Electrophysiology is the science and branch of physiology 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 and the potential changes related to them.
At the cellular level, these include so-called passive recording as well "patch-clamp" technique, which "clamp" or maintain the cell potential (voltage-clamp ) or current (current-clamp ) at a level the experimenter may specify. This control is established using feedback through an operational amplifier circuit. The control of the membrane potential using voltage-clamp is most obviously of value in the study of voltage-gated ion channels, but also aids in characterizing conductance. Current-clamp, on the other hand, is used to study cell excitability by analyzing the action potentials produced by a cell.
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 capillar glass tube, which is then pulled to an even finer (but still hollow of about 1 micrometer diameter for patch-clamp) 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 pipet fluid and the tissue or cell into which the pipet is inserted (typically with the aid of a microscope and finely adjustable pipet holders, known as micromanipulators). The chloride-coated silver wire connects back to the amplifier. Classically, electrophysiologists watched biological currents/voltages on an oscilloscope and recorded them onto chart paper/screen, 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 cellular electrophysiological recordings:
1. Intracellular recording. This technique entails impaling a cell, usually a neuron, with a sharp glass electrode and recording either the currents (voltage-clamp) or the voltage (current-clamp) across the membrane. This technique is widely used when recording from brain slices or when performing "in-vivo" recording from live animals.
2. Extracellular recording. In this technique an electrode is placed on the extracellular medium and field-potentials contributed by the action potentials of many neurons are recorded. Some popular clinical applications of extracellular recording are the electrocardiogram (ECG) and the electroencephalogram (EEG).
3. The patch-clamp technique. With this technique it is possible to clamp the cell potential (voltage-clamp) or the cell current (current-clamp) using a glass micropipette as explained previously. Current-clamp recordings allow the detection and measurement of action potentials in excitable cells such as neurons and the beta cells of the pancreas. Voltage-clamp recordings are very popular for measuring macroscopic currents in which the activity of many ion channels is occurring at the same time. However with this powerful technique it is also possible to measure the current flowing through a single ion channel and study its behavior. There are different modalities of the patch-clamp technique.
- Advantages: Single channels can be recorded and channel properties are not changed.
- Disadvantages: Poor pharmacology.
- Advantages: Good pharmacology, large current is recorded because it is the whole cell.
- Disadvantages: The cell is perforated so some cell contents are diluted.
- Excised patch (inside-out or outside-out 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.
- Advantages: It is possible to obtain large, whole cell currents without washing out the intracellular medium.
- Disadvantages: Impossible to record from single channels.
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
Amperometry is another technique of electrophysiology, which uses a carbon electrode and is typically used to detect and record changes in the chemical composition of the oxidized components inside of biological solution being studied. It is typically employed for studing the neural exocytosis of some neurotransmitters.