Effect of external resistance on propagation of action potentials in cardiac muscle and visceral smooth muscle in PSpice simulation

Propagation of action potentials in cardiac muscle and visceral smooth muscle were simulated using the PSpice program. Excitation was transmitted from cell to cell along a strand of six cells (cardiac muscle) or ten cells (smooth muscle) not connected by low-resistance tunnels (gap-junction connexons). The entire surface membrane of each cell fired nearly simultaneously, and nearly all the propagation time was spent at the cell junctions, thus, giving a staircase-shaped propagation profile. The junctional delay time was about 0.2-0.5 msec in cardiac muscle and 2–6 msec in smooth muscle. A significant negative cleft potential develops in the narrow junctional clefts, and its magnitude depends on several factors, including the radial cleft resistance (Rjc). The cleft potential (VC) depolarizes the postjunctional membrane (post-JM) to threshold by a patch-clamp action. Therefore, one mechanism for the transfer of excitation from one cell to the next is by the electric field (EF) that is generated in the junctional cleft when the pre-JM fires. When the external resistance (Ro) was increased over a very wide range (e.g., the radial (ror) and longitudinal (rol) components increased over 106 times) (from 1 KΩ to 1000 MΩ), propagation still occurred. At Rjc values between 5 MΩ and 40 MΩ, the propagation velocity (θ) actually increased in cardiac muscle. At lower Rjc: values (1, 0.1, 0.01MΩ), propagation was not only facilitated at high Ro values, but also enabled because partial or complete block occurred at the lower Ro values. Similar results were found for smooth muscle. These data underscore the fact that propagation, in this simulation model, does not occur by means of local-circuit current. Instead, transmission across the junctional cleft occurs by the EF in the cleft.