The purpose of this page is to summarize Hodgkin and Huxley's second paper. In this paper, they examined the currents that were created by the movement of sodium ions and potassium ions trhough the neuron membrane. Their article was concerned with finding out which ionic current was associated with each phase of the membrane current. This information could later be used to created a mathmatical equation to describe the ionic currents. These mathmatical equations could then be used in Hodgkin and Huxley's model.

Experiment

Results

Ionic Current

The Experiment

This article is concerned with identifying which ions are associated with different phases of the action potential. This was accomplished using the voltage clamp method they had developed previously. It was found that when the membrane potential was lowered from its resting value by an amount between 10mV and 100mV, the initial current was inward, opposite of the direction of current that had been noted in earlier experiments. Hodgkin and Huxley thought that this change in direction of current could be induced by changing the concentration of sodium in the external environment. Previously, there had been some evidence that the rising phase of the action potential was caused by the influx of sodium. This may explain the phenomenon they observed. In order to test their hypothesis, Hodgkin and Huxley carried out a voltage clamp experiment where they altered the external solution the axon was immersed in. The control was normal sea water. The altered solution had a reduced sodium concentration.

(Chemicals in sea water outside of sodium)

Results
When the external sodium concentration was reduced to zero, the inward current disappeared and was replaced by an early hump of the outward current. The late outward current was only slightly altered, with a steady level that was 15-20% less in the sodium free environment. In sum, the initial phase of inward current that is normally associated with depolarization was reversed when the sodium in the external environment was lowered to zero. These results qualitatively agree with their hypothesis that the inward current is carried by sodium ions. A reason that the delayed outward current is unaffected by changes in sodium concentration could be that this component in the action potential is due largely to the movement of potassium ions.

In a solution that has a reduced sodium concentration, the results are intermediate between a sodium free solution and a sodium rich solution, as is expected. The Nerst equation can be used to quantify how the sodium potential is influenced by the sodium concentration. It was found that there was a lot of agreement between the experimental sodium potential results and the theoretical results based off the Nerst equation. Provided that there was initially sodium in the external environment, it is possible to find a critical potential above which the initial phase of the ionic current is inward and below which the initial phase is outward. Normally, this critical potential was at 100mV although it varied with external sodium concentration in the same way as the potential of a sodium electrode. These results suggest that depolarization leads to a rapid change in permeability to sodium ions. The movement of sodium ions after this change in permeability carries the initial phase of the ionic current.

Seperating Ionic Current into Components
Some assumptions needed to be made in order to be able to separate the ionic current into its components, the sodium current and the potassium current. First, the time course of the potassium current must be the same in the case where there is low sodium as well as in the case where is high sodium concentration. Secondly, the time course of the sodium current is similar in the two cases (high and low sodium). The amplitude and the direction may be changed between the two cases but the time scale and the form of the time course should remain the same. Thirdly, the rate of change of the potassium current must be zero for about 1/3 of the period taken by the sodium current to reach its maximum.

Using these assumptions, a set of experiments was designed for this analysis. The time course of the sodium or potassium permeability when the axon is held in the depolarized condition is found by using conductance as a measure of permeability. Three series of voltage clamp records were taken. The first was in the solution in question. The second was in the opposite solution. The third was again in the solution in question. The records of the voltage clamps in this series were then compared. The membrane capacitor current was subtracted out. Each pair of records (the first and the third) was averaged in order to take into account aging of the neuron over time.

The difference in resting potential was accounted for by interpolation. It was found that the sodium conductance rises rapidly to a maximum and then declines along an exponential curve. The potassium conductance rises more slowly along an S-shaped curve and is maintained at a high level for a longer period of time. The maximum sodium and potassium conductance were about 30 mmho/cm2 at a depolarization of 100mV.

Figure 1: Illustration of the seperation of ionic current into sodium current and potassium current. I is in sea water solution. I' is in 10% sodium sea water solutions.

Figure 2: Sodium (a) and Potassium (b) conductances. Displacement of membrane potential for each record is given along the y-axis. Axon in sea water.