 Dear students, in this topic we shall discuss the properties of receptor cells. The general properties which are common to all sensory receptor cells are their selectivity, their capacity to transduce, their high sensitivity and their capacity to transmit signals. We shall discuss these properties one by one in detail. First, the selectivity of receptor cells. Receptor cells are highly selective for a specific kind of stimulus energy. Receptor cells have certain proteins which are present in their membrane and these proteins have a special affinity for a specific stimulating agent. We shall take a few examples of selectivity of receptor cells. In the first example we see that light may strike at any part of our body but only the eyes have special photoreceptors that can perceive light because these photoreceptors are activated by the stimulus of light. They have special proteins which are called rhodopsins which can transduce light energy into the energy of nerve impulses. The rest of the body cells do not have this capacity so the photoreceptors are selective for receiving the light energy in the form of photons. In the second example we see that mechanoreceptors are selective for those stimuli which cause slight distortion in the membrane. These special proteins are activated by physical stimulation. When they get physical stimulation, they get stretch or pressure or vibration as a result these proteins become active and their channels open and the eye currents flow and the signals generate. Dear students, the second property of receptor cells is the capacity of transduction. Receptor cells convert a physical or chemical stimulus energy into electrical energy of nerve impulses. This capacity is called transduction. Dear students, the third property of receptor cells is the sensitivity of receptors. Receptor cells are extremely sensitive to their stimuli. They can even receive a very weak stimulus that is very near to the threshold level minimum level of energy which is detectable. It is because they can amplify the received signal during the process of transduction. Dear students, we can see from some examples that how sensitive receptors are. For example, photoreceptors can be activated by a single photon of energy. Similarly, the mechanoreceptors can be activated by displacement of membrane for only about the distance of diameter of hydrogen atom. If we look at it in nanometer, the mechanoreceptors are activated by the displacement of 0.1 nanometer. Such small stimulus is also received. Similarly, order receptors which feel smell can detect only few molecules of an order end in the air. Dear students, the fourth property of receptor cells is the ability to transmit signals. Receptor cells are used to receive the stimulus, process it, and give it an electrical energy form. After that, they have the capacity to transmit the signal to the central nervous system so that they can generate a response. For this capacity, receptor cells are closely associated with nerve cells, or neurons' nerve endings are only present in receptor cells, that is, innervated with nerve endings. As a result, these receptor cells are able to transmit nerve impulses into the sensory systems directly.