Action potential and Na/K pump

[iamsoysauce]

i wanted to clarify this topic. please respond with any corrections and such.

resting potential is when the inside of the cell has a net (-) charge because it is high in K+. the outside of the cell has net (+) charge from the Na+ ions. even though K is a cation, does the inside have a (-) charge because it is a "net" value? resting potential is regulated because K is able to diffuse in/out along its gradient. does Na/K+ act here via active transport to move 3 Na in /2 K out or is that only in restoring the resting potential after an action potential has occurred?

threshold- initiates an action potential. the inside has become sufficiently (+)

depolarization- Na channels open allowing Na to move down its gradient (hi to lo) and enter the cell which becomes more and more (+).

repolarization- as Na channels close, K channels open and let K move down its concentration gradient and leave the cell which becomes more (-) again

hyperpolarization- more K leaves the cell making it even more (-) than its resting potential

i think i have the steps here but mainly got confused about when the Na/K pump is operating.

also, i really don't get the part about the dorsal root ganglion and the reflex arc. anyone out there who understands this, please explain. thanks.

[ayushman80]

This is a confusing topic so lets lay down some facts (you'll have to push the "I Believe" button on this).

1) The inside of a cell is comparatively more negative than the outside of a cell. This takes into account the effect of the Na+ and K+ ions on either side. Just realize that there are more Na+ on the outside than the inside and more K+ on the inside of the cell compared to the outside.
2) The Na/K Pumps maintain this net negative potential difference by counteracting the diffusion of Na+ and K+ via leakage channels.
3) The net negative charge is due to various negatively charged ions (such as Cl-) and also due to negatively charged proteins. Even though most protiens are neutral, their 3D shape makes their negative ends face outward; thus causing a net negative charge. Other types of molecules contribute as well.

Now that we have that down let talk about how an action potential is generated. Since the Na/K Pump keeps the Na out and K in, we have to have some way to overpower those pumps and make the inside of the cell relatively more positive than in the resting phase. This is the job of the ligand based channels (e.g. acetylcholine channels at muscle synapse). Once these channels open, Na molecules are allowed to flood the inside of the cell. They simply open and let Na+ follow its concentration gradient.

Since we let a whole bunch of Na+ ions in the cell, the inside becomes more positive and the action potential carries on via VOLTAGE GATED channels. Now, if we want to quickly restore the voltage back to normal we have to let some sort of positive ion back into the environment so we can become more negative (and resemble resting membrane potential). K+ ions serve ths purpose. Because there is a higher concentration on the inside, they will automatically flow outward when their respective channels are open.

After the charge is restored via outflow of K+ ions. The Na/K pumps return the ions to their proper place and we're ready to go again:)