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Axon terminals (also called synaptic boutons, terminal boutons, or end-feet) are distal terminations of the
telodendria An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
(branches) of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell, or
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...
, that conducts electrical impulses called
action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
s away from the neuron's cell body, or soma, in order to transmit those impulses to other neurons, muscle cells or glands. Neurons are interconnected in complex arrangements, and use electrochemical signals and
neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neu ...
chemicals to transmit impulses from one neuron to the next; axon terminals are separated from neighboring neurons by a small gap called a
synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...
, across which impulses are sent. The axon terminal, and the neuron from which it comes, is sometimes referred to as the "presynaptic" neuron.


Nerve impulse release

Neurotransmitters are packaged into synaptic vesicles that cluster beneath the axon terminal membrane on the presynaptic side of a synapse. The axonal terminals are specialized to release the neurotransmitters of the presynaptic cell. The terminals release transmitter substances into a gap called the synaptic cleft between the terminals and the dendrites of the next neuron. The information is received by the dendrite receptors of the postsynaptic cell that are connected to it. Neurons don't touch each other, but communicate across the synapse. The neurotransmitter molecule packages (vesicles) are created within the neuron, then travel down the axon to the distal axon terminal where they sit '' docked''. Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic membrane and releasing their contents to the synaptic cleft within 180  µs of calcium entry. Triggered by the binding of the calcium ions, the synaptic vesicle proteins begin to move apart, resulting in the creation of a fusion pore. The presence of the pore allows for the release of neurotransmitter into the synaptic cleft. The process occurring at the axon terminal is
exocytosis Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use ...
, which a cell uses to exude secretory
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...
out of the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
. These membrane-bound vesicles contain soluble
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
to be secreted to the extracellular environment, as well as membrane proteins and
lipids Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
that are sent to become components of the cell membrane. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling.


Mapping activity

Wade Regehr, a
Professor Professor (commonly abbreviated as Prof.) is an academic rank at universities and other post-secondary education and research institutions in most countries. Literally, ''professor'' derives from Latin as a "person who professes". Professo ...
of Neurobiology at
Harvard Medical School Harvard Medical School (HMS) is the graduate medical school of Harvard University and is located in the Longwood Medical Area of Boston, Massachusetts. Founded in 1782, HMS is one of the oldest medical schools in the United States and is cons ...
's Department of Neurobiology, developed a method to physiologically see the synaptic activity that occurs in the brain. A dye alters the fluorescence properties when attached to calcium. Using fluorescence-microscopy techniques calcium levels are detected, and therefore the influx of calcium in the presynaptic neuron. Regehr's laboratory specializes in pre-synaptic calcium dynamics which occurs at the axon terminals. Regehr studies the implication of
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
Ca2+ as it affects synaptic strength. By studying the physiological process and mechanisms, a further understanding is made of neurological disorders such as
epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrica ...
,
schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...
and
major depressive disorder Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Intro ...
, as well as
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
and
learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machines; there is also evidence for some kind of lea ...
.


See also

*
Endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ...
*
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...
*
Micelle A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated coll ...
*
Membrane nanotube A tunneling nanotube (TNT) or membrane nanotube is a term that has been applied to protrusions that extend from the plasma membrane which enable different animal cells to touch over long distances, sometimes over 100 μm between T cells. Two typ ...
* Endocytosis * Vesicular monoamine transporter


Further reading

* * * * *LTP promotes formation of multiple spine synapses between a single axon terminal and a dendrite.


References

{{Nervous tissue Neurohistology