Hodgkin and Huxley developed a set of equations to describe how a 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 the 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 afterhyperpolarization.
||The basic equations describing this process are
where Iinj is the injected current.
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:
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.
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
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