Neurotransmitters are chemical messengers in the nervous system. Neurons consist of receptor, body cell, and dendrite. The axon is housed in the dendrite and this extends the neurotransmitter to connect to the receptor of the adjacent neuron. Synaptic transmission is a process which begins with the production and storage of the neurotransmitters. These neurotransmitters are synthesized in the terminal lodged in the axon.
Precursors required for the synthesis of these neurotransmitters are available in the body or administered from outside. They are reached to the membrane of the terminal by special transporters. Many precursors are available as by-products of the processes occurring in the neuron cells. Enzymes catalyze the interaction amongst the precursors. These enzymes are generated in the neuron cell and transported to the axon end.
Acetylcholine is a small-molecule, excitatory variety of neurotransmitter made up of choline and acetate. This is available at all the junctions of neurotransmitters and all over the CNS and PNS. The enzyme required for the synthesis of Acetylcholine and the process itself takes place in the terminal of the nerve.
Neuropeptides are large size neurotransmitters. Their formation occurs in the body of the cell since the process requires formation of peptide bond. This process of synthesis is more extensive than that of the small-molecule neurotransmitter. The process includes transcription of genes in the nucleus. Specific sequence of peptide coding of the DNA is made use of to construct a strand of RNA which will correspond to the DNA strand. The mRNA strand reaches the ribosome where its translation occurs. The sequence of nucleotides which constitute the mRNA behave like a code and strings along a sequence of amino acids which finally forms the neuropeptide required at the axon end. Once the neuropeptide is processed in endoplasmic reticulum, packed in the golgi and transported to the axon terminal it is ready to be released in to the synoptic gap for transmission of electric signal from one neuron to the adjacent one.
The synthesized neurotransmitters whether they are small molecule variety or neuropeptides are stored in sacs called vesicles, which are, in turn, housed in the axon terminals. They get released when the action-potential or electric signal reaches the axon. The small-molecules are stored mostly in vesicles which are 40 to 60mm in diameter. They also seem to have clear centers when viewed under electron micro-graphs. The neuropeptides are stored in larger vesicles which are 90 to 250mm in its diameter. When viewed under the electron micrograph, these vesicles seem dense with electron and dark as well.
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Williams College: Synthesis and Storage of Neurotransmitters