. Biophysics: concepts and mechanisms. Biophysics. 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
. Biophysics: concepts and mechanisms. Biophysics. 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 â (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 u
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