CONDUCTION OF THE NERVE IMPULSE 



85 



The electrode set shown in the figure consists of one 

 wire with a long (about 1 2 mm) exposed surface and 

 the other with a short (i mm) exposed surface. The 

 long wire is used to send a constant current into the 

 axon and the other for recording potential changes 

 caused by the current. The short electrode has its 

 exposed (uninsulated) surface in the middle of the 

 long one. The remaining surface of each electrode is 

 insulated with a layer of enamel. A pulse of constant 

 current can be generated by connecting a high 

 voltage source to the current electrode through a 

 high resistance. 



When the sign of the applied current is such that 

 the axon membrane is traversed by an inward di- 

 rected current, the potential inside the membrane is 

 found to be lowered by the current. However, as can 

 be seen in the record in the figure, the potential 

 change at the onset of the current is gradual — 

 mathematically speaking, exponential. The potential 

 change varies roughly proportionately with the in- 

 tensity of the applied current. When the current is 

 reversed, the sign of the potential change is simply 

 reversed, provided that the change in the resting 

 potential does not exceed about 5 mv. 



This behavior of the axon can be readily under- 

 stood if one assumes that the axon membrane con- 

 sists of a condenser with a parallel resistance (fig. gA). 

 As is well known, the current flowing through a con- 

 denser of a capacity, C, is given by C dV/dt, where 

 dV/dt is the rate of change in the potential difference, 

 V, across the condenser. The current, /, through a 

 system of a conden.ser and a parallel resistance is 



given by the expression 



A 



B 





il 



out 





T 



[) i (!) in "5 ~ 



r,flx V(x-4X,t) V(x,t) y(x+ftx,t) 



dV V 



I = C 1- - , 



dt R 



(4-0 



i.e. by the sum of the capacitati\'e current and the 

 ohmic current. When the current, the capacity and 

 the resistance, /?, are all constant, the time course of 

 the potential is given by 



IR (i - e-"«'0, 



(4-0 



Cm-AX rm/4X 



v = o 



where I is the time after the onset of the current. By 

 comparing equation (4-2) with the observed result of 

 figure 8B, the values of R and C can be determined. 

 The capacity, C, of the giant axon membrane deter- 

 mined by this method is approximately i /xf/cm- 

 and the membrane resistance is between i and 

 2.5 kl2-cm-. [cf. Hodgkin et al. (61), p. 440]. The 

 time constant of the membrane, RC, is, therefore, 

 I to 2.5 msec.-' 



In the argument developed abo\e, the resistances 

 of both the axoplasm and the sea water have been 

 ignored. Cole & Hodgkin (20) and Schmitt (ro6) 

 have shown that the axoplasm is a homogeneous con- 

 ductor with a specific resistance of about 40 ohm -cm 

 at 2o°C. The specific resistance of the sea water is 

 approximately 20 ohm -cm at the same temperature. 

 These resistances are too small to have any observable 

 effect upon the measurement of figure QB. 



Now the cjuestion arises of how the voltage .source 

 representing the resting membrane potential fits in 

 the system of a capacity and a parallel resistance of 

 figure 9.-I. It is po.ssible to draw a continuous current 

 from the resting membrane; therefore, it is legitimate 

 lo represent the source of the resting potential by a 

 battery. There are obviously two simple wa\s, B and 

 C in the figure, of connecting a battery in the circuit 

 ot A. Both ways fit with the obser\ed data. There is 

 at present no direct experimental procedure that can 

 serve to determine which one of them represents the 

 axon membrane better. In the sodium theory (cf 

 p. 118), the electromotive force of the membrane is 

 assumed to be connected in parallel with the con- 

 denser as in B. 



As the result of the above discussion, it has become 

 clear that a squid axon behaves like the core-conduc- 

 tor of Hermann (see p. 75) or like a submarine cable. 

 Using elementary calculus, we may proceed slightly 



FIG. 9. Structure of the squid giant axon revealed by the 

 use of intracellular electrodes. C, capacity, and R, resistance 

 of the membrane. Two possible ways of connecting the source 

 of the resting potential in the circuit of R and C arc shown by 

 diagrams B and C. (Further detail in te.xt.) 



^ These figures were obtained by eliminating the effect of 

 the current flowing near the end of the current electrode by 

 the technique described by Marmont (84). The reader is 

 reminded in this connection to pay attention also to the di- 

 mensions of these figures. 



