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Last updated: Sat, Aug 17, 2024
Now imagine that you have just twisted your ankle1. As with the cutaneous scratch, there is a quick, intense pain that is easy for you to localize. This soon subsides and is replaced by a different aching, sickening type of pain. It is steady and spreading. Your ankle will swell and become tender, then soon your lower leg will become tender and stiff. You know that this aching and tenderness will persist for days or longer. During this entire period, motions that are normally pain-free will be painful. The inflammatory response described for the scratch will occur if you have stretched your ankle ligaments to the point of cell damage (The Pain System in Acute Injury).
Experiments have demonstrated that this additional long-lasting protective phenomenon is the result of changes in the spinal cord that are caused by signals received through C fibers. This helps to explain some facts about the C fibers. They seem like poor relations of the A-delta fibers—both types transmit in response to noxious stimuli, but the C fibers are much slower than the A-deltas. Yet the C fibers are the most common sensory fibers in both cutaneous and deep tissues in all mammals. Unlike A fibers, C fibers use peptides, a class of small proteins, as neurotransmitters. The specific peptides they produce depends on the type of tissue that they innervate, meaning that they can "tell" the spinal neurons what type of tissue they provide signals from.
In a sense, your spinal cord "knows" that your ligaments are damaged. The signals it has received through the C fibers have caused changes to the spinal cord that will affect the sensations from a large part of your lower leg for an extended period.