Acetylcholine receptors are embedded in the sarcolemma at neuromuscular junctions. A neuromuscular junction is where a motor neuron makes contact with a muscle fiber. The sarcolemma is the cell membrane of a muscle cell.
Acetylcholine is the neurotransmitter released by motor neurons. It binds to the receptors in the sarcolemma at the neuromuscular junction. When acetylcholine binds to receptors, the receptors form a channel through the sarcolemma that allows sodium to flow into the muscle cell and potassium to flow out of the muscle cell. The flow of sodium ions creates an action potential that travels down the sarcolemma into the T-tubules, triggering a muscle contraction.