Neurotransmitters and Neurohormones
A multitude of chemicals called neurotransmitters mediate intercellular communication in the nervous system. These include acetylcholine, the catecholamines, serotonin, glutamate, GABA, glycine, and a wide variety of neuropeptides. Although they exhibit great diversity in many of their properties, all are stored in vesicles in nerve terminals and are released to the extracellular space via a process requiring calcium ions. Their actions are terminated by reuptake into the presynaptic terminal or nearby glial cells by specific transporter proteins or by their destruction in the extracellular space. The role of neurotransmitters is to alter the properties—chemical, electrical, or both—of some target cell. With the arrival on the scene of the neuropeptides, it has become evident that signaling in the nervous system occurs through the use of rich and varied forms of chemical currency, and that some neurons use more than one type of currency simultaneously.