250 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



current would be required. In practice the membrane 

 is charged by a large, brief current. In the feed-back 

 system used by Hodgkin and co-workers, the surge of 

 capacitative current lasted less than 30 /usee. Following 

 an abrupt hyperpolarization, there is a small negative 

 (inward) current of the size to be expected from the 

 initial membrane resistance. A depolarization, how- 

 ever, produces a large, diphasic variation in current 

 (upper current curve, fig. 4B). If the membrane 

 resistance remained constant, the expected effect 

 would be a small positive current just equal and 

 opposite to that produced by the hyperpolarization. 

 This is the case initially, but the small initial step of 

 outward current quickly changes into a large inward 



mA/crrf 







mSEC 



FIG. 5. Voltage clamp current records obtained when £ 

 was depolarized to values near the early current reversal 

 potential. The early current was inward at depolarizations 

 of 91 and 104 mv and outward at 130 and 143 mv. The current 

 was small and nearly constant for 0.5 msec following a 117 

 mv depolarization. Normal [Na+]o; temperature, 3.5°C. 

 [After Hodgkin el al. (62).] 



current, which reaches a peak in less than i msec. 

 The current then falls gradually toward zero, reverses 

 sign and levels ofT at a large, maintained, positive 

 value. The curves obtained for other depolarizations 

 are much the same shape if £ < Sni- If S > fix„, the 

 sign of the early current surge changes from negative 

 to positive, whereas the late current is relatively un- 

 changed. This behavior can be seen in figure 6 and is 

 shown clearly in figure 5, where a series of currents 

 resulting from a number of depolarizations to a value 

 near Snu are superimposed. For a depolarization of 

 1 1 7 mv it is seen that, aside from the initial step, the 

 current is unchanging for about 0.5 msec. This curve 

 contrasts with the records for other depolarizations. 

 Since the resting potential was about —65 mv, 

 depolarization of 117 mv reversed the membrane 

 potential by about 52 mv, a value equal to SNa within 

 the limits of experimental error. 



The effects of changes in [Xa+]o furnish convincing 

 evidence that the initial current is carried by Na+ 

 ions flowing down their electrochemical gradient. 

 Figure 6 shows the effects of substituting choline"*" 

 for 30 per cent of the Na"*". The left and right hand 

 columns are membrane ionic current records taken 

 in 30 per cent [Na"'"]o before and after the records 

 of the center column which were obtained in normal 

 [Na+Jo. In this figure, the capacitative surges have 

 been subtracted from each current record. The 

 curves in the two solutions are quite similar; the 

 major difference is in the depolarizations at which 

 the initial current re\'erses sign, approximately 70 



FIG. 6. Effects of altered [Na"*"]o and 

 clamping voltage on the early ionic 

 current. Records in left and right hand 

 columns were obtained when choline 

 was substituted for 7oT[' of the Na"^ 

 in the external bathing medium. Rec- 

 ords in middle column were obtained 

 with axon in normal [Na+Jo. The num- 

 ber by each curve indicates the amount 

 by which the membrane voltage was 

 depolarized from the resting potential 

 at time zero. Note that in the left and 

 right columns the early current was 

 negativ'e for small depolarizations and 

 positive at large depolarizations; also 

 that there was little early current for 

 depolarizations of 79 mv. In normal 

 [Na'*']„ solutions, the sequence of 

 changes in early currents was the 

 same, but the reversal voltage was 108 

 mv. Temperature, 63 °C. [After Hodg- 

 kin & Huxley (57).] 



30% [Na+] 



:oo% [no*] 



30% [No*] 



I 2 3 4 5 



J I 1 1 1 I 



12 3 4 5 



J i_ 



■ . 1 I 



2 3 4mSEC 



