Neurotransmitters are endogenous chemicals that transmit signals across a synapse from one neuron (nerve cell) to another “target” neuron. Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by receptors on other synapses. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available from the diet and only require a small number of biosynthetic steps to convert them. Neurotransmitters play a major role in shaping everyday life and functions. Their exact numbers are unknown but more than 100 chemical messengers have been identified.