io6 



HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY 1 



The final case to be discussed is the potential field 

 produced by a uniform axon suspended in a large 

 volume of conducting fluid. In this case, the potential 

 in the fluid at a great distance away from the axon is 

 not influenced by the nerve impulse; therefore, the 

 electrode on such a point is truly "indifferent'. Under 

 such circumstances, the potential in the space is in- 

 vensely proportional to the distance from the source 

 of current. Since there is a line source in the present 

 case, the potential at point P in the fluid medium is 

 given by 



f/. 



4^ J 





d.v, 



where .S' is the specific resistance of the fluid medium 

 and /?,,(.v) is the distance between point P and point x. 

 If point P is on the surface of the axon (at x = p), 



U,. cc /,„(/) + ,.t\ 



since the source in the immediate neighborhood ol the 

 recording electrode is expected to have an over- 

 whelmingly large effect in determining U,,. The time 

 course of Up is now triphasic as is the time course of 

 the membrane current in figures 23 and 24. Under 

 these circumstances it is incorrect to say that the sur- 

 face of the active region of the axon is 'electrically 

 negative'. 



More complicated cases of the volume conductor 

 problems can be solved by finding the solution of 

 Laplace's equation AT' = o under the boundary con- 

 dition described roughly by ( — i /.S) (3 r/c)«) = 

 /,„(.v + r/), where n is the normal to the surface of the 

 axon. To apply this concept of volume conductors to 

 the potential field in the body, one has to consider 

 both the nonuniformity of the excitable tissues and 

 the nonhomogeneity of the conducting medium. The 

 arguments described above on the potential field 

 cau.sed by nerve impulses are based on the work of 

 Craib (21), Marmont (83), Lorente de No (77), 

 Tasaki & Takeuchi (136) and others. 



NERVOUS CONDUCTION IN MYELINATED NERVE 

 FIBER (SALT.ATORY CONDUCTION) 



The mode of propagation of a nerve impulse in 

 the vertebrate myelinated nerve fil)er is expected to 

 be somewhat diflferent from that in the invertebrate 

 nerve fiber because of the structural discontinuities 

 along the myelinated nerve fiber. We have seen that 

 the myelin sheath of the vertebrate nerve fiber shows 

 an cxtremelv hia;h electric resistance to a direct cur- 



rent (p. 87). We have also become acquainted with 

 the experimental evidence indicating that the elec- 

 tric response of the nerve fiber derives from physio- 

 logical activity localized at nodes of Ranvier of the 

 fiber (p. 88). The myelinated nerve fiber has a 

 cable structure; when one of the nodes of the fiber is 

 thrown into action, there is a local current which 

 tends to raise the membrane potential of the adjacent 

 node to a level higher than the threshold potential. 

 When all the nodes of the fiber are excitable, there- 

 fore, it is expected that the activity will spread from 

 node to node indefinitely along the fiber. We shall 

 examine the line of evidence indicating that this is 

 actually the mode of nervous conduction in the 

 mvlinated nerve fiber. 



Effect of Increase of External Resistance 



It is a fairly difficult problem to demonstrate that 

 an increase in the resistance of the external fluid 

 medium does affect propagation of a nerve impulse 

 in the myelinated nerve fiber. The reason is that the 

 resistance per unit length of the axis cylinder is very 

 high (150 to 250 Mfl per cm) even in the largest 

 nerve fiber in the frog .sciatic nerve. Unless the ex- 

 ternal resistance is raised above this level of the in- 

 ternal resistance, it would not be possible to demon- 

 strate a clear effect upon the process of nervous 

 conduction. 



The first piece of evidence along this line was ob- 

 tained in the nerve fiber of which a portion was 

 rendered inexcitable by narcosis (117, 135). The 

 upper part of figure 26 shows the experimental 

 arrangement employed. An isolated nerve fiber of 

 the toad is mounted across three pools of Ringer's 

 fluid separated by two narrow air-gap partitions. A 

 portion of the fiijer, including two nodes of Ranvier, 

 is introduced into the small middle pool, and the 

 remaining portions of the fiber are immersed in the 

 large lateral pools. In each of the three pools, an 

 electrode of Ag-AgCl Ringer (agar) type is im- 

 mersed. The electrode in one of the lateral pools is 

 connected to a low input amplifier, and the remaining 

 two electrodes are grounded. 



With all three pools filled with normal Ringer's 

 solution, the nerve impulse arising at E in the figure 

 alwavs travels across the two narrow partitions (record 

 A). When the portion of the fiber in the middle pool 

 is treated with a cocaine-Ringer's solution (0.2 per 

 cent), the impulse fails in some preparations to 

 propagate beyond the narcotized region (record B). 

 When the electrode in the small middle pool is 



