Synaptic vesicle

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 'terminal bouton'. Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the visual cortex of the human brain, synaptic vesicles have an average diameter of 39.5 nanometers (nm) with a standard deviation of 5.1 nm. 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 'terminal bouton'. Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the visual cortex of the human brain, synaptic vesicles have an average diameter of 39.5 nanometers (nm) with a standard deviation of 5.1 nm. Synaptic vesicles are relatively simple because only a limited number of proteins fit into a sphere of 40 nm diameter. Purified vesicles have a protein:phospholipid ratio of 1:3 with a lipid composition of 40% phosphatidylcholine, 32% phosphatidylethanolamine, 12% phosphatidylserine, 5% phosphatidylinositol, and 10% cholesterol. Synaptic vesicles contain two classes of obligatory components: transport proteins involved in neurotransmitter uptake, and trafficking proteins that participate in synaptic vesicle exocytosis, endocytosis, and recycling. The stoichiometry for the movement of different neurotransmitters into a vesicle is given in the following table. Recently, it has been discovered that synaptic vesicles also contain small RNA molecules, including transfer RNA fragments, Y RNA fragments and mirRNAs. This discovery is believed to have broad impact on studying chemical synapses. Some neurotoxins, such as batrachotoxin, are known to destroy synaptic vesicles. The tetanus toxin damages vesicle-associated membrane proteins (VAMP), a type of v-SNARE, while botulinum toxins damage t-SNARES and v-SNARES and thus inhibit synaptic transmission. A spider toxin called alpha-Latrotoxin binds to neurexins, damaging vesicles and causing massive release of neurotransmitters. Vesicles in the nerve terminal are grouped into three pools: the readily releasable pool, the recycling pool, and the reserve pool. These pools are distinguished by their function and position in the nerve terminal. The readily releasable pool are docked to the cell membrane, making these the first group of vesicles to be released on stimulation. The readily releasable pool is small and is quickly exhausted. The recycling pool is proximate to the cell membrane, and tend to be cycled at moderate stimulation, so that the rate of vesicle release is the same as, or lower than, the rate of vesicle formation. This pool is larger than the readily releasable pool, but it takes longer to become mobilised. The reserve pool contains vesicles that are not released under normal conditions. This reserve pool can be quite large (~50%) in neurons grown on a glass substrate, but is very small or absent at mature synapses in intact brain tissue. The events of the synaptic vesicle cycle can be divided into a few key steps: Synaptic vesicle components are initially trafficked to the synapse using members of the kinesin motor family. In C. elegans the major motor for synaptic vesicles is UNC-104. There is also evidence that other proteins such as UNC-16/Sunday Driver regulate the use of motors for transport of synaptic vesicles.