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HARRY GRUNDFEST AND JOHN P. REUBEN 



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50msec 



Fig. 9. Large e.p.s.p.'s evoked in muscle fiber that had been soaked in a Ringer's 

 solution in which Cs+ had replaced K+. The e.p.s.p.'s showed only small facili- 

 tation on repetitive stimulation, but the increase was sufficient to evoke graded 

 responses in the electrically excitable membrane. Note the rapid repolarization 

 when the graded responses were produced. 



fibers exposed for a short time to zero K+ (Grundfest et a!., 1959; Reuben, 

 1959). The equilibrium potential of the i.p.s.p.'s is about 7 mV more negative, 

 also as in the case of muscle fibers soaked for a short time in zero K+, al- 

 though as shown, the responses in Cs+ are maximal with single stimuli. 

 When muscles are soaked for some hours in a K+-free solution the resting 

 potential becomes much more inside-negative (average: — llOmV) and the 

 equilibrium potential for the i.p.s.p.'s is at — 120mV (average). The indivi- 

 dual i.p.s.p.'s are nevertheless small and show marked faciUtation. The 

 different effects on the resting and equilibrium potentials produced by 

 absence of K+ or its replacement with Cs+ appear to be due to the different 

 modes of redistribution of Cl~ in the two experimental situations. The 

 physiological manifestations in the synaptic responses, characterized by the 

 presence or absence of augmentation of the single i.p.s.p., and by the absence 

 or presence of facilitation, are nevertheless evidenced independently of the 

 electrochemical conditions. 



