A question about the effects of potassium on the heart



Hello forum members,

So I'm reading this chapter on heart physiology and I have a question on 2 paragraphs I came across. The text is as follows:

"Excess potassium in the extracellular uids causes the heart to become dilated and accid and also slows the heart rate. Large quantities of potassium also can block conduction of the cardiac impulse from the atria to the ventricles through the A-V bundle. Elevation of potassium concentration to only 8 to 12 mEq/L—two to three times the normal value—can cause severe weakness of the heart, abnormal rhythm, and death.
These effects result partially from the fact that a high potassium concentration in the extracellular fluids decreases the resting membrane potential in the cardiac muscle fibers, as explained in Chapter 5. That is, high extracellular fluid potassium concentration partially depolarizes the cell membrane, causing the membrane potential to be less negative. As the membrane potential decreases, the intensity of the action potential also decreases, which makes contraction of the heart progressively weaker."

This is where I'm lost:

1. Does the extracellular potassium cause the cell to be less negative by flowing into the cell, forcing itself inside?
2. The last sentence says that the membrane potential decreases, but doesn't the potential increase if the membrane potential becomes less negative?

Can someone maybe explain this text?

Thanks so much in advance.


Experiencing a pancake aversion
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May 10, 2018
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1. No. Increasing the extracellular K concentration alters the Nernst potential of potassium, reducing the electrochemical gradient that normally causes K to flow out of the cell. The increased Nernst potential of K will cause the overall membrane potential to become less negative, because K is the primary ion responsible for making the membrane potential more negative. K normally has a Nernst potential around -90mV, while Na is around +60mV (or something like that). The cell sits around -70mV because K has greater permeability than Na at rest and pulls the overall membrane potential closer to its Nernst potential. If you increase the extracellular concentration of K, it’s Nernst potential will be >-90mV and pull the cells membrane potential along with it.

2. Membrane potential is actually a potential difference, i.e. the difference between the intracellular and extracellular environments. By raising membrane potential, greater than -70, the difference in membrane potential decreases, which is why the book says membrane potential (difference) decreases.

I hope that makes sense!
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