Final answer:
An action potential is necessary for muscle contraction because it triggers the release of calcium ions from the sarcoplasmic reticulum, enabling the muscle to contract through a process called excitation-contraction coupling.
Step-by-step explanation:
An action potential is necessary in muscle contraction because it stimulates the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR). This release enables the muscle contraction process by acting on the troponin-tropomyosin complex, thus allowing myosin heads to form cross-bridges with actin, leading to muscle fiber shortening. The process of action potential triggering a muscle contraction is part of what is known as excitation-contraction coupling, which is essential for voluntary muscle movement and is influenced by the neurotransmitter acetylcholine (ACh) at the neuromuscular junction (NMJ).
When ACh binds to receptors on the muscle cell, it causes depolarization that generates an action potential. This action potential then travels along the sarcolemma and down the T-tubules, triggering the release of Ca2+ ions into the sarcoplasm. The presence of Ca2+ ions is mandatory for the conformational changes that lead to myosin head binding on actin sites, initiating contraction through the sliding filament model. ATP is required at multiple stages, including for the detachment of myosin heads and re-cocking for the next cycle of contraction.