Active Potential.....

[sps27]

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I had a few questions on Active Potentials, basically trying to understand the sequence of events and trying to corrrelate the gates and channels that get activated or opened or closed in the depolarization, repolarization, hyperpolarization cycle.

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1) The values of resting potential which is about -70mV inside the cell, threshould potential, about -55mV inside and then it goes up to 35mV +ve potential ---- is all of this only for Neurons and Muscle cells. I mean, are these potential values present across the membrane of most cells or just Neurons and Sarcomeres. Also, the smooth muscle cell of the heart has another kind of activation, like not active potential based. Is that correct?

2) So AcH binds to the receptor, which allows Na+ to get in, aka facilitated diffusion. So that is fine. Na+ is not leaking in via leakage channels, it is getting in via facilitated diffusion through AcH. Now when Na+ gets in, that activates the 'voltage gated Na+ channels in the neighbouring section of axon hillock' and they open up. Also as Na+ is gettting in, it is slowly activating the K+ voltage gated channels as well, for the same section of Axon hillock through which Na+ got in. So as the Na+ is getting in, the potential goes past the threshold and then to 35+ve. So this would be depolarization. At that point, K+ voltage gated channels open up in the same section, AcH sterase breaks up AcH and Na+ channels close down. Na+ is not getting in anymore. And now the stage is set for repolarization. So repolarization takes place primarily due to K+ voltage gated channels which have recently opened up. Meanwhile the Na+ voltage gated channels of the neighbouring section are depoloarizing the neighbouring section.

3) Since the affinity of K+ voltage gated channels is far more than Na+ and since K+ channels are far more leaky, so they typically allow more K+ to get in the cell, thereby causing it to hyperpolarize.

4) As soon as hyperpolarization sets in, then the Na/K pump protein kicks into gear and starts throwing 2 K+ ions out and takes 3 Na+ ions in, to reset the balance. So I guess my question is, can we safely assume that Na/K pump protein gets activated only to bring back the hyperpolarization back to resting potential for the membrane?????

Thanks!!!

 

[monkeyvokes]

I had a few questions on Active Potentials, basically trying to understand the sequence of events and trying to corrrelate the gates and channels that get activated or opened or closed in the depolarization, repolarization, hyperpolarization cycle.

------------------------------------------------------------------------

1) The values of resting potential which is about -70mV inside the cell, threshould potential, about -55mV inside and then it goes up to 35mV +ve potential ---- is all of this only for Neurons and Muscle cells. I mean, are these potential values present across the membrane of most cells or just Neurons and Sarcomeres. Good question. I have only heard these specific numbers used for neurons and myocytes (remember, sarcomere isn't a cell it is the distance from one Z-disc to the next Z-disc). I have to believe that it is not universal, but am not sure.
Doubt that it'd be on the mcat though.

Also, the smooth muscle cell of the heart has another kind of activation, like not active potential based. Is that correct? Well, the heart has cardiac (not smooth) muscle. Contractile myocytes are activated differently than skeletal muscle cells yes. Myocytes are not innervated by alpha motor neurons, so acytlcholine isn't released onto nicotinic receptors. They are depolarized via calcium-induced calcium release on the inside, but this occurs due to conduction pathway cells at the AV and SA nodes. These cells depolarize automatically (autorhythmic cells). They use something called funny channels I believe, and I think it's because the channels have a good sense of humor.

2) So AcH binds to the receptor, which allows Na+ to get in, aka facilitated diffusion. So that is fine. Na+ is not leaking in via leakage channels, it is getting in via facilitated diffusion through AcH. Now when Na+ gets in, that activates the 'voltage gated Na+ channels in the neighbouring section of axon hillock' and they open up.
Talking about ACh release from an alpha motor neuron at the neuromuscular junction, right? Yes ACh is released (due to electric current opening voltage gated calcium channels, remember Ca entry leads to exocytosis of ACh). What I am wonder about here is what you mean by neugboring section of axon hillock? The nicotinic receptors (sodium channel) that ACh binds to are on the motor end plate of a skeletal muscle cell, so there isn't an axon hillock. Waht do you mean?
Also as Na+ is gettting in, it is slowly activating the K+ voltage gated channels as well, for the same section of Axon hillock through which Na+ got in. So as the Na+ is getting in, the potential goes past the threshold and then to 35+ve. So this would be depolarization. At that point, K+ voltage gated channels open up in the same section, AcH sterase breaks up AcH and Na+ channels close down. Na+ is not getting in anymore. And now the stage is set for repolarization. So repolarization takes place primarily due to K+ voltage gated channels which have recently opened up. Meanwhile the Na+ voltage gated channels of the neighbouring section are depoloarizing the neighbouring section.
Are you mixing parts of skeletal muscle stimulation with parts of action potential propogation along the axon of a neuron? Or maybe I am just getting confused

3) Since the affinity of K+ voltage gated channels is far more than Na+ and since K+ channels are far more leaky, so they typically allow more K+ to get in the cell, thereby causing it to hyperpolarize.


4) As soon as hyperpolarization sets in, then the Na/K pump protein kicks into gear and starts throwing 2 K+ ions out and takes 3 Na+ ions in, to reset the balance. remember, the sodium potassium pump (Na/K ATPase) is always working.
Doesn't turn on or off at any point.

So I guess my question is, can we safely assume that Na/K pump protein gets activated only to bring back the hyperpolarization back to resting potential for the membrane?????
well, there you have it, the sodium potassium pump is always on..

Thanks!!!

 

[sps27]

There is a passage in TBR Bio book 1 psg 3 page 44, questions 16 and 17 in particular - which is specifically based on questions like, during depolarizing or repolarizing phase which channels are opening and closing. So that got me into thinking that Na/K pump would also be getting triggered in some way by these voltages. And I reasoned that perhaps after hyperpolarization, the Na/K pump gets into action to throw out 3 Na ions outside the cell and take in 2 K ions inside the cell. What I was forgetting is, there is nothing static in a cell. Stuff is always constantly moving across the cell membrane in both directions, more like a fluid. I understand now.

Regarding the axon hillock, well the "trigger zone" for an action potential is the axon hillock -- simply because the Na channels are numerous in that region. The synapse could be like axo-somatic or axo-dendritic or axo-axonic. And so once initiated at the hillock, how will that action potential travel along the length of axon. So the action potential travels like a wave along the axon with these Na and K channels acting like dominos along the length - opening and closing, till it reaches the other synapse or a neuromuscular joint, whatever the case might be. So that is what I meant.