HANDBOOK OF PH^■SK5I,0^,Y 



NEUROPHYSIOLOGY I 



FIG. 3 1 . Effect of short polarizing current pulses upon the 

 action potential of a single node of Ranvier. The arrangement 

 shown in the upper part of fig. 16 was used. The action poten- 

 tial was initiated by a short stimulating pulse approximately i 

 Qejl) and 4 msec. (jighQ after the start of the polarizing pulse. 

 Voltage calibration, 50 mv; time marks, i msec. .\ toad nerve 

 liber at 1 1 °C. [From Tasaki (126).] 



When a pulse of outward subthreshold current is 

 applied through the nodal membrane, the potential 

 inside the node rises above the resting level, resulting 

 in an upward deflection in the record. The threshold 

 membrane potential measured during the period of 

 current flow (of about 10 msec.) is nearly identical 

 with the level before the start of the subthreshold 

 pulse. In other words, a weak additional current, 

 which is sufficient to raise the membrane potential 

 from the new level reached by application of the sub- 

 threshold pulse to the normal threshold level, re- 

 leases a full-sized action potential. The membrane 

 potential at the peak of the action potential is also 

 unaff"ected by the constant current. 



When the polarity of the constant current is re- 

 versed, a stronger additional stimulating pulse is re- 

 quired to raise the membrane potential to the 

 threshold level. The membrane potential at the peak 

 of the action potential is not affected by application 

 of a constant inward current of about 10 msec, 

 duration. 



In the experiment just described, if one regards the 

 threshold for the short (additional) current pulse as a 

 function of the rectangular polarizing current, one 

 finds that the threshold is lowered by an outward 

 polarizing current and raised by a current of opposite 

 polarity. Similarly, if one measures action potentials 

 from the level immediately before the delivery of the 

 short stimulating pulse, it is found that the amplitude 

 is reduced by an outward (or cathodally polarizing) 

 current and increased iiy an inward (or anodally 

 polarizing) current. This is the direct, or primary 

 eff'ect of the polarizing current upon the threshold 

 and the action potential. 



A long polarizing current brings about a secondary 

 change in the membrane. A strong maintained 



cathodal polarization caused an additional decrease 

 in the amplitude of the action potential (cathodal 

 depression) accompanied by changes in the mem- 

 brane conductance and probably in its emf The 

 effect of a strong anodal polarization is somewhat ob- 

 scured ijy the strong stimulating current required to 

 raise the membrane potential up to the threshold 

 level. Using intact sciatic nerves, Lorente de No (77) 

 made an extensive investigation on the changes in 

 the membrane potential caused by long polarizing 

 currents. 



Now let us disctiss in this connection the well-known 

 experiment iiy Erlanger & Blair (28) who in 1934 

 discovered the electric sign of the discontinuous na- 

 ture of nervous conduction in the myelinated nerve 

 fiber. They applied anodal polarization to the por- 

 tion of the nerve under the recording electrode (mono- 

 phasic lead) and found that, when the intensity of 

 the polarizing current was gradually increased, the 

 configuration of the action potential of a single nerve 

 fiber in the nerve underwent a sudden discontinuous 

 change. Figure 32 furnishes an example of their 

 record. In record B the intensity of the polarizing 

 current was maintained at the critical level for the 

 discontinuous change. The action potential showed 

 in one sweep a distinct notch in its rising phase, and 

 in the next .sweep (superposed on the same film) 



FIG. 32. Changes in the configuration of a monophasic 

 action potential of a single nerve fiber in an intact nerve trunk 

 produced by anodal polarization at the proximal recording 

 lead. .-1. The normal spike potential. B. The spike under anodal 

 polarization just strong enough to block at the most accessible 

 node; two action potentials superposed. C. Further increase 

 in the polarizing current to the next critical strength. [From 

 Erlanger & Blair (28).] 



