In a skeletal muscle fiber, the sarcolemma forms tubules that penetrate into the cell and contact the sarcoplasmic reticulum.

Yes, in a skeletal muscle fiber, the sarcolemma, which is the cell membrane, does form tubules known as transverse or T-tubules. These T-tubules penetrate into the cell and are in close contact with the sarcoplasmic reticulum, which is a specialized type of smooth endoplasmic reticulum that stores, releases, and retrieves calcium ions. The T-tubules and the sarcoplasmic reticulum work together to transmit signals from the outside of the muscle cell to the proteins inside the cell that control muscle contraction. When an electrical signal, or action potential, travels along the sarcolemma, it moves down into the T-tubules and stimulates the sarcoplasmic reticulum to release calcium ions into the sarcoplasm, the cytoplasm of the muscle fiber. This increase in calcium ion concentration triggers the muscle fiber to contract.