134 PHYSIOLOGICAL TRIGGERS 



The 'slow' muscle fibers of the frog (132) do not respond to electrical stimuli 

 (31), but do so to neural stimulation. The graded, summative depolarization of 

 these 'junctional potentials' (132) is therefore also a p.s.p. As is to be expected 

 for the response of electrically inexcitable membrane, the p.s.p. reverses in 

 sign when strong outward currents are applied across the membrane (32; cf. 



below, ELECTROGENIC ACTION OF SYNAPTIC TRANSDUCERS). 



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A A 9 A. ^^^- ^- Electrical inexcitability 



of the post-synaptic membrane of 

 apical dendrites of the cat cortex. 

 A: Stimulating and recording from 

 the cortex. The large normal sur- 

 face negativity which is the re- 

 sponse of cortical dendrites (/) is 

 p decreased 50 sec. after intravenous 



injection of 3 mg/kg rf-tubocurarine 

 chloride (2). At 70 sec, it is gone 

 (j), to return after 5 min. (4) and 

 20 min. (5). Four consecutive 

 records superposed in each set. B: 

 Stimulating about 0.8 mm below 

 the cortical surface, recording as in 

 A. The dendritic negativity (/) 

 2 3 "" disappeared and only the positive 



response, representing the activity 

 of the directly excited spikes, re- 

 mained 45 sec. after an injection of 

 drug {2). Recovery was rapid, 

 although not complete at 90 sec. 

 (j). Three consecutive records 

 superposed in each set. O Re- 

 sponse of motor cortex to stimulation of the pyramidal tract (/) is composed of two posi- 

 tive deflections preceding the negative dendritic potential. Two minutes after the injection of 

 drug all synapticall}' induced activity (the second positive component and the negative 

 dendritic response) disappeared, leaving only the first positivity which is ascribed to the 

 antidromic response of pyramidal axons and/or cells in the deep cortical laj-ers {2). Recovery 

 after 20 min. (j). 4-6: From the same experiment, stimulating the cortex and recording in the 

 pyramidal tract. The normal response is composed of a directly excited component and sev- 

 eral synaptically evoked repetitions (.;). The latter are absent 5 min. after drug was ad- 

 ministered (5) and recovered somewhat after 20 min. Five superposed responses in each 

 set. Times 20 msec, for A and B, 10 msec, for C (from ref. 105). 



Invertebrate neuronal junctions. Evidence for electrical inexcitability of 

 invertebrate neurons is chiefly indirect. The large cells of the lobster cardiac 

 ganglion develop a graded response when excited synaptically by the smaller, 

 pacemaker neurons of the ganglion, but the soma membrane is not capable of 

 supporting a spike, that response occurring in the axon of the cell (106). The 

 synaptic potential of the squid third order giant axon may be elicited while the 



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