Exercise increases heart rate, and heart rate returns to baseline over time after exercise.

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.

Correct answer: B.

B: Ventricular depolarization is the electrical activation of the ventricles that triggers ventricular contraction. It occurs after the impulse travels from the sinoatrial . On an electrocardiogram (ECG), ventricular depolarization is represented by the QRS complex, labelled as B in the diagram.

D: artrial depolarization - Atrial depolarization initiates atrial contraction, driven by impulses from the SA node. This electrical activity spreads across atrial myocardium, generating the P wave on the ECG and enabling ventricular filling.

E: ventricular repolarization - Ventricular repolarization restores the ventricles to their resting state after contraction. Represented by the T wave, it prepares the heart for the next depolarization, ensuring rhythmic cardiac cycles and effective pumping.