The answer is acetylcholine.
A neuromuscular junction is a contact between a neuron and a sarcolemma (muscle fiber). When an action potential reaches presynaptic terminal on a motor neuron, neurotransmitter acetylcholine is released from the motor neuron. Neurotransmitters pass through the synaptic cleft and bind to the receptors on the muscle fiber. These receptors are nicotinic acetylcholine receptors.
