Editing Action potential (section)

==Experimental methods==
{{See also|Electrophysiology}}

[[Image:Loligo forbesii.jpg|thumb|right|250px|Giant axons of the longfin inshore squid (''[[Doryteuthis pealeii]]'') were [[Marine Biological Laboratory#Neuroscience, neurobiology, and sensory physiology|crucial for scientists]] to understand the action potential.<ref>{{cite book |url=https://books.google.com/books?id=SDi2BQAAQBAJ |title=The Brain, the Nervous System, and Their Diseases |first=Jennifer L. |last=Hellier | name-list-style = vanc |year=2014 |pages=532 |publisher=ABC-Clio |isbn=9781610693387}}</ref>|alt=Illustration of the longfin inshore squid.]]

The study of action potentials has required the development of new experimental methods. The initial work, prior to 1955, was carried out primarily by [[Alan Lloyd Hodgkin]] and [[Andrew Fielding Huxley]], who were, along [[John Carew Eccles]], awarded the 1963 [[Nobel Prize in Physiology or Medicine]] for their contribution to the description of the ionic basis of nerve conduction. It focused on three goals: isolating signals from single neurons or axons, developing fast, sensitive electronics, and shrinking [[electrode]]s enough that the voltage inside a single cell could be recorded.

The first problem was solved by studying the [[Squid giant axon|giant axons]] found in the neurons of the [[squid]] (''[[Loligo forbesii]]'' and ''[[Doryteuthis pealeii]]'', at the time classified as ''Loligo pealeii'').<ref name="keynes_1989" group=lower-alpha>{{cite journal | vauthors = Keynes RD | title = The role of giant axons in studies of the nerve impulse | journal = BioEssays | volume = 10 | issue = 2–3 | pages = 90–3 | year = 1989 | pmid = 2541698 | doi = 10.1002/bies.950100213 }}</ref> These axons are so large in diameter (roughly 1&nbsp;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.<ref name="hodgkin_1952" group=lower-alpha /><ref name=Meunier group=lower-alpha>{{cite journal | vauthors = Meunier C, Segev I | title = Playing the devil's advocate: is the Hodgkin-Huxley model useful? | journal = Trends in Neurosciences | volume = 25 | issue = 11 | pages = 558–63 | date = November 2002 | pmid = 12392930 | doi = 10.1016/S0166-2236(02)02278-6 | s2cid = 1355280 }}</ref> However, they are not representative of all excitable cells, and numerous other systems with action potentials have been studied.

The second problem was addressed with the crucial development of the [[voltage clamp]],<ref name="cole_1949" group=lower-alpha>{{cite journal | vauthors = Cole KS | year = 1949 | title = Dynamic electrical characteristics of the squid axon membrane | journal = Arch. Sci. Physiol. | volume = 3 | pages = 253–8| author-link = Kenneth Stewart Cole }}</ref> which permitted experimenters to study the ionic currents underlying an action potential in isolation, and eliminated a key source of [[electronic noise]], the current ''I<sub>C</sub>'' associated with the [[capacitance]] ''C'' of the membrane.{{sfn|Junge|1981|pp=63–82}} Since the current equals ''C'' times the rate of change of the transmembrane voltage ''V<sub>m</sub>'', the solution was to design a circuit that kept ''V<sub>m</sub>'' fixed (zero rate of change) regardless of the currents flowing across the membrane. Thus, the current required to keep ''V<sub>m</sub>'' at a fixed value is a direct reflection of the current flowing through the membrane. Other electronic advances included the use of [[Faraday cage]]s and electronics with high [[input impedance]], so that the measurement itself did not affect the voltage being measured.{{sfn|Kettenmann|Grantyn|1992}}

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,<ref name="ling_1949" group=lower-alpha>{{cite journal | vauthors = Ling G, Gerard RW | title = The normal membrane potential of frog sartorius fibers | journal = Journal of Cellular and Comparative Physiology | volume = 34 | issue = 3 | pages = 383–96 | date = December 1949 | pmid = 15410483 | doi = 10.1002/jcp.1030340304 }}</ref> which was quickly adopted by other researchers.<ref name="nastuk_1950" group=lower-alpha>{{cite journal | vauthors = Nastuk WL, Hodgkin A | year = 1950 | title = The electrical activity of single muscle fibers | journal = Journal of Cellular and Comparative Physiology | volume = 35 | pages = 39–73 | doi = 10.1002/jcp.1030350105 }}</ref><ref name="brock_1952" group=lower-alpha>{{cite journal | vauthors = Brock LG, Coombs JS, Eccles JC | title = The recording of potentials from motoneurones with an intracellular electrode | journal = The Journal of Physiology | volume = 117 | issue = 4 | pages = 431–60 | date = August 1952 | pmid = 12991232 | pmc = 1392415 | doi = 10.1113/jphysiol.1952.sp004759 }}</ref> Refinements of this method are able to produce electrode tips that are as fine as 100 [[Ångström|Å]] (10 [[nanometre|nm]]), which also confers high input impedance.<ref>Snell, FM in {{harvnb|Lavallée|Schanne|Hébert|1969|loc=''Some Electrical Properties of Fine-Tipped Pipette Microelectrodes''.}}</ref> Action potentials may also be recorded with small metal electrodes placed just next to a neuron, with [[neurochip]]s containing [[EOSFET]]s, or optically with dyes that are [[Calcium imaging|sensitive to Ca<sup>2+</sup>]] or to voltage.<ref name="dyes" group=lower-alpha>{{cite journal | vauthors = Ross WN, Salzberg BM, Cohen LB, Davila HV | title = A large change in dye absorption during the action potential | journal = Biophysical Journal | volume = 14 | issue = 12 | pages = 983–6 | date = December 1974 | pmid = 4429774 | pmc = 1334592 | doi = 10.1016/S0006-3495(74)85963-1 | bibcode = 1974BpJ....14..983R }}<br />* {{cite journal | vauthors = Grynkiewicz G, Poenie M, Tsien RY | title = A new generation of Ca2+ indicators with greatly improved fluorescence properties | journal = The Journal of Biological Chemistry | volume = 260 | issue = 6 | pages = 3440–50 | date = March 1985 | doi = 10.1016/S0021-9258(19)83641-4 | pmid = 3838314 | doi-access = free }}</ref>

[[Image:Single channel.png|thumb|left|As revealed by a [[patch clamp]] electrode, an [[ion channel]] has two states: open (high conductance) and closed (low conductance).|alt=Plot of membrane potential versus time. The channel is primarily in a high conductance state punctuated by random and relatively brief transitions to a low conductance states ]]

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 discovery, they were awarded the [[Nobel Prize in Physiology or Medicine]] in 1991.<ref name="Nobel_1991" group=lower-Greek>{{cite press release | url = http://nobelprize.org/nobel_prizes/medicine/laureates/1991/press.html | title = The Nobel Prize in Physiology or Medicine 1991 | publisher = The Royal Swedish Academy of Science | year = 1991 | access-date = 2010-02-21 | url-status = live | archive-url = https://web.archive.org/web/20100324031907/http://nobelprize.org/nobel_prizes/medicine/laureates/1991/press.html | archive-date = 24 March 2010 | df = dmy-all }}</ref> 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. Using [[Potentiometric dyes|voltage-sensitive dyes]], action potentials have been optically recorded from a tiny patch of [[cardiomyocyte]] membrane.<ref name="pmid19289075" group=lower-alpha>{{cite journal | vauthors = Bu G, Adams H, Berbari EJ, Rubart M | title = Uniform action potential repolarization within the sarcolemma of in situ ventricular cardiomyocytes | journal = Biophysical Journal | volume = 96 | issue = 6 | pages = 2532–46 | date = March 2009 | pmid = 19289075 | pmc = 2907679 | doi = 10.1016/j.bpj.2008.12.3896 | bibcode = 2009BpJ....96.2532B }}</ref>