When the action potential reaches the axon ending (the axon bulb), the membrane’s permeability to calcium ions increases. Calcium diffuses into the neuron and this causes vesicles filled with neurotransmitter to move to the membrane and expel their contents into the synaptic cleft. Neurotransmitter molecules diffuse across the cleft to the postsynaptic membrane and bind to receptors. The neurotransmitter acts for only a short time before being absorbed or broken down by enzymes.

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