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2130-Module Four Summary

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Western University
Physiology 3120
Anita Woods

Weekly Thoughts MODULE 4 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 potentials. 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 channels. 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
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