Voltage and the Axon

In the axon, the distribution of ions during the resting potential causes there to a difference in the amount of positively and negatively charged ions on the inside of the axon relative to the outside of the axon. This difference is measured as a voltage. If one compares the region with more positive charges to a region with more negative charges one has a positive voltage. If the comparison is the other direction, a region with more negative charges to a region with more positive charges, the voltage is negative. Note that the measurement of voltage is always relative - comparing two regions to each other. Now in the axon, there are more negative charges on the inside than on the outside during the resting potential giving the axon a negative voltage when comparing the inside to the outside (Figure 6).
Figure 6. A volt meter measures the voltage difference across the membrane of an axon during a resting potential. The value depicted in the window of the volt meter, - 70 mV, is typical of some axons. Each axon has its characteristic resting potential voltage and in each case the inside is negative relative to the outside.

The Role of Voltage in an Action Potential

When an action potential begins, the channels that allow sodium ions, Na+, to cross open up. Now during the resting potential phase, the outside of the axon is positively charged relative to the inside of the axon (Figure 7).
Figure 7.A representation of a sodium ion at the gate of a sodium channel right after the channel has opened.

So what effect will the voltage difference have on the Na+ ions when the Na+ channels first open?


Click on the correct answer in the image below.   


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