Final answer:
The contraction cycle in muscle fibers is initiated by the release of calcium ions (Ca2+) from the sarcoplasmic reticulum after an action potential triggers the process.
Step-by-step explanation:
The rise in calcium ions (Ca²+) released from the sarcoplasmic reticulum (SR) triggers the contraction cycle. When an action potential is generated, it triggers the opening of calcium channels in the membrane of the adjacent SR, causing Ca²+ to diffuse out and into the sarcoplasm. The presence of Ca²+ in the sarcoplasm initiates contraction of the muscle fiber.
The contraction cycle is triggered by the rise in calcium ions (Ca2+) released from the sarcoplasmic reticulum (SR).
In skeletal muscle fibers, an action potential triggers sodium ions (Na+) to enter the cell, leading to depolarization that spreads across the membrane. This depolarization reaches the T-tubules, which in turn prompts the opening of calcium channels in the SR.
The release of Ca2+ ions from the SR into the sarcoplasm is the critical step that initiates muscle contraction. These ions interact with troponin, displacing the tropomyosin and revealing the binding sites on actin filaments. The myosin heads then bind to these sites and, using ATP, pull the actin filaments toward the center of the sarcomere, thereby shortening the muscle fiber and causing contraction.