The Action Potentials and Currents on the I-V Plane in the Molluscan Neuron |
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| Institution: | 1. Department of Civil and Environmental Engineering, The University of Western Ontario, London, Ontario N6A 5B9, Canada;2. Department of Civil Engineering, Lassonde School of Engineering, York University, Toronto, Ontario M3J 1P3, Canada;3. Department of Civil, Environmental and Geomatic Engineering, University College London, London WC1E 6BT, UK;1. Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA;2. SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA;3. Chair of Space Propulsion, Technical University of Munich, Garching 85748, Germany |
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| Abstract: | The action potentials and the corresponding transmembrane currents, directly recorded in the F1 neuron of Helix aspersa by the Self-clamp Technique, were plotted on the I-V plane to represent the real electrical cycle of the cell membrane during activity. The membrane electrical cycle, experimentally obtained, agreed in several aspects with a similar cycle obtained from calculated data on the giant axon of Loligo, but not for the sign, with the consequence of a different localization, as far as voltage and time are concerned, of the negative impedance period. The negative impedance proved to be −614 ± 181 Ω cm2 and corresponded to the late phase of the repolarization after the action potential peak. A constant positive impedance was found of 522 ± 131 Ω cm2 during the ascending tract of the action potential. These two results are in contrast with previous analyses. The simultaneous availability of the conjugate voltage and current directly measured signals led to the immediate representation of the membrane total conductance in its real time course during activity, in agreement with the Hodgkin and Huxley predictive model. The peak conductance was 1.9 ± 0.7 mmho/cm2 in this preparation. The electrical work spent to sustain a single active event proved to be 70 ± 19 nJ/cm2. A vectorial representation of the membrane electrical activity is proposed to describe analytically the characteristic behaviour of excitable cells, as well as a new method that utilizes the only action potential to measure the threshold potential in spontaneously discharging cells. The proposed new experimental protocol, based on the use of the Self-clamp Technique, proved to be faster, easier, more productive when compared with the conventional methods; it could be used advantageously in the electrophysiological studies on excitable cells both to define the basic conditions of the investigated preparation and to directly evaluate the effects of subsequent pharmacological stimulations. |
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