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Voltage-Gated Channels and the Action Potential
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How is charge generated across a membrane? Gradients across a membrane can be both chemical and electrical. There is a separate chemical gradient for each molecule and ion. The electrical gradient is the sum total of the charge differences caused by the concentration gradients of the various ions. When a neuron is at rest electrical and chemical gradients are maintained by differences in the ability of different ions to cross the membrane. If one ion is free to move across the membrane and another is not then the one that is free to move will tend to move towards equilibrium, with equal concentrations on both sides. As the other ion is not free to move this may lead to the build up of an electrical gradient. Eventually the two forces acting on the system will balance out leading to electrical and chemical gradients that counteract one another.

View the animation below, then complete the quiz to test your knowledge of the concept.





1Depolarization occurs because...
A)potassium ions continue to diffuse out of the cell after the inactivation gates of the voltage-gated sodium ion channels begin to close.
B)the extra efflux of potassium ions causes the membrane potential to become slightly more positive than the resting value.
C)the increased potassium ion permeability lasts slightly longer than the time required to bring the membrane potential back to its resting level.
D)more sodium ions diffuse into the cell than potassium ions diffuse out of it.
E)the inactivation gates of the voltage-gated sodium ion channels begin to open and the diffusion of sodium ions decreases.


2Repolarization occurs because...
A)potassium ions continue to diffuse out of the cell after the inactivation gates of the voltage-gated sodium ion channels begin to close.
B)the extra efflux of potassium ions causes the membrane potential to become slightly more positive than the resting value.
C)the increased potassium ion permeability lasts slightly longer than the time required to bring the membrane potential back to its resting level.
D)more sodium ions diffuse into the cell than potassium ions diffuse out of it.
E)the inactivation gates of the voltage-gated sodium ion channels begin to open and the diffusion of sodium ions decreases.


3Hyperpolarization, or afterpotential occurs because...
A)potassium ions continue to diffuse out of the cell after the inactivation gates of the voltage-gated sodium ion channels begin to close.
B)the extra efflux of potassium ions causes the membrane potential to become slightly more positive than the resting value.
C)the increased potassium ion permeability lasts slightly longer than the time required to bring the membrane potential back to its resting level.
D)more sodium ions diffuse into the cell than potassium ions diffuse out of it.
E)the inactivation gates of the voltage-gated sodium ion channels begin to open and the diffusion of sodium ions decreases.


4After the passage of the action potential, the sodium-potassium pump reestablishes the resting membrane potential.
A)True
B)False


5The sodium-potassium pump uses bulk transport to move the sodium and potassium ions.
A)True
B)False





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