 Hello and welcome to Physiology Open. Think about this question. Can you imagine which sensation you will perceive? If hypothetically we replace the cutaneous mechanoreceptors with photoreceptors and somehow make the photons reach to the receptors, you can pause the video and think about the answer. We will come back to the question at the end of the video again. Okay, so here we are seeing a diagram which is showing two different nerve fibers, one in red color and one in blue color carrying information from one area of the skin. Red pathway is for touch sensation while blue pathway is for pain sensation. Now if you trace the pathways, each of these paths end at a specific point in the somatosensory cortex. This also applies to other sensations. The receptors at the peripheral end of these pathways are mechanoreceptor in touch pathway and nosisceptor in pain pathway. So these receptors convert only their specific stimulus modality to electrical stimulus. But it is not only the conversion of one modality into electrical energy, but also the entire path which is important called perception. Now the doctrine of specific nerve energy says modality specific receptors transmit the information through nerves to a modality specific area of the sensory cortex. So there is a dedicated line which belongs to a particular sensation modality. This is also known as labelled line principle. Basically the modality is the label and the connection is the line. That means no matter how this red pathway is stimulated always you will feel touch sensation. If blue pathway is stimulated you will always feel pain sensation. You can think of this as electrical connections in your home. Each electrical line is connected to a switch at the end and line goes and ends in a specific electrical equipment, say different bulbs. If the switch is stimulated always that bulb which is connected to that line will work. Now if an electrician removes a switch and directly stimulates the wire then also that bulb will light. So point is no matter how the line is stimulated you will always feel that sensation only. It gives us an insight how modality of stimulus is coded. Labelled line principle also helps us understand about how brain gets information about the location of a stimulus. Whenever a line is stimulated the brain projects as if the stimulus is coming from the area of the body from where the line started. This has been confirmed in experiments where cortical regions are stimulated and the person feels sensations at the site from where that line is coming from. Despite there being no stimulus externally this is known as law of projection. That is no matter where a particular line of the sensory pathways is stimulated along its course to cortex, conscious sensation is perceived as if it is coming from the original location of the receptor. It is an offshoot of labelled line principle only. Another implication of the same is the phenomena of phantom limb. This phenomena occurs in patients with limb amputation. If after the limb is amputated and the left over nervous thumb gets irritated the action potentials reach to the brain. So despite the limb not being there the brain projects as if the stimulus is coming from the original area of the body from where the line started. This is strange but this is how brain works. Now let's come back to our original question. So if we replace mechanoreceptors with the photoreceptors what sensation will we perceive? So in this case basically we have changed a switch which now does not respond to your pressing the switch instead it responds to maybe remote signals. But the line is the same. So the same bulb will light. So here even the photoreceptors convert energy of photons into potential but the line is of that sensation. And any action potential which reaches brain via this line it perceives it as touch. So basically you will start touching the energy of photons which you normally perceive as light. Amazing isn't it? Okay I'll give you one another twist. Let's keep the mechanoreceptors intact and instead change the connection and connect it to visual cortex. It's like keeping the switch intact instead you are connecting the line to another bulb. So in this case the receptors will detect mechanical stimulus coming in contact with the skin. However since it is going to the visual cortex you will start seeing that mechanical energy. I know it's difficult to comprehend but this is what CNS is all about. Okay thanks for watching the video. If you liked it do not forget to subscribe to the channel Physiology Open. Thank you.