What is the neurotransmitter of the neuromuscular junction? How does the nervous system trigger muscle contraction?

The nervous cells that trigger the muscle contraction are the motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited the depolarizing current flows along the membrane of its axon unless reaching the synapse at the neuromuscular junction (the neural impulse passage zone among the axon extremity and the sarcolemma). Near the axonal extremity the depolarization permits the entrance of calcium ions into the axon (note that calcium also has a relevant role here). The calcium ions stimulate the neuron to release acetylcholine in the synapse.