Describe what occurs at a neuromuscular junction during transmission of a nerve impulse

When an action potential arrives at the presynaptic terminal of a neuromuscular junction, the depolarisation stimulates voltage-gated Ca2+ channels to open, and calcium ions flow into the neurone. This flood of positive charge stimulates pre-packaged vesicles filled with acetylcholine to move towards the presynaptic membrane - these vesicles then undergo exocytosis and the neurotransmitter moves into the synaptic cleft. Once in the synapse, acetylcholine diffuses across to bind to (nicotinic) receptors on the postsynaptic membrane. The neurotransmitter binding stimulates postsynaptic Na+ channels to open and results in an influx of sodium ions, depolarising the neurone and creating an action potential which is then transmitted to the muscle membrane to eventually create movement. Acetylcholinesterase is the enzyme responsible for breaking down acetylcholine in the synaptic cleft after a nerve impulse has been transmitted, to prevent excessive stimulation.