CONDUCTION OF THE NERVE IMPULSE 



99 



anthropomorphized meaning [e.g. Verworn (140)] 

 and the procedure of measuring it was more-or-less 

 comparable to determining a man's ability by 

 mental tests. Such a concept of excitability has no 

 clear physiological meaning. 



Here, we shall discuss the significance of the method 

 of using test shocks to explore the state of the nerve 

 fiber. This method has been used mainly on verte- 

 brate nerve fibers. 



In the arrangement illustrated in the inset in figure 

 19, an isolated nerve fiber is mounted across two pools 

 of Ringer's solution. The narrow air gap is located 

 between nodes Ni and No. Through the electrodes im- 

 mersed on the pools, short pulses superposed on long 

 rectangular \-oltage pulses are applied to the fiber. 

 The intensity of the short pulse, .S', the voltage of the 

 long pulse, V, and the time interval, /, from the begin- 

 ning of the long pulse to the start of the short pulse 

 are three variables in this experiment. The data pre- 

 sented in A were obtained by fixing voltage, v, at one 

 of four different values ( — 20, —10, 10 and 20 mv) 

 and adjusting S to make the composite stimulating 

 pulses barely eflfective in eliciting a nerve impulse at 

 varying values of /. The data in B were obtained by 

 fixing t at 2 msec, and adjusting v and .S* to make the 

 pulses barely effective. Thresholds were determined 

 by taking the response of the muscle innervated by 

 the nerve fiber as an index of initiation of an im- 

 pulse; the same result, however, can \)e obtained by 



taking the action potential of the nerve fiber as an 

 index. 



In B, the observed point for .S' = o is at j) = 30 to 

 31 mv, indicating that the rheobasic voltage of the 

 fiber under these experimental conditions was about 

 30 mv. The threshold for the brief shock depends on 

 the duration of the shock; for durations shorter than 

 about 30 /isec, the threshold rises inversely as the du- 

 ration. The shock used in the present experiment was 

 within this range and its threshold was taken as unity. 



The curves in A show how the threshold for the test 

 shock, S, is modified by the subthreshold pulse, v. At 

 any fixed value of /, the change in S is roughly pro- 

 portional to I', except when v is greater than about 

 50 per cent of the rheobase (5). One thing that looks 

 strange in this figure at first sight is the change in 

 threshold observed at / = o and for negative values of 

 /. This is a constant finding in single fiber experiments 

 and has also been observed by Erlanger & Blair in 

 their experiments with nerve trunks (27). If the test 

 shocks measure the state of the nerve fiber at the 

 moment when the shocks are delivered, it is obviously 

 absurd that the threshold starts to change before the 

 beginning of the subthreshold pulse used to modify 

 the state of the fiber. 



There are two factors that serve to explain this 

 strange fact. One factor is the time required for the 

 spread of membrane potential along the myelin 

 sheath, and the other factor is the production of a 



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FIG. 19. Changes in threshold for a brief shock (S) caused by application of a subthreshold rec- 

 tangular voltage pulse (v). The sign of the stimulating \oltage pulse is positi\ e when the pulse induces 

 an outward current through node N-j in the diagram. S(, represents the threshold for the brief shock 

 alone. 



