Final answer:
Calcium is released from the sarcoplasmic reticulum during skeletal muscle contraction by membrane depolarization, which is part of the excitation-contraction coupling process. Option d. Membrane depolarization is the correct answer.
Step-by-step explanation:
During the process of excitation-contraction coupling in skeletal muscle, calcium is released from the sarcoplasmic reticulum by d. Membrane depolarization. When a skeletal muscle fiber has to contract, its membrane gets excited by an action potential that travels along the sarcolemma.
This excitation triggers the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). The released calcium interacts with the shielding proteins troponin and tropomyosin, which move aside to expose the actin-binding sites for myosin heads attachment, leading to muscle contraction.
The mechanism starts with a neural signal which leads to depolarization as sodium ions (Na+) enter the muscle fiber, prompting an action potential. This action potential then propagates along the sarcolemma and T-tubules, reaching the SR and causing the release of stored Ca++, which is crucial for muscle contraction.