NEUROMUSCULAR SYNAPTIC ACTIVITY IN TIIF CRAB 109 



by attenuation of junctional potentials in most fibers, and where attenuation 

 did occur it never exceeded about 15% (Fig. 2). 



To determine the membrane resistance change under the influence of 

 GABA, a hyperpolarizing square pulse was applied across the membrane. 

 The magnitude of this pulse was reduced only very sHghtly, never more than 

 12%, when GABA (10"^ g/ml) was apphed (Fig. 1). This is in marked contrast 

 to the results obtained in the lobster muscle. In spite of making penetrations of 

 several hundreds of muscle fibers very few were found in which membrane 

 potential changes could be detected during inhibitory action. Even when the 

 membrane potential was displaced by many millivolts with depolarizing or 

 hyperpolarizing current, the inhibitory potential changes were less than 

 1 mV (Fig. 3). 



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Fig. 5. Cancer magister, sequence of five intracellular records from muscle fiber 

 in closer muscle of walking leg, obtained a few minutes after washing off pre- 

 viously applied picrotoxin (lO^-^ g/ml). Note the strong summation of junction 

 potentials which in two instances leads to production of spike. Time base 60 cycle, 

 voltage calibration 10 mV. 



Under the influence of picrotoxin (10 ■* g/ml) a hyperpolarizing square 

 pulse was increased in magnitude at the recording electrode by about 15%, 

 indicating an increase in membrane resistance (Fig. 4). At the same time the 

 junctional potentials were reduced. As the picrotoxin was washed out the 

 reappearing excitatory junctional potentials had a greatly increased decay 

 time, so that they summated to a much greater extent than normally. In 



