Neurotransmitters are perhaps the main chemical messengers in the brain. They are the means by which neurons communicate with each other. It is through neurotransmitters that one neuron tells another neuron to fire. If we think of the neuronal networks of the brain as a vast economy, neurotransmitters can be seen as the currency of that economy. The exchange of neurotransmitters stimulates neurons to act.

Neurotransmitters are stored in sac-like vesicles in the axon terminals of the neuron. When the neuron fires, its axon terminals release neurotransmitters into the synaptic cleft, the space between the pre-synaptic and post-synaptic neuron. When the neurotransmitters bind to the receptor sites on the post-synaptic neuron, they impact the likelihood that the neuron will fire. Excitatory neurotransmitters increase the likelihood of firing while inhibitory neurotransmitters decrease it.