Microelectrodes electrical noise

This technique allows the study of single-ion channels as well as whole-cell ion channel currents. Essentially, the patch-clamp technique is an improved and refined version of the voltage-clamp technique. It requires a low electrical noise borosiUcate glass electrode, also known as a patch electrode or patch pipette, with a relatively large tip (>1 pm) that has a smooth surface rather than a sharp tip as with the conventional microelectrodes. This is a major difference between the patch electrode and the sharp electrode used to impale cells directly through the cell membrane (Figure 16.20). 

This relation was proved by Nyquist (10) to be a consequence of basic thermodynamics laws and, except for quantum corrections, was never really challenged. Studies performed with glass microelectrodes (II) and heterogeneous ionic systems (12) showed that for zero ionic gradients and zero applied currents, the measured levels of noise were in agreement with noise levels calculated from the impedance according to eq 1. Hence, a study of electrical noise of a system under equilibrium conditions can be initiated for only two reasons. First, if there is some a priori information that the system is in equilibrium, then measurements of the system impedance or temperature can be performed without external perturbations (quantum effects are not considered here). Second, if impedance and temperature are measured independently by some other techniques, noise measurements can verify that the system under study is in an equilibrium state. 

Metallic microelectrodes have the advantage over glass micropipettes that they produce lower amounts of electrical noise. Gesteland et al. (1959) have shown that noise in metal microelectrodes is approximately that for a pure resistance that is. 

A recent study by DeFelice and Firth (1971) has shown that the electrical noise present in glass microelectrodes is in excess of the Johnson noise predicted by the Nyquist formula given in Section 4.4.3. If the microelectrode lumen is filled with an electrolyte of concentration ri2 and the external electrolyte in which the electrode is immersed has the concentration when = ri2 2i noise voltage is observed as predicted by the Nyquist formula. When Ml offset voltage is noted (tip potential) because of the... 

The electrode arrangement and dimensions are displayed in Fig. 5-4. The two rotating electric contacts (disk and microelectrode) are ensured by two mercury contactors [58], allowing the EHD impedance to be measured under satisfactory signal-to-noise conditions. The system Fe(CN)g /Fe(CN)ft in the reduction direction was used. 

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