Voltage clamping circuit

Record response to applied ligand with voltage clamp circuit... 

Figure 7.25 Intracellular three-electrode voltage clamp circuits, (a) ellipsoid cell form, (b) nerve...

The voltage-clamp technique was introduced by Cole in 1949. A general survey of the principle of operation and various shortcomings is presented by Moore and Cole (1963). A voltage-clamp circuit provides a means for holding constant the potential difference across the resting or active plasma membrane of a neuron (axon), within the limitations of practical electrodes. The discussion here is concerned primarily with the amplifier system being used. 

Fig. 5.17 A four-electrode potentiostatic circuit (voltage clamp). and R2 are reference electrodes with Luggin capillaries (arrows) attached as close as possible to the membrane or ITIES (dashed line), B, and B2 are auxiliary electrodes, P and P2 potentio-stats, G programmed voltage generator and Z recorder... 

Voltage clamp This refers to the experimental procedure of using active electronic circuits to hold the transmembrane potential difference at a fixed value by pumping current into the cell. 

U sing OAs allows construction of potential and current control circuitry that operates more ideally than the simple voltage and current sources of Fig. 1 [2]. In voltage control appKcations, the use of an OA buffer between a voltage source and the cell replaces the internal resistance of the voltage source with the low-output impedance of the OA. However, a simple extension of the OA buffer permits elimination of a large fraction of the solution resistance of the electrochemical cell. This OA circuit is called a potentiostat or voltage clamp. 

Figure 7.25 shows a similar three-electrode voltage intracellular clamp circuit. A problem with the circuits is that two micro-electrodes must be introduced through file cell membrane. 

Figure 8.19 Three noise reduction approaches. Left grounded patient. Middle floating patient with instrumentational amplifier. Right ground-clamping circuit. Vp is typical patient AC voltage with respect to ground. Vp is in practice nonsinusoidal. Because of the high-pass filtering effects, the harmonics of the mains supply are expanded.

Additionally, another often used approach is the application of a voltage clamped stimulus and converting die current through the MUT into a voltage by means of a transimpedance circuit as shown in Figure 8.32. 

For the chamber, see Rehm WS, Dennis WH, Schlesinger H. Electrical resistance of the mammalian stomach. Am J Physiol 181 451-470, 1955. For attempts to modify the blood, see Thull NB, Rehm WS. Composition and osmolarity of gastric juice as a function of plasma osmolarity. Am J Physiol 185 317-324,1956. For early isolated mucosa work, see Rehn WS. Acid secretion, resistance, short-circuit current and voltage clamping in frog s stomach. Am J Physiol 203 63-72, 1962. 

Figure 7.32 illustrates the basic clamp circuit. A differential amplifier is used. The mathematical relations which describe voltage-clamp operation are... 

After this preliminary stage the voltage clamp was turned on using the output of the compensating circuit as a measure of the membrane potential. The holding potential was then set to - 100 mV and small voltage steps were given with the microelectrode still in the fibre in order to check that the membrane was clamped properly. The microelectrode was then removed and the measures initiated. 

Soy-bean lecithin and oxidized cholesterol membranes were also used in some experiments. The conductance was measured with two Ag-AgCl electrodes connected to an electrical circuit similar to that previously described (Ciani et al., 1975). Current fluctuations measurements were performed in voltage-clamp. The value of the current was inferred from the potential drop on a resistance R, in series with the membrane. In most experiments was smaller than 10 and the time constant, t, of the circuit was less than 10 ms. 

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