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HARRY GRUNDFEST AND JOHN P. REUBEN 



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Fig. 7. Localized activity of transmissional presynaptic membrane. Intracellular 

 recordings simultaneously from two muscle fibers, registered with inkwriter. 

 Amplitude calibration applies to upper trace of each set. Lower trace at 1/10 gain. 

 A, B. E.p.s.p.'s recorded on stimulating the excitatory axon were produced 

 simultaneously in both muscle fibers, c, d. Ten minutes after applying serotonin. 

 The e.p.s.p.'s had increased markedly in amplitude, e, f. At 45 min. Spontaneous 

 activity had increased markedly, and large e.p.s.p.'s are seen on lower trace which 

 are independent of activity in muscle fiber recorded on upper trace. The latter also 

 shows large potentials that do not occur on lower trace. 



similar to that on frog skeletal muscle (Furukawa et ai, 1957). These actions 

 are confined to the local transmissional sites of the presynaptic terminals, 

 since involvement of conductile activity in the axons should have caused 

 synchronous activity in the various muscle fibers. Other agents affect only 

 the terminals of the excitatory axon. Thus, serotonin increases the spon- 

 taneous e.p.s.p.'s to the point where they are ahnost of the same magnitude 

 as evoked e.p.s.p.'s (Fig. 7), but it does not increase the magnitude or fre- 

 quency of spontaneous or evoked i.p.s.p.'s (Reuben and Grundfest. 1960c). 

 The foregoing data therefore demonstrate that a transmissional component, 

 which is probably secretory in function (Grundfest, 1957a, 1959), has 

 different pharmacological properties from a conductile component, and that 

 the latter differs from the electrically excitable membrane of the rest of the 



