Final answer:
Option C). The threshold for stimulation in a muscle fiber leads to the fiber contracting, initiated by the neurotransmitter acetylcholine at the neuromuscular junction and followed by a cascade of events involving action potentials, calcium ion release, and the sliding filament mechanism.
Step-by-step explanation:
A muscle fiber will contract when it reaches its threshold for stimulation. The process begins at the neuromuscular junction, where acetylcholine (ACh), a neurotransmitter released by a motor neuron, binds to ACh receptors on the muscle fiber, causing the muscle cell membrane, or sarcolemma, to depolarize. The release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm is caused by this depolarization, which sets off an action potential that travels through the sarcolemma and down into the T-tubules. An increase in calcium ions in the sarcoplasm causes muscle fibers to contract as they bind to troponin. This causes tropomyosin to move away from actin-binding sites, allowing myosin heads to form cross-bridges with actin and producing an ATP-powered sliding filament motion. This process shortens the sarcomeres within the muscle fibers and causes muscle contraction.