How do synapses work?

When an action potential reaches a presynaptic cell, it causes voltage-gated ion channels in the membrane to open, which allows Ca2+ to enter the cell. This Ca2+ diffuses into the cell from high to low concentration. This causes the neurotransmitter, normally acetylcholine, to get packaged into vesicles by the Golgi apparatus. These vesicles then fuse with the cell membrane, and the NT diffuses into and across the synaptic cleft to the postsynaptic cell, where it binds to receptors on the postsynaptic cell surface membrane. This causes ligand gated ion channels to open on the postsynaptic cell membrane, therefore producing a new action potential which will travel to the effector organ.