Editing Axon (section)

====Axonal initial segment====
The '''axonal initial segment''' (AIS) is a structurally and functionally separate microdomain of the axon.<ref name="Nelson">{{cite journal | vauthors = Nelson AD, Jenkins PM | title = Axonal Membranes and Their Domains: Assembly and Function of the Axon Initial Segment and Node of Ranvier | journal = Frontiers in Cellular Neuroscience | volume = 11 | pages = 136 | date = 2017 | pmid = 28536506 | pmc = 5422562 | doi = 10.3389/fncel.2017.00136 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Leterrier C, Clerc N, Rueda-Boroni F, Montersino A, Dargent B, Castets F | title = Ankyrin G Membrane Partners Drive the Establishment and Maintenance of the Axon Initial Segment | language = en | journal = Frontiers in Cellular Neuroscience | volume = 11 | pages = 6 | date = 2017 | pmid = 28184187 | pmc = 5266712 | doi = 10.3389/fncel.2017.00006 | doi-access = free }}</ref> One function of the initial segment is to separate the main part of an axon from the rest of the neuron; another function is to help initiate action potentials.<ref>{{cite journal | vauthors = Leterrier C | title = The Axon Initial Segment: An Updated Viewpoint | journal = The Journal of Neuroscience | volume = 38 | issue = 9 | pages = 2135–2145 | date = February 2018 | pmid = 29378864 | pmc = 6596274 | doi = 10.1523/jneurosci.1922-17.2018 }}</ref> Both of these functions support neuron [[cell polarity]], in which dendrites (and, in some cases the [[Soma (biology)|soma]]) of a neuron receive input signals at the basal region, and at the apical region the neuron's axon provides output signals.<ref>{{cite journal | vauthors = Rasband MN | title = The axon initial segment and the maintenance of neuronal polarity | language = En | journal = Nature Reviews. Neuroscience | volume = 11 | issue = 8 | pages = 552–62 | date = August 2010 | pmid = 20631711 | doi = 10.1038/nrn2852 | s2cid = 23996233 }}</ref>

The axon initial segment is unmyelinated and contains a specialized complex of proteins. It is between approximately 20 and 60&nbsp;μm in length and functions as the site of action potential initiation.<ref name="Jones">{{cite journal | vauthors = Jones SL, Svitkina TM | title = Axon Initial Segment Cytoskeleton: Architecture, Development, and Role in Neuron Polarity | journal = Neural Plasticity | volume = 2016 | pages = 6808293 | date = 2016 | pmid = 27493806 | pmc = 4967436 | doi = 10.1155/2016/6808293 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Clark BD, Goldberg EM, Rudy B | title = Electrogenic tuning of the axon initial segment | journal = The Neuroscientist | volume = 15 | issue = 6 | pages = 651–68 | date = December 2009 | pmid = 20007821 | pmc = 2951114 | doi = 10.1177/1073858409341973 }}</ref> Both the position on the axon and the length of the AIS can change showing a degree of plasticity that can fine-tune the neuronal output.<ref name="Jones"/><ref name="Yamada">{{cite journal | vauthors = Yamada R, Kuba H | title = Structural and Functional Plasticity at the Axon Initial Segment | journal = Frontiers in Cellular Neuroscience | volume = 10 | pages = 250 | date = 2016 | pmid = 27826229 | pmc = 5078684 | doi = 10.3389/fncel.2016.00250 | doi-access = free }}</ref> A longer AIS is associated with a greater excitability.<ref name="Yamada"/> Plasticity is also seen in the ability of the AIS to change its distribution and to maintain the activity of neural circuitry at a constant level.<ref name="Susuki"/>

The AIS is highly specialized for the fast conduction of [[Action potential|nerve impulses]]. This is achieved by a high concentration of [[Voltage-gated sodium channel|voltage-gated sodium channels]] in the initial segment where the action potential is initiated.<ref name="Susuki">{{cite journal | vauthors = Susuki K, Kuba H | title = Activity-dependent regulation of excitable axonal domains | journal = The Journal of Physiological Sciences | volume = 66 | issue = 2 | pages = 99–104 | date = March 2016 | pmid = 26464228 | doi = 10.1007/s12576-015-0413-4 | s2cid = 18862030 | doi-access = free | pmc = 10717305 }}</ref> The ion channels are accompanied by a high number of [[cell adhesion molecule]]s and [[scaffold protein]]s that anchor them to the cytoskeleton.<ref name="Jones"/> Interactions with [[Ankyrin-3|ankyrin-G]] are important as it is the major organizer in the AIS.<ref name="Jones"/>

In other cases as seen in rat studies an axon originates from a dendrite; such axons are said to have "dendritic origin". Some axons with dendritic origin similarly have a "proximal" initial segment that starts directly at the axon origin, while others have a "distal" initial segment, discernibly separated from the axon origin.<ref name="Höfflin-2017" /> In many species some of the neurons have axons that emanate from the dendrite and not from the cell body, and these are known as axon-carrying dendrites.<ref name=Triarhou/> In many cases, an axon originates at an axon hillock on the soma; such axons are said to have "somatic origin". Some axons with somatic origin have a "proximal" initial segment adjacent the axon hillock, while others have a "distal" initial segment, separated from the soma by an extended axon hillock.<ref name="Höfflin-2017">{{cite journal | vauthors = Höfflin F, Jack A, Riedel C, Mack-Bucher J, Roos J, Corcelli C, Schultz C, Wahle P, Engelhardt M | display-authors = 6 | title = Heterogeneity of the Axon Initial Segment in Interneurons and Pyramidal Cells of Rodent Visual Cortex | language = en | journal = Frontiers in Cellular Neuroscience | volume = 11 | pages = 332 | date = 2017 | pmid = 29170630 | pmc = 5684645 | doi = 10.3389/fncel.2017.00332 | doi-access = free }}</ref>