The influence of t-system excitability upon the t-system response to a sarcolemmal AP. The response of E_t(n) (thin lines) and E_A (broken lines) to E_m changes (thick lines) during an AP is depicted for a model fiber with a t-system simulated as a single homogenous compartment (A and B) and for a model fiber with a 20-shell t-system (C and D). In A and C, the t-system lacks voltage-gated ion channels and therefore the voltage response is entirely passive. B and D demonstrate the response of a fiber with physiological voltage-gated ion channel densities within the t-system. Surface E_m changes are similar in each case, but the resulting tubular excitation is enhanced by the presence of voltage-gated ion channels within the t-system. (E) A summary of peak changes in fiber core E_t during the passage of a sarcolemmal AP. (F) Surface conduction velocity for fibers with (squares) and without (triangles) voltage-gated ion channels in the t-system, in the homogenous t-system model (dashed lines) and in the 20-shell t-system model, for model fibers with a range of values of R_A (solid lines). The vertical line in each case denotes the value of R_A used for the simulations in A–D. Increases in R_A reduce the passive electrical response of the t-system to the sarcolemmal AP (E) while accelerating sarcolemmal conduction velocity (F). A physiological value of R_A thus necessitates voltage-gated channels to produce a full t-system voltage response (E). Such channels have no direct effect on sarcolemmal conduction velocity (F).