 Hello and welcome to Physiology Open. Before we discuss signal transduction by receptors, try to solve this question. You can pause the video and think about the answer. In the end, we will come back to the question again. Signal transduction is the process of converting one form of energy into other forms. Sensory receptors which are modified sensory nerve endings convert the energy of various stimuli which impinge upon them to electrical energy. This is possible because of the presence of ion channels on these receptors. When the stimulus impinges upon the receptors, these stimulus specific ion channels open and cause entry of sodium ions inside them. This leads to a change in the potential known as receptor potential or generator potential. Receptor potential is a graded potential. That is, as the stimulus intensity increases, the magnitude of change in receptor potential increases. And once it reaches a threshold, action potential is generated which is conveyed via afro-neuron to the central nervous system. Now once the action potential fires, the neuron will be refracted for some time. But as soon as the tripole rises and by that time the receptor potential is still above the threshold, another action potential will be fired. So you can understand that if stimulus intensity is more, magnitude of receptor potential is more. Hence more and more action potentials will be fired which will reach central nervous system. Thus we can directly say that as stimulus intensity increases, frequency of action potential increases. This is a mechanism by which stimulus intensity is coded. That is whenever CNS receives more action potentials from a particular sensory neuron, you will perceive it as a higher intensity of stimulus. Now we will see action potential generation in two types of receptors. That is slowly adapting receptors and fast adapting receptors, taking the example of cutaneous mechanoreceptors. So in cutaneous mechanoreceptors, mergers, zis and rafne endings are slowly adapting receptors while mesmos corpuscles and pasenium corpuscles are fast adapting receptors. And remember, adaptation means decrease in the response of the receptor with time. If we consider this diagram as a representation of the stimulus, upward limb represents start of the stimulus, the top plateau as sustenance of the stimulus and downward limb as end of the stimulus. The below diagram is showing the receptor potential and the third diagram is the action potential in the sensory neuron. In a slowly adapting receptor, the receptor potential will be sustained and hence action potentials will be continuously generated as long as the receptor potential is above the threshold. So they continuously tell CNS about the presence of stimulus. Because of the sustained response, slowly adapting receptors are also known as tonic receptors. For fast adapting receptors, we will take the example of pasenium corpuscle. So this diagram is showing a pasenium corpuscle which is a modified nerve ending with various connective tissue layers and inside there is unmyelinated nerve ending with ion channels. In between the layers, there is fluid. Now when a stimulus comes, it distorts these connective tissue layers causing the movement of fluid in between the layers which in turn deforms the membrane of nerve ending present inside these lamella. This leads to opening of ion channels and entry of sodium causing change in the receptor potential. Now, if the stimulus is sustained, what happens is that the fluid in between the layers settle down into a new position thus there is no more distortion of the membrane. Basically it appears as if there is no stimulus. Thus the receptor potential comes back to resting state and hence there is no more generation of the action potential. So despite sustained presence of a stimulus, there is no action potential generation. Only at the beginning of the stimulus, this action potential generation takes place. Now when the stimulus is removed, the lamella comes back to its original position thus disturbing the fluid which has settled down. So this disturbance again causes opening of the ion channels. This leads to generation of action potential when the stimulus is being removed. Thus fast adapting receptors detect the stimulus when it comes and then when it goes. Because of this basic response which occurs when the stimulus is coming and going, the rapidly adapting receptors are also known as basic receptors. Take one real life example. When a mother keeps her hand under her crying baby's head, they may calm down and fall asleep and then forget about the stimulus basically. But as soon as the mother removes the hand from under the baby's head, the action potential again reaches CNS and that's when baby starts crying again. So it's tricky right? That's why a mother removes her hand very gently to decrease the intensity of the stimulus and thus preventing change in the receptor potential to the threshold. Well, let's come back to our original question. So an increase in the intensity of a stimulus will lead to increase in which of the following? See here first option is correct that is magnitude of receptor potential increases. But the second option as stimulus intensity increases the magnitude of receptor potential increases and not the frequency. In contrast when magnitude of receptor potential increases the frequency of action potential increases and not the magnitude. Action potential is an all or none phenomena so its magnitude won't change if the baseline conditions are same. Okay? Thanks for watching the video. 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