NEUROMUSCULAR SYNAPTIC ACTIVITY IN LOBSTER 101 



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Isec 



Fig. 10. Differential effects on i.p.s.p.'s in muscle fiber soaked in K+-free Ringer's 

 solution (right) and in Cs+-Ringer's solution {left). (Right) — In the K+-free 

 solution, the individual i.p.s.p.'s were small, but summation and facihtation at 

 higher frequencies of stimulation drove the potential to its equilibrium level 

 about 10 mV negative to the resting potential which was — llOmV. (Left) — In 

 the Cs+-Ringer"s solution the individual i.p.s.p.'s were large, facilitation was 

 absent and the equilibrium potential was equal to the peak single i.p.s.p. It was 

 about 6 mV negative to the resting potential which was only —85 mV. 



Augmentation of the e.p.s.p. to nearly maximal responses by Cs makes 

 possible an estimate with some degree of accuracy of the reversal potential 

 of the e.p.s.p. (Fig. 11). Furthermore, the change in the ampUtude of the 

 e.p.s.p. with the membrane potential has a slope of about 0-5. Thus, the 

 conductance change during activity of the excitatory postsynaptic membrane 

 is much less than is the conductance change during maximal activity in the 

 inhibitory postsynaptic membrane. When the latter is excited by GABA the 

 membrane resistance of the normal muscle fiber falls some three- to six-fold, 

 and in Ba+-treated fibers the resistance decreases some twenty-five- to thirty- 

 fold (Grundfest et al., 1959). 



CONCLUSION 



The foregoing brief survey describes some of the complications that must 

 arise in the functioning of a synaptic system of even moderate degree of 

 organizational complexity. The synaptic membranes themselves and the 

 electrically excitable components of the postsynaptic unit have been neglected 

 at this time to emphasize factors that are ordinarily not considered or in- 

 sufficiently regarded. Further details of the pharmacological and electro- 



