Please cite as Berridge, M.J. (2011) Cell Signalling Biology; doi:10.1042/csb0001007
Excitationcontraction coupling in the junctional zone of ventricular cells.
Module 7: Figure ventricular cell E-C coupling (animated)
Depolarization of the T-tubule activates the α1 subunit of the Cav
1.2 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 Cav
1.2 channel is removed from the cell by the Na+
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.