 Dear students, in this topic we shall discuss the phenomena of range fractionation. Dear students, the sensory receptor cells in a sense organ are arranged in an order of increasing sensitivity to different intensities of the stimulus. This hierarchical arrangement of receptor cells is known as range fractionation. You know that most stimuli have a wide range of intensities. Each individual receptor cell in a sense organ is sensitive to and detects only a fraction of the total dynamic range of such intensities range. This is why it is necessary to collect information about all the intensities of the stimulus in order to create a sense of discrimination and to create a system of receptor cells in a sensory organ. Dear students, because of hierarchical arrangement, i.e. range fractionation, a very important system is produced on a practical basis called recruitment phenomena. In this recruitment phenomena, the sensory receptor cells arranged in an order of increasing sensitivity work on a special pattern. Similarly, when the intensity of the stimulus is lowest, the most sensitive receptors are activated and receive the lowest intensity of the stimulus. As the intensity of the stimulus increases, the less sensitivity receptors are activated. When the intensity is high, the most sensitive receptors are activated. When the intensity is a little high, the less sensitive receptors are activated. If the intensity increases further, the less sensitive receptors will also be activated and receive the stimulus. This level of intensity will be reached to a point where the high intensity and the sensitive receptors will receive it. At such a level, finally the system becomes saturated so that all the receptors have now become activated and are receiving the highest intensity or all intensity range of the stimulus. So, we can say that as the intensity of the stimulus is increased, less and less sensitive receptors become activated i.e. they are being recruited. This amal me shamal ho rahe hain, this ko kate hain recruitment and this phenomena is called recruitment phenomena. At this level, when all the receptors have become saturated, the system responds maximally to the stimulus. This phenomena in which the activation of receptor cells takes place in a graded fashion or stepwise fashion is called recruitment phenomena. Dear students, the range fractionation is very important phenomena. It increases the range of a multi-neuronal sensory system. It increases the range of overall whole system much more as compared to the range of a single receptor cell. It also increases the precision and correction of the sense organ. It enables the central nervous system to discriminate among different intensities of the stimulus. Dear students, we shall take an example of the photoreceptors which are the rod and cone cells. The cone cells have low sensitivity while the rod cells have high sensitivity for the stimulus of light. At low intensity, that is dimmer light, only rod cells are activated which are highly sensitive. As a result, we can see dimly, that is only black and white, a chromatic region. As the intensity of light increases, the cone cells which are less sensitive also start to become active. This is where the reception of colors begins and we are getting the color vision. As the brightness increases, more and more cone cells are activated. When more cone cells are activated, we see bright colors. After a highest limit, the increasing of intensity further does not increase our ability to distinguish more colors or see more bright. This is the level where the system has become saturated. This has occurred because of recruitment of first rod cells than the cone cells.