Module 7: Figure junctional zone Ca2+ channels
Distribution of Ca2+ channels in the ventricular cell junctional zone.
The top left of the figure illustrates the organization of the junctional zones that are distributed along the length of the T-tubule in register with the Z disc of the myofibrils. Junctional zones are connected to each other by the tubular network of free sarcoplasmic reticulum (SR). Approximately ten L-type channels are found in each T-tubule region that faces a junctional zone. The latter has approximately 100 type 2 ryanodine receptors (RYR2s) that are packed together in a pseudo-crystalline array. During excitation–contraction (E-C) coupling, approximately one L-type channel opens to create a sparklet, which then activates 10–15 RYR2s (e.g. those coloured red). The free SR contains the sarco/endo-plasmic reticulum Ca2+-ATPase (SERCA) pumps that transfer Ca2+ into the lumen, from which it diffuses towards the junctional zone.