NEUROMUSCULAR SYNAPTIC ACTIVITY IN LOBSTER 



99 



axon. Furthermore, the terminals of the excitatory and inhibitory axons 

 have some phamiacological similarities and also differences. 



INTERACTIONS OF THE TR ANSM ISSION AL COMPONENTS 

 AT THE SYNAPSE 



The large e.p.s.p.'s produced by the action of serotonin show marked 

 facilitation (Fig. 8). Like a number of other agents (Werman and Grundfest, 



lOOmsec 



Fig. 8. Facilitation and summation of e.p.s.p.'s in serotonin-treated muscle fibers. 

 (Upper set) — Two e.p.s.p.'s evoked by stimulating axon with paired pulses at 

 difterent intervals. The closely spaced pairs of e.p.s.p.'s evoked a small graded 

 response of the electrically excitable membrane. (Below) — Three sweeps super- 

 imposed in each record. A single stimulus evoked a relatively large e.p.s.p. com- 

 pared with that in untreated muscle fibers, but not large enough to evoke a 

 response of the electrically excitable membrane. The second stimulus produced a 

 facilitated synaptic response which caused a graded response and a spike in the 

 electrically excitable membrane. 



1961; Reuben and Grundfest, 1960a), serotonin may convert the gradedly 

 responsive electrically excitable membrane to all-or-none responsiveness, 

 and the summated e.p.s.p.'s after a few nerve volleys evoke spikes. Both 

 e.p.s.p.'s and i.p.s.p.'s are enhanced even more by soaking the preparations 

 in solutions that have the K+ replaced with Cs^ (Reuben and Grundfest, 

 1960d). The p.s.p.'s are now nearly maximal in amplitude (Figs. 9, 10), and 

 show very little facilitation. 



The site of action of Cs^ has not yet been completely identified, but one 

 factor, the electrically excitable membrane may be excluded (Fig. 10). The 

 resting potential of the muscle fibers (— 85 mV) is about like that of muscle 



