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
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?