Evaluation of electric field changes in the cleft between excitable cells.

Abstract Two contiguous myocardial cells with a narrow junctional cleft between them (intercalated disk) and bathed in a large volume conductor were modelled by an electrical analog circuit. Both a circuit analysis and a computer simulation were done. Each cell was divided into four lumped regions (units): (1) junctional membrane (JM) at left end of cell, (2) surface membrane (SM) at left half of cell, (3) SM at right half, and (4) JM at right end. Each region (unit) was represented by two parallel legs: (1) a K + resistance in series with a K + battery, and (2) a Na + resistance in series with a Na + battery. The JMs were given the same resistivity as the SM. The radial resistance of the cleft was varied to represent different cleft widths. Firing an action potential in one JM bordering the cleft (cell 1), by lowering its Na + resistance, caused the potential in the cleft to swing negative with respect to ground (fluid bathing the cells). Although the inner surface of the JM of cell 2 remained at nearly constant potential with respect to ground, the membrane was depolarized by the same degree as the negativity at its outer surface. The depolarization for a 200 A cleft was beyond the hypothetical threshold potential. The narrower the cleft, the greater the electric field effect. The activated JM, in turn, activated the remainder of cell 2. Thus, virtually no local-circuit current flowed through cell 2, and it fired without low-resistance connections between the cells. However, because simultaneous firing of the right half SM of cell 1 (unit 3) causes complete cancellation of the cleft potential, for the present model to give a workable basis for the transfer of excitation from cell to cell, it is required that unit 4 fire a fraction of a millisecond before unit 3. (Unit 2 causes partial cancellation.) This is possible for several reasons: (a) the depolarization of unit 4 (when unit 1 is fired) is slightly larger than that of unit 3, (b) K + accumulation in the cleft and (c) the threshold of the JM may be slightly lower. Thus, intercellular communication can occur electrically without low-resistance connections.

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