The sarcolemma has along its surface many tubelike invaginations celled transverse or T tubules which are located at regular intervals along the muscle fiber and wrap around sarcomeres where the actin and myosin myofilaments overlap. The T tubules are assoiated with a highly organized, smooth endoplasmic reticulum called the sarcoplasmic reticulum. T tubules connect the sarcolemma to the sarcoplasmic reticulum. The sarcoplasmic reticulum has a relatively high concentration of calcium ions, which play a major role in muscle contractions.