Final answer:
Excitation-contraction coupling in skeletal muscle contraction requires both calcium ions (Ca²⁺) and ATP. Calcium ions released from the SR enable myosin to form cross-bridges with actin, while ATP provides the energy necessary for myosin head movement and detachment, as well as the reuptake of calcium ions during muscle relaxation. The correct answer is option: A. Ca²⁺ and ATP.
Step-by-step explanation:
Excitation-contraction coupling is the physiological process where an electrical stimulus triggers a muscle contraction. This process involves several steps, with the crucial players being calcium ions (Ca²⁺) and ATP. When an action potential reaches the muscle fiber, it causes the sarcoplasmic reticulum (SR) to release Ca²⁺ into the cytoplasm.
These ions then bind to troponin, causing a conformational change that moves tropomyosin, thereby exposing binding sites on actin for the myosin heads. The myosin heads attach to actin to form cross-bridges, but in order to do so and to subsequently “pull” the actin and achieve contraction, myosin heads require energy, provided by ATP.
The myosin head releases from actin after the ATP is hydrolyzed into ADP and inorganic phosphate. Additionally, ATP is necessary for the active transport of Ca²⁺ back into the SR during relaxation.
So the substances required for excitation-contraction coupling are A. Ca²⁺ and ATP.