Provided by Walter Sipe, M.D., clinical fellow of pediatric gastroenterology, hepatology, and nutrition, University of California, San Francisco (former HHMI medical fellow)
Before we get to the science part of your question, let me start by saying: Do not put salt on your cut. The single most important aspect of wound care in the backcountry is vigorous and copious irrigation with clean water (filtered or chemically treated so it is drinkable). You can generate a high-pressure stream by filling a zip-top plastic bag with water, poking a tiny hole in a bottom corner of it with a needle, and then squeezing the bag so water comes out through the hole. For dirty wounds, vigorous scrubbing to remove foreign bodies is also important. Exposing wounds to iodine, alcohol, peroxide, and pure salt is no more effective than water irrigation at preventing infection and can potentially damage tissues. The safest way to slow bleeding is to hold direct pressure on the wound until the bleeding stops.
The reason that salt stings a cut is that as the salt dissolves, it causes the fluid surrounding damaged tissues to become extremely hypertonic (which means that the concentration of salt and other electrolytes is higher than it is in normal body fluids).
Pain-sensing neurons have receptors on them that respond to a variety of stimuli. For example, there is a specific receptor that responds vigorously to capsaicin, which is the substance that gives jalapeño and habanero peppers their kick. So, quite literally, when you are adding hot pepper to food, you are (carefully, I hope!) inducing the sensation of pain to complement the other flavors of the meal.
More recently identified is a receptor that responds to changes in electrolyte concentration—such as the change that a large amount of salt induces. Thus, putting salt on a wound stimulates pain-sensing neurons in much the same way hot pepper does.
When tissue is damaged, many pain receptors become sensitized—that is, they need a much lower level of stimulation to respond than they normally would. (For example, after you have burned your mouth, try eating some spicy food that you could normally handle—ouch!) Salt may then compound the pain by further damaging injured tissues and making them more sensitive in general.
10/16/06
