What happens when acetylcholine stimulates its receptors in the neuromuscular junction?

• A. Calcium ions are released from the presynaptic neuron into the synaptic cleft.
• B. Acetylcholine is converted into dopamine, which then activates the muscle fiber.
• C. A muscle action potential is generated, leading to muscle contraction.
• D. The postsynaptic membrane becomes hyperpolarized, inhibiting muscle contraction.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the role of acetylcholine in the neuromuscular junction. Acetylcholine is a neurotransmitter released by the presynaptic neuron into the synaptic cleft, where it binds to receptors on the postsynaptic membrane of the muscle fiber.
2. Step 2: Recognize the effect of acetylcholine binding to its receptors. When acetylcholine binds to nicotinic acetylcholine receptors on the postsynaptic membrane, it causes ion channels to open, allowing sodium ions (Na⁺) to flow into the muscle cell and potassium ions (K⁺) to flow out.
3. Step 3: Explain the generation of a muscle action potential. The influx of sodium ions depolarizes the postsynaptic membrane, creating an electrical signal known as the muscle action potential. This depolarization is the key event that triggers muscle contraction.
4. Step 4: Describe the role of calcium ions in muscle contraction. The muscle action potential travels along the sarcolemma (muscle cell membrane) and into the T-tubules, leading to the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the cytoplasm of the muscle cell.
5. Step 5: Connect calcium ion release to muscle contraction. Calcium ions bind to troponin, a regulatory protein on the actin filaments, causing a conformational change that allows myosin heads to interact with actin filaments. This interaction initiates the sliding filament mechanism, resulting in muscle contraction.