1. The brain is made up of neurons that process and transmit information by electrochemical signaling. Neurotransmitters are endogenous chemicals which relay, amplify, and modulate signals between a neuron and another cell. Chemical messengers must fulfill four criteria to be considered a transmitter. These four steps include the synthesis of a transmitter substance, the storage and release of the substance, its interaction with a receptor in the membrane, and the removal of the transmitter form the synaptic cleft. The nervous system makes use of two types of neurotransmitters: small-molecule transmitters (acetylcholine, amines and A.A) and neuroactive transmitters(peptides)
2. Neurons have developed a sophisticated ability to regulate the synthesis, storage, release, and degradation (or removal) of neurotransmitters to achieve the desired levels of transmitter molecules. In general, each of these processes is specific to the transmitter involved, requiring enzymes found only in neurons that use the transmitter at their synapses. The synthesis of small-molecule neurotransmitters occurs within presynaptic terminals. The enzymes needed for transmitter synthesis are synthesized in the neuronal cell body and transported to the nerve terminal cytoplasm by a mechanism called slow axonal transport. The enzymes generate a cytoplasmic pool of neurotransmitter that must then be loaded into synaptic vesicles by transport proteins in the vesicular membrane. The mechanisms responsible for the synthesis and storage of peptide transmitters are fundamentally different from those used for the smallmolecule neurotransmitters. Peptide-secreting neurons generally synthesize polypeptides in their cell bodies that are quite large. Processing these polypeptides, which are called pre-propeptides takes place by a sequence of reactions in several intracellular organelles.