Final answer:
The T tubules communicate with the sarcoplasmic reticulum at the triad to release calcium ions (Ca++). These ions are stored in the sarcoplasmic reticulum.
Step-by-step explanation:
The T tubules communicate with the sarcoplasmic reticulum at a specialized structure known as the triad. This triad consists of a T tubule flanked by terminal cisternae of the sarcoplasmic reticulum. It is in this area that the action potential triggers the release of calcium ions (Ca++) from the sarcoplasmic reticulum. In terms of the options provided, the correct answer is not explicitly listed, but the most closely related term to the triad would be the myofibril connection, as the triad surrounds the cylindrical structure called a myofibril, which contains actin and myosin. Answering the second question, the Ca++ ions are stored in the sarcoplasmic reticulum, thus making option (d) sarcoplasmic reticulum the correct choice.