be smaller or larger than the amplitude of the first response.