Figure 4-5. Cellular structures and pathways involved in calcium cycling during excitation-contraction coupling. Calcium channels shown in light blue. RYR, ryanodine receptor calcium release channels; NCX, sodium-calcium exchanger; PLB, phospholamban; ATP, adenosine triphosphate; sarcoplasmic reticular calcium ATPase (SERCA2a), transmembrane ATP-dependent calcium pump shown in purple and sodium-potassium ATPase shown in purple.