spleenharvester
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Can someone help me with action potentials? Going back over my notes, here are the fragments that don't really make sense, see the bold type:

For resting potential - voltage-gated Na+ channels closed, K+ channels open.

For action potential - 1) depolarisation - (non-voltage gated) Na+ channels open and Na+ enters via facilitated diffusion - doesn't this happen at rest too? Or is this referring to generator potentials from sensory receptors, eg pressure on pacinian corpuscles opening the channels?

K+ channels are mostly closed - when does this happen? K+ are freely diffusing back out at rest so this doesn't make sense?

2) repolarisation - Na+ channels close and K+ channels open - is it just the voltage-gated Na+ channels that close or all of them? Also, does the K+ channel opening happen suddenly?

K+ ions can now diffuse out of the cell by facilitated diffusion - wouldn't this make the membrane pd more positive, since there are only Na+ ions? Or is it the presence of negatively charged proteins with the loss of positive ions that makes it more negative? I've never understood this.

I don't have any textbooks anymore so would appreciate some help.

Cheers!
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Joshalos
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(Original post by spleenharvester)
Can someone help me with action potentials? Going back over my notes, here are the fragments that don't really make sense, see the bold type:

For resting potential - voltage-gated Na+ channels closed, K+ channels open.

For action potential - 1) depolarisation - (non-voltage gated) Na+ channels open and Na+ enters via facilitated diffusion - doesn't this happen at rest too? Or is this referring to generator potentials from sensory receptors, eg pressure on pacinian corpuscles opening the channels?

K+ channels are mostly closed - when does this happen? K+ are freely diffusing back out at rest so this doesn't make sense?

2) repolarisation - Na+ channels close and K+ channels open - is it just the voltage-gated Na+ channels that close or all of them? Also, does the K+ channel opening happen suddenly?

K+ ions can now diffuse out of the cell by facilitated diffusion - wouldn't this make the membrane pd more positive, since there are only Na+ ions? Or is it the presence of negatively charged proteins with the loss of positive ions that makes it more negative? I've never understood this.

I don't have any textbooks anymore so would appreciate some help.

Cheers!
During the resting potential, both voltage gated Na+ and K+ channels remain closed. However, there exist leakage channels for both Na+ and K+ (mostly K+ channels) that allow the flow of ions into and out of the cell still. The Na+/K+ ATPase pump maintains the resting potential.

1) Depolarisation - A stimulus is received which causes some of the Na+ channels to open and so Na+ flows into the cell which begins to depolarise it. If it is depolarised sufficiently (past the threshold value) an action potential is formed.

Most Na+ voltage gated channels open in response to the depolarisation causing a large influx of Na+. K+ channels remain closed. This causes the inner membrane to become positive with respect to the outside of the membrane.

2) Repolarisation - Inactivation gates on the Na+ channels then close, halting Na+ influx and K+ gates open, causing efflux of K+. Now the inner membrane becomes more negative.

3) Hyperpolarisation - K+ channels are slow to close and so more K+ leaves the cell, causing the membrane to become even more negative. The K+ channels close and the resting membrane potential is returned and maintained by the Na+/K+ pump.


With regards to your last question. The inside of the membrane becomes less positive (or more negative) as the positively charged K+ ions leave the cell. It is negative relative to the outside of the membrane.
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spleenharvester
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(Original post by Joshalos)
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That's fantastic mate, thankyou very much!
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Joshalos
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(Original post by spleenharvester)
That's fantastic mate, thankyou very much!
You're welcome! :cookie:
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