Answer:
b. store and release calcium ions
Step-by-step explanation:
The two major functions of the T - tubules are as follows -
1. the T - tubules propagates the action potential of the plasma membrane into the muscle cells interior through the voltage - gated Na ⁺ and K⁺ channels .
2. The cation potential which is carried by the T - tubules helps to regulate the opening and the closing of the calcium ion channels present in the sarcoplasmic reticulum . The resulting change in the cytosolic calcium ions concentration triggers the contraction of the myofibrils .