160 



Lawrence S. Frishkopf and Walter A. Rosenblith 



They regard these potentials as being the result of spontaneous firings in the 

 fine terminal branches of a motor nerve. The occurrence of an impulse in the 

 nerve causes simultaneous firing in about a hundred such teitninals, giving 

 rise to the normal end-plate potential. Spontaneous firing implies the existence 

 of a local source of varying excitation. Fatt and Katz compute that for 

 fibers with a diameter of 0.1 /,( thermal fluctuations in ionic concentrations 



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O " 2 4 6 8 10 12 14 16 18 20 22 24 26 28 

 POPULATION RESPONSE AMPLITUDE 

 1.0 

 0,9 

 0.8 

 0.7 

 0.6 

 0.5 

 0.4 

 0.3 

 0.2 



0.1 

 O 



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Fig. 8. Top: the upper curve is a histogram of population response amplitudes 

 obtained as in Fig. 7 from triceps surae motoneurons by delivering repeated 

 identical shock stimuli to the gastrocnemius nerve. The lower curve (shaded) was 

 obtained from single-unit recordings like those shown in Fig. 7 ; the number of 

 single-unit responses associated with population responses in each amplitude 

 interval is plotted. Bottom : for a given population amplitude interval the number 

 of single unit responses is divided by the total number of trials in that interval, 

 and the ratio plotted as a function of population amplitude. The interpolated 

 solid curve is a sigmoid fit to the data points and approximates the probability of 

 unit response as a function of population amplitude. Note that when the popula- 

 tion amplitude is large, the probability of unit response is large, and when the 

 population response is small, the single unit probability is small, thus signifying a 

 high degree of correlation among the thresholds of different units of the popula- 

 tion. From Rall and Hunt (6). 



could cause variations of resting potential of 1 mV to 2 mV. Though probably 

 insufficient to produce excitation, such a variation would cause threshold 

 fluctuations and contribute to spontaneous firing. 



Both Pecher (2) and Hunt (8) have discussed possible sources of threshold 

 fluctuation. Pecher considers in detail the apparent threshold variation that 

 would result from statistical variations in the number of ions traversing the 

 axon membrane when a constant potential is applied across it. Assuming 

 that the excitatory current that he uses is uniformly distributed over a cross 

 section of the nerve trunk, he concludes that at threshold about a million ions 

 traverse a single nerve fiber. The statistical variation in this number of ions is 



