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|Electrophysiology is the science and branch of
neurophysiology that pertains to the flow of ions in biological tissues and, in particular, to the electrical recording techniques that enable the measurement of this flow.
These include so-called passive recording as well as the "voltage clamp" and "patch clamp" techniques, which "clamp" or maintain the cell potential at a level the experimenter may specify. This control is established using feedback through an operational amplifier circuit. Control of the membrane potential is most obviously of value in the study of voltage-gated ion channels, but also aids in characterizing conductance.
The most common electrophysiological recording techniques establish electrical contact with the inside of a cell or tissue with a "glass electrode." Such an electrode is fashioned by the experimenter from a fine glass tube of about 1 mm diameter, which is then pulled to an even finer (but still hollow) tip under heat and allowed to cool. This glass "micropipette" is then filled with a chloride-based salt solution, and a chloride-coated silver wire is inserted to establish an electrochemical junction with the pipette fluid and the tissue or cell into which the pipette is inserted (typically with the aid of a microscope and finely adjustable pipette holders, known as micromanipulators). The chloride-coated silver wire connects back to the amplifier. Classically, neurophysiologists watched biological currents on an oscilloscope and recorded them onto chart paper, but now the vast majority use computers. Other requirements are an air or sand table to reduce vibration, and a Faraday cage to eliminate outside interference from the tiny measured currents.
Where experiments require low impedance measurements and no ionic contribution from the microelectrode, the chloride solution is replaced with cerralow, a low melting temperature alloy. The tip is electroplated with soft gold and platinum black, from chloroplatinic acid. Electrodes of this type are used to measure electrical pulses in unmyelinated axons down to 100 nm.
There are four main types of electrophysiological recording:
1. Cell attached mode - which records channel activity.
Advantages: Single channels can be recorded and channel properties are not changed.
Disadvantages: Poor pharmacology.
2. Whole cell recording.
Advantages: Good pharmacology, large current is recorded because it is the whole cell.
Disadvantages: The cell is perforated so cell contents are lost.
3. Excised patch.
Advantages: Recordings can be taken from individual channels, good pharmacology and the inside/outside solutions can be changed.
Disadvantages: Risk that channel properties are changed.
4. Axon recording.
Advantages: Chemistry of cell is unchanged, axon pulses are discriminated from the less frequent retrograde cell action potentials.
Disadvantages: Experimental protocol requirements are strict