ECCLES: ELECTRICAL THEORIES OF TRANSMISSION 439 



over-all resistance offered to the penetrating current will be much lower 

 in the second phase than in the first, the current being, as it were, 

 canalized through the localized low resistance at the cathodal focus. 

 It should be noted that, in this way, rectification will diminish, at the 

 synapse, the depressing action of the relatively high intensity Ai, and 

 then increase the stimulating action of the relatively low intensity Cz 

 (cf. FIGURE 7b). The effectiveness of this discriminative action of 

 rectification is illustrated for bipolar stimulation, by Cole.* It should 

 be even more effective for the unipolar type of stimulation that occurs 

 at the synapse. It is evident that, if the membrane had a high induct- 

 ance in series with R (figure 1), the brief penetrating currents would 

 be much less intense, and the rectification correspondingly less effective. 

 The polarization of the membrane, in the first and second stages, is 

 shown diagrammatically in figures 6a and 6b. Note the wider spread 

 of anelectrotonus, Ai, than C2, and the reversal of potential gradients 

 along the inner side of the membrane, corresponding to the reversal of 

 the "core currents" (cf. figures 5a and 5b). Note, also, that, at the 

 dotted lines separating the anelectrotonic and catelectrotonic areas, the 

 curves of the inner and outer membrane potentials are inflected, as 

 would be expected for zero density of penetrating current. The catelec- 

 trotonic focus shown in figure 6b will not immediately develop the 

 pre-synaptic current flow reverses. The anodal polarization in figure 

 6a takes some time to be removed by the local current flow, as well as 

 by the reversed penetrating currents, and further time is needed to 

 charge the membrane condensers to the fully-developed cathodal focus 

 in figure 6b (cf. figure 7b). If, at this latter stage, the external elec- 

 tric field, applied by the impulse in the pre-synaptic terminal, were sud- 

 denly removed, the membrane would immediately revert to the poten- 

 tial distribution of figure 6c (assuming that the internal and external 

 media have equal longitudinal resistances; i.e., that ri and r2 of figure 

 1 are equal). If no local response is set up, i.e., if the membrane ex- 

 hibits only its electrotonic properties, local current flow would quickly 

 cause the anodal surround to discharge into, and repolarize, the 

 cathodal focus, and the membrane would quickly revert to the normal, 

 uniformly charged, condition. Thus, under such circumstances, with 

 the usual disposition of electrodes for recording responses at the 

 synaptic region (one close to the synapse and one distally on the post- 

 synaptic cell) , there would be recorded merely a brief diphasic poten- 

 tial, attributable to currents generated by the pre-synaptic impulse and 

 but little modified by the passive properties of the post-synaptic cell. 



* Cole. K. S." FiCDKE 3. 



