Editing Axon (section)

==Action potentials==
{{Main|Action potential}}
{{Further |Neural coding|Active zone}}
[[File:Chemical_synapse_schema_cropped.jpg|thumb|upright=1.2|[[Synapse|Synaptic connections from an axon]]]]
[[Image:SynapseSchematic en.svg|thumb|260px|Neurotransmitter released from presynaptic axon terminal, and transported across synaptic cleft to receptors on postsynaptic neuron|alt=The pre- and post-synaptic axons are separated by a short distance known as the synaptic cleft. Neurotransmitter released by pre-synaptic axons diffuse through the synaptic cleft to bind to and open ion channels in post-synaptic axons.]]

Most axons carry signals in the form of action potentials, which are discrete electrochemical impulses that travel rapidly along an axon, starting at the cell body and terminating at points where the axon makes synaptic contact with target cells. The defining characteristic of an action potential is that it is "all-or-nothing"{{Snd}}every action potential that an axon generates has essentially the same size and shape. This [[All-or-none law|all-or-nothing]] characteristic allows action potentials to be transmitted from one end of a long axon to the other without any reduction in size. There are, however, some types of neurons with short axons that carry graded electrochemical signals, of variable amplitude.

When an action potential reaches a presynaptic terminal, it activates the synaptic transmission process. The first step is rapid opening of calcium ion channels in the membrane of the axon, allowing calcium ions to flow inward across the membrane. The resulting increase in intracellular calcium concentration causes [[synaptic vesicle]]s (tiny containers enclosed by a lipid membrane) filled with a neurotransmitter chemical to fuse with the axon's membrane and empty their contents into the extracellular space. The neurotransmitter is released from the presynaptic nerve through [[exocytosis]]. The neurotransmitter chemical then diffuses across to receptors located on the membrane of the target cell. The neurotransmitter binds to these receptors and activates them. Depending on the type of receptors that are activated, the effect on the target cell can be to excite the target cell, inhibit it, or alter its metabolism in some way. This entire sequence of events often takes place in less than a thousandth of a second. Afterward, inside the presynaptic terminal, a new set of vesicles is moved into position next to the membrane, ready to be released when the next action potential arrives. The action potential is the final electrical step in the integration of synaptic messages at the scale of the neuron.<ref name="Debanne"/>

Extracellular recordings of action potential propagation in axons has been demonstrated in freely moving animals. While extracellular somatic action potentials have been used to study cellular activity in freely moving animals such as [[place cells]], axonal activity in both [[White matter|white]] and [[gray matter]] can also be recorded. Extracellular recordings of axon action potential propagation is distinct from somatic action potentials in three ways: 1. The signal has a shorter peak-trough duration (~150μs) than of [[pyramidal cell]]s (~500μs) or [[interneuron]]s (~250μs). 2. The voltage change is triphasic. 3. Activity recorded on a tetrode is seen on only one of the four recording wires. In recordings from freely moving rats, axonal signals have been isolated in white matter tracts including the alveus and the corpus callosum as well hippocampal gray matter.<ref>{{cite journal | vauthors = Robbins AA, Fox SE, Holmes GL, Scott RC, Barry JM | title = Short duration waveforms recorded extracellularly from freely moving rats are representative of axonal activity | journal = Frontiers in Neural Circuits | volume = 7 | issue = 181 | pages = 181 | date = Nov 2013 | pmid = 24348338 | pmc = 3831546 | doi = 10.3389/fncir.2013.00181 | doi-access = free }}</ref>

In fact, the generation of action potentials in vivo is sequential in nature, and these sequential spikes constitute the [[neural coding|digital codes]] in the neurons. Although previous studies indicate an axonal origin of a single spike evoked by short-term pulses, physiological signals in vivo trigger the initiation of sequential spikes at the cell bodies of the neurons.<ref>Rongjing Ge, Hao Qian and Jin-Hui Wang* (2011) Molecular Brain 4(19), 1~11</ref><ref>Rongjing Ge, Hao Qian, Na Chen and Jin-Hui Wang* (2014) Molecular Brain 7(26):1-16</ref>

In addition to propagating action potentials to axonal terminals, the axon is able to amplify the action potentials, which makes sure a secure propagation of sequential action potentials toward the axonal terminal. In terms of molecular mechanisms, [[voltage-gated sodium channel]]s in the axons possess lower [[Threshold potential|threshold]] and shorter [[Refractory period (physiology)|refractory period]] in response to short-term pulses.<ref>{{cite journal | vauthors = Chen N, Yu J, Qian H, Ge R, Wang JH | title = Axons amplify somatic incomplete spikes into uniform amplitudes in mouse cortical pyramidal neurons | journal = PLOS ONE | volume = 5 | issue = 7 | pages = e11868 | date = July 2010 | pmid = 20686619 | pmc = 2912328 | doi = 10.1371/journal.pone.0011868 | bibcode = 2010PLoSO...511868C | doi-access = free }}</ref>