When an action potential arrives at the neuromuscular junction, the most immediate result is __________.

• A. the release of acetylcholine from the motor neuron
• B. the depolarization of the sarcolemma
• C. a rise in intracellular Ca²⁺ levels in the muscle fiber
• D. the formation of actin-myosin cross bridges

Answer: (A)

Verified step by step guidance

1. Understand the sequence of events at the neuromuscular junction: An action potential travels down the motor neuron to the axon terminal.
2. Recognize that the arrival of the action potential at the axon terminal causes voltage-gated calcium channels to open, allowing Ca²⁺ ions to enter the neuron.
3. Identify that the influx of Ca²⁺ ions into the neuron triggers synaptic vesicles to release acetylcholine (ACh) into the synaptic cleft.
4. Note that acetylcholine diffuses across the synaptic cleft and binds to receptors on the sarcolemma (muscle cell membrane), leading to depolarization.
5. Understand that the depolarization of the sarcolemma eventually leads to a rise in intracellular Ca²⁺ levels and the formation of actin-myosin cross bridges, but these are subsequent steps following the release of acetylcholine.