Module 7: Figure ventricle and atrial cell organization
Comparison of the structural organization of cardiac ventricular and atrial cells.
In ventricular cells, T-tubules extend deep into the cell, where they make regular contact with the junctional sarcoplasmic reticulum (SR) to form junctional zones (see boxed area) that are responsible for excitation–contraction (E-C) coupling. The blue spheres are the type 2 ryanodine receptors (RYR2s) that release a Ca2+ spark that can occur spontaneously when the cells are at rest, as shown here. In atrial cells, there are no T-tubules, and the junctional zones are lined up on the sarcolemma at the cell surface. This junctional zone contains both RYR2s and type 2 inositol 1,4,5-trisphosphate receptors (InsP3R2s). In addition to the RYR2s in the junctional zone, the SR that extends into the atrial cell also contains these release units that are critical for amplifying the Ca2+ emanating from the sparks. This activation of ventricle and atrial cells is shown in Module 7: Figure ventricular and atrial cell kinetics.