Neurotransmitter

A neurotransmitter is a chemical messenger that a neuron releases across a synapse to excite, inhibit or modulate the next cell.

Neurons store most neurotransmitters in small vesicles at their terminals. When an electrical impulse arrives, calcium enters, the vesicles fuse with the membrane and the transmitter crosses the synaptic gap in a fraction of a millisecond, binding receptors on the receiving cell. Its action ends when it is pumped back into the sending neuron, broken down by an enzyme or diffuses away. Major examples include glutamate and GABA, which set the balance of excitation and inhibition; dopamine, serotonin and norepinephrine, which modulate mood, motivation and arousal; and acetylcholine, which drives muscle contraction and attention.

Endocannabinoids are unusual because they travel backward, from the receiving neuron to the sending one, and reduce how much conventional transmitter is released next. Through that mechanism the endocannabinoid system adjusts the release of many neurotransmitters at once. CBD's effects on individual transmitter systems, including serotonin and adenosine, have been studied mainly in laboratory settings, and their significance in people is still being worked out.