CONDUCTION OF THE NERVE IMPULSE 



87 



The width of the nodal membrane uncovered by the 

 myehn sheath is roughly 0.5 to i m- The distance be- 

 tween the nodes has been discussed on p. 78. 



The first experimental evidence indicating that the 

 myelin sheath has a high resistance to a direct current 

 was obtained in Tokyo in 1934 [Kubo, Ono & Tasaki 

 cited in (70)]. When the threshold of an isolated 

 single nerve fiber was determined with a small 

 electrode placed near the fiber, it was found that the 

 threshold varied regularly with the distance from the 

 nodes of the fiber (fig. 10). In these early experiments 

 the threshold was determined by taking twitches of 

 the muscle innervated by the isolated fiber as an index 

 that a nerve impulse had been initiated in the fiber. 

 Later, measurements were made by taking electric 

 responses of the fiber as an index [e.g. fig. i in Tasaki 

 (123)]. All these experiments indicate that the 

 threshold is lowest when the small electrode (stimu- 

 lating cathode) is placed directly on one of the nodes 

 (the other electrode placed in the fluid medium away 

 from the fiber), and is highest when the electrode is at 

 the point half way between two neighboring nodes. 

 These findings have been interpreted as indicating 

 that, because of the high (d.c.) resistance of the 

 myelin sheath, the stimulating current enters and 

 leaves only at the nodes and consequently that the 

 nerve fiber is excited only at the nodes. A further 

 discussion on this subject may be found elsewhere 

 (71, 124). 



It was found later that the myelin sheath is not a 

 perfect insulator but that short current pulses can flow 



readily through this sheath (66, 1:24, 125, 136). To 

 illustrate this point, we shall mention an observation 

 published in Germany during World War II (136). 

 The diagram in figure 11. 4 illustrates the experi- 

 mental arrangement used. 



A single nerve fiber of the toad is mounted across 

 three small pools of Ringer's solution divided by two 

 narrow air gaps of o.i to 0.3 mm width. The pool in 

 the middle is about i mm wide and contains only the 

 myelin covered part of the fiber. All the nodes (Ni, 

 N2 and others) are kept in the larger, lateral pools. In 





\ 



\/ 



FIG. 10. Threshold strength of a long stimulating current 

 (in amperes) plotted against distance from a node of Ranvier, 

 Ni. Motor nerve fiber of the toad immersed in a shallow pool 

 of Ringer. Black circles show the results obtained with the 

 cathode of the battery connected to the microelectrode, and 

 the crosses with current flowing in the opposite direction. 

 Temperature, 23°C. [From Tasaki (124).] 



_n_ 



-T 





No N, N2 



FIG. II. .-i. Membrane current led through i mm long myelin covered portion of toad motor nerve 

 fiber. B. Similar to .1 ; there is a node (Ni) in the middle pool. The fibers were stimulated through 

 the electrode on the nerve trunk. Note that the action potential at the node is about 0.9 msec, dura- 

 tion at 24°C. (The nerve fiber in the diagram is disproportionately thick and short.) 



