Some of the presynaptic receptors are from other neurons. Other presynaptic receptors, called autoreceptors, respond to the transmitter released by that presynaptic neuron. Therefore, the presynaptic neuron's reaching the state to generate an action potential and releasing neurotransmitters is under exquisite control. The scope of the problem is vast because thousands of synapses can converge on a single postsynaptic neuron and a single presynaptic cell can affect many postsynaptic cells. And it was thought to be a rule that there was only one neurotransmitter produced and released by an axon.
Receptors are another important control point for the effectiveness of synapses.
Receptor number and subtype on a membrane can vary and there are many genes involved in their synthesis. Many substances can regulate gene expression. Drugs and disease mainly affect synaptic mechanisms.