|ENGR 093: Biomedical Directed Reading Spring 2004|
It was found during the course of their work that the membrane would increase permeablity to sodium quickly but it would be take a little longer for the sodium permeabilty to decrease after the initial rise associated with depolarization. This page attempts to explore this phenomenon further. By further exploring inactivation, Hodgkin and Huxley were able to expand their model to include an inactivation term in describing the sodium current during an action potential.
This paper deals with the inactivation process which gradually reduces sodium permeability after it has undergone the initial rise associated with depolarization. Again the voltage clamp apparatus was used.
A series of experiments were designed to look into the inactivation process. In the first experiment, the membrane potential was changed in two steps. The amplitude of the first step was –8mV. It’s duration was between 0 and 50 ms. This is the conditioning voltage. The second step is the test voltage and had an amplitude of –44mV. The second experiment involved raising the membrane potential to +30 mV and then lowering it to –44mV.
The results of these experiments revealed that small changes in the membrane
potential of the giant axon are associated with large changes in the ability
of the neuron membrane to undergo its normal increase in sodium conductance.
A steady depolarization of 10mV reduces the sodium current associated
with a sudden 45mV depolarization by 60%. A steady rise of 10mV increases
the sodium current associated with a subsequent depolarization by about
50%. These effects can be described by realizing that depolarization gradually
inactivates the system that allows sodium ions to pass through the membrane.
In steady state, inactivation appears to be complete if the membrane potential
is reduced by 30mV and is almost absent if increased by 30mV. Between
these two extremes, the % inactivation is determined by a smooth symmetrical
Figure 1: Development of inactivation: The left hand column represents the time course of the membrane potential. The right hand column represents the time course of the membrane current. Inward current has upward deflection.
Figure 2: Removal of inactivation
Figure 3: Influence of membrane potential on inactivation at steady state
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