HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



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FIG. 29. A. Time courses of the longitudinal current at two points in one internode of Lillie's salta- 

 tory nerve model. [From Franck (35).] B. Time courses of the longitudinal current recorded at two 

 extreme ends in one internode of a frog nerve fiber. Stimulus at E. [From Hodler ei al. (64).] 



recognized by color changes and bubbling on the 

 surface spreads from node to node. The trace repro- 

 duced in the figure is the time course of the longitudi- 

 nal current taken from a recent article by Franck (35). 

 Since the glass tubing is a perfect insulator of elec- 

 tricity, the time courses of the longitudinal currents 

 recorded at two different points in one internode are 

 undoubtedly the same. 



Figure 29^ shows a corresponding observation on 

 the real myelinated nerve fiber. By the arrangement 

 illustrated at the top, the longitudinal current is re- 

 corded at two points in one internodal segment. As 

 can be seen in the tracing below, there is a large dif- 

 ference between the longitudinal currents recorded 

 at two points which are about 1.5 mm apart in this 

 case. The difference between the two longitudinal 

 currents represents the double peaked membrane cur- 

 rent recorded through the myelin sheath (fig. 11 A). 



We see in figure 2(jB that the two longitudinal 

 currents recorded at two different points in one 

 internode rise at different rates, reach the peaks at 

 different moments and fall at different rates. This is 

 a direct consequence of the existence of a large capaci- 

 tative flow of current through the myelin sheath. 

 Like a signal travelling along a submarine cable, the 

 longitudinal current spreads along the axis cylinder 

 at a finite rate.'" Because of this slow spread of the 

 membrane potential (cf. p. 100) and of the longi- 



'" A different viewpoint is stated in a previous paper by 

 Huxley & Stampfii (66). The slight difference between 

 their experimental results and the results described in the text 

 is probably due to their use of a high input resistance in their 

 amplifier which tends to lower the time resolution in recording 

 [cf. footnote on p. 11, Tasaki (124)]. 



tudinal current along the internode, it is not legiti- 

 mate to state that a nerve impulse jumps from node 

 to node without spending any time in the internode. 

 This point has been stressed in an article by Hodler 

 et al. (64) [cf. also Stampfli (i 14)]. It has been pointed 

 out (125) that the major portion of the temperature 

 dependence of the conduction velocity (Qio of about 

 1.8) can ije attributed mainly to a change in the 

 cable properties of the nerve fiber [cf. also Schmitt 

 C106)]. 



Field of Piitential Produced by a .\enr Impulse 



We have discussed in the preceding section the 

 field of potential produced in the surrounding fluid 

 medium by a nerve impulse travelling along a uni- 

 form invertebrate nerve fiber. Because of the struc- 

 tural discontinuities along the myelinated nerve 

 fiber, the statements made in the preceding section 

 are not in a strict sense applicable to the myelinated 

 nerve fiber. However, there is a special case in which 

 the effect of the discontinuities is very small. 



Let us consider the case in which a single nerve 

 fiber of a uniform diameter is enclosed in a glass 

 tubing of a uniform diameter filled with Ringer's 

 .solution (as in fig. 25.-1). In this ca.se, the longitudinal 

 current at one point along the fiber is equal in in- 

 tensity and opposite in sign to the current flowing 

 through the medium at the same point. From the 

 argument described in the preceding section, it is 

 found that the spatial distribution of the potential 

 along the fluid medium in the glass tubing is a mir- 

 ror image of the potential inside the axis cylinder, 

 its absolute value being determined by the ratio of 



