Final answer:
When a nerve impulse reaches the neuromuscular junction during muscle contraction, signaling begins as the motor neuron releases acetylcholine (ACh) that binds to ACh receptors on the muscle fiber, causing muscle contraction.
Step-by-step explanation:
When a nerve impulse reaches the neuromuscular junction (NMJ) during muscle contraction, signaling begins. The motor neuron releases a neurotransmitter called acetylcholine (ACh) into the synaptic cleft. ACh binds to ACh receptors on the muscle fiber, causing depolarization of the sarcolemma and initiation of muscle contraction. This process ensures that every nerve impulse from a motor neuron leads to muscle fiber contraction.