reduction in the membrane impedance associated 

 with production of an action potential. 



The temporal relation between the action potential 

 and the bridge unbalance shown in record C is similar 

 to that observed by Cole & Curtis with their external 

 impedance electrodes. They explained their data as 

 indicating that at the peak of activity there occurs a 

 200-fold increase in the membrane conductance. In 

 the squid giant axon, the membrane conductance 

 stays above the resting level for some time after the 

 end of the falling phase of the action potential. 



In the myelinated nerve fiber of the frog, the im- 

 pedance measurement is complicated by the fact 

 that the change in the membrane impedance takes 

 place only at the node (133). An example of simul- 

 taneous recording of the action current and of the 

 membrane impedance in a single node is shown in 

 figure 1 3. A quantitati\'e analysis of this data is com- 

 plicated by the fact that the bridge a.c. flows readily 

 through the myelin sheath because of its capacity. 

 .Some quantitative information in regard to the con- 

 ductance at the peak of activity can be obtained by 

 passing testing current pulses through the node and 

 comparing the change in the membrane potential due 

 to the current pulse before and during activity. It has 

 been shown by this method that at the peak of ac- 

 tivity the membrane conductance increases approxi- 

 mately 10 times. In the nodal membrane, there is a 

 close parallelism between the time course of the action 

 potential and the time course of the loss in the mem- 

 brane impedance (129, 133); in this respect the nodal 

 membrane is in sharp contrast with the squid axon 

 membrane. 



More recently, Hodgkin, Huxley & Katz (57, 58, 



FIG. 13. Simultaneous recording of action potentials and 

 changes in the membrane impedance during activity of a 

 single node of Ranvier. In the left-hand record, the bridge was 

 balanced for the impedance at rest; in the right-hand record, 

 the best balance was obtained near the peak of activity. [From 

 Tasaki & Freygang (129).] 



CONDUCTION OF THE NERVE IMPULSE 9 1 



OUT, , IN 



o I o— -oJT. 





-!-^==^^^"»-r 



A2 



XT 



^xouyz^v^^v^ V 



i L 



I 1 



o o— 



Ai 



4: 



FIG. 14. L'/i/)fr.- Arrangement used for clamping the membrane 

 potential of a squid giant axon along rectangular time courses. 

 This circuit is slightly different from that used by Hodgkin 

 et al. (61), but the principle is the same. Ai is a low-gain differ- 

 ential amplifier; An, a high-gain differential amplifier (1000 

 times). The thick portions of the lines in the axon represent 

 the exposed surface of the metal wire electrodes. The distance 

 between the two partitions (P) was 8 mm. (The diameter of 

 the axon and the wire drawn in the diagram is dispropor- 

 tionately large.) Resistance r was 2.5 (sometimes 50 or 250) 

 ohms. Lower: Relation between the membrane depolarization 

 (F) and the membrane current at the peak of the inward 

 surge (/). Near V = o, the V-I relationship is roughly linear, 

 but its slope is about '250 °f ''^'" °f 'he straight line on the 

 right-hand side. Temperature, 2 2°C. The labile portion of 

 the V-I relation shown by the broken line represents either all- 

 or-none (probably nonsynchronous) responses in some parts 

 of the membrane (the patch theory), or a partial increase in 

 the conductance uniformly all o\'er the membrane (the sodium 

 theory). 



61) measured in a series of beautiful experiments the 

 conductance of the squid axon membrane by a very 

 direct, theoretically simple method, often referred to 

 as the ' method of voltage clamp'. The diagram in the 

 upper part of figure 14 illustrates the principle of the 

 method. 



A giant axon is placed across three pools of sea 

 water separated by two narrow partitions. A pair of 

 metal wire electrodes is thrust through the axon; one 

 is used for measuring the membrane potential (F) 

 and the other for passing currents through the axon 



