88 PAUL FATT 



bathing solution with respect to K+ and CI content was varied, and the 

 changes produced in the resting potential and in the level of reversal of the 

 inhibitory resp3nse was observed. The observations were made with two 

 microele:trodes inserted close together in a muscle fibre, one being used to 

 apply current while the steady level and transients of membrane potential 

 were recorded by the other. When the K* concentration was raised there 

 was an immediate fall in resting potential, roughly in accord with the view 

 that in this range of membrane potentials the conductance was predominantly 

 due to K+ ions. At this reduced level of membrane potential the inhibitory 

 response appeared as a transient hyperpolarization. Its reversal could be 

 effected by applying an inward current to the muscle fibre, and by this 

 means displace the membrane potential back to about the level existing 

 before the application of the high K+ solution. The true value of the equi- 

 librium potential, at which junctional acti\'ity would produce no additional 

 current flow, would, however, correspond to a somewhat smaller displace- 

 ment, because of the fact that the shift in membrane potential produced by 

 the current is not uniform throughout the region of the muscle fibre in which 

 junctional activity takes place, but is largest at the position of recording. 

 An additional complication is introduced by the influx of K+ and Ch ions 

 into the muscle fibre in the high K+ solution, causing the internal CI con- 

 centration to rise gradually. Nevertheless the experiment clearly shows that 

 variation in the external K+ concentration has a greater effect on the resting 

 potential than on the equilibrium potential for inhibition. From this it follows 

 that as a result of junctional activity there is a reduction in the relative 

 contribution to membrane conductance made by K+ ions. In the com- 

 plimentary experiment the Cb of the bathing solution was replaced by a 

 large organic anion (pyroglutamate and acetylglycine having been used). 

 When Cb was completely removed the resting potential fell by from 10 to 20 

 mV and the inhibitory response appeared as a transient depolarization. The 

 application of an outward current producing a further decrease in membrane 

 potential of several tens of millivolts was necessary to reverse the response, 

 and it was then usually found that on continuing the current the reversal 

 level was driven to a still lower level of membrane potential. Such reversal 

 as was effected is liable to be due to Ch ions which had leaked out of the 

 fibre and had accumulated in low concentration immediately outside the 

 membrane. It is concluded from these observations on the effects of the 

 external K+ and Ch concentrations that the inhibitory process involves an 

 increase in the relative contribution of Ch to the total membrane con- 

 ductance. Experiments on varying the K+ concentration in a CI -free solution 

 failed to give evidence of a possible increase in K+ conductance; the results 

 continued to depend on variations in the movement of Ch, now entirely 

 in the outward direction. In this material any increase in membrane per- 

 meability toward K+ produced by inhibitory activity must be of a smaller 



