TRANSIENT BIOELECTRICS IN NERVE 



269 



drolysis of ATP are probably the source of energy for this process, as they 

 are for many other biological processes. 



(3) Voltage Clamp: This is a technique, rather simple in principle, in com- 

 mon use in solid-state research and in electrochemical research. In short, 

 electric current is passed between two electrodes maintained at some con- 

 stant potential difference, or voltage. A steady current is a measure of the 

 rate of the steady-state which is operating within the system. Suddenly the 

 voltage is changed to another value, and "clamped" there; and the current 

 is followed closely as it changes toward a new steady-state value. The shape 

 of the current versus time curve (Figure 10-4; top right) is diagnostic. Il- 

 lustrated in the figure is evidence that the first part of the action spike is due 

 to rapid sodium ion transport through the membrane: the inward current 

 disappears if the electrolyte in which the axon is bathed contains no sodium. 

 Of course, propagation of the impulse disappears under the same conditions 

 also. 



Theories: Quantitative descriptions of the electrical phenomena have been 

 attempted, it being variously assumed that deviations from the Nernst equa- 

 tion (see Table 7-12) are due to (a) poor knowledge of the activity at the 



6 



^y 



-®- 



no current 



( switches 

 open) 



<h 



\-®~ 



e 



axon 



current inwordl current outwardf 



■(cell shorted (voltage clamped) 

 out ) 



(a) 



(b) 



(c) 



start 



Figure 10-4. Voltage Clamp Technique. Two reversible electrodes, one inside and one out- 

 side the axon are used, (a) Natural voltage (50-100 millivolts) measured across axon. 

 Current (top, right) is zero; voltage (bottom, right) is steady, (b) Membrane voltage is 

 short-circuited through external resistive load. Positive current (due to Na ' ) flows inward. 

 Voltage and current both decay toward zero as energy is dissipated as heat in external load, 

 (c) Voltage is "clamped" at unnatural value by connection to a potentiostat, a source of 

 constant voltage. After the first millisecond, positive current (due to K + ) flows outward. 

 Within the first millisecond, inward current (due to Na + ) flows because, the membrane's per- 

 meability to K + is still small. The inward current is completely absent if external fluid has 

 no sodium in it (top, right; broken curve). 



