For instance, when pressure is applied to the skin, what determines how much pressure results in nociceptor stimulation. And when a sharp object pierces the skin, why is pain, rather than simply touch, perceived? Also, what causes nociceptors to sometimes be stimulated when there is no painful stimulus present, such as during a migraine when the skin feels painful to touch and in chronic pain?
Thank you in advance :)
All of these are affected by specific characteristics of the nociceptors.
Nociceptors are only sensitive to high pressure. The low pressure stimulus would only stimulate mechanoresceptors which would not cause a perception of pain. Receptors like $TRPA1$ sense mechanical stimulus of a higher degree. $P2X/P2Y$ sense ATP which is released due to mechanical tissue injury, as mediated by high pressure or a sharp object injury. Further more, several cells die as a result of nociceptive mechanical stimuli releasing mediators the receptors to which also act as nociceptors.
Exaggerated response to a noxious stimuli is called Hyperalgesia. A sensation of pain in response to a normal stimulus is called Allodynia. Both of these are mediated by specific sensitizing receptors on the nerve endings sensitive to Bradykinin, %HT, Serotonin, Substance P etc. All these are commonly found in conditions where these two are observed, as in the case of burning on touching following sunburns, or increased pain adjacent to an inflammatory foci.
Source:- Ganong's Review of Medical Physiology
