Action Potential Laboratory

Hodgkin and Huxley developed a set of equations to describe the ways in which cell produces an action potential upon stimulation, through interactions of voltage-dependent Na⁺ and K⁺ channels. !

When a depolarizing current is injected into a 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 rise of the membrane potential ultimately triggers the process of Na⁺ channel inactivation, which prevents further membrane depolarization. At the same time, the voltage-dependent K⁺ channels are activated, repolarizing the cell and producing the falling phase of the action potential and the after hyperpolarization.

The basic equations describing this process are

H-H integration equation

where I_inj is the injected current.
The conductance of the Na⁺ channel is governed by an activation variable m and an inactivation variable h,

g_Na = g_Namaxm³h

and the conductance of the K⁺ channel is governed by a single activation variable n,

g_K = g_Kmax n⁴

The default parameters are:

Maximum value of voltage-dependent Na⁺ conductance: 1500 uS
Maximum value of voltage-dependent K⁺ conductance: 360 uS
Na⁺ equilibrium potential: 55 mV
K⁺ equilibrium potential: -75 mV

The resting membrane potential of the modeled neuron is -60 mV and the current injection is a 10-msec pulse starting at 5 msec (400 nA by default). The simulation results using the default parameters are displayed in red for your reference. Please use the slider bars to make adjustments in the parameters and observe the ways in which they affect the different phases of the action potential. For more details on simulation of action potentials, see: SNNAP Home Page.