What is the role of acetylcholine (ACh) in muscle contraction?

Acetylcholine (ACh) plays a crucial role in initiating muscle contraction. When a nerve impulse reaches the neuromuscular junction, ACh is released from the motor neuron and binds to specific receptors on the muscle cell's sarcolemma (the cell membrane of the muscle fiber). This binding triggers the opening of sodium channels, leading to depolarization of the muscle membrane, which is a key step in the excitation-contraction coupling process.

Once the muscle membrane is depolarized, it results in the opening of voltage-gated calcium channels in the sarcoplasmic reticulum, allowing calcium ions to flood into the cytosol. The increase in intracellular calcium concentration is what ultimately initiates the sliding filament mechanism that leads to muscle contraction.

Thus, the activation of receptors by ACh is essential for starting the cascade of events that leads to muscle contraction, making this choice the correct answer.