96 



Harry grundfest and john p. reuben 



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Fig. 4. Effect of GABA on repetitive activity in presynaptic terminals. Intra- 

 cellular recording from a muscle fiber. (Left column; upper trace) — Three control 

 e.p.s.p.'s evoked by stimulating the axon. {Middle) — Repetitive activity develop- 

 ing soon after applying PEA and picrotoxin. {Bottom) — Further development 

 of repetitive activity in presynaptic terminals indicated by greater summation of 

 e.p.s.p.'s which were produced at higher frequency. Note that repetitive activity 

 disappeared in the second evoked response and that only one repetitive e.p.s.p. 

 occurred in the third sequence. (Right column) — Another sequence of repetitive 

 activity following five stimuli to the axon. Recording on a slower time base. 

 Note again the modification in frequency of repetitions and absence after the 

 second orthodromic stimulation. However, the single response was facilitated 

 as were the subsequent responses to the orthodromic stimuli. (Middle) — Repetitive 

 activity was abolished on adding GABA. The agent also activated the inhibitory 

 membrane which had been blocked by the picrotoxin. Consequently the e.p.s.p.'s 

 were diminished markedly in amplitude. Note the presence of facilitation despite 

 their eftects on both presynaptic terminals and synaptic membrane. (Bottom) — 

 Repetitive activity was restored on adding picrotoxin. 



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Fig. 5. Spontaneous miniature p.s.p.'s both depolarizing and hyperpolarizing in 

 lobster muscle fiber. Intracellular recording. The hyperpolarizing responses were 

 consistently smaller than the depolarizing responses since the equilibrium 

 potential of the inhibitory synaptic membrane is close to the resting potential. 



