Which of the following is false about the left ventricle?

A. Potassium efflux and membrane repolarization: Potassium efflux occurs during the repolarization phase of the cardiac action potential, restoring the resting membrane potential. While necessary for resetting the cell electrically, it does not directly trigger the contractile machinery or link the action potential to muscle contraction.

B. Inhibition of myosin heads: Myosin head inhibition occurs when tropomyosin blocks binding sites on actin in relaxed muscle. This is the resting state prior to contraction, not the process that couples the action potential to contraction.

C. Closing of sodium channels: Sodium channels close immediately after depolarization, contributing to the refractory period and preventing further Na+ influx. While important for the timing of action potentials, this does not directly initiate contraction or activate contractile proteins.

D. Calcium entry and activation of contractile proteins: During excitation-contraction coupling in cardiac muscle, calcium ions enter the cell through voltage-gated L-type calcium channels during the plateau phase. This calcium triggers release of Ca2+ from the sarcoplasmic reticulum, allowing calcium to bind to troponin. Troponin undergoes a conformational change, which permits myosin cross-bridge cycling ultimately producing muscle contraction.

A. Myoglobin and lipid inclusions: Both skeletal and cardiac muscle fibers contain myoglobin, which stores oxygen for aerobic metabolism, and lipid inclusions as energy reserves. These features support high metabolic demands in both muscle types and are not unique to skeletal muscle.

B. A single nucleus per cell: Cardiac muscle cells are typically uninucleated, whereas skeletal muscle fibers are multinucleated. However, the presence of a single nucleus is characteristic of cardiac myocytes, not a distinguishing feature of skeletal muscle.

C. Triads formed by long T tubules and cisternae of the sarcoplasmic reticulum: Skeletal muscle fibers have well-organized triads, where a T tubule is flanked by two terminal cisternae of the sarcoplasmic reticulum, allowing rapid calcium release for synchronous contraction. In contrast, cardiac contractile cells have diads (one T tubule with one adjacent cisterna) and smaller T tubules, reflecting slower calcium handling.

D. Sarcomeres along myofibrils: Both skeletal and cardiac muscle fibers contain sarcomeres arranged along myofibrils, giving them striated appearance under microscopy. Sarcomeres are essential for contraction in both types of striated muscle and are not a distinguishing structural feature.