EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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Cable ModelVoltage Clamp
Propagation of an Action Potential
Illustrations are taken from:J. Malmivuo, R. Plonsey, Bioelectromagnetism, Oxford Press, 1995http://butler.cc.tut.fi/~malmivuo/bem/book/
EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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Selective measurement of sodium and potassium currents by selective blocking of Na and K
channels
Control measurement without pharmacological agents.
After tetrodotoxin (TTX). After tetraethylammonium (TEA).
EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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The potassium ions cross the membrane only through channels that are specific for potassium. Hodgkin and Huxley supposed that the opening and closing of these channels are controlled by electrically charged particles called n-particles.
These may stay in a permissive (i.e., open) position (for instance inside the membrane) or in a nonpermissive (i.e., closed) position (for instance outside the membrane), and they move between these states (or positions) with first-order kinetics.
The probability of an n-particle being in the open position is described by the parameter n, and in the closed position by (1 - n), where 0 n 1. Thus, when the membrane potential is changed, the changing distribution of the n-particles is described by the probability of n relaxing exponentially toward a new value.
EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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The process determining the variation of K
conductance with depolarization
and repolarization
EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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The behavior of sodium conductance is initially similar to that of potassium conductance, except that the speed of the conductance increase during depolarization is about 10 times faster. The rise in sodium conductance occurs well before the rise in potassium conductance becomes appreciable.
Hodgkin and Huxley assumed again that at the sodium channels certain electrically charged particles called m-particles exist whose position control the opening of the channel.
Thus they have two states, open (permissive) and closed (nonpermissive); the proportion m expresses the fraction of these particles in the open state (for instance inside the membrane) and (1 - m) the fraction in the closed state (for instance outside the membrane), where 0 m 1.
EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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The process determining the variation of K conductance
with depolarization
and repolarization
EE-515 Bioelectricity & Biomagnetism 2002 Fall Murat Eyuboglu
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In addition to the variables discussed above, the constants of the Hodgkin-Huxley model are as shown:
Cm = 1 µF/cm²Vr - VNa = -115 mVVr - VK= +12 mVVr - VL= -10.613 mVGNa max = 120 mS/cm²GK max = 36 mS/cm²GL = 0.3 mS/cm²
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