Acetylcholine released by the motor neuron at the neuromuscularjunction changes the permeability of the cell membrane at the motor and plate.The permeability change allows the influx of positive charges, which in turntriggers an electrical event called an action potential. The action potentialspreads across the entire surface of the muscle fiber and into the interior via Ttubules. The cytoplasmic concentration of calcium ions (released fromsarcoplasmic reticulum) increases, triggering the start of a contraction. Thecontractions ends when the Ach has been removed from the synaptic cleftand motor end plate by AChE.