 Dear students, in this topic we shall discuss the mechanism of sensory adaptation. Sensory adaptation is a decrease in the perceived sensation of a stimulus in response to a stimulus that is continuously present. This happens because of reduction in the frequency of sensory response during a continuous stimulation. Dear students, the adaptation is shown by all components of the sensory system. It is shown by the receptor cells, by the neurons and by the cells of central nervous system. This phenomena is of high significance because it allows the detection of new stimuli in the presence of an ongoing persistent stimulus. We shall take few examples of adaptation. First, adaptation to the touch stimuli. You know that our clothing also touches our skin. The stimulus of touch because of clothes is perceived but our nervous system adapts to this stimulus quickly so that we are able to detect new touch stimuli even in the presence of this continuous or persistent stimulation of our clothes. This is a very high example of adaptation phenomena. Apart from this, there are few examples that come into our common feelings. There is sense of smell adaptation. For example, we apply a perfume and after a while our nervous system adapts and we don't feel it. But the stimulus is continuously present and the perfume we applied is applied. This is another example of adaptation to a continuous stimulation. Similarly, hot and cold water. Similarly, there is an adaptation for dim and bright light. Now we shall discuss the mechanism of adaptation. The sensory components of nervous system adapt through a variety of mechanisms at all levels of sensory transduction pathway. Most of these mechanisms take place at the receptor cell level while some also take place at the neuron and central nervous system level. We shall discuss the mechanisms of adaptation at receptor cell level. The first mechanism operates at the membrane of receptor cell when the mechanical properties of membrane act as a filter because the membrane cannot pass sustained stimuli. It can pass only transient stimuli. The second mechanism operates at the receptor protein level because the transducer molecules may run down or used up due to continuous stimulation. For example, visual pigments are bleached on exposure to continuous light. The third mechanism operates at the enzymatic level when enzyme cascade may be inhibited due to accumulation of a product that is the negative feedback mechanism. The fourth mechanism in the receptor cell operates at the receptor channels because of accumulation of free calcium ions. When free calcium ions increase, the result in the closure of channels as a result more response is reduced. The fifth mechanism at the receptor level takes place at the spike initiating zone that becomes less sensitive. You know that the place for action potential to generate in neurons is the spike initiating zone. If this becomes less excitable, then the capacity to generate action potential is reduced. Dear students, except for receptor cells, the components of the central nervous system are also adapted. If the muscle cell signals go to the central nervous system from the receptor cell, then the central nervous system is adapted and they stop responding.