Synaptic vesicle
Encyclopedia
In a neuron
, synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitter
s 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.
in which transport protein
s specific to each type of neurotransmitter are inserted. Neurotransmitters are moved from the cell's cytoplasm
into the vesicles by vesicular transporter
s that rely on active transport
mechanisms involving an exchange of proton
s (H+ ions). The necessary proton gradient is created by V-ATPase
, which breaks down ATP
for energy. Vesicular glutamate transporter
s, for example, sequester glutamate into vesicles by this process.
The stoichiometry
for the movement of different neurotransmitters into a vesicle is given in the following table.
membranes are tethered to SNAP via ATPase.
s, 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 toxin
s damage t-SNARES and v-SNARES and thus inhibit synaptic transmission. A spider
toxin called α-Latrotoxin binds to neurexin
s, damaging vesicles and causing massive release of neurotransmitters.
, 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 constitutes the vast majority of vesicles in the nerve terminal, but it is not clear that vesicles in this pool are released under normal conditions. Under experimental conditions, this pool is mobilised by intense stimulation, and might occur only once the other two pools are exhausted. When there are no longer readily releasable pools of vesicles to be released into the synaptic cleft, the neuron begins to experience synaptic fatigue
.
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
, synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s that are released
Exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...
at the synapse
Chemical synapse
Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie...
. The release is regulated by a voltage-dependent calcium channel
Voltage-dependent calcium channel
Voltage-dependent calcium channels are a group of voltage-gated ion channels found in excitable cells with a permeability to the ion Ca2+...
. Vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
are essential for propagating nerve impulses between neurons and are constantly recreated by the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. The area in the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
that holds groups of vesicles is an axon terminal
Axon terminal
Axon terminals are distal terminations of the branches of an axon. An axon nerve fiber is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body, or soma, in order to transmit those impulses to other neurons.Neurons are...
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.
Formation
Synaptic vesicles are made of a lipid bilayerLipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
in which transport protein
Transport protein
A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane. Transport proteins are integral membrane proteins; that is they exist within and span the membrane across which they...
s specific to each type of neurotransmitter are inserted. Neurotransmitters are moved from the cell's cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
into the vesicles by vesicular transporter
Vesicular transport protein
A vesicular transport protein is a transmembrane or membrane associated protein. It regulates or facilitates the movement by vesicles of the contents of the cell.-Types:Examples include:* Archain* ARFs* Clathrin* Caveolin...
s that rely on active transport
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
mechanisms involving an exchange of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s (H+ ions). The necessary proton gradient is created by V-ATPase
V-ATPase
Vacuolar-type H+-ATPase is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons across the plasma membranes of numerous cell types...
, which breaks down ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
for energy. Vesicular glutamate transporter
Glutamate transporter
Excitatory amino-acid transporters , formerly known as Glutamate transporters, belong to the family of neurotransmitter transporters. They serve to terminate the excitatory neurotransmitter signal by removal of glutamate from the neuronal synapse into Glia cells.In details, the EAATs are...
s, for example, sequester glutamate into vesicles by this process.
The stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
for the movement of different neurotransmitters into a vesicle is given in the following table.
| Neurotransmitter type(s) | Inward movement | Outward movement |
|---|---|---|
| norepinephrine Norepinephrine Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter... , dopamine Dopamine Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their... , histamine Histamine Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by... , serotonin Serotonin Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans... and acetylcholine Acetylcholine The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans... |
neurotransmitter+ | 2 H+ |
| GABA Gabâ Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions... and glycine Glycine Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid... |
neurotransmitter | 1 H+ |
| glutamate | neurotransmitter- + Cl- | 1 H+ |
membranes are tethered to SNAP via ATPase.
Effects of neurotoxins
Some neurotoxinNeurotoxin
A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...
s, such as batrachotoxin
Batrachotoxin
Batrachotoxins are extremely potent cardiotoxic and neurotoxic steroidal alkaloids found in certain species of frogs , melyrid beetles, and birds...
, are known to destroy synaptic vesicles. The tetanus
Tetanus
Tetanus is a medical condition characterized by a prolonged contraction of skeletal muscle fibers. The primary symptoms are caused by tetanospasmin, a neurotoxin produced by the Gram-positive, rod-shaped, obligate anaerobic bacterium Clostridium tetani...
toxin damages vesicle-associated membrane proteins (VAMP), a type of v-SNARE, while botulinum toxin
Botulinum toxin
Botulinum toxin is a protein produced by the bacterium Clostridium botulinum, and is considered the most powerful neurotoxin ever discovered. Botulinum toxin causes Botulism poisoning, a serious and life-threatening illness in humans and animals...
s damage t-SNARES and v-SNARES and thus inhibit synaptic transmission. A spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
toxin called α-Latrotoxin binds to neurexin
Neurexin
A neurexin is a presynaptic protein that helps to glue together neurons at the synapse. Neurexins are type I membrane proteins that can be classified into two types, α-NRXNs and β-NRXNs. The α-NRXNs are larger and have different amino-terminal extracellular sequences...
s, damaging vesicles and causing massive release of neurotransmitters.
Vesicle pools
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 membraneCell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, 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 constitutes the vast majority of vesicles in the nerve terminal, but it is not clear that vesicles in this pool are released under normal conditions. Under experimental conditions, this pool is mobilised by intense stimulation, and might occur only once the other two pools are exhausted. When there are no longer readily releasable pools of vesicles to be released into the synaptic cleft, the neuron begins to experience synaptic fatigue
Synaptic fatigue
Synaptic fatigue, or short-term synaptic depression, is an activity-dependent form of short-term plasticity that affects neuronal efficacy and results in the temporary inability to fire and therefore transmit an input signal. It is thought to be a form of negative feedback in order to...
.
See also
- Vesicular monoamine transporterVesicular monoamine transporterThe vesicular monoamine transporter is a transport protein integrated into the membrane of intracellular vesicles of presynaptic neurons. It acts to transport monoamines into the synaptic vesicles.-Monoamines:...
- Vesicle (biology)Vesicle (biology)A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
- SynapsinSynapsinThe synapsins are a family of proteins that have long been implicated in the regulation of neurotransmitter release at synapses. Specifically, they are thought to be involved in regulating the number of synaptic vesicles available for release via exocytosis at any one time.Current studies suggest...
s - Vesicle fusionVesicle fusionVesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis at the porosome...