Bridge balance in intracellular recording; introduction of the phase-sensitive method

Passing current through the microelectrode during intracellular recording produces an artifactual voltage-drop across the electrode's resistance. In modern biological amplifiers, the means of removing this artifact is through what is no more than an analog arithmetic unit which subtracts a voltage, having some fixed proportion to the current being injected, from the voltage appearing at the input to the amplifier. Various techniques have evolved for determining that fixed proportion, an operation which has come to be referred to as bridge balance, and these are reviewed. A new method is presented for determining bridge balance and another amplifier parameter which affects current injection, capacitance neutralization. Essentially, a sinusoidal current is passed through the electrode and preparation at a frequency high enough so that the cellular membrane presents negligable impedance but low enough so that the electrode appears as nearly a pure resistance. A set of analytical formulations is presented which describes the system and which is useful in choosing the best frequency of the test sinusoidal signal. This choice, part of the experimental design, is based on the predicted time constants of the recorded cell and the recording electrode. In using the phase-sensitive method, the capacitance neutralization of the recording amplifier is adjusted until there is no phase error in the injected current. Bridge balance can then be adjusted until no sinusoidal signal is visible at the output of the amplifier. Both bridge balance and correction of capacitance compensation can be automatically and continuously performed during intracellular recording by incorporating a phase-sensitive alternating current (AC) voltmeter (i.e. a lock-in amplifier) into the system. This permits accurate measurement of intracellular potential during current injection even though the resistance of the electrode might be changing.