NEUROMUSCULAR SYNAPTIC ACTIVITY IN LOBSTER 95 



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Fig. 3. Differential effects of picrotoxin and PEA on the electrically excitable 

 and synaptic membranes of a muscle fiber. Lower half in each set is of intracellular 

 recording. {Top row) — Absence of effects on membrane resistance. Left, control; 

 middle, after picrotoxin; right, after PEA. Upper trace shows the hyperpolarizing 

 current applied through another intracellular microelectrode. The membrane 

 potential change was about the same in all three records. {Middle row) — The 

 e.p.s.p.'s evoked by three stimuli to the axon under the same conditions. After 

 picrotoxin the e.p.s.p.'s increased, indicating that there probably had been 

 considerable spontaneous activity of the inhibitory synaptic membrane which 

 was blocked by the drug. After PEA the e.p.s.p.'s become small. Note that facili- 

 tation was present in all conditions indicating that the processes causing this 

 were not affected by the drugs. (Bottoui row) — Three series of records as PEA 

 and picrotoxin developed their maximal effect on the presynaptic repetitive 

 firing. Note that the pattern of postsynaptic responses to the three orthodromic 

 stimuli was disrupted by the repetitive activity. 



repetitive activity (cf. Riker et al., 1959). However, the agents which are 

 effective in these cases did not produce repetitive activity in the lobster axons. 

 GABA which excites the inhibitory synaptic membrane (Grundfest et al., 

 1959) also acts antagonistically to picrotoxin on the presynaptic terminals 

 (Fig. 4). However, it does not block the responses of the axons, but eliminates 

 the repetitive activity. The latter is again brought about by adding picrotoxin. 

 Similar effects are also observed on recording from the excitatory and inhibi- 

 tory axons. They obviously occur at the presynaptic terminals, and therefore 

 the conductile component of the membrane at the terminals is affected 

 differently by the drugs than is the conductile membrane along the axon. It 

 is likely that the repetitive activity of the terminals is a manifestation of 

 prolonged depolarization, either by some type of after-potential, or by pro- 

 longed spikes (Reuben and Grundfest, 1960a; Werman and Grundfest, 1961). 

 However, since the details of the electrical activity of the terminals are still 

 unknown, the nature of the actions of the drugs cannot be fully analyzed as 



