Definition
A neurotransmitter is a chemical messenger that carries a signal across the synapse. When a neuron fires, it converts its electrical impulse into a release of these molecules, which drift across the synaptic gap and deliver the message to the next cell. Neurotransmitters are the brain's vocabulary.
There are many — dopamine, serotonin, norepinephrine, glutamate, GABA, and dozens more. Each is associated with particular circuits and roles, though Behave is careful to warn against the cartoon idea that any one chemical simply equals a single mood.
Why it matters
How it works
A neurotransmitter is synthesized inside the sending neuron and stored in tiny packets at the axon terminal. When an action potential arrives, those packets release their contents into the synapse. The molecules cross the gap and bind to receptors on the receiving neuron, either exciting it or inhibiting it.
What a neurotransmitter does depends less on the chemical itself than on which receptor it meets — the same molecule can excite one cell and quiet another. After the signal is delivered, the transmitter is cleared from the synapse by reuptake or enzymatic breakdown. Behave stresses that behavior cannot be read off a single neurotransmitter level; what matters is the whole pattern across circuits and contexts.