 Afrin neurons transmit action potential from the periphery to the central nervous system. That is to spinal cord and the brain. These Afrin neurons have sensory receptors at their peripheral endings. Receptors as the term indicates receives information or we can say characteristics of a stimulus from the surrounding. Stimulus is a change that is detectable by the body. It can be the internal change within the body or external that is outside the body. So these receptors can respond to stimuli in the external world or in the internal world. Now the stimuli are of various types depending on the type of energy chain. For example they can be either mechanical stimulus, chemical stimulus or thermal stimulus. So that means to detect these various types of stimuli there should be various types of receptors right? Yes, body has receptors for detecting different types of energies. But once the receptors detect the stimulus it has to be transmitted to the CNS in a language which CNS understands that is action potential. So receptors act as translators converting various forms of energy of the world into electrical signals. This conversion of one form of energy into another form is known as signal transaction. So basically the receptors do two things that is they respond to a specific type of stimulus which may be either internally or externally located and convert the energy form into electrical energy telling the information to CNS about the stimuli. Now the information which a receptor provides about the stimulus is dependent on three things. Where the receptor is located, which type of stimulus it is detecting and how fast they adapt. Thus receptors are classified based on these three categories which we will discuss one by one. So first is classification of the receptors based on their location. Based on the location receptors are classified into exteroceptors, interoceptors and proprioceptors. Exteroceptors as the term indicates detect changes in the external world. For example touch pain, temperature changes while interoceptors are located inside the body and sense a change inside the body. For example chemoreceptors which detect changes in oxygen, carbon dioxide in blood and in turn effect respiration, then baroreceptors for detecting changes in blood pressure, then there are thermal receptors also inside the body which detect a core body temperature. Then third type of receptors based on the location are proprioceptors which detect the changes in the position of the body. So suppose if you do an experiment and I ask you to close your eyes and keep on hand in one particular position and then keep the other hand in the same position or with your eyes closed like this, will you be able to do that? Yes, you will be able to do that even with your eyes closed because the information about the position of your body is reaching to the central nervous system via proprioceptors which are present in joints, muscle bellies and tendons. So that means location of the receptor is important in order to provide information about the location of the stimulus, right? Second classification of the receptor is based on which type or modality of stimulus they detect. So based on this receptors are classified into mechanoreceptors, chemoreceptors, thermoreceptors, no-c-ceptors and electromagnetic receptors. The mechanoreceptors detect deformation, for examples receptors for touch, pressure, vibration and also hair cells of cochlea for auditory stimulus. Then chemoreceptors detect change in chemical energy like oxygen and carbon dioxide concentration, chemical compounds in food by smelling and tasting, thermoreceptors detect change in temperature. Electromagnetic receptors are photoreceptors that is rods and cones which detect different frequencies in electromagnetic spectrum. No-c-ceptors detect physical and chemical damage. So how come different receptors detect different energies? Well this happens because the sensory nerve endings are modified into many types. Thus it is the design of the receptor and the channels which are present on the receptor which determine to which modality of stimulus it will respond. So say suppose for mechanoreceptors whenever there is deformation these mechanosensitive channels will open cause entry of ions and cause change in potential in the receptor. Well now coming to third classification of the receptors. Third classification is based on how fast or slowly the receptors adapt. Adaptation is a decrease in the response of the receptor with time despite the stimulus being continuously present. This is important for detecting the duration of the stimulus that is how long the stimulus is staying. So we will take some example to understand all this. Basically we will take the example of mechanoreceptors on a skin. Now in the skin there are various mechanoreceptors but mechanical stimulus may be of various types that is either it may be a sustained mechanical stimulus which may be light or very deep pressure or maybe it comes and goes very fast like this which we call vibration. So how do receptors detect all these various kinds of mechanical stimuli? Now see this diagram. This diagram is showing various receptors that are present on the skin. Now because they are located on the skin all of them are extra receptors right. However they are located in various layers of skin. See this one that is the Merkel cells are located superficially in dermis while this one Ruffiny ending is deeper in dermis. And now if I ask you which of these receptors was made responsible for detecting light deformation that is touch and which one for greater deformation that is pressure. You can say that deeper one will be for pressure because pressure means more deformation of the skin right. Similarly the various mechanoreceptors have different rates of adaptation. The ones which adapt very slowly detect sustained pressure like this Ruffiny ending that means the stimulus is detected throughout the time it is present right and the receptor keeps on responding to the presence of the stimulus. Similarly hair follicle receptors adapt very fast that means that when the stimulus comes they will detect the presence of the stimulus but with time they will stop responding to the presence of the stimulus right. So that is adaptation and that is why we feel the touch at the beginning of the stimulus and then it is gone that is why you do not feel your clothes whole day ok. Now also appreciate the difference between Miesner's corpuscles versus Prasenian corpuscles. Both these receptors adapt very fast thus they detect vibration sense that is only coming and going of the stimulus but Miesner's corpuscles respond to slow vibration while Prasenian corpuscles respond to fast vibration because Prasenian corpuscles in comparison to Miesner's corpuscles adapt very very fast plus Prasenian corpuscles are located deep so they respond to deep pressure vibration while Miesner's has to be superficial because it is located superficially so it detects something like a flutter on the skin. Also Miesner's corpuscles detects a sensation from small superficial area see the size of the Miesner's corpuscles compared to Prasenian corpuscles it is small right. So only stimulus in this area will stimulate Miesner's corpuscles while Prasenian corpuscles respond to the stimulus even when it is present here, here or here that is from a little larger area. This is known as receptive field of the receptor so the receptor response only when the stimulus is in its receptive field well there is more to receptive field which we will discuss in other videos of sensory system. So anyways this is a great example to understand how location of the receptor, design of the receptor and adaptation rate of the receptor tell us about the characteristics of our stimulus. Okay thanks for watching the video hopefully we answered some of the questions which we raised in the beginning. If you like the video do not forget to subscribe to the channel Physiology Open, thank you.