Synaptic vesicles

Synaptic vesicle
Neuron A (transmitting) to neuron B (receiving). 1. ; 2. Synaptic vesicle with neurotransmitters; 3. Autoreceptor 4. Synapse with neurotransmitter released (serotonin); 5. Postsynaptic receptors activated by neurotransmitter (induction of a postsynaptic potential); 6. Calcium channel; 7. Exocytosis of a vesicle; 8. Recaptured neurotransmitter.
Details
Identifiers
Latin	vesicula synaptica
Code	TH H2.00.06.2.00004
Anatomical terminology []

In a neuron, synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell. The area in the axon that holds groups of vesicles is an axon terminal or "bouton". Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the human brain region V1 synaptic vesicles have an average diameter of 39.5 nanometers with a standard deviation of 5.1 nanometers.

With the advent of modern electron microscopic techniques in the early 1950s, nerve endings were found to contain a large number of electron-lucent vesicles. The term synaptic vesicle was first introduced by De Robertis and Bennett in 1954. This was shortly after transmitter release at the frog neuromuscular junction was found to induce postsynaptic miniature end-plate potentials that were ascribed to the release of discrete packages of neurotransmitter (quanta) from the presynaptic nerve terminal. It was thus reasonable to hypothesize that the transmitter substance (acetylcholine) was contained in such vesicles, which by a secretory mechanism would release their contents into the synaptic cleft (vesicle hypothesis).

The missing link was the demonstration that the neurotransmitter acetylcholine is actually contained in synaptic vesicles. About ten years later, the application of subcellular fractionation techniques to brain tissue permitted the isolation first of nerve endings (synaptosomes), and subsequently of synaptic vesicles from mammalian brain. Two competing laboratories were involved in this work, that of Victor P. Whittaker at the Institute of Animal Physiology, Agricultural Research Council, Babraham, Cambridge, UK and that of Eduardo de Robertis at the Instituto de Anatomía General y Embriología, Facultad de Medicina, Universidad de Buenos Aires, Argentina. Whittaker's work demonstrating acetylcholine in vesicle fractions from guinea-pig brain was first published in abstract form in 1960 and then in more detail in 1963 and 1964, and the paper of the de Robertis group demonstrating an enrichment of bound acetylcholine in synaptic vesicle fractions from rat brain appeared in 1963. Both groups released synaptic vesicles from isolated synaptosomes by osmotic shock. The content of acetylcholine in a vesicle was originally estimated to be 1000–2000 molecules. Subsequent work identified the vesicular localization of other neurotransmitters, such as amino acids, catecholamines, serotonin, and ATP. Later, synaptic vesicles could also be isolated from other tissues such as the superior cervical ganglion, or the octopus brain. The isolation of highly purified fractions of cholinergic synaptic vesicles from the ray Torpedo electric organ was an important step forward in the study of vesicle biochemistry and function.