Editing Action potential (section)

===Initiation===
Before considering the propagation of action potentials along [[axon]]s and their termination at the synaptic knobs, it is helpful to consider the methods by which action potentials can be initiated at the [[axon hillock]]. The basic requirement is that the membrane voltage at the hillock be raised above the threshold for firing.{{sfn|Bullock|Orkand|Grinnell|1977|pp=150–151}}{{sfn|Junge|1981|pp=89–90}}{{sfnm|1a1=Purves|1a2=Augustine|1a3=Fitzpatrick|1a4=Hall|1y=2008|1pp=49–50|2a1=Bullock|2a2=Orkand|2a3=Grinnell|2y=1977|2pp=140–141|3a1=Schmidt-Nielsen|3y=1997|3pp=480-481}}{{sfn|Schmidt-Nielsen|1997|pp=483-484}} There are several ways in which this depolarization can occur.
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[[Image:SynapseSchematic en.svg|thumb|right|300px|When an action potential arrives at the end of the pre-synaptic axon (top), it causes the release of [[neurotransmitter]] molecules that open ion channels in the post-synaptic neuron (bottom). The combined [[excitatory postsynaptic potential|excitatory]] and [[inhibitory postsynaptic potential]]s of such inputs can begin a new action potential in the post-synaptic 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 clef to bind to and open ion channels in post-synaptic axons.]]