The neuronal action potential, a spike in membrane voltage, travels down the length of an unmyelinated axon by diffusion alone.
The action potential is always of the same height and shape; otherwise, it does not self-reinforce as it travels, and dies out. Thus it is a robust method of transmitting a signal. Qualitative information is encoded by the frequency of spiking.
The action potential is generated when the voltage across the membrane is raised far enough past resting potential, past the 'threshold potential'. (As resting potential is negative due to electrochemical gradients of K+ and Na+, this event is called a depolarization.)
---After depolarization, the membrane voltage spikes as Na+ floods in, producing the upstroke. A downstroke follows, as more ion channels slowly open to let K+ exit, whilst others become inactivated to prevent more Na+ from entering.
While the Na+ channels are inactivated, the neuron cannot respond to further stimuli; this is the absolute refractory period. Additionally, the neuron overshoots its resting potential during the downstroke (the sweep of red); this is the relative refractory period.
Continuous stimulation causes the neuron to spike with a certain frequency. In this simple model, that frequency is fixed.
---This model, based on the generalized FitzHugh-Nagomo equations, characterizes the axon as many segments linked by diffusion, each of which tracks its own excitation and inactivation.
The change in excitation is a cubic function of excitation. It is negative for both low and high excitation, but positive for excitation between the threshold and peak potential. It turns out that excitation always trends towards zero (absent external stimulus), but if it is pushed past threshold potential, it takes a long detour towards peak potential.
The change in inactivation is a linear function of both exictation and inactivation. It is positive for high excitation, but quickly becomes negative as inactivation rises. So inactivation tends to peak slightly after excitation does.
Notice that no specific count is kept of the two most important species of charge carrier, K+ and Na+. In this model, diffusion between segments is treated as a linear function of the difference in excitement between them, which does not seem to be a disastrous oversimplification.
---pending:
in-applet graph of voltage.
controls for threshold potential, rate of recovery, and # segments.
graph of phase plane.