There are conditions where the dragging of a feather across the skin, normally a soft and pleasurable experience, is felt as a painful stimulus. It is important to understand just how this process of transmitting a signal works because it is in the process of transmitting signals that provides the greatest opportunity for problems, but also for treatments. Lets start with the stimulating signal. Stimulation of nerves can occur from basically two sources, external and internal. Examples of external stimulation are obvious, the 5 senses, taste, touch, smell, sight, and hearing.
External stimulations are generally physical while internal stimulations are generally chemical.
I don't want to get into the sordid details of all these chemical reactions, but I do want to focus on one standard mechanism. Recognize that the signal must be processed inside of the initial stimulated nerve then get transmitted outside of that nerve, carried between that nerve and the next, and then passed into the next nerve where the signal is passed on again until it reaches its final destination. It has a specific receptor. Both substance P and its receptor are used by the types of nerves which are responsible for sending pain signals. I will refer to substance P as SP and the receptor for SP as SPR from now on.