CONDUCTION OF THE NERVE IMPULSE I I I 



the resistance per unit length of tlie outside fluid to 

 that of the axis cylinder (equation 9-3). This field of 

 potential travels along the fiber at the average ve- 

 locity of the impulse. Insofar as one disregards the 

 variations in the potential that occur within one 

 internodal distance (about 2 mm) or within one 

 internodal conduction time (about o. i msec), the 

 potential field produced by a myelinated nerve fiber 

 in the fluid medium is similar to that produced by a 

 uniform invertebrate axon. 



The distribution of the potential on the surface of 

 a uniform nerve trunk produced by a nerve impulse 

 travelling along a single nerve fiber in the trunk can 

 be regarded as analogous to the case described above. 

 To the approximation that the potential variations 

 within 0.1 msec, are disregarded, therefore, the 

 principle of 'diphasic recording of the action poten- 

 tial' described in the preceding section is applicable 

 to this case. A further discussion on this problem can 

 be found elsewhere (124). Frankenhauser (36), 

 Hodler el al. (64), Stampfli & Zotterman (i 15) and 

 others have investigated the details of the potential 

 variations occurring within one internodal conduction 

 time and also within one internodal length. 



When a myelinated nerve fiber is immersed in a 

 two-dimensional or three-dimensional volume con- 

 ductor, the potential field produced ijy a nerve im- 

 pulse is very different from the field produced by an 

 impulse of a uniform invertebrate axon. As has been 

 shown in figure 1 1, strong sinks of electric current are 

 localized at the nodes while the sources are distributed 

 along the internodes as well as at the nodes. There- 

 fore, the time course of the potential picked up by a 

 recording electrode placed near one of the nodes is 

 expected to be very different from the record ob- 

 tained with the electrode on the myelin co\ered por- 

 tion of the fiber. 



Figure 30 shows the time courses of the action 

 potentials recorded with a metal microelectrode 

 placed at various points near a node of Ranvier of 

 an isolated single nerve fiber immersed in a thin 

 layer of Ringer's solution. The vertical straight line 

 in the middle of the figure represents the course of 

 the fiber, and the center of the two concentric circles 

 represents the position of the node under study. It is 

 seen in the figure that the largest negative potential 

 is observed when the recording electrode is placed in 

 the immediate neighborhood of the node. The ampli- 

 tude of the negative component of the action poten- 

 tial decreases as the distance from the node increases, 

 and this decrease is roughly independent of the direc- 

 tion in which the electrode is moved awav from the 



FIG. 30. Records of action potentials taken with a small 

 metal electrode placed around a node of Ranvier. The nerve 

 fiber was immersed in a shallow layer of Ringer's solution. 

 The vertical line represents the nerve fiber, and the center 

 of the two concentric circles the node under study. The im- 

 pulse travels downward. Five records on the vertical line were 

 taken with the electrode along the fiber and slightly to one side. 

 Other nodes of the fiber were not exposed in the operated 

 region of the preparation. The conduction distance was about 

 45 mm. Temperature, 2o°C. [From Tasaki (137).! 



fiber. For further details of this experiment see Tasaki 



(124). 



Conduct uin in a Polarized Nerve Fiber 



When a direct current is applied to a nerve trunk 

 through a pair of nonpolarizable electrodes in con- 

 tact with its surface, the portion of the nerve fiber 

 near the anode is traversed by a continuous inward 

 membrane current, and the region near the cathode 

 is subjected to an outward membrane current. The 

 behavior of the nerve impulse in such ' polarized' 

 regions of the nerve fiber was discussed by Pfliiger 

 (too) more than a half century ago. A nerve fiber 

 modified by a constant current is said to be in an 

 'electrotonic' state. In order to understand the be- 

 havior of a nerxe impulse in the nerve fiber under 

 'electrotonus', it is desirable to investigate the be- 

 havior of a single node preparation under influence of 

 a constant current. 



Figure 31 shows the effect of a passage of a short 

 rectangular current pulse upon the threshold and the 

 action potential of the single node. The arrangement 

 employed is the same as that for figure 16 (p. 94). 



