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29 novembre 2011 2 29 /11 /novembre /2011 02:54

Neuromuscular junctions

A special case of a chemical synapse is the neuromuscular junction, in which the axon of a motor neuron terminates on a muscle fiber. In such cases, the released neurotransmitter is acetylcholine, which binds to the acetylcholine receptor, an integral membrane protein in the membrane (the sarcolemma) of the muscle fiber.Sony VAIO VPCF116FGBI Battery

However, the acetylcholine does not remain bound; rather, it dissociates and is hydrolyzed by the enzyme,acetylcholinesterase, located in the synapse. This enzyme quickly reduces the stimulus to the muscle, which allows the degree and timing of muscular contraction to be regulated delicately. Sony VAIO VPCF117FJ/W Battery

Some poisons inactivate acetylcholinesterase to prevent this control, such as the nerve agents sarin and tabun, and the insecticidesdiazinon and malathion.

The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual.Sony VAIO VPCF117HG/BI Battery

This plateau is due to the action of slower calcium channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated.

The cardiac action potential plays an important role in coordinating the contraction of the heart.Sony VAIO VPCF118FJ/W Battery

The cardiac cells of thesinoatrial node provide the pacemaker potential that synchronizes the heart. The action potentials of those cells propagate to and through the atrioventricular node (AV node), which is normally the only conduction pathway between the atria and the ventricles. Sony VAIO VPCF119FC Battery

Action potentials from the AV node travel through the bundle of His and thence to the Purkinje fibers.[note 1] Conversely, anomalies in the cardiac action potential—whether due to a congenital mutation or injury—can lead to human pathologies, especially arrhythmias.Sony VAIO VPCF119FC/BI Battery

Several anti-arrhythmia drugs act on the cardiac action potential, such as quinidine, lidocaine, beta blockers, and verapamil.

Muscular action potentials

The action potential in a normal skeletal muscle cell is similar to the action potential in neurons. Sony VAIO VPCF119FJ/BI Battery

Action potentials result from the depolarization of the cell membrane (the sarcolemma), which opens voltage-sensitive sodium channels; these become inactivated and the membrane is repolarized through the outward current of potassium ions. The resting potential prior to the action potential is typically ?90mV, somewhat more negative than typical neurons.Sony VAIO VPCF11AFJ Battery

The muscle action potential lasts roughly 2–4 ms, the absolute refractory period is roughly 1–3 ms, and the conduction velocity along the muscle is roughly 5 m/s. The action potential releases calcium ions that free up the tropomyosin and allow the muscle to contract.Sony VAIO VPCF11AGJ Battery

Muscle action potentials are provoked by the arrival of a pre-synaptic neuronal action potential at the neuromuscular junction, which is a common target for neurotoxins.

Plant action potentials

Plant and fungal cells  are also electrically excitable. The fundamental difference to animal action potentials is, that the depolarization in plant cells is not accomplished by an uptake of positive sodium ions, but by release of negative chloride ions.Sony VAIO VPCF11AHJ Battery

Together with the following release of positive potassium ions, which is common to plant and animal action potentials, the action potential in plants infers, therefore, an osmotic loss of salt (KCl), whereas the animal action potential is osmotically neutral, when equal amounts of entering sodium and leaving potassium cancel each other osmotically. Sony VAIO VPCF11JFX/B Battery

The interaction of electrical and osmotic relations in plant cells   indicates an osmotic function of electrical excitability in the common, unicellular ancestors of plants and animals under changing salinity conditions, whereas the present function of rapid signal transmission is seen as a younger accomplishment of metazoan cells in a more stable osmotic environment.Sony VAIO VPCF11M1E Battery

It must be assumed that the familiar signalling function of action potentials in some vascular plants (e.g. Mimosa pudica), arose independently from that in metazoan excitable cells.

Taxonomic distribution and evolutionary advantages

Action potentials are found throughout multicellular organisms, including plants, invertebrates such as insects, and vertebrates such as reptiles and mammals.Sony VAIO VPCF11M1E/H Battery

Sponges seem to be the main phylum of multicellular eukaryotes that does not transmit action potentials, although some studies have suggested that these organisms have a form of electrical signaling, too. The resting potential, as well as the size and duration of the action potential, Sony VAIO VPCF11MFX/B Battery

have not varied much with evolution, although the conduction velocity does vary dramatically with axonal diameter and myelination.

Given its conservation throughout evolution, the action potential seems to confer evolutionary advantages. Sony VAIO VPCF11S1E Battery

One function of action potentials is rapid, long-range signaling within the organism; the conduction velocity can exceed 110 m/s, which is one-third the speed of sound. For comparison, a hormone molecule carried in the bloodstream moves at roughly 8 m/s in large arteries. Part of this function is the tight coordination of mechanical events, such as the contraction of the heart.Sony VAIO VPCF11S1E/B Battery

A second function is the computation associated with its generation. Being an all-or-none signal that does not decay with transmission distance, the action potential has similar advantages to digital electronics. The integration of various dendritic signals at the axon hillock and its thresholding to form a complex train of action potentials is another form of computation, Sony VAIO VPCF11Z1E Battery

one that has been exploited biologically to form central pattern generators and mimicked in artificial neural networks.

Experimental methods

The study of action potentials has required the development of new experimental methods. Sony VAIO VPCF11Z1E/BI Battery

The initial work, prior to 1955, focused on three goals: isolating signals from single neurons or axons, developing fast, sensitive electronics, and shrinkingelectrodes enough that the voltage inside a single cell could be recorded.

The first problem was solved by studying the giant axons found in the neurons of the squid genus Loligo.Sony VAIO VPCF11ZHJ Battery

These axons are so large in diameter (roughly 1 mm, or 100-fold larger than a typical neuron) that they can be seen with the naked eye, making them easy to extract and manipulate. However, the Loligo axons are not representative of all excitable cells, and numerous other systems with action potentials have been studied.Sony VAIO VPCF127HGBI Battery

The second problem was addressed with the crucial development of the voltage clamp, which permitted experimenters to study the ionic currents underlying an action potential in isolation, and eliminated a key source of electronic noise, the currentIC associated with the capacitance C of the membrane.Sony VAIO VPCF137HG/BI Battery

Since the current equals C times the rate of change of the transmembrane voltage Vm, the solution was to design a circuit that kept Vm fixed (zero rate of change) regardless of the currents flowing across the membrane. Thus, the current required to keep Vm at a fixed value is a direct reflection of the current flowing through the membrane. Sony VAIO VPCS111FM/S Battery

Other electronic advances included the use of Faraday cages and electronics with highinput impedance, so that the measurement itself did not affect the voltage being measured.

The third problem, that of obtaining electrodes small enough to record voltages within a single axon without perturbing it, was solved in 1949 with the invention of the glass micropipette electrode, Sony VAIO VPCS115EC Battery

which was quickly adopted by other researchers. Refinements of this method are able to produce electrode tips that are as fine as 100 Å (10 nm), which also confers high input impedance. Action potentials may also be recorded with small metal electrodes placed just next to a neuron, with neurochips containing EOSFETs, or optically with dyes that are sensitive to Ca2+ or to voltage.Sony VAIO VPCS115FG Battery

While glass micropipette electrodes measure the sum of the currents passing through many ion channels, studying the electrical properties of a single ion channel became possible in the 1970s with the development of the patch clamp by Erwin Neher and Bert Sakmann. For this they were awarded the Nobel Prize in Physiology or Medicine in 1991.Sony VAIO VPCS117GG Battery

Patch-clamping verified that ionic channels have discrete states of conductance, such as open, closed and inactivated.

Optical imaging technologies have been developed in recent years to measure action potentials, either via simultaneous multisite recordings or with ultra spatial resolution. Sony VAIO VPCS117GGB Battery

Using voltage-sensitive dyes, action potentials have been optically recorded from a tiny patch of cardiomyocyte membrane.

Neurotoxins

Several neurotoxins, both natural and synthetic, are designed to block the action potential. Sony VAIO VPCS118EC Battery

Tetrodotoxin from the pufferfish andsaxitoxin from the Gonyaulax (the dinoflagellate genus responsible for "red tides") block action potentials by inhibiting the voltage-sensitive sodium channel; similarly, dendrotoxin from the black mamba snake inhibits the voltage-sensitive potassium channel. Sony VAIO VPCS119FJ/B Battery

Such inhibitors of ion channels serve an important research purpose, by allowing scientists to "turn off" specific channels at will, thus isolating the other channels' contributions; they can also be useful in purifying ion channels by affinity chromatography or in assaying their concentration.Sony VAIO VPCS119GC Battery

However, such inhibitors also make effective neurotoxins, and have been considered for use as chemical weapons. Neurotoxins aimed at the ion channels of insects have been effective insecticides; one example is the synthetic permethrin, which prolongs the activation of the sodium channels involved in action potentials. Sony VAIO VPCS11AFJ Battery

The ion channels of insects are sufficiently different from their human counterparts that there are few side effects in humans. Many other neurotoxins interfere with the transmission of the action potential's effects at the synapses, especially at the neuromuscular junction.Sony VAIO VPCS11AGJ Battery

The role of electricity in the nervous systems of animals was first observed in dissectedfrogs by Luigi Galvani, who studied it from 1791 to 1797.  Galvani's results stimulatedAlessandro Volta to develop the Voltaic pile—the earliest-known electric battery—with which he studied animal electricity (such as electric eels) and the physiological responses to applied direct-current voltages.Sony VAIO VPCS11AHJ Battery

Scientists of the 19th century studied the propagation of electrical signals in whole nerves (i.e., bundles of neurons) and demonstrated that nervous tissue was made up of cells, instead of an interconnected network of tubes (a reticulum). Sony VAIO VPCS11AVJ Battery

Carlo Matteucci followed up Galvani's studies and demonstrated that cell membranes had a voltage across them and could produce direct current. Matteucci's work inspired the German physiologist, Emil du Bois-Reymond, who discovered the action potential in 1848.Sony VAIO VPCS11J7E/B Battery

Theconduction velocity of action potentials was first measured in 1850 by du Bois-Reymond's friend, Hermann von Helmholtz. To establish that nervous tissue is made up of discrete cells, the Spanish physician Santiago Ramón y Cajal and his students used a stain developed by Camillo Golgi to reveal the myriad shapes of neurons, which they rendered painstakingly. Sony VAIO VPCS11M1E/W Battery

For their discoveries, Golgi and Ramón y Cajal were awarded the 1906 Nobel Prize in Physiology. Their work resolved a long-standing controversy in the neuroanatomy of the 19th century; Golgi himself had argued for the network model of the nervous system.Sony VAIO VPCS11V9E Battery

The 20th century was a golden era for electrophysiology. In 1902 and again in 1912, Julius Bernstein advanced the hypothesis that the action potential resulted from a change in thepermeability of the axonal membrane to ions. Bernstein's hypothesis was confirmed by Ken Cole and Howard Curtis, who showed that membrane conductance increases during an action potential.Sony VAIO VPCS11V9E/B Battery

In 1907, Louis Lapicque suggested that the action potential was generated as a threshold was crossed, what would be later shown as a product of the dynamical systems of ionic conductances. In 1949, Alan Hodgkin and Bernard Katz refined Bernstein's hypothesis by considering that the axonal membrane might have different permeabilities to different ions;Sony VAIO VPCS11X9E/B Battery

in particular, they demonstrated the crucial role of the sodium permeability for the action potential. This line of research culminated in the five 1952 papers of Hodgkin, Katz and Andrew Huxley, in which they applied the voltage clamp technique to determine the dependence of the axonal membrane's permeabilities to sodium and potassium ions on voltage and time, from which they were able to reconstruct the action potential quantitatively.Sony VAIO VPCS123FGB Battery

Hodgkin and Huxley correlated the properties of their mathematical model with discrete ion channels that could exist in several different states, including "open", "closed", and "inactivated". Their hypotheses were confirmed in the mid-1970s and 1980s by Erwin Neher and Bert Sakmann, who developed the technique of patch clamping to examine the conductance states of individual ion channels.Sony VAIO VPCS125EC Battery

In the 21st century, researchers are beginning to understand the structural basis for these conductance states and for the selectivity of channels for their species of ion,[105] through the atomic-resolution crystal structures,[106] fluorescence distance measurements[107] and cryo-electron microscopy studies.Sony VAIO VPCS128EC Battery

Julius Bernstein was also the first to introduce the Nernst equation for resting potential across the membrane; this was generalized by David E. Goldman to the eponymous Goldman equation in 1943. The sodium–potassium pump was identified in 1957 and its properties gradually elucidated,culminating in the determination of its atomic-resolution structure by X-ray crystallography.Sony VAIO VPCS129GC Battery

The crystal structures of related ionic pumps have also been solved, giving a broader view of how these molecular machines work.

Quantitative models

Mathematical and computational models are essential for understanding the action potential, and offer predictions that may be tested against experimental data, providing a stringent test of a theory. Sony VAIO VPCS12C7E/B Battery

The most important and accurate of these models is the Hodgkin–Huxley model, which describes the action potential by a coupled set of four ordinary differential equations (ODEs).  Although the Hodgkin–Huxley model may be a simplification of a realistic nervous membrane as it exists in nature, Sony VAIO VPCS12L9E/B Battery

its complexity has inspired several even-more-simplified models, such as the Morris–Lecar model and the FitzHugh–Nagumo model, both of which have only two coupled ODEs. The properties of the Hodgkin–Huxley and FitzHugh–Nagumo models and their relatives, such as the Bonhoeffer–van der Pol model,  have been well-studied within mathematics,Sony VAIO VPCS12V9E/B Battery

computation and electronics. More modern research has focused on larger and more integrated systems; by joining action-potential models with models of other parts of the nervous system (such as dendrites and synapses), researches can study neural computation and simple reflexes, such as escape reflexes and others controlled by central pattern generators.Sony VAIO VPCY115FGS Battery

Electrophysiology (from Greek ????????, ?lektron, "amber" [see the etymology of "electron"]; ?????, physis, "nature, origin"; and -?????, -logia) is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart. Sony VAIO VPCY115FX/BI Battery

In neuroscience, it includes measurements of the electrical activity of neurons, and particularlyaction potential activity. Recordings of large-scale electric signals from the nervous system such as electroencephalography, may also be referred to as electrophysiological recordings.Sony VAIO VPCY115FXBI Battery

Classical electrophysiological techniques

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. Classical electrophysiology techniques involve placing electrodes into various preparations of biological tissue. Sony VAIO VPCY118EC Battery

The principal types of electrodes are: 1) simple solid conductors, such as discs and needles (singles or arrays, often insulated except for the tip), 2) tracings on printed circuit boards, also insulated except for the tip, and 3) hollow tubes filled with an electrolyte, such as glass pipettes filled with potassium chloride solution or another electrolyte solution. Sony VAIO VPCY118GX/BI Battery

The principal preparations include 1) living organisms, 2) excised tissue (acute or cultured), 3) dissociated cells from excised tissue (acute or cultured), 4) artificially grown cells or tissues, or 5) hybrids of the above.

If an electrode is small enough (micrometers) in diameter, then the electro-physiologist may choose to insert the tip into a single cell. Sony VAIO VPCY119FJ/S Battery

Such a configuration allows direct observation and recording of the intracellular electrical activity of a single cell. However, at the same time such 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. Sony VAIO VPCY11AFJ Battery

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 pressure to the electrolyte in order to rupture the small patch of membrane encircled by the pipette rim (whole-cell recording).Sony VAIO VPCY11AGJ Battery

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).Sony VAIO VPCY11AHJ Battery

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, and is termed single-unit recording. Sony VAIO VPCY11AVJ Battery

Depending on the preparation and precise placement, an extracellular configuration may pick up the activity of several nearby cells simultaneously, and this is 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. Sony VAIO VPCY11M1E/S Battery

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.Sony VAIO VPCY11S1E Battery

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.Sony VAIO VPCY11V9E/S Battery

Essentially, 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.Sony VAIO VPCY218EC/BI Battery

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 involves measuring voltage and/or current across the membrane of a cell. Sony VAIO VPCY218EC/G Battery

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. Sony VAIO VPCY218EC/L Battery

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), Sony VAIO VPCY218EC/P Battery

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 megaohms. The micropipettes are filled with a solution that has a similar ionic composition to the intracellular fluid of the cell.Sony VAIO VPCY21S1E/L Battery

A chlorided silver wire inserted in to 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. Sony VAIO VPCY21S1E/P Battery

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).Sony VAIO VPCY21S1E/SI Battery

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. Sony VAIO VPCCW2S5C CN1 Battery

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.Sony VAIO VPCEA20 Battery

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,Sony VAIO VPCEB10 Battery

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 neurotransmittersthat act by opening membrane ion channels.Sony VAIO VPCEB11FM Battery

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 job is to change the nature of small signals (in the mV range) produced by cells so that they can be accurately recorded by low-impedanceelectronics.Sony VAIO VPCEB11FM/BI Battery

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 M? of resistance. The electrometer changes this "high impedance signal" to a "low impedance signal" by using a voltage follower circuit. Sony VAIO VPCEB11FM/T Battery

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.Sony VAIO VPCEB11FM/WI Battery

The patch-clamp technique

This technique was developed by Erwin Neher and Bert Sakmann who received the Nobel Prize in 1991.[2] 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. Sony VAIO VPCEB11FX Battery

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. Sony VAIO VPCEB11FX/BI Battery

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. Sony VAIO VPCEB11FX/T Battery

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 minimise these problems.Sony VAIO VPCEB11FX/WI Battery

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. Sony VAIO VPCEB11GX Battery

This approach enables the membrane properties of the patch to be analysed pharmacologically.

Sharp electrode technique

In situations where one wants to record the potential inside the cell membrane with minimal effect on the ionic constitution of the intracellular fluid a sharp electrode can be used.Sony VAIO VPCEB11GX/BI Battery

These micropipettes (electrodes) are again like those for patch clamp pulled from glass capillaries, but the pore is much smaller so that there is very little ion exchange between the intracellular fluid and the electrolyte in the pipette. The resistance of the micropipette electrode is tens or hundreds of M?.Sony VAIO VPCEB11GX/T Battery

Often the tip of the electrode is filled with various kinds of dyes like Lucifer yellow to fill the cells recorded from, for later confirmation of their morphology under a microscope. The dyes are injected by applying a positive or negative, DC or pulsed voltage to the electrodes depending on the polarity of the dye.Sony VAIO VPCEB11GX/WI Battery

Single-unit recording

An electrode introduced into the brain of a living animal will detect electrical activity that is generated by the neurons adjacent to the electrode tip. If the electrode is a microelectrode, with a tip size of about 1 micrometre, the electrode will usually detect the activity of at most one neuron. Recording in this way is in general called "single-unit" recording. Sony VAIO VPCEB12FX Battery

The action potentials recorded are very much like the action potentials that are recorded intracellularly, but the signals are very much smaller (typically about 1 mV). Most recordings of the activity of single neurons in anesthetized animals are made in this way, and all recordings of single neurons in conscious animals. Sony VAIO VPCEB12FX/BI Battery

Recordings of single neurons in living animals have provided important insights into how the brain processes information. For example, David Hubel and Torsten Wiesel recorded the activity of single neurons in the primary visual cortex of the anesthetized cat, and showed how single neurons in this area respond to very specific features of a visual stimulus.Sony VAIO VPCEB12FX/BIC Battery

Hubel and Wiesel were awarded the Nobel Prize in Physiology or Medicine in 1981. If the electrode tip is slightly larger, then the electrode might record the activity generated by several neurons. This type of recording is often called "multi-unit recording", and is often used in conscious animals to record changes in the activity in a discrete brain area during normal activity.Sony VAIO VPCEB12FX/T Battery

Recordings from one or more such electrodes that are closely spaced can be used to identify the number of cells around it as well as which of the spikes come from which cell. This process is called spike sorting and is suitable in areas where there are identified types of cells with well defined spike characteristics. Sony VAIO VPCEB14FX Battery

If the electrode tip is bigger still, in general the activity of individual neurons cannot be distinguished but the electrode will still be able to record a field potential generated by the activity of many cells.

Field potentials

Extracellular field potentials are local current sinks or sources that are generated by the collective activity of many cells. Sony VAIO VPCEB14FX/BI Battery

Usually, a field potential is generated by the simultaneous activation of many neurons by synaptic transmission. The diagram to the right shows hippocampal synaptic field potentials. At the right, the lower trace shows a negative wave that corresponds to a current sink caused by positive charges entering cells through postsynaptic glutamate receptors,Sony VAIO VPCEB14FX/T Battery

while the upper trace shows a positive wave that is generated by the current that leaves the cell (at the cell body) to complete the circuit. For more information, see local field potential.

Amperometry

Amperometry uses a carbon electrode to record changes in the chemical composition of the oxidized components of a biological solution. Sony VAIO VPCEB14FX/WI Battery

Oxidation and reduction is accomplished by changing the voltage at the active surface of the recording electrode in a process known as "scanning". Because certain brain chemicals lose or gain electrons at characteristic voltages, individual species can be identified. Amperometry has been used for studying exocytosis in the neural and endocrine systems. Sony VAIO VPCEB15FM Battery

Many monoamineneurotransmitters; e.g., norepinephrine (noradrenalin), dopamine, and serotonin (5-HT) are oxidizable. The method can also be used with cells that do not secrete oxidizable neurotransmitters by "loading" them with 5-HT or dopamine.

Planar patch clamp is a novel method developed for high throughput electrophysiology. Sony VAIO VPCEB15FM/BI Battery

Instead of positioning a pipette on an adherent cell, cell suspension is pipetted on a chip containing a microstructured aperture.

A single cell is then positioned on the hole by suction and a tight connection (Gigaseal) is formed. The planar geometry offers a variety of advantages compared to the classical experiment: -Sony VAIO VPCEB15FM/T Battery

it allows for integration of microfluidics, which enables automatic compound application for ion channelscreening. - the system is accessible for optical or scanning probe techniques - perfusion of theintracellular side can be performed.Sony VAIO VPCEB15FM/WI Battery

Solid-supported membrane (SSM) based

With this electrophysiological approach, proteoliposomes, membrane vesicles, or membrane fragments containing the channel or transporter of interest are adsorbed to a lipid monolayer painted over a functionalized electrode. This electrode consists of a glass support, a chromium layer, a gold layer, and an octadecyl mercaptanemonolayer. Sony VAIO VPCEB15FX Battery

Because the painted membrane is supported by the electrode, it is called a solid-supported membrane. It is important to note that mechanical perturbations, which usually destroy a biological lipid membrane, do not influence the life-time of an SSM.Sony VAIO VPCEB15FX/BI Battery

The capacitive electrode (composed of the SSM and the absorbed vesicles) is so mechanically stable that solutions may be rapidly exchanged at its surface. This property allows the application of rapid substrate/liga

nd concentration jumps to investigate the electrogenic activity of the protein of interest, measured via capacitive coupling between the vesicles and the electrode.Sony VAIO VPCEB15FX/T Battery

Bioelectric recognition assay (BERA)

The bioelectric recognition assay (BERA) is a novel method for determination of various chemical and biological molecules by measuring changes in the membrane potential of cells immobilized in a gel matrix. Apart from the increased stability of the electrode-cell interface, immobilization preserves the viability and physiological functions of the cells. Sony VAIO VPCEB15FX/WI Battery

BERA is used primarily in biosensor applications in order to assay analytes that can interact with the immobilized cells by changing the cell membrane potential. In this way, when a positive sample is added to the sensor, a characteristic, ‘signature-like’ change in electrical potential occurs. Sony VAIO VPCEB16FX Battery

BERA has been used for the detection for human viruses (Hepatitis B and C viruses, herpes viruses) and veterinary disease agents (foot and mouth disease virus, prions, blue tongue virus) and plants (tobacco and cucumber viruses) in a highly specific, rapid (1–2 minutes), reproducible and cost-efficient fashion. Sony VAIO VPCEB16FX/B Battery

The method has also been used for the detection of environmental toxins, such as herbicides and the determination of very low concentrations of superoxide anion in clinical samples.

A recent advance in the evolution of the BERA technology is the development of a technique called molecular identification through membrane engineering (MIME). Sony VAIO VPCEB16FX/G Battery

This technique allows for building cells with absolutely defined specificity for virtually any molecule of interest, by embedding thousand of artificial receptors into the cell membrane.

Reporting guidelines for electrophysiology experiments

Minimum Information (MI) standards or reporting guidelines specify the minimum amount of meta data (information) and data required to meet a specific aims or aims. Sony VAIO VPCEB16FX/L Battery

Usually the aim is to provide enough meta data and data to enable the unambiguous reproduction and interpretation of an experiment. MI guidelines are normally informal human readable specifications that inform the development of formal data models (e.g. XML or UML), data exchange formats (e.g. FuGE, MAGE-ML, MAGE-TAB) or knowledge models such as an ontology (e.g. OBI, MGED-Ontology).Sony VAIO VPCEB16FX/P Battery

The Minimum Information about a Neuroscience investigation (MINI) family of reporting guideline documents, produced by community consultation and continually available for public comment aims to provide a consistent set of guidelines in order to report an electrophysiology experiment.Sony VAIO VPCEB16FX/W Battery

A MINI module represents the minimum information that should be reported about a dataset to facilitate computational access and analysis to allow a reader to interpret and critically evaluate the processes performed and the conclusions reached, and to support their experimental corroboration. Sony VAIO VPCEB17FX Battery

In practice a MINI module comprises a checklist of information that should be provided (for example about the protocols employed) whena data set is described for publication. The full specification of the MINI module can be found here.Sony VAIO VPCEB17FX/B Battery,Sony VAIO VPCEB17FX/G Battery,Sony VAIO VPCEB17FX/L Battery,Sony VAIO VPCEB17FX/P Battery,Sony VAIO VPCEB17FX/W Battery

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