When an action potential reaches the pre-synaptic bulb, calcium ion channels open. The diffusion of calcium ions into the axon causes vesicles containing neurotransmitters to move to the pre-synaptic membrane, fuse with the membrane and release their contents by exocytosis. The neurotransmitters diffuse across the synaptic cleft and bind to receptors on the post-synaptic membrane. These receptors are also sodium ion channels which open and allow sodium ions to diffuse in and depolarise the membrane.