Fig. 7.

Proposed mechanisms of Ca²⁺ transients. (A) Schematic of imaged region. Five different regions are distinguished: (1) couplon with triggered activation, (2) adjacent to sarcolemma, (3) distant to sarcolemma, (4) RyR clusters not activated, and (5) RyR clusters activated through couplon or other release site. (B) Onset time is dependent on distance to early activated couplon. Only couplons are associated with an early onset time. (C) The relationship between distance and maximal upstroke velocity is dependent on the region. Only regions with couplons or indirectly activated RyR clusters exhibit a high upstroke velocity.