How is an itch relieved by scratching? I understand the evolutionary reason for the physical response, but what's the mechanism by which an itch is relieved by scratching?
I suspect that there is more involved than just this, but one explanation is as follows. There are two types of pain pathways to the brain (actually more, but two is good for this discussion). One is acute pain. Acute pain sends its signal to the brain via the nerve pathway, but it also sends a collateral message to an inhibitory neuron which silences the pain signal after a brief period of time. Thus, you have a short, stabbing pain and then the pain ends. The second type of pain is a chronic persistent pain. This pain also sends a collateral to an inhibitory neuron, but instead of stimulating it... it inhibits it. Thus, inhibiting the inhibitor and the pain continues. Now. When you apply acute pain stimulus to an area that is experiencing persistent pain, then the acute pathway fires up and also fires up the inhibitory neuron which cancels out the pain for a brief period. Thus. Scratching the itch... causes a brief cessation of the chronic itch. The same thing occurs in throbbing muscles. When your muscles throb and you squeeze them or massage them violently, part of what is going on is this firing up of the acute pain pathway to stimulate the inhibitory neurons to silence the persistent pain. Weird, eh?
good explanation. i always find it annoying and rather humorous when i have an itch i can't find. assuming neural impulses getting crossed somewhere. but i'll be scratching my leg for an hour, it won't go away and then i find the actual itch is on my pinky toe or something. it's odd. it makes me laugh.
Pain is not well understood. Usually certain nerves are involved more than others, but pain like all other "qualias" such as "tired," "hungry," "redness" etc. are mysteries of perception. I hope you do not really think there is "an inhibitory neuron" somewhere. What if it were to die? Would you be in pain for the rest of your life? By "silences the pain signal" do you think the neural stimlation coming to the brain from the site of the acute pain stops? Where in body is your "pain signal" when brick falls on big toe? If it is "moving" from toe, where does it terminate? Why does anticipated injection hurt more than accidental pin stick even if the perferial nerves are stimulated identically? Conversely, why is pain often non existent until the wound is noticed? For example, leg cut and bleeding in a car accident with no leg pain until you notice the blood wetting your pants? Was sight of blood necessary to send the "pain signal" towards the brain? (Many times a child riding bike will fall, skin a knee, get up, resume riding and later start to cry when the wound is seen, so the "distraction" of the fall from bike is over before the crying starts.) Pain is even very difficult to measure and this is big problem for companies trying to show their new pain killer is better than an old one. (Like all qualias, pain is subjective.) SUMMARY: Pain is much more complex than your simple theory would suggest.
Not an inhibitory neuron. But several. This is the way the nervous system is set up. With loopbacks and inhibitory circuits. My explanation was taken directly from a lecture by Robert Sapolsky, a professor of biological sciences at Stanford University. He's also Professor of Neurology and Neurosurgery at the same university. So. Take it up with him, I guess. I'm sure he'd say the same thing that I did and do. That the explanation given is a simplified one. But stands. I'm not entirely certain where the inhibitory signal takes place. If it is in the brain, spinal cord, or close to the site of sensation... That's something to look into. At the exact moment the brick falls on your toe? Nowhere. But, after the pain sensors have a chance to respond, then it is travelling up the axon to the brain. Where else would it be? Hmm. Mechanics. And I'm not entirely sure. However. What do you mean by terminate? I would think that the signal from the pain sensor travels to the spinal cord and releases its message there. That's usually the way it works, I believe. Then, there is usually a loopback circuit there so that the limb can be pulled back instinctively without having to travel all the way up the brain and back. So. It would go from pain sensor to spinal cord. There it would split. One signal going back to the limb for automatic reaction while another signal is sent up the spinal cord to the brain. It would most likely then end in the thalamus of the brain. Then get redirected to the somatosensory cortex for one. But I imagine that it would also get sent to a few limbic structures as well. Also. I believe that the cerebellum actually gets sensory information that bypasses the thalamus altogether... But. What's your point exactly? Why go into all the nitty gritty details? The general explanation was good enough, yes? Especially with the caveat that there is more to it than that... Well. One mechanism I can think of right off-hand is that anticipated stimulation is fed through a feedback loop which amplifies the signal. Hell. These feedback loops can even make a nonexistant signal seem to be real... And then when the needle actually does its job, the guy's like... "Oh. That was it?" Hmm. Strange nervous system you have if you have too see the wound to feel pain. Leprosy or anything? I will say that it usually takes a moment for the pain to hit. Why? Several reasons, I suppose. Number one would be the suddeness of certain types of injury. The pain receptors require a moment to get their shit together? Probably a lot more to it than that though. I'll also say that oftentimes the pain seems worse when the wound is noticed. But, that would be that feedback loop. Wouldn't it? When the pain is coming in strictly from the pain receptors, that's one modality. Much of which is processed subconsciously. But, when it is then processed visually (or even with the other senses) then the modality increases and the feedback possibilities skyrocket. The consciousness plays tricks with the signal and la la la. Shock? Massive injury overriding the nervous system's ability to cope? Endorphins? Adrenaline? Hell. Lots of reasons. Why, exactly, do you put quotes around pain signal? Do you think that pain and other nervous system events are transmitted in some other fashion? Hmm. However they'd be transmitted they'd still be a 'signal'. Of some sort. Would you prefer that I use action potential? Nerve impulse? Sodium cascade along the axonal fiber? Hmm. Anyway. Maybe the skinned knee wasn't that big a deal then? And the child was not, in fact, in pain? But rather just scared by the thought of pain? True. But nerve pathways are not. The conscious experience provided by said pathways varies, I'm sure. But, there is some sort of... median in how the nervous system operates, yes? Of course it is. I didn't go into an explanation of all the different pain pathways and sensors. I mentioned this in my post...
Interesting! My experience confirms this. For example, an itching mosquito bite can be temporarily soothed by pressing hard or squeezing that part of the skin. Also wounds, by pressing them, they hurt less for the time being.
