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An <action potential> is a brief regenerative excursion of <membrane potential>, produced by changing <electrical resistance and conductance> through <voltage-gated ion channels>. In a typical fast neuronal <action potential>, <sodium> entry raises <membrane potential>, activating further <sodium> channels. This feedback produces the rapid rising phase. Inactivation of <sodium> channels and increased conductance to <potassium> then lower <membrane potential> again; persisting <potassium> conductance can produce an undershoot below the resting level. See https://www.ncbi.nlm.nih.gov/books/NBK10897/[Purves and colleagues: the ionic basis of action potentials].

For an illustrative electrical calculation, write inward <sodium> <electric current> as $I_{\mathrm{Na}}=g_{\mathrm{Na}}(E_{\mathrm{Na}}-V)$. At $V=-50\,\mathrm{mV}$, taking $E_{\mathrm{Na}}=60\,\mathrm{mV}$ and $g_{\mathrm{Na}}=10\,\mathrm{nS}$ gives $I_{\mathrm{Na}}=1.1\,\mathrm{nA}$ inward. A <potassium> current $I_{\mathrm{K}}=g_{\mathrm{K}}(E_{\mathrm{K}}-V)$ with $E_{\mathrm{K}}=-80\,\mathrm{mV}$ and $g_{\mathrm{K}}=20\,\mathrm{nS}$ gives $-0.6\,\mathrm{nA}$, outward. These illustrative values expose opposing driving forces; a waveform requires the conductances to evolve with <membrane potential> and time, as in the <Hodgkin-Huxley model>.

An <action potential> propagates along an <axon> because local <electric currents> bring neighboring regions of <cell membrane> into the regime where their <voltage-gated ion channels> regenerate the signal. It is a traveling pattern of excitation, not a packet of <sodium> <ions> traversing the entire <axon>. Regeneration allows an <action potential> to travel without the passive attenuation of a small local voltage change. See <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Ion Channels and the Electrical Properties of Membranes>.

The <neuronal refractory period> constrains how soon another <action potential> can follow. Under comparable conditions, successive <action potentials> have roughly stereotyped shapes, making event count and timing useful for <neural coding>; waveform details can nevertheless change with channel state and conditions. The <leaky integrate-and-fire model> abstracts each <action potential> to a threshold event and reset. Its reset jump is a mathematical rule, not the actual regenerative waveform produced by <sodium> and <potassium> currents.


 Action potential (source code)