At a synapse (the small gap across which impulses are transmitted from one neuron to another), a neuron releases chemicals that affect another neuron, these chemicals are known as neurotransmitters. A hundred or more chemicals are known or suspected to be neurotransmitters.
Ctenophores, possibly representative of the oldest, most primitive animals, apparently have only one neurotransmitter, glutamate. Most of the rest of the animal kingdom has all or nearly all of the same transmitters like we have.
The oddest transmitter is nitric oxide, a gas released by many small local neurons. Nitric oxide is poisonous, yet, many neurons contain an enzyme that enables them to make it efficiently. Many neurons release nitric oxide when they are stimulated.
The neurotransmitter ensures excitatory and inhibitory effects. The important neurotransmitters of human and other high animals are glutamate, GABA, Actetycholin, Dopamine, Serotonin, Norepinephrine, Amino acids, Glycin, Peptides, Lipids, and certain Nucleosides.