Neurotransmitters are chemical messengers released by neurons throughout the nervous system, transmitting an impulse from one cell to the next. Neurotransmitters are made in the brain, but also by the bacteria in our gut. Neurotransmitters are either inhibitory (calming), excitatory (stimulating) or modulating (regulating), and this determines the effect they have on the cell they are communicating with.[1][2][3]