electrophysiology electrophysiology electrophysiology (from greek ἥλεκτ, ēlektron, "amber" ; φύσις, physis, "nature, origin"; and -λογία, -logia) 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. "current clamp" is a common technique in electrophysiology. this is a whole-cell current clamp recording of a neuron firing due to its being depolarized by current injection definition and scope 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 agent 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- + + -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: modalityabbreviationbody partprevalence in clinical use electrocardiographyecg or ekgheart (specifically, the cardiac muscle), with cutaneous electrodes (noninvasive)1—very common electroatriographyeagatrial cardiac muscle3—uncommon electroventriculographyevgventricular cardiac muscle3—uncommon intracardiac electrogramegmheart (specifically, the cardiac muscle), with intracardiac electrodes (invasive)2—somewhat common electroencephalographyeegbrain (usually the cerebral cortex), with extracranial electrodes2—somewhat common electrocorticographyecog or ieegbrain (specifically the cerebral cortex), with intracranial electrodes2—somewhat common electromyographyemgmuscles throughout the body (usually skeletal, occasionally smooth)1—very common electrooculographyeogeye—entire globe2—somewhat common electroretinographyergeye—retina specifically2—somewhat common electronystagmographyengeye—via the corneoretinal potential2—somewhat common electroolfactographyeogolfactory epithelium in mammals3—uncommon electroantennographyeagolfactory receptors in arthropod antennae4—not applicable clinically electrocochleographyecog or ecochgcochlea2—somewhat common electrogastrographyeggstomach smooth muscle2—somewhat common electrogastroenterographyegegstomach and bowel smooth muscle2—somewhat common electroglottographyeggglottis3—uncommon electropalatographyepgpalatal contact of tongue3—uncommon electroarteriographyeagarterial flow via streaming potential detected through skin3—uncommon electroblepharographyebgeyelid muscle3—uncommon electrodermographyedgskin3—uncommon electropancreatographyepgpancreas3—uncommon electrohysterographyehguterus3—uncommon electroneuronographyeneg or enognerves3—uncommon electropneumographyepglungs (chest movements)3—uncommon electrospinographyesgspinal cord3—uncommon electrovomerographyevgvomeronasal organ3—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. 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 pipet 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). voltage clamp main article: voltage clamp the voltage clamp uses a negative feedback mechanism. the membrane potential amplifier measures membrane voltage and sends output to the feedback amplifier. the feedback amplifier subtracts the membrane voltage from the command voltage, which it receives from the signal generator. this signal is amplified and returned into the cell via the recording electrode. 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. most current-clamp amplifiers provide little or no amplification of the voltage changes recorded from the cell. the "amplifier" is actually an electrometer, sometimes referred to as a "unity gain amplifier"; its main purpose is to reduce the electrical load on the small signals (in the mv range) produced by cells so that they can be accurately recorded by low-impedance electronics. the amplifier increases the current behind the signal while decreasing the resistance over which that current passes. consider this example based on ohm's law: a voltage of 10 mv is generated by passing 10 nanoamperes of current across 1 mω of resistance. the electrometer changes this "high impedance signal" to a "low impedance signal" by using a voltage follower circuit. a voltage follower reads the voltage on the input (caused by a small current through a big resistor). it then instructs a parallel circuit that has a large current source behind it (the electrical mains) and adjusts the resistance of that parallel circuit to give the same output voltage, but across a lower resistance. patch-clamp recording the cell-attached patch clamp uses a micropipette attached to the cell membrane to allow recording from a single ion channel. main article: patch clamp this technique was developed by erwin neher and bert sakmann who received the nobel prize in 1991. conventional intracellular recording involves impaling a cell with a fine electrode; patch-clamp recording takes a different approach. a patch-clamp microelectrode is a micropipette with a relatively large tip diameter. the microelectrode is placed next to a cell, and gentle suction is applied through the microelectrode to draw a piece of the cell membrane (the 'patch') into the microelectrode tip; the glass tip forms a high resistance 'seal' with the cell membrane. this configuration is the "cell-attached" mode, and it can be used for studying the activity of the ion channels that are present in the patch of membrane. if more suction is now applied, the small patch of membrane in the electrode tip can be displaced, leaving the electrode sealed to the rest of the cell. this "whole-cell" mode allows very stable intracellular recording. a disadvantage (compared to conventional intracellular recording with sharp electrodes) is that the intracellular fluid of the cell mixes with the solution inside the recording electrode, and so some important components of the intracellular fluid can be diluted. a variant of this technique, the "perforated patch" technique, tries to minimize these problems. instead of applying suction to displace the membrane patch from the electrode tip, it is also possible to make small holes on the patch with pore-forming agents so that large molecules such as proteins can stay inside the cell and ions can pass through the holes freely. also the patch of membrane can be pulled away from the rest of the cell. this approach enables the membrane properties of the patch to be analyzed pharmacologically. patch-clamp may also be combined with rna sequencing in a technique known as patch-seq by extracting the cellular contents following recording in order to characterize the electrophysiological properties relationship to gene expression and cell-type. sharp electrode recording in situations