Page 92 - YORAM RUDY BOOK FINAL

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If the degree of inhomogeneity is increased by reducing gap junction conductance in the

poorly coupled segment to 0.07 μS (Figure 3.12), conduction fails in the transition from this
segment to the well coupled segment (panel A). In the transition zone, SF falls sharply from 2.8
and stays below 1 beyond cell 79 (panel B), resulting in propagation failure. The mismatch

between source and sink at the transition is too large for compensation by I to restore
Ca,L
conduction. In the opposite direction (panels C and D) conduction is maintained and occurs with
high SF along the entire fiber. Thus, unidirectional block is established by the asymmetry in tissue
structure. Of course, success or failure of conduction depends also on the strength of the source.
If I is reduced by 60% in the fiber of Figure 3.11, conduction fails at the transition from the poorly
Na
coupled to the well coupled segment but not in the reverse direction (not shown). A similar
unidirectional block occurs when I is reduced by 30%, highlighting again the important role of
Ca,L
this current where long delays are encountered by the propagating action potential.

Figure 3.12. Unidirectional block caused by inhomogeneous coupling. Top diagram: in this
simulation, g in the poorly coupled segment is further decreased to 0.07μS. Other conditions are
j
the same as in Figure 3.11. A and C: AP; numbers indicate selected cells. B and D: SF along the
fiber. g , gap-junction conductance. From Wang and Rudy [218] courtesy of The American
j
Physiological Society.