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HANDBOOK OF PHYSIOLOGY' 



NEUROPHYSIOLOGY I 



membrane. The uvo lateral pools are directly 

 grounded with large silver wire electrodes. The middle 

 pool is also grounded but through a resistor (r) of a 

 few ohms. When a current is sent through the l^ng 

 internal electrode, this resistor (/■} is traversed by a 

 current (/) passing through the axon membrane in 

 the middle pool; the small potential drop (/r) is 

 amplified and is taken as the measure of the membrane 

 current. The membrane potential is measured across 

 the axon membrane in the middle pool. The circuits 

 connected to the axon are constructed in such a 

 manner that the membrane potential (f) can be 

 maintained at any desired level by an automatic 

 adjustment of the membrane current (/). 



The principle of the automatic control of the mem- 

 brane current by the feed-back mechanism is as 

 follows. In the diagram of figure 14, Ai is a preampli- 

 fier which transmits the membrane potential (I) at 

 its input to one of the inputs of a differential amplifier 

 A-). The other input of A., marked i in the figure, is 

 connected to a source of rectangular (or other) voltage 

 pulses. The output of amplifier A2 has the .same 

 phase as that of input i and opposite to that of 

 input 2. 



First let us consider the case in which input i is 

 grounded. When membrane potential (T) tends to 



rise by some intrinsic process in the axon, the poten- 

 tial of input 2 starts to rise immediately. This po- 

 tential is then amplified and, after reversing its 

 polarity, transmitted to the long wire electrode in the 

 axon. This immediately causes a flow of an inward 

 membrane current which lowers the membrane po- 

 tential (r). As a consequence, if the gain of Ao is 

 sufficiently high, any change in the membrane po- 

 tential (r) can be almost completely suppressed by an 

 automatic control of the membrane current (/). In 

 practice, the over-all gain of this feed-back amplifier 

 was 1000 to 3000. 



Next, we consider the case in which the potential 

 of input I of amplifier A2 varies along a rectangular 

 time course. .Xt the moment when the potential of 

 input I starts to rise, thert is a sudden flow of an out- 

 ward current through the axon membrane. This flow 

 immediately raises the membrane potential (!'). The 

 rise in Fis transmitted to input 2, tending to lower the 

 output voltage of A-i. In the steady state there is a flow 

 of a constant membrane current which is sufficient to 

 maintain the membrane potential at the constant 

 level. If the gain of Aj is unity, the time course of the 

 membrane potential (T) reproduces the potential 

 applied to input i fairly accurately. 



The records furnished in figure 15 show the rela- 



FiG. 15. Relationship between the membrane potential (dotted trace) and the membrane current 

 (continuous trace) obser\ed with the arrangement of fig. 14. In records A to D, the membrane po- 

 tential was 'clamped' along rectangular time courses by automatic adjustment of the membrane 

 current. In E and F, rectangular current pulses were applied through the current electrode and the 

 variation in the membrane potential was recorded with the other internal electrode; the defection 

 sensitivity of the current trace is 20 times as high as in other records. Blanking of the potential trace 

 indicates 0.25 msec. Temperature, 22 °C. 



