Neurology Edited by Professor Emeritus Desire' Dubounet, IMUNE

This situation has an important implication when an action potential ismeasured with one electrode on the surface of an irritable tissue and the other in anarea of injury. Suppose that before excitation, the resting membrane potential is -70mV, that electrode A is on the intact surface of the irritable tissue, and that electrodeB is in the site of injury. Under this condition the potential difference between theelectrodes may be thirty-five percent of the membrane potential and amount toabout -25 mV. Now if the tissue is stimulated to the left of electrode A, whenexcitation reaches this electrode the potential difference measured between theelectrodes will be the algebraic sum of the potentials at the two electrodes. Forexample, assume that the membrane depolarizes and reverse polarizes to +20 mV;the potential difference was -25 mV just before depolarization and +65 mV at thepeak of reverse polarization. It will then return to -25 mV when the wave of excitationpasses the surface electrode. This sequence illustrates that a fair representation ofthe wave form of the transmembrane action potential can be obtained by injuring thetissue under one electrode. Important to note that, although the magnitude of thereverse polarization of the membrane amounted to only 20 mV, in the record itshowed up as a much larger potential of +65 mV. This situation probably serves toexplain the considerable reverse potential observed by Bernstein (1871) when hemeasured the nerve action potential with the resistance meter (see Hoff andGeddes, 1957).There is another point to consider when the action potential is measured withone electrode on an intact membrane and the other in a region of injury. Beforeexcitation there will be a standing potential difference (the injury potential), whosemagnitude will depend primarily on the location of the electrode at the site of injury.If electrode B is over the injured area, an appreciable percentage of the membranepotential may be detected; if it is moved a short distance from the site of injury andis over-excitable tissue, the steady (injury) potential difference between theelectrodes will be less. Now if the tissue is excited and excitation and recoverypasses under the surface electrode, the usual monophasic action potential willoccur, superimposed on a baseline of the injury potential. If the strip of irritabletissue is long with respect to the time of propagation of the impulse and the amountof tissue occupied by excitation is small with respect to the inter-electrode distance,excitation and recovery will take place under the first electrode before it enters theregion of electrode B, which is near the area of injury. Electrode B may also beclose to uninjured tissue, and therefore detect not only the injury potential but also anattenuated action potential as it advances toward the area of injury. Thus theresulting action potential measured between the two electrodes will be diphasic,consisting of a large monophasic action potential superimposed on the injurypotential, followed by a smaller monophasic action potential in the oppositedirection reflecting what electrode B detects from the depolarization andrepolarization of normal tissue near the site of injury. This is a factor used by QXCImachinery to find improper reactivity or to correlate proper reactivity.7