Module 7: Figure ventricular cell E-C coupling
Excitation–contraction coupling in the junctional zone of ventricular cells.
Depolarization of the T-tubule activates the α1 subunit of the Cav1.2 L-type channel that introduces a small amount of Ca2+ to form a sparklet. Ca2+ diffuses across the 20 nm gap in the junctional zone to activate the type 2 ryanodine receptors (RYR2s) that release Ca2+ to form a much larger spark. The latter then diffuses away from the junctional zone to activate contraction in the neighbouring myofibrils. After releasing a package of Ca2+, the RYRs inactivate, and the released Ca2+ is then removed. During this recovery phase, a proportion similar to that entering through the Cav1.2 channel is removed from the cell by the Na+/Ca2+ exchanger (NCX), while the remainder is taken back into the sarcoplasmic reticulum (SR) by the sarco/endo-plasmic reticulum Ca2+-ATPase (SERCA) pumps distributed over the longitudinal free SR. The Ca2+ entering the latter is then transferred back to the junctional SR through a process of tunnelling. Ca2+ homoeostasis is maintained by virtue of the fact that the Ca2+ fluxes across the T-tubule and the SR are balanced during the excitation and recovery phases. An animated version of this figure is available.