Predicted delays in the activation of the contractile system.

The capacitance C'(e), presumed to be located across the walls of the transverse tubules of twitch fibers, was identified in earlier impedance measurements by virtue of having a resistance in series with it. When the voltage V(m) across the surface membrane is made to vary, the voltage V(c) across C'(e) will be delayed with respect to V(m), the extent of the delay depending on the location of the series resistance. Model 1 assumes that the resistivity of the lumen of the tubules is negligible; model 2 assumes that the series resistance arises entirely in the tubular lumen; model 3 assumes that the resistivity of the tubular lumen is small, but not negligible and that the bulk of the resistance arises in a structure directly in series with C'(e) and having a similar geometric distribution. If V(m) varies sinusoidally, the relative value of V(c(max)) will fall with increasingly higher powers of the frequency at the center of the fiber if model 2 is applicable, whereas models 1 and 3 predict that V(c(max)) will fall at high frequency only in proportion to the frequency everywhere in the cross-section of the fiber. Equations have been derived for the voltage change V(c) in response to a step change of V(m) and during an action potential. On the assumption that contraction is initiated when V(c) reaches mechanical threshold, the delay between the activation of myofibrils on the axis of the fiber and at the surface would amount to 2.6 msec in model 2 and 0.25 msec in model 3 for frog fibers of about 100 mum diameter during a twitch.