K+ channel open or closed at resting potential

hi,
at resting potential the na+ channel closes right and thats why na cannot enter the axon/cell. flow of k+ is changed from going in to going our right? but in my note there is no reference as to what happens to the k+ chanells later on uptil action potential. i do have here that the na+ chanell reopens obviously for action potential.

Reply 1

Jpw1097

19

Original post by ra1500

hi,
at resting potential the na+ channel closes right and thats why na cannot enter the axon/cell. flow of k+ is changed from going in to going our right? but in my note there is no reference as to what happens to the k+ chanells later on uptil action potential. i do have here that the na+ chanell reopens obviously for action potential.

At the resting membrane potential, voltage-gated sodium and potassium channels are closed. However, there are so-called 'leak' sodium and potassium channels which are always open, and so there is always some flow of sodium into and potassium out of the cell along their concentration gradients. The membrane is much more permeable to potassium ions (because there are more leak potassium channels) than sodium ions, and this is the main reason why the resting membrane potential is around -70 mV.

When the threshold potential is reached, voltage-gated sodium channels open and there is an influx of sodium ions into the cell along their electrochemical gradient - causing the cell to depolarise. Once the membrane potential reaches around +40 mV, sodium channels close (and are inactivated). Also, voltage-gated potassium channels open and potassium ions leave the cell down their electrochemical gradient - causing the cell to repolarise. Once the cell membrane reaches the resting membrane potential, these voltage-gated potassium channels are slow to cause and actually remain open - causing the cell to become hyperpolarised. The potassium channels eventually close and the membrane potential returns to the resting membrane potential.

So to summarise, leak channels are always open - allowing sodium and potassium ions to cross the membrane. Voltage-gated sodium and potassium channels are CLOSED at the resting membrane potential. During depolarisation, voltage-gated sodium channels open (voltage-gated potassium channels are closed); during repolarisation, voltage-gated sodium channels close and voltage-gated potassium ions are open.

(edited 5 years ago)

Reply 2

ra1500

OP

11

Original post by Jpw1097

At the resting membrane potential, voltage-gated sodium and potassium channels are closed. However, there are so-called 'leak' sodium and potassium channels which are always open, and so there is always some flow of sodium into and potassium out of the cell along their concentration gradients. The membrane is much more permeable to potassium ions (because there are more leak potassium channels) than sodium ions, and this is the main reason why the resting membrane potential is around -70 mV.

When the threshold potential is reached, voltage-gated sodium channels open and there is an influx of sodium ions into the cell along their electrochemical gradient - causing the cell to depolarise. Once the membrane potential reaches around +40 mV, sodium channels close (and are inactivated). Also, voltage-gated potassium channels open and potassium ions leave the cell down their electrochemical gradient - causing the cell to repolarise. Once the cell membrane reaches the resting membrane potential, these voltage-gated potassium channels are slow to cause and actually remain open - causing the cell to become hyperpolarised. The potassium channels eventually close and the membrane potential returns to the resting membrane potential.

So to summarise, leak channels are always open - allowing sodium and potassium ions to cross the membrane. Voltage-gated sodium and potassium channels are CLOSED at the resting membrane potential. During depolarisation, voltage-gated sodium channels open (voltage-gated potassium channels are closed); during repolarisation, voltage-gated sodium channels close and voltage-gated potassium ions are open.

Thanks soo much for the time to write that up, appreciate it! makes so much, ive never come across the leak chanels thing, do i need to know this detail for alevel btw??

Reply 3

Jpw1097

19

Original post by ra1500

Thanks soo much for the time to write that up, appreciate it! makes so much, ive never come across the leak chanels thing, do i need to know this detail for alevel btw??

Probably not at A level. I think if you just remember when voltage-gated channels open and close, that should be fine. I just wanted to make it clear that there is always a flow of sodium and potassium ions into and out of the cell respectively. The membrane potential is a dynamic equilibrium, not a static equilibrium.

Reply 4

macpatgh-Sheldon

20

Excellent explanation by @Jpw1097: I would add a couple of minor points to make it a bit easier for OP to understand this complex topic.

1. As JPW points out, the resting potential is created due to the difference in permeability to Na+ and K+. It is worth knowing that MAINTENANCE of this resting potential is contributed to by the Na+, K+ - ATPase action, which "kicks out" 3 K+ for every Na+ going in.
2. The normal situation in the human body is that Na+ is a mainly extracellular cation (serum Na+ = 135-150 mM/l) and K+ is a mainly intracellular cation (Serum K+ (extracellular) = 3.5-5.0 mM/l). You do not need to know at A level why this is so, but if you are curious, google "Nernst equation".
3. In the pacemaker cells of the heart (normally the sino-atrial node in the right atrium), the RESTING POTENTIAL SLOPES slightly UPWARD with time, partly due to an outward leak of Ca++ (enough for A level), and so the resting potential reaches the threshold to fire an action potential SPONTANEOUSLY (this is the basis of the intrinsity of cardiomyocytes I.e. their propensity to depolarize without external nervous input).

Although some of this is beyond A level, there is a good chance of a Q being asked at A level, perhaps telling you the action of a drug on these mechanisms e.g. a group of drugs called calcium channel blockers that do just that, and asking you to predict/work out the answers to Qs that are derived from this basic info.

M

K+ channel open or closed at resting potential

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