So, in the last module you learned about ion concentration and membrane potentials. Almost all cells have
membrane potentials that are generated by the electrochemical gradient of the major ions. Why is the
resulting membrane potential so very important?
Membrane potentials are essential for generating action potentials, the language of the brain, communicated
via neurons. Every sense we detect, every movement we make, requires the generation of action
You should know the different phases of the action potential;
1. Depolarization- the resting membrane potential goes from -70 mV to + 35 mV (this is very quick,
1/1000 of a second)
2. Repolarization- the return of the membrane potential back to -70 mV
3. Hyperpolarization- the membrane potential becomes more negative (-90 mV) for a period of time
and the slowly returns to -70 mV.
How are the different phases of the action potential generated? The answer is specialized channels. These
channels respond (open) due to changes in voltage; that is why they are called voltage-gated channels.
There are two types of voltage-gated channels involved in action potentials, Na+ and K+ voltage-gated
A) Voltage Gated Sodium Channels
These channels open quickly in response to the inside of the cell become more positive (depolarization) (for
example, as the inside of the cell goes from -70 mV to -60 mV).
When the Na+ Voltage Gated (V. G.) channels open, which way does the Na+ flow? Na+ comes flooding
into the cell (remember, Na+ is higher outside of the cell), making the inside of the cell become more
positive, to +35 mV.
Why doesn’t the inside of the cell continue to becoming more positive (remember, the equilibrium potential
for Na+ is 60 mV)?
i) One reason is that the Na+ V. G. channels also close very quickly and become “locked up” by an
inactivation gate that prevents the channel from reopening for a set amount of time. This is an
important design of this channel which we will understand better when we discuss the directional
flow of the action potential.
ii) Secondly, the K+ V.G. channels are a little slow to open, so as Na+ V.G. channels are closing,
K+ V. G. channels are beginning to open. Which way does the K+ flow? Because the inside of the
cell has a higher concentration of K+, K+ will then flow out of the cell, making the cell repolarize.
B) Voltage Gated Potassium Channels
These channels are slower to open in response to the inside of the cell becoming more positive
(depolarization). As mentioned above, K+ ions will begin to flow causing the repolarization phase (the
membrane potential returns to -70 mV) of the action potential. So realize that because of the different
Reponses of the Na+ V.G. channel to a change in voltage (fast to open) and K+ V. G. channel (slow to
open), there is some overlap at the peak part of the action potential. Some Na+ V. G. channels are starting
to close and some K+ V. G. channels are starting to open.
Why does the resting membrane potential hyperpolarize (become more negative than -70 mV)? It is
because of the slow to open K+ channels are also slow to close. The K+ ions will continue leaving until the
equilibrium potential is reached (remember