What is the role of the sarcoplasmic reticulum in cardiac muscle?

What is the role of the sarcoplasmic reticulum in cardiac muscle?

• A. It surrounds myofilaments and regulates calcium ion concentration for contraction.
• B. It stores ATP for energy.
• C. It generates action potentials.
• D. It pumps blood out of the cell.

Answer: (A)

The key idea is that calcium regulation inside the cell controls contraction. The sarcoplasmic reticulum surrounds the myofilaments and acts as the intracellular store for Ca2+. When cardiac muscle is excited, calcium enters the cell and triggers the SR to release more Ca2+ into the cytoplasm. The increased cytosolic Ca2+ binds to troponin C on the thin filaments, moving tropomyosin away from the myosin-binding sites on actin, which allows cross-bridge cycling and contraction to occur. After contraction, calcium is pumped back into the SR by SERCA pumps, lowering cytosolic Ca2+ and allowing relaxation. So the SR’s role is to regulate calcium levels to control contraction. ATP storage, action potential generation, and pumping blood out of the cell are not functions of the sarcoplasmic reticulum: ATP is derived mainly from mitochondria and glycolysis, action potentials come from membrane ion channels and propagation, and pumping blood out is the mechanical job of the heart muscle during contraction.

The key idea is that calcium regulation inside the cell controls contraction. The sarcoplasmic reticulum surrounds the myofilaments and acts as the intracellular store for Ca2+. When cardiac muscle is excited, calcium enters the cell and triggers the SR to release more Ca2+ into the cytoplasm. The increased cytosolic Ca2+ binds to troponin C on the thin filaments, moving tropomyosin away from the myosin-binding sites on actin, which allows cross-bridge cycling and contraction to occur. After contraction, calcium is pumped back into the SR by SERCA pumps, lowering cytosolic Ca2+ and allowing relaxation. So the SR’s role is to regulate calcium levels to control contraction.

ATP storage, action potential generation, and pumping blood out of the cell are not functions of the sarcoplasmic reticulum: ATP is derived mainly from mitochondria and glycolysis, action potentials come from membrane ion channels and propagation, and pumping blood out is the mechanical job of the heart muscle during contraction.