Electrophysiology (from [see the etymology of "electron"]; ; and ) 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 (ion current) 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 (singles or arrays, often insulated except for the tip), 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.
The principal preparations include: living organisms (example in insects), excised tissue (acute or cultured), dissociated cells from excised tissue (acute or cultured), artificially grown cells or tissues, or hybrids of the above. Neuronal electrophysiology is the study of electrical properties of biological cells and tissues within the nervous system. With neuronal electrophysiology doctors and specialists can determine how neuronal disorders happen, by looking at the individual's brain activity. Activity such as which portions of the brain light up during any situations encountered. If an electrode is small enough (micrometers) in diameter, then the electrophysiologist may choose to insert the tip into a single cell. Such a configuration allows direct observation and intracellular recording 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 (hollow) 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 (whole-cell recording). 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 (perforated patch recording). Finally, the patch may be left intact (patch recording).
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 (from electro- + -graphy, "electrical recording"), with the record thus produced being an electrogram. However, the word electrography has other senses (including electrophotography), and the specific types of electrophysiological recording are usually called by specific names, constructed on the pattern of electro- + [body part combining form] + -graphy (abbreviation ExG). Relatedly, the word electrogram (not being needed for those other senses) often carries the specific meaning of intracardiac electrogram, which is like an electrocardiogram but with some invasive leads (inside the heart) rather than only noninvasive leads (on the skin). 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 (specifically, the cardiac muscle), with cutaneous electrodes (noninvasive) 1—very common electroatriography EAG atrial cardiac muscle 3—uncommon electroventriculography EVG ventricular cardiac muscle 3—uncommon intracardiac electrogram EGM heart (specifically, the cardiac muscle), with intracardiac electrodes (invasive) 2—somewhat common electroencephalography EEG brain (usually the cerebral cortex), with extracranial electrodes 1—very common electrocorticography ECoG or iEEG brain (specifically the cerebral cortex), with intracranial electrodes 2—somewhat common electromyography EMG muscles throughout the body (usually skeletal, occasionally smooth) 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 (chest movements) 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 (sharp) 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 (Loligo pealei), and were among the first applications of the "voltage clamp" technique.Movie featuring Alan Hodgkin recording action potentials from a squid axon https://www.youtube.com/watch?v=k48jXzFGMc8
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 (small enough to penetrate a single cell with minimum damage to the cell) and resistance (low enough so that small neuronal signals can be discerned from thermal noise in the electrode tip).
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.
Voltage clamp
The voltage clamp technique allows an experimenter to "clamp" the cell potential at a chosen value. This makes it possible to measure how much ionic current crosses a cell's membrane at any given voltage. This is important because many of the ion channels in the membrane of a neuron are voltage-gated ion channels, which open only when the membrane voltage is within a certain range. Voltage clamp measurements of current are made possible by the near-simultaneous digital subtraction of transient capacitive currents that pass as the recording electrode and cell membrane are charged to alter the cell's potential.
Current clamp
The current clamp technique records the membrane potential by injecting current into a cell through the recording electrode. Unlike in the voltage clamp mode, where the membrane potential is held at a level determined by the experimenter, in "current clamp" mode the membrane potential is free to vary, and the amplifier records whatever voltage the cell generates on its own or as a result of stimulation. This technique is used to study how a cell responds when electric current enters a cell; this is important for instance for understanding how neurons respond to neurotransmitters that act by opening membrane ion channels.