Two molecules must bind for a post synaptic receptor to activate a channel. Acetyl cholinesterase destroys the acetylcholine after it binds to a receptor. Removal of transmitter is important. The system has to be reset. Transmitters open ligand gated channels on the post synaptic membrane.
Ions flow through the open channel and change the post synaptic potential.
Post synaptic potentials are graded. Synapses are also the site of action of many mind altering drugs. Depression linked to low levels of seretonin. Dopamine thought to be involved in schizophrenia and Parkinson's disease . Amphetamines increases norephinephrine in the brain.