EXPERIMENTS ON THE CHEMICAL STIMULATION OF PAIN-ENDINGS ASSOCIATED WITH SMALL BLOOD-VESSELS

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Abstract
Pain which follows the intra-arterial injection of various solns. results from stimulation of afferent endings located in proximity to the capillary bed. Solns. as weakly acid as pH 6.0 provide stimulation and solns. as alkaline as pH 9.2 have a similar effect. Neutral solns. do not produce pain unless they possess a molar ionic concn. of 0.7-0.8 or contain some substance which operates to exert an irritant action upon the pain-endings concerned. The cations of Na, Li, Ca, and Cs have relatively little effect upon the endings, whereas the ions of Sr, K, Rb, and Ba are increasingly irritating in the order named. The chloride is least effective of the anions and the citrate the most irritating of those tested. When two irritating ions are combined in soln. there is a summation of effect. When the nerve-endings are bathed in a hypotonic solution their irritability is thereby increased. The relation of these findings to current ideas of pain-production in inflammation and in ischemia is discussed. It is suggested that local increase in K ion or H ion may be of importance in producing the pain of inflammatory conditions. It is believed that pain in ischemia results from a local accumulation of acid metabolites.

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