How does a microelectrode work?

Question

On Wikipedia, the entire microelectrode page states only the following:

A microelectrode is an electrode of very small size, used in electrophysiology for either recording of neural signals or electrical stimulation of nervous tissue. Initially, pulled glass pipette microelectrode was used with later introduction of insulated metal wires. These microelectrodes are made from inert metals with high Young modulus such as tungsten, stainless steel, platinum and iridium oxide and coated with glass or polymer insulator with exposed conductive tips. More recent advances in lithography yielded to silicon based microelectrodes.

Can someone provide a more elaborate explanation of how it works?

Like ideally, I would like something as specific as the usual descriptions you see in introductory chemistry books for how emf works in a battery. In other words, I know how electric potential works, but I am not understanding the exact mechanism of the microelectrode works.

If there are many types of microelectrodes, just give me the properties of one involving small shafts of hollow glass filled with a conductive salt solution.

Pictures would be great.

Answer 1

A microelectrode is quite literally a small electrode and they come in a variety of shapes.

The glass pipette electrode you are specifically referring to is mostly used for patch clamp experiments. Patch clamp experiments are performed using various configurations:

^{Source: Leica}

So basically there is the cell-attached configuration, where a patch of cell membrane is sucked slightly into the pipette. Upon stronger suction the membrane is ruptured and the whole-cell patch clamp configuration is obtained. Here, access is gained to the cell, but a high-impedance seal is maintained between pipette and cell membrane. Lastly, there is a configuration where the patch is broken off the cell to exclude effects of the entire cell. This patch configuration is often used to obtain single-channel recordings. Dependent on the exact technique an inside-out or outside-out patch configuration is reached (see the picture above).

Hence, the glass pipette is basically only there to obtain a proper seal (cell-attached), to gain access to the cell (whole-cell), or to break off a piece of membrane. Notably, in whole-cell mode the pipette provides a means to administer compounds into the cell (think ions or drugs).

The recording electrode is situated inside the pipette and is a simple metal wire (platinum or platinum/iridium) and nothing different from any microelectrode as you rightfully state in your question. The electrode wire simply picks up a voltage difference (or current flow) between the interior of the cell (in whole-cell mode) and the exterior. The exterior is measured using a reference electrode outside the cell.

Furthermore, current can be injected to stimulate the cell, or to current-clamp the cell at a specific current. Likewise, voltage can be applied to voltage-clamp the cell. Therefore, an inert material is often chosen such as platinum or platinum/iridium to maintain integrity and prevent electrolytic breakdown. Electrodes are made out of metal to provide a low-resistance means to pick up currents or voltage-differences.

Voltage differences or currents can be amplified with an amplifier, registered and analyzed. See the following picture for the basic recording setup:

^{Source: Robinson et al. (2013)}

<sub>Reference
- Robinson et al. Front Neur Circuits 2013; 7: 1-7</sub>