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 until reaching the synapse at the neuromuscular junction (the neural impulse passage zone between the axon extremity and the sarcolemma). Near the axonal extremity the depolarization allows 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.
Acetylcholine then binds to special receptors in the outer surface of the sarcolemma, the permeability of this membrane is altered and an action potential is created. The depolarization is then conduced along the sarcolemma to the sarcoplasmic reticulum that thus releases calcium ions for the sarcomere contraction.