Action Potentials

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What is it that causes Na+ channels to close at the peak of an action potential? TBR mentions that there is no more influx of Na+ once the membrane potential reaches the equilibrium potential for Na+. At this point the membrane potential is balanced by the concentration gradient. Can someone explain what it means when they say that the membrane potential is balanced by the concentration gradient?

Why do larger diameter neurons conduct depolarization further and faster than a small diameter neuron? Is this because they allow for more ions to flow down the axon?

I am not clear on why myelinated nerves conduct action potentials faster. TBR states that the myelin that surrounds the axon acts as an electrical insulator and prevents the transfer of ions across the plasma membrane and I think that this has something to do with it. How does the myelin act as an electrical insulator? Is it an electrical insulator because it blocks ions from leaving?

 

[echoyjeff222]

concentration gradient is caused by the sheer # of Na+ ions floating around i.e., moving along its concentration gradient from high to low; this is balanced by the voltage gradient/force that attracts/repulses ions (i.e., positive moves to negative etc) -- at the equilibrium potential, these two forces are equal so there is no net movement. the reason why the channels actually close is because the sodium inactivation gates eventually close after a period of time when depolarized

so, i think the VNa is around +55 mV, right (for a typical neuron, changes depending on other factors)? So we have a ton of Na+ outside trying to come in (concentration gradient) while we have the + voltage (inside +) pushing the Na+ out, preventing it from coming in. this is the balance i'm talking about 🙂

larger diameter neurons conduct faster because there is less resistance on the inside of the membrane ... it's actually a bit more complicated because you compare membrane resistance with internal resistance, but i think what I said is a good simplified way to think about it.

myelin prevents electrical charge from leaving, as you said -- it's an insulator. Since less charge flows outward, more can be available to flow down the axon 🙂

 

[orangetea]

This guy does wonders with a lot of the bio topics



best 10 minutes of my life. He also has more on nervous system. The one I posted is the one I used to understand resting potential concepts

You can find his channel Armando Hasudungan

 

[sillyjoe]

best 10 minutes of my life.

Ha