Final answer:
Skeletal muscle fibers contain a plasma membrane called the sarcolemma, cytoplasm known as sarcoplasm, and myofibrils made of sarcomeres that hold myofilaments. The sarcoplasmic reticulum manages calcium ions for contraction, and T-tubules help convey signals triggering calcium release. Motor end-plates receive input from nerves, but do not store calcium.
Step-by-step explanation:
Structure of Skeletal Muscle at the Cellular Level
Skeletal muscle fibers are long, multinucleated cells encased by the sarcolemma, which is the plasma membrane. Within these muscle fibers is the cytoplasm, known as sarcoplasm. Contained within the sarcoplasm is a network of myofibrils that are responsible for the muscle's contraction. These myofibrils are composed of repeating units called sarcomeres, which contain thick and thin myofilaments, made up of the proteins myosin and actin respectively.
The sarcoplasmic reticulum (SR) is a specialized form of the endoplasmic reticulum that stores, releases, and retrieves calcium ions essential for muscle contraction. Transverse tubules, commonly referred to as T-tubules, are infoldings of the sarcolemma that penetrate the cell, aiding in the conduction of electrical signals to the SR, and triggering the release of calcium. At the neuromuscular junction, the motor end-plate receives nervous system signals, initiating muscle contraction.
Clarifying the parts of the question: A) T-tubules are indeed invaginations of the sarcolemma that conduct signals to the SR. B) Motor end-plates do not store calcium ions; they are the region of the muscle membrane that receives the signal from the nerve. C) Myofibrils are indeed composed of sarcomeres, and each sarcomere contains multiple myofilaments. D) The sarcoplasmic reticulum stores calcium ions, not muscle glycogen, which is found throughout the sarcoplasm.