128 PHYSICAL CHEMISTRY 



Stimulation 



Modern theories of electrical stimulation depend on membrane 

 polarization. Nernst accidentally passed through his body a high 

 frequency Tesla current of enormous voltage, such as is used in 

 wireless telegraphy, and observed no stimulation. This led him 

 to formulate his theory of electric stimulation, according to which 

 the current must carry a certain number of coulombs of electricity 

 per unit cross section, before it is reversed, in order to stimulate. 

 In other words, stimulation is due to polarization of membranes, 

 and the polarization must reach a certain minimal value. From 

 this it is evident that the higher the frequency of an alternating 

 current (the shorter the time the current flows continuously in 

 one direction) the greater the amperage required in order to 

 stimulate. Using sine wave alternating currents, this was found 

 to be true within certain limits of frequency, but it appeared im- 

 possible to stimulate. if frequency reached 100,000 per second. 

 Nernst and Barrat (1904) found that the current required to 

 stimulate = .079 X the square root of the frequency. In order 

 to extend the limits of frequency complicated formulae have 

 been developed to include accessory factors, such as backward 

 diffusion of ions (A. V. Hill, 1910). According to Bethe (1916) 

 the facts fit better the hypothesis that only H ions are concerned 

 in the stimulation. He had found that the passage of a current 

 through a membrane causes the accumulation of H ions on one 

 side and OH ions on the other and supposes that the accumula- 

 tion of H ions on the outer surface of the plasma membrane 

 causes stimulation. He says that the bare nerve fibers exposed 

 at the cut end of a nerve, or which have grown out of the end 

 on regeneration, can be stimulated by acid that is not hyper- 

 tonic, and hence does not stimulate osmotically. 



The relation of ions to irritability has been much studied, but 

 the problem has not been entirely cleared up. An ion may have 

 one effect at a certain concentration and a different effect at an- 

 other. K ions are said to depress irritability, but according to 

 Beccari (191 5) they increase irritability, when applied within 

 physiological limits to skeletal muscle. It is well known that Na 

 increases and Ca decreases irritability. But irritability is lost in 

 the pure solution of any salt. If muscle is placed in isotonic 



