|ENGR 093: Biomedical Directed Reading Spring 2004
This page is devoted to examining the effects of changing sodium and potassium ion concentrations in order to better understand sodium and potassium conductances. Hodgkin and Huxley ran two sets of experiments, one that was concerned with the movement of sodium ions and the other devoted to the movement of potassium ions. It was found that both sodium and potassium conductance rise along an inflected curve during depolarization and fall without any inflection during repolarization. However, the rate that the sodium conductance rises and falls is at least 10 times faster than the rate of rise and fall for potassium conductance. Also, if the axon is depolatized, the potassium conductance is maintained but the sodium conductance declines after reaching it's peak. The identity of the effects of ion conductances on the membrane current were later used to help Hodgkin and Huxley develop their mathmatical model of the action potential in a neuron.
From “The Components of Membrane Conductance in the Giant Axon of Loligo” (third paper)
This article is concerned with the situation where the membrane potential is suddenly restored from the depolarized level back to the resting potential. The experiments that Hodgkin and Huxley ran can be characterized into two categories. The first is largely concerned with the movement of sodium ions and the second is concerned with the movement of potassium ions. Again, the voltage clamp apparatus was used in this set of experiments.
In one set of experiments Hodgkin and Huxley inputted a single step down from 0mV to –41mV into the membrane of the axon. As expected, they noticed that the membrane current had a wave of inward current followed by a maintained phase of outward current. They then inputted a step down from 0 to –41mv followed by a step up in the middle of the action potential from –41 mV to 0mV. The sudden change in potential is associated with a change in the capacity current followed by a “tail” of ionic current. Repolariztion during the period of high sodium permeability is associated with a large inward current with declines rapidly along an exponential curve. The tail disappears if sodium ions are removed from the external medium. These results can be explored quantitatively by supposing that the sodium conductance is continuous which rises when the membrane is depolarized and falls when the membrane is repolarized. The total period of inward current is greatly reduced by cutting the period of depolarization. This suggests that the process underlying sodium permeability is reversible and repolarization causes the sodium current to fall more rapidly than it would with a maintained depolarization. It was also found that large discontinuties could be induced with a sudden change in membrane potential (see figure2) Previous experimentation revealed that the inward current is carried by sodium ions. THe tail of inward current can also be associated with the sodium current. (see figure3) The time course of the sodium conductance during voltage clamp can be calculated the variation of the tail of inward current during depolarization.
Potassium ions are largely responsible for maintaining the outward current during a prolonged depolarization of the membrane. In order to fully investigate the instantaneous relationship between potassium current and membrane potential, longer depolarization must be used. Like sodium potassium also returns to the same lower conductance when the membrane is repolarized. Repolarization of the membrane during high potassium permeability is associated with a tail of outward current at the resting potential and an inward current above the critical potential. The relationship between the potassium current and the membrane potential is linear passing through zero at 10-20mV above the resting potential. This suggests that potassium conductance is a continuous function of time which rises when the nerve is depolarized and falls when the nerve is repolarized. The rate at which the potassium conductance is reduced on repolarization increases with membrane potential. The critical potential at which the potassium current appears to reverse sign varies with the external concentration of potassium but less steeply than in the case of a potassium electrode.
The Final Conclusions
(Figure 1: On the left hand column, time course of the potential difference between the internal and external electrode. The right hand column shows the membrane current time course for the associated membrane potential shown in the left hand column)
(Figure 2: Membrane current time courses associated with depolarization of 97.5mV lastking 0.05, 0.08, 0.19, 0.32, 0.91, 1.6, and 2.6 msec)
(Figure 3: Membrane current associated with depolarization of 110mV, lasting 0.28 msec. Nerve A is in sea water. Nerve B is in chlorine water. Nerve C had a depolarization of 220mV and is in chlorine water)
(Figure 4:Time course of the sodium conductance estimated from records C and D from figure 2)
(Figure 5: A is the ionic current associated with depolarization of 25mV, lasting 4.9 msec. B is the potassium conductance estimated from A.)
|Send an email Alexis Kristina Erik
|Swarthmore College Department of Engineering Erik Cheever's Homepage