The Hodgkin-Huxley experiment

The Hodgkin-Huxley experiment measured the membrane conductances of sodium and potassium ions in the giant axon of the squid Loligo. This demonstration reproduces the essentails of the original experimental procedure:
1) A constant voltage step of adjustable magnitude depolarizes the membrane at 1 ms using a voltage clamp. The circuit features an amplifier in the form of an op-amp, whose gain may be varied to explore the quality of the resulting potential step, shown in the bottom left panel. The total membrane current is recorded and shown in blue in the bottom middle panel.
2) In a separate trial, the ionic currents are determined. The external concentration is adjusted such that the Nernst potential of sodium is equal to the voltage step, eliminating the sodium current. The resultant measured current consists principally of the current, shown in the bottom middle panel in orange (leak and capacitative currents occur on a shorter timescale and thus here neglected). Subtracting the current from the total current in (1) gives the current, shown in green.
3) The and conductances are computed from the respective currents, voltage changes and reversal potentials, and are shown in the right column. The conductance plots may be saved to show their dependence on the voltage step.
  • Contributed by: Oliver K. Ernst



  • [Snapshot]
  • [Snapshot]
  • [Snapshot]


Snapshot 1: the sodium and potassium conductances may be determined by following the experimental procedure of Hodgkin, Huxley and Katz.
Snapshot 2: the dependence of the conductances on the voltage step may be explored.
Snapshot 3: the Hodgkin-Huxley model for the conductances may be shown for valdiating the theory.
The Hodgkin-Huxley neuron model is a principal discovery in neuroscience and electrophysiology. It describes the ionic currents and resulting voltage changes that occur across a neuron's membrane. Practically, it was developed by measuring the membrane conductance of sodium and potassium ions, from which the governing differential equations are deduced. The essential procedure of the original experiment [1] is described below.
(1) A voltage and current electrode, wrapped in a glass capillary, are inserted in the the axon of the giant squid. The axon itself is held in place in a grounded cylinder, originally made of silver sheet. The potential across the membrane is stepped and held at a constant value using a voltage clamp [1].
The high impedance op-amp draws minimal current from the membrane, and outputs approx. the membrane voltage . The amplifier has an adjustable gain , and outputs voltage . The resultant current is applied across the membrane using the current electrode, leading to a feedback circuit that at high gain fixes the membrane voltage [2].
When the potential across the membrane is constant, the sum of the ionic currents must be balanced by the current applied by the voltage clamp. Thus, the measured current applied by the clamp is the same magnitude as the total ionic current across the membrane (and opposite in sign). The total ionic current is divided into 3 components: and leak (other ions).
(2) Next, in a separate experiment, the components of the total current are determined. The extracellular sodium concentration is adjusted such that sodium current does not occur, given by
where is the Nernst reversal potential. Clearly, at .
The reversal potential is given by
where denote the variable extracellular and fixed intracellular concentrations, is the charge of , and at C [2].
When , as can be seen in the lower left panel, the sodium current disappears. Since the leak and capacitative currents occur on a short timescale ms (seen in the blue spike at 1 ms in the bottom middle panel), prinicpally only the potassium current remains. The measured total current is now , shown in orange. The sodium current is obtained as , with from (1), shown in green.
(3) Once the ionic currents have been determined, the conductances are given by
and similarly for [2].
The time dependant behavior of the conductances is explained by the transitions of activation and inactivation particles ,
where the particle dynamcis are described by
for , and are transition rates between open and closed states. For further information, see [2,3].
The true conductances may be shown on the plots for validation of the theory. Combined with the dynamics of the membrane voltage
these equations constitute the Hodgkin-Huxley model [2,3].
[1] A. L. Hodgkin, A. F. Huxley, and B. Katz, "Measurement of current-voltage relations in the membrane of the giant axon of Loligo," The Journal of Physiology, 116(4), 1952 pp. 424–448.
[2] C. Koch, Biophysics of Computation: Information Processing in Single Neurons, New York: Oxford University Press, 1999.
[3] A. L. Hodgkin and A. F. Huxley, "A Quantitative Description of Membrane Current and Its Application to Conduction and Excitation in Nerve," The Journal of Physiology, 117(4), 1952 pp. 500–544.