Final answer:
The rise in calcium ions (Ca++) released from the sarcoplasmic reticulum (SR) triggers the contraction of muscle fibers.
Step-by-step explanation:
The rise in calcium ions (Ca++) released from the sarcoplasmic reticulum (SR) triggers the contraction of muscle fibers. When the muscle fiber depolarizes and an action potential spreads, it causes calcium ions to be released from storage in the SR. The presence of calcium ions initiates muscle contraction by binding to troponin, which keeps the actin-binding sites exposed and allows the cross-bridge cycling between actin and myosin. This contraction is sustained by ATP.