Figure 11. Schematic representation depicting our explanation for the mechanism of re-excitation and reflection at the isthmus.

A, as the depolarizing wavefront (blue) enters the distal expansion there is a strong voltage gradient, and repolarizing electrotonic current flows from the expansion into the isthmus, which leads to conduction slowing and block. B, upon depolarization of the distal region, the voltage gradient is reversed because there is a large area at the depolarized state, while the small area within the isthmus is beginning to repolarize, resulting in APD prolongation at the expansion, and depolarizing electrotonic current flows into the cells within the isthmus, which face a high input impedance and membrane resistance. Consequently, there is re-excitation at the distal expansion, which leads to reflection.