CONDUCTION OF THE NERVE IMPULSE 



93 



tionslup between the membrane putential and the 

 membrane current as revealed Ijy the method of 

 voltage clamp. When the membrane potential is 

 raised suddenly from its resting le\el to a new level 

 slightly above the ordinary threshold (i.e. abo\e 12 

 to 15 mv) and is maintained at this constant le\el 

 (record ^4), it is found that the membrane is trax'crsed 

 by a current which flows first outward, then inward 

 and finally outward again. The first phase of the out- 

 ward current is .so short that it is .seen as a mere break 

 in (he upper (current) trace in the record. The second 

 phase of an inward current is seen as a downward 

 deflection in the record. The third phase of a steady 

 flow of an outward current is shown by the current 

 trace staying above the zero level in the right-hand 

 side of the record. 



The obvious explanation of the time course of the 

 membrane current in records A and B is as follows. 

 The a.xon membrane has a capacity of the order of 

 I ^f per cm- (p. 85). In order to shift the membrane 

 potential suddenly by an amount (', a total charge of 

 C- r (where C is the capacity of the memijrane in the 

 middle pool) has to be supplied by the current 

 electrode. This capacitative flow of current takes 

 place within the extremely short period of time during 

 which the membrane potential is actually rising. The 

 second phase is related to the ability of the membrane 

 to produce an action potential in response to a sudden 

 rise in the axoplasm potential. If the membrane 

 potential had not been clamped (as in fig. 15/^), the 

 potential inside the axon should start a rapid rise; an 

 inward membrane current is needed to counteract this 

 potential ri.se during activity and to maintain the 

 membrane potential at the constant level. The third 

 phase of the membrane current reflects the situation 

 in which a relatively strong continuous current is 

 needed to maintain the membrane at a steady 

 'depolarized' level. 



When the voltage step in the clamping rectangular 

 pulse is increased, the intensity of the inward mein- 

 brane current is found to decrease. The relation 

 between the depolarizing voltage step and the peak of 

 the inward surge of current is plotted in the lower 

 part of figure 14. When the voltage step is approxi- 

 mately equal to the peak value of the meinbrane 

 action potential, the peak of the inward surge is 

 found to reach zero (fig. 15C). As the voltage step is 

 increased further, the peak stays above the zero 

 level; i.e. even at the peak of the inward surge of 

 current, the membrane current is in the direction 

 imposed by the applied voltage. As can be seen in 

 the figure, the relation between the voltage step V 



and the current / at the peak of the inward surge is 

 represented by a straight line in a wide range of 

 voltage. 



The fact thai the \oltage-current relation is linear 

 can he taken as indicating that, in thi^ range of 

 membrane depolarization, the axon membrane be- 

 haves like a ' battery' with a definite electromotive 

 force (emf) and a definite internal resistance. The 

 voltage at which there is no current flow represents 

 the emf of this i)attery and the slope of the voltage- 

 current straight line corresponds to the internal 

 resistance. The membrane emf at the peak of the 

 inward surge of current coincides with the peak of 

 the membrane action potential. In the experiments 

 of Hodgkin & Huxley (57, p. 465), the membrane 

 resistance determined from the slope of the ] -I rela- 

 tion is about 30 ohm -cm'-. The figure obtained 

 recently by several investigators from the National 

 Institutes of Health is 7 to 12 ohm -cm- (at i5to22°C). 

 The resistance of the resting membrane, measured 

 with small voltage steps (less than 5 mv or negative 

 voltages) is 2 to 3 kl2-cm- (61, p. 440). At the peak of 

 activity, therefore, the membrane conductance is 

 increased by a factor of one to three hundred.^ 



In agreement with the notion that the inward surge 

 of current is associated with the ability of the inem- 

 brane to develop an action potential, narcosis of the 

 axon with ethanol or urethane is known to eliminate 

 the inward surge reversibly. A recently popularized 

 method of reversible elimination of the action po- 

 tential is to reduce the sodium concentration of the 

 .surrounding sea water. 



The finding that sodium ions are necessary in the 

 process of excitation is not new. More than half a 

 century ago, Overton (97) pointed out that the frog 

 nerve-mu.scle preparation loses its ability to respond 

 to stimuli unless there are .sodium or lithium ions in 

 the medium. He also pointed out that chloride ions in 

 Ringer can be replaced with bromide, nitrate, ace- 

 tate, salicylate, etc. without eliminating the excita- 

 bilit\-. Recently Hodgkin & Katz (62) have shown the 

 importance of sodium ions in a inore quantitative 

 manner [cf also Huxle>- & Stampfli (68) ]. They have 

 found that the spike amplitude of the squid giant axon 



* Quite recently similar voltage-clamp experiments were 

 carried out on single node preparations of the toad. It was 

 observed that the voltage-current relationship obtained was 

 similar to that shown in figure 1 4 except that the labile portion 

 of the cur%'e indicated by the broken line was limited in a 

 narrower voltage range. The membrane conductance deter- 

 mined by this method was approximately 10 times as high as 

 that of the resting nodal membrane. 



