Electrophysiology
Electrophysiology is the branch of physiology that studies the electrical properties of biological cells and tissues. It involves measurements of voltage changes or electric current or manipulations on a wide variety of scales from single ion channel proteins to whole organs like the heart. In neuroscience, it includes measurements of the electrical activity of neurons, and, in particular, action potential activity. Recordings of large-scale electric signals from the nervous system, such as electroencephalography, may also be referred to as electrophysiological recordings. They are useful for electrodiagnosis and monitoring.
Definition and scope
Classical electrophysiological techniques
Principle and mechanisms
Electrophysiology is the branch of physiology that pertains broadly to the flow of ions in biological tissues and, in particular, to the electrical recording techniques that enable the measurement of this flow. Classical electrophysiology techniques involve placing electrodes into various preparations of biological tissue. The principal types of electrodes are:- Simple solid conductors, such as discs and needles,
- Tracings on printed circuit boards or flexible polymers, also insulated except for the tip, and
- Hollow, often elongated or 'pulled', tubes filled with an electrolyte, such as glass pipettes filled with potassium chloride solution or another electrolyte solution.
- living organisms,
- excised tissue,
- dissociated cells from excised tissue,
- artificially grown cells or tissues, or
- hybrids of the above.
If an electrode is small enough in diameter, then the electrophysiologist may choose to insert the tip into a single cell. Such a configuration allows direct observation and of the intracellular electrical activity of a single cell. However, this invasive setup reduces the life of the cell and causes a leak of substances across the cell membrane.
Intracellular activity may also be observed using a specially formed glass pipette containing an electrolyte. In this technique, the microscopic pipette tip is pressed against the cell membrane, to which it tightly adheres by an interaction between glass and lipids of the cell membrane. The electrolyte within the pipette may be brought into fluid continuity with the cytoplasm by delivering a pulse of negative pressure to the pipette in order to rupture the small patch of membrane encircled by the pipette rim. Alternatively, ionic continuity may be established by "perforating" the patch by allowing exogenous pore-forming agents within the electrolyte to insert themselves into the membrane patch. Finally, the patch may be left intact.
The electrophysiologist may choose not to insert the tip into a single cell. Instead, the electrode tip may be left in continuity with the extracellular space. If the tip is small enough, such a configuration may allow indirect observation and recording of action potentials from a single cell, termed single-unit recording. Depending on the preparation and precise placement, an extracellular configuration may pick up the activity of several nearby cells simultaneously, termed multi-unit recording.
As electrode size increases, the resolving power decreases. Larger electrodes are sensitive only to the net activity of many cells, termed local field potentials. Still larger electrodes, such as uninsulated needles and surface electrodes used by clinical and surgical neurophysiologists, are sensitive only to certain types of synchronous activity within populations of cells numbering in the millions.
Other classical electrophysiological techniques include single channel recording and amperometry.
Electrographic modalities by body part
Electrophysiological recording in general is sometimes called electrography, with the record thus produced being an electrogram. However, the word electrography has other senses, and the specific types of electrophysiological recording are usually called by specific names, constructed on the pattern of electro- + + -graphy. Relatedly, the word electrogram often carries the specific meaning of intracardiac electrogram, which is like an electrocardiogram but with some invasive leads rather than only noninvasive leads. Electrophysiological recording for clinical diagnostic purposes is included within the category of electrodiagnostic testing. The various "ExG" modes are as follows:| Modality | Abbreviation | Body part | Prevalence in clinical use |
| electrocardiography | ECG or EKG | heart, with cutaneous electrodes | 1—very common |
| electroatriography | EAG | atrial cardiac muscle | 3—uncommon |
| electroventriculography | EVG | ventricular cardiac muscle | 3—uncommon |
| intracardiac electrogram | EGM | heart, with intracardiac electrodes | 2—somewhat common |
| electroencephalography | EEG | brain, with extracranial electrodes | 1—very common |
| electrocorticography | ECoG or iEEG | brain, with intracranial electrodes | 2—somewhat common |
| electromyography | EMG | muscles throughout the body | 1—very common |
| electrooculography | EOG | eye—entire globe | 2—somewhat common |
| electroretinography | ERG | eye—retina specifically | 2—somewhat common |
| electronystagmography | ENG | eye—via the corneoretinal potential | 2—somewhat common |
| electroolfactography | EOG | olfactory epithelium in mammals | 3—uncommon |
| electroantennography | EAG | olfactory receptors in arthropod antennae | 4—not applicable clinically |
| electrocochleography | ECOG or ECochG | cochlea | 2—somewhat common |
| electrogastrography | EGG | stomach smooth muscle | 2—somewhat common |
| electrogastroenterography | EGEG | stomach and bowel smooth muscle | 2—somewhat common |
| electroglottography | EGG | glottis | 3—uncommon |
| electropalatography | EPG | palatal contact of tongue | 3—uncommon |
| electroarteriography | EAG | arterial flow via streaming potential detected through skin | 3—uncommon |
| electroblepharography | EBG | eyelid muscle | 3—uncommon |
| electrodermography | EDG | skin | 3—uncommon |
| electropancreatography | EPG | pancreas | 3—uncommon |
| electrohysterography | EHG | uterus | 3—uncommon |
| electroneuronography | ENeG or ENoG | nerves | 3—uncommon |
| electropneumography | EPG | lungs | 3—uncommon |
| electrospinography | ESG | spinal cord | 3—uncommon |
| electrovomerography | EVG | vomeronasal organ | 3—uncommon |
Optical electrophysiological techniques
Optical electrophysiological techniques were created by scientists and engineers to overcome one of the main limitations of classical techniques. Classical techniques allow observation of electrical activity at approximately a single point within a volume of tissue. Classical techniques singularize a distributed phenomenon. Interest in the spatial distribution of bioelectric activity prompted development of molecules capable of emitting light in response to their electrical or chemical environment. Examples are voltage sensitive dyes and fluorescing proteins.After introducing one or more such compounds into tissue via perfusion, injection or gene expression, the 1 or 2-dimensional distribution of electrical activity may be observed and recorded.
Intracellular recording
Intracellular recording involves measuring voltage and/or current across the membrane of a cell. To make an intracellular recording, the tip of a fine microelectrode must be inserted inside the cell, so that the membrane potential can be measured. Typically, the resting membrane potential of a healthy cell will be -60 to -80 mV, and during an action potential the membrane potential might reach +40 mV.In 1963, Alan Lloyd Hodgkin and Andrew Fielding Huxley won the Nobel Prize in Physiology or Medicine for their contribution to understanding the mechanisms underlying the generation of action potentials in neurons. Their experiments involved intracellular recordings from the giant axon of Atlantic squid, and were among the first applications of the "voltage clamp" technique.
Today, most microelectrodes used for intracellular recording are glass micropipettes, with a tip diameter of < 1 micrometre, and a resistance of several megohms. The micropipettes are filled with a solution that has a similar ionic composition to the intracellular fluid of the cell. A chlorided silver wire inserted into the pipette connects the electrolyte electrically to the amplifier and signal processing circuit. The voltage measured by the electrode is compared to the voltage of a reference electrode, usually a silver chloride-coated silver wire in contact with the extracellular fluid around the cell. In general, the smaller the electrode tip, the higher its electrical resistance. So an electrode is a compromise between size and resistance.
Maintaining healthy brain slices is pivotal for successful electrophysiological recordings. The preparation of these slices is commonly achieved with tools such as the Compresstome vibratome, ensuring optimal conditions for accurate and reliable recordings. Nevertheless, even with the highest standards of tissue handling, slice preparation induces rapid and robust phenotype changes of the brain's major immune cells, microglia, which must be taken into consideration when using this model.