OSMT (Ontario Society of Medical Technologists) Practice Exam

Which structure stores and releases calcium ions in muscle cells?

1. Myofibrils

2. T-tubules

3. Sarcomeres

4. Sarcoplasmic reticulum

The correct answer is: Sarcoplasmic reticulum

The correct answer is the structure known as the sarcoplasmic reticulum. This organelle plays a crucial role in muscle contraction by storing calcium ions, which are essential for the contraction process. When a muscle cell is stimulated, the sarcoplasmic reticulum releases these calcium ions into the cytoplasm of the muscle cell. This release triggers the interaction between actin and myosin filaments, leading to muscle contraction. In contrast, myofibrils are the contractile fibers within muscle cells, composed of repeating units known as sarcomeres, where the actual contraction occurs. T-tubules are extensions of the cell membrane that help transmit the electrical signal deep into the muscle fiber, facilitating the rapid release of calcium from the sarcoplasmic reticulum. Sarcomeres represent the functional units of muscle contraction and are organized structures made of myofilaments; however, they do not store or release calcium themselves. Therefore, the sarcoplasmic reticulum is specifically responsible for the storage and release of calcium ions, making it the correct answer.