From USMLE Step 1 226 to Step 2CK 260CHECK IT OUT
From USMLE Step 1 226 to Step 2CK 260

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Equilibrium/Nernst Potential for the USMLE

 

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USMLE Step 1 Equilibrium Potential

Presented here is Part 1 of membrane/equilibrium potentials, with specific focus on the topics most likely to be found on the USMLE Step 1.  As always, we will walk through the topic sequentially, while giving you questions at the end that will force you to integrate and apply the knowledge you’ve obtained, which is exactly how it will be tested on the USMLE Step 1 exam.  As a teaser, here are the questions you will be able to answer by the end of this:

  1. How is the resting membrane potential set for a typical human cell?

  2. Where would the majority of Na+ have to be in order for its Nernst potential to be positive – intracellular or extracellular?

  3. Where would the majority of Cl- have to be in order for its Nernst potential to be negative – intracellular or extracellular?

  4. Where would the majority of Ca++ have to be in order for its Nernst potential to be +125 mV – intracellular or extracellular?

  5. If the Nernst potentials are positive for Na+ and Ca++, and negative for Cl- and K+, which ions would move into the cell if you opened up all 4 classes of ion channels?

Ready for the recall-type USMLE Step 1 questions?

Cell membrane potential – why does it exist?

There is an accumulation of (usually) negatively-charged particles on the inside of the cell, and positively-charged particles on the outside

Is the majority of K+ intracellular or extracellular?

Intracellular

Recall that on the basic metabolic panel (aka CHEM-7), which is drawn from people’s BLOOD (and is thus measuring the extracellular [K+]), that “normal” K+ is 4.0 mEq

Is the majority of Na+ intracellular or extracellular?

Extracellular

Recall that on the basic metabolic panel, that “normal” Na+ is 140 mEq

Normally, if you have a high ion concentration on one side of a membrane, and a low ion concentration on the other, what direction would the ion typically flow if you opened up an ion channel to it?

From areas of high concentration → low concentration

This is exactly what happens when you open an ion channel (e.g. when a Na+ channel is open → Na+ ions influx, since Na concentration is HIGHER outside the cell)

USMLE Step 1 Equilibrium Potential

Ready to move on?

In the hypothetical situation above, where Na+ and K+ concentrations are equal, but on opposite sides of the membrane, what direction would these ions flow if no Na+ or K+ channels were open?

There would be NO flow

Because they are charged, ions must travel through their respective ion channels, since they are charged and cannot cross the nonpolar plasma membrane by themselves. If their ion channels are closed, they cannot cross.

What would happen if the K+ channels were open?

Because of its high concentration inside the cell, and low concentration outside, K+ would move OUTSIDE the cell, down its concentration gradient.

USMLE Step 1 Equilibrium Potential

If K+ moves outside, what charge will form across the membrane?

Negative charge INSIDE the cell (K+ is positively charged, and is moving outside the cell, carrying its positive charge with it)

USMLE Step 1 Equilibrium Potential

If there is a negative charge on the inside of the cell, would this draw K+ inside, or push it outside the cell?

Draw it inside the cell

Are the concentration gradient and electrical gradient pushing K+ in the same or opposite directions?

Opposite directions

Concentration gradient pushes K+ OUTSIDE the cell (there is more K+ inside than outside)

Electric gradient pulls K+ INSIDE the cell (when K+ leaves the cell → negative charge inside, which will draw the positive K+ cation)

When will K+ STOP moving outside the cell?

When the forces pushing it OUTSIDE the cell (concentration gradient) = forces pulling it INSIDE the cell (electrical gradient)

It does NOT stop when the K+ concentration are equal on both sides (it will stop much sooner than this)

USMLE Step 1 Equilibrium Potential

Equilibrium potential – what is it?

Membrane potential at which there is NO ion flow

What is another term for the equilibrium potential when considering a single ion?

Nernst potential

If I opened the Na+ channels instead, what potential difference would form across the membrane?

Na+ would move INSIDE the cell (down its concentration gradient) → positive intracellular potential

What would the membrane potential be if there were MANY K+ channels open, but only a few Na+ channels open?

The membrane potential would be closest to that of K+, since the majority of ions traveling across the membrane would be K+.

Thus, the membrane potential would be negative.

Wait!

I strongly recommend that you attempt to answer the final questions by yourself first, before looking at the answers.  Remember, the USMLE Step 1 exam will test your ability to make connections on the spot. The more practice you have, the higher your score!  Then, when you think you might know the answer (or are completely stumped), look at the answers below!

Ready for the answers?

How is the resting membrane potential set for a typical human cell?

Similarly to the previous scenario. At rest, the majority of ion channels open are K+ channels, thus the membrane potential is closest to that of the Nernst potential for K+.

Where would the majority of Na+ have to be in order for its Nernst potential to be positive – intracellular or extracellular?

Extracellular

If I have LOTS of Na+ outside the cell, then open ONLY Na+ channels → Na+ will move inside the cell → create a positive charge on the inside of the cell relative to outside

Where would the majority of Cl- have to be in order for its Nernst potential to be negative – intracellular or extracellular?

Extracellular

If I have LOTS of Cl- outside the cell, then open ONLY Cl- channels → Cl- will move inside the cell → create a negative charge on the inside of the cell relative to outside

Where would the majority of Ca++ have to be in order for its Nernst potential to be positive – intracellular or extracellular?

Extracellular

If I have LOTS of Ca++ outside the cell, then open ONLY Ca++ channels → Ca++ will move inside the cell → create a positive charge on the inside of the cell relative to outside

If the Nernst potentials are positive for Na+ and Ca++, and negative for Cl- and K+, which ions would move into the cell if you opened up all 4 classes of ion channels?

Na+, Ca++, Cl- would all move INSIDE the cell

Note that Na+ and Ca++ both move inside the cell because their membrane potential is positive, but Cl- moves inside of the cell because it is negative.

K+ would move outside the cell (moving outside the cell generates its negative Nernst potential)

If you’ve made it this far, congratulations!! Equilibrium/Nernst potentials are some of the most dreaded topics on the USMLE Step 1, especially since it has been so long since many of us have taken a basic science course. Here, we’ve highlighted the fundamental principles underlying equilibrium potentials, and demonstrated in a practical way the kinds of ways this information can be tested on the USMLE Step 1.

What should you do next?

  1. Turn the narrative, “Pathogenesis to Presentation” questions into Anki cards by copy/pasting the question/answer into the “Front” and “Back” fields in Anki.  Do the same for the fundamental facts that you were unfamiliar with, to maximize your chances of USMLE Step 1 success!  Remember: the USMLE is a test of understanding, so the better you can understand these questions, the better your score!
  2. Add reverse cards when appropriate to your Anki cards
  3. Re-word the questions/explanations as desired, and BOLD the important text to make it easier to review in the future
  4. Learn something new?  Something unclear?  Comment below!
  5. If you liked this post, please consider sharing it on Facebook/Twitter!  I judge the utility of these posts by the number of comments / shares they receive, so if you’d like more, or would like a particular topic addressed, please let us know!
 

ROCK MICROBIOLOGY, even if you SUCK AT MEMORIZING!

With the FREE micro deck of more than 130+ Anki cards you will be practically BEGGING to get as many micro questions on your exam as possible.

it's free!
100% privacy guaranteed, no messin' around