Editing Chemical synapse (section)

===Neurotransmitter release===
[[File:Neuro Muscular Junction.png|thumb|Release of neurotransmitter occurs at the end of axonal branches.]]
The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or [[action potential]]) and occurs through an unusually rapid process of cellular secretion ([[exocytosis]]). Within the presynaptic nerve terminal, [[vesicle (biology)|vesicle]]s containing neurotransmitter are localized near the synaptic membrane. The arriving action potential produces an influx of [[second messenger|calcium ions]] through [[Voltage-dependent calcium channel|voltage-dependent, calcium-selective ion channels]] at the down stroke of the action potential (tail current).<ref name="Llinás81">
{{cite journal |vauthors=Llinás R, Steinberg IZ, Walton K |title=Relationship between presynaptic calcium current and postsynaptic potential in squid giant synapse |journal=Biophysical Journal |volume=33 |issue=3 |pages=323–351 |year=1981 |pmid=6261850 |doi=10.1016/S0006-3495(81)84899-0 |pmc=1327434 |bibcode=1981BpJ....33..323L }}</ref> Calcium ions then bind to [[synaptotagmin]] proteins found within the membranes of the synaptic vesicles, allowing the vesicles to fuse with the presynaptic membrane.<ref>{{Cite journal|last=Chapman|first=Edwin R.|date=2002|title=Synaptotagmin: A Ca2+ sensor that triggers exocytosis?|journal=Nature Reviews Molecular Cell Biology|language=En|volume=3|issue=7|pages=498–508|doi=10.1038/nrm855|pmid=12094216|s2cid=12384262|issn=1471-0080}}</ref> The fusion of a vesicle is a [[stochastic]] process, leading to frequent failure of synaptic transmission at the very small synapses that are typical for the [[central nervous system]]. Large chemical synapses (e.g. the [[neuromuscular junction]]), on the other hand, have a synaptic release probability, in effect, of 1. [[Vesicle fusion]] is driven by the action of a set of proteins in the presynaptic terminal known as [[SNARE (protein)|SNAREs]]. As a whole, the protein complex or structure that mediates the docking and fusion of presynaptic vesicles is called the active zone.<ref>Craig C. Garner and Kang Shen. Structure and Function of Vertebrate and Invertebrate Active Zones. Structure and Functional Organization of the Synapse. Ed: Johannes Hell and Michael Ehlers. Springer, 2008.</ref> The membrane added by the fusion process is later retrieved by [[endocytosis]] and [[Endocytic cycle|recycled]] for the formation of fresh neurotransmitter-filled vesicles.

An exception to the general trend of neurotransmitter release by vesicular fusion is found in the type II receptor cells of  mammalian [[taste bud]]s. Here the neurotransmitter [[Adenosine triphosphate|ATP]] is released directly from the cytoplasm into the synaptic cleft via voltage gated channels.<ref name="RomanovLasher2018">{{cite journal|last1=Romanov|first1=Roman A.|last2=Lasher|first2=Robert S.|last3=High|first3=Brigit|last4=Savidge|first4=Logan E.|last5=Lawson|first5=Adam|last6=Rogachevskaja|first6=Olga A.|last7=Zhao|first7=Haitian|last8=Rogachevsky|first8=Vadim V.|last9=Bystrova|first9=Marina F.|last10=Churbanov|first10=Gleb D.|last11=Adameyko|first11=Igor|last12=Harkany|first12=Tibor|last13=Yang|first13=Ruibiao|last14=Kidd|first14=Grahame J.|last15=Marambaud|first15=Philippe|last16=Kinnamon|first16=John C.|last17=Kolesnikov|first17=Stanislav S.|last18=Finger|first18=Thomas E.|title=Chemical synapses without synaptic vesicles: Purinergic neurotransmission through a CALHM1 channel-mitochondrial signaling complex|journal=Science Signaling|volume=11|issue=529|year=2018|pages=eaao1815|issn=1945-0877|doi=10.1126/scisignal.aao1815|pmid=29739879|pmc=5966022}}</ref>