nerve impulses also known as action potentials travel from the brain or spinal cord to trigger the contraction of skeletal muscles an action potential propagates down a motor neuron to a skeletal muscle fiber the site where a motor neuron excites a skeletal muscle fiber is called the neuromuscular Junction this Junction is a chemical synapse consisting of the points of contact between the axon terminals of a motor neuron and the motor endplate of a skeletal muscle fiber the events at the neuromuscular Junction occur in seven coordinated steps step one an action potential travels the length of the axon of a motor neuron to an axon terminal step2 voltage-gated calcium channels open and calcium ions diffuse into the terminal step3 calcium entry causes synaptic vesicles to release acetylcholine via exocytosis step four acetylcholine diffuses across a synaptic cleft and binds to acetylcholine receptors which contain ligand gated cation channels five these ligand-gated cation channels open step 6 sodium ions shown here in red enter the muscle fiber and potassium ions shown here in blue exit the muscle fiber the greater inward flux of sodium ions relative to the outward flux of potassium ions causes the membrane potential to become less negative step 7 once the membrane potential reaches a threshold value an action potential propagates along the sarcolemma neural transmission to a muscle fiber ceases when acetylcholine is removed from the synaptic cleft this removal occurs in two ways one acetylcholine diffuses away from the synapse to acetylcholine is broken down by the enzyme acetylcholine esterase to acetic acid and choline choline is then transported into the axon terminal for the resynthesis of acetylcholine you