Final answer:
Calcium release channels in the SR release Ca2+ due to the action potential carried by T-tubules; this release of Ca2+ initiates muscle fiber contraction by enabling myosin to bind with actin.
Step-by-step explanation:
When a muscle is stimulated to contract, calcium release channels in the sarcoplasmic reticulum (SR) will release Ca2+ by the action of the T-tubules. These T-tubules carry the action potential into the interior of the muscle cell, which triggers the opening of calcium channels in the membrane of the adjacent SR. The release of Ca2+ into the sarcoplasm from its storage in the SR is the key event that initiates the contraction of the muscle fiber's contractile units or sarcomeres. This excitation-contraction coupling involves the binding of calcium to the troponin-tropomyosin complex, resulting in the exposure of actin-binding sites, enabling the myosin heads to attach and produce the power stroke that leads to muscle fiber shortening and contraction, all of which is sustained by ATP.