When a depolarizing current is injected into a nerve cell, the resultant decrease in the membrane potential activates voltage-dependent Na+ channels. The activation of the Na+ channels in turn accelerates the depolarization process, producing the rising phase of the action potential. The depolarization associated with the rising phase of the action potential ultimately triggers the process of Na+ channel inactivation, which prevents further membrane depolarization and initiates the repolarization phase of the action potential. At the same time, the voltage-dependent K+ channels are activated, also contributing to the repolarizing the cell, and, in addition, producing the after hyperpolarization.
Change the values of the default parameters in each box above using the increase and decrease sliders OR by entering a numerical value to visualize the effect on the action potential. The basic equations used in the Action Potential Simulator are: $$V_m = \frac{1}{C_m}\int(I_{inj} - (V_m - E_{Na})g_{Na} - (V_m - E_K)g_K - (V_m - E_{l})g_{l})dt$$where Vm is the membrane potential, Cm is the membrane capacitance, Iinj is the injected current, gNa is the Na+ conductance, gK is the K+ conductance, and gl is the leakage conductance. The conductance of the Na+ channel is governed by an activation variable m and an inactivation variable h, gNa = gNamax m3h and the conductance of the K+ channel is governed by a single activation variable n, gK = gKmax n4
The default parameters are: