This document defines neurotransmitters as endogenous chemicals that transmit signals between neurons across synapses. It lists important neurotransmitters like GABA, serotonin, acetylcholine, catecholamines, and glutamate. It provides details on the synthesis, release, and actions of several key neurotransmitters including GABA, glutamate, serotonin, acetylcholine, glycine, and nitric oxide. It also discusses some disorders related to issues with neurotransmitters like deficiencies or excess levels.