In todays session about the “gate theory” I would probably start at “just under the skin” to try and summarise the sensory perception and the “receivers” of what happens over the skin.
It is a very interesting area to start with, but can be very complicated as well. I will try to make it simple and easy to visualise as much as I can. Let me mention at first the “nociceptors” which are responsible for pain. Let me then mention the rather more friendly receptors “the Meisner’s corpuscles” or otherwise “the touch receivers”; here I am talking about that kind of “gentle”, and friendly touch. These corpuscles are in places where “touch” is enjoyable, and feels rather “very nice” and “calming”. We then walk through the other receivers.. “the Pacinian Corpuscles” which are responsible for pressure feeling. There are many other receivers such as “thermo-receptors” which are specialised in receiving heat & cold. There are many other receivers as appeared on the attached figure, including Messner’s corpuscles, Ruffini endings, Merkel disks, Krause end pulps, and root hair plexuses, etc.
The talk here will have no end, and if I let myself to keep talking about under-skin receivers I may write pages and pages. I rather try hard to concentrate here and be more precise. My mission is “pain” and its receivers… at least for now !.
Let me mention here two types of “pain”: the sharp and annoying pain, and the gentle and pleasurable pain. The sharp and nasty annoying pain comes from sharp and crushing objects. That kind of pain is received by nociceptors under the skin, then the sharp message will take the “A” road in its way towards the centre. “A” fibres are specialised for that kind of pain, and they are prepared for that job judging by their size, length, and the myelinated status. They very quickly pass the pain signals to the posterior(back) horn of the spinal cord. Spinal cord local centres are not designed to deal with that kind of strong signals. It just let the signals go up on the same side to the hypothalamus, then thalamus, then the sensory neurons in the post-central gyrus in the brain.
What about the “gentle” pain, the kind of touch, tickle, gentle rub, or a smooth message?. That kind of “pleasant” pain is carried out by very small, none-myelinated fibres(B or C) to the gentle pain receivers in the back of the spinal cord, and there the “gentle pain” receivers study the nature and the sender and act accordingly. If the source of that pain is “B” fibres, then it’ll be dealt with locally in the spinal cord without any need to send it higher up. The back of the spinal cord will just liaise with the frontal motor receptors, and those motor receptors they produce an electric impulse to be sent through the motor fibres to the related muscle or muscles to simply just move the part of the body affected away from the source of pain(the source of attack) and remain calm and quiet !!.
Let me now talk about the “friendly pain”, that kind of “pleasurable pain”. That pain once received by gentle pain receptors, it’ll just go up to the back of the spinal cord via “C” fibres, and there instead of handing the pain signals to the motor nerve fibres and send them down to the appropriate muscle or muscles to act; the gentle pain receivers decide at that moment to send the signals to very specialised neurons “small generators” which will produce a kind of “inhibitory electricity” to pass it to “A” fibres and disable them temporarily for them not to pass any signals higher and instead they tickle “the joy receptors” under skin which make you then feel that kind of relieve, joy, calming… etc. Just remember that little spoiled cat in your lap when you pass your finders gently over its chest or between her frontal legs. The kitten will simple start purring !!.
Here I will stop, to leave a space for another episode of this series to just talk about the “gate theory” and its uses in relieving pain and generating joy and relaxation.