The brain is composed of billions of cells called neurons. Synapses are the connections between neurons or the space between neurons that allows one to pass a signal to another. Neurons communicate using chemicals called neurotransmitters. There are many different kinds of neurotransmitters. Neurons have receptors that only recognize specific ones. Without the corresponding receptor, the neuron will be unaffected by the neurotransmitter’s release.
Neurotransmitters find the right receptor by random collision. The greater the amount of neurotransmitter passing a receptor, the greater the chance of the receptor being activated. At a synapse, activation of the receptor opens a channel for the flow of charged ions inside the neuron. When enough positively-charged ions have flowed into a neuron, it will spike and release its own neurotransmitter.