During the contraction of a vertebrate skeletal muscle fiber, calcium ions a. break cross-bridges as a cofactor in hydrolysis of ATP. b. bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed. c. transmit action potentials from the motor neuron to the muscle fiber. d. spread action potentials through the T tubules.

Verified step by step guidance

1. Understand the role of calcium ions in muscle contraction: Calcium ions play a crucial role in the contraction of skeletal muscles by interacting with specific proteins within the muscle cells.
2. Identify the proteins involved: In muscle cells, calcium ions interact mainly with the proteins troponin and tropomyosin, which are located on the actin filaments.
3. Recognize the mechanism of action: When calcium ions bind to troponin, it causes a conformational change in troponin. This change moves tropomyosin away from the myosin-binding sites on actin filaments.
4. Link the action to muscle contraction: Once the myosin-binding sites are exposed, myosin heads can bind to these sites, which is essential for the cross-bridge cycle that leads to muscle contraction.
5. Choose the correct option: Based on the understanding of the mechanism, the correct answer is 'b. bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.'

Key Concepts

Calcium Ions in Muscle Contraction

Calcium ions play a crucial role in muscle contraction by binding to troponin, a regulatory protein on the actin filaments. This binding causes a conformational change in troponin, which subsequently moves tropomyosin away from the myosin-binding sites on actin. This exposure allows myosin heads to attach to actin, facilitating the cross-bridge cycle necessary for muscle contraction.

Cross-Bridge Cycle

The cross-bridge cycle is the process by which myosin heads attach to actin filaments, pull them inward, and then detach, powered by ATP hydrolysis. This cycle is essential for muscle contraction and involves several steps: attachment, power stroke, detachment, and re-cocking of the myosin head. The cycle repeats as long as calcium ions are present and ATP is available.

Action Potentials and Muscle Fiber Activation

Action potentials are electrical signals that trigger muscle contraction. They are generated in motor neurons and travel to the muscle fibers, causing the release of calcium ions from the sarcoplasmic reticulum. This release initiates the contraction process by enabling the interaction between actin and myosin, ultimately leading to muscle shortening and force generation.