At the end of an axon are closed calcium voltage gated channels (similar to K+ and Na+). If positive enough near the voltage gated channel (or high concentration of calcium on outside), it open up and Ca2+ flood into cell. The calcium ions then bond to other proteins.
Synapse: Place where axon meets dendrite (connection point). Pre-Synaptic neuron is at the end of the axon. Post-Synaptic neuron is at the end of the dendrite. Synaptic cleft is the place b/w the two neurons.
At the pre-synaptic neuron are vesicles that that contain neurotransmitters that they train. ‘
After the flood of calcium ions, these ions bond to SNARE proteins in the cell. These proteins snare the vesicles to the pre-synaptic membrane. The ions change the SNARE protein conformations and the proteins bring the vesicles closer to the membrane by pulling both membranes closer together. The vesicles then undergo exocytosis and release neurotransmitters (like Acetycholine) into synaptic cleft outside the cell.
These neurotransmitters then bond to post-synaptic membrane of the dendrite at ligand-gated sodium ion channels. It then triggers neurotransmitter-gated channels to open as Na+ channel get excited with the positive charge. Membrane potential is more positive as Na+ ions flood in (diffuses). There is an electrotonic effect at the axon hillock. Action potential occurs as the potential at point increases (when membrane potential reaches threshold level).
If the K+ voltage-gated channel was triggers, then K goes out of cell due to concentration gradient being high inside cell. This is an inhibitory chemical synapse.
McGraw-Hill Animation with Quizzes:
Transmission Across a Synapse: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__transmission_across_a_synapse.html