USMLE Step 1 Physiology Review 52 12 Resting Potentials

USMLE Step 1 Physiology Review 52 12 Resting Potentials

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Play USMLE Audio MP3 52 12 Resting Potentials Below

Begin 52 12 Resting Potentials Transcription

Okay, now we’re going to talk about resting potentials.

What two kinds of gradients control ion movement or flux?

  • Concentration and electrical.

If these gradients are equal but opposite in direction, what would be the value of the total electrochemical gradient?

  •  Zero.

Would there be a net current flow?

  • No.

What is this situation called?

  • Electrochemical equilibrium.

Compared to high extracellular fluid, do cells contain high or low concentrations of potassium?

  • High.

How about sodium, high or low?

  • Low.

What does the Nernst equation calculate?

  • The Nernst equation calculates the electrical gradient for an ion that would exactly balance the force of a concentration gradient in the opposite direction.

State the Nernst equation.  Please pause the tape.

  • Equilibrium potential equals logarithm of sixty-one divided by the charge of the ion, all multiplied by the quotient of the ion concentration outside, divided by the ion concentration inside.

How much would the potential difference be for a ten-fold difference in ion concentration?

  • Sixty-one millivolts.

And for a one hundred fold difference of ion concentration?

  • One hundred and twenty two millivolts.

Are the Nernst calculated values for the equilibrium potentials for potassium and sodium ever achieved?

  • No, they are approached but never achieved.

Why?

  • Because membranes are never permeable to only one kind of ion.

Student Doctor, please pause the tape and summarize the information about resting potential discussed thus far.

  • Concentration and electrical gradients control ion movement or flux.  If these gradients are equal but opposite in direction, the total electrochemical gradient would be zero and there be no net current flow.  This situation called electrochemical equilibrium.  Compared to high extracellular fluid, cells contain high concentrations of potassium and low concentrations of sodium  The Nernst equation calculates the electrical gradient for an ion that would exactly balance the force of a concentration gradient in the opposite direction.  The Nernst equation is as follows: equilibrium potential equals logarithm of sixty-one divided by the charge of the ion, all multiplied by the quotient of the ion concentration outside, divided by the ion concentration inside.  The potential difference for a ten-fold difference of ion concentration would be sixty-one millivolts.  And for a one hundred fold difference of ion concentration, it would be one hundred and twenty two millivolts.  The Nernst calculated values for the equilibrium potentials for potassium and sodium are approached but never achieve because membranes are never permeable to only one kind of ion.

The resting potential is due to a high resting conductance to what ion?

  • Potassium.        

As potassium ions diffuse out, what happens to the cell interior within one nanometer of the membrane?

  • It becomes electronegative.

The resting potential approaches another potential. In other words, the resting potential comes very close in value to another potential.  What other potential does the resting potential approach?

  • The equilibrium potential of potassium.

Why is the resting potential slightly less than the equilibrium potential of potassium?

  • Because of the slight sodium leak across the membrane.

What is the cord conductance equation?  Please pause the tape.

  • It is an average of equilibrium potentials weighted by ionic conductances.

What can you calculate with it?

  • The exact values of membrane potentials.

What maintains the steady state concentrations of ions within the cell, in spite of sodium and potassium leaks?

  • The sodium potassium pump.         

What two things determine the magnitude of the membrane potential?

  • The concentration differences of various ions across the membrane and the membranes relative permeability to these ions.

Student Doctor, please pause the tape and summarize the information on resting potentials discussed since the last summary.  The first question in this section was about the conductance of potassium determining the resting potential.

  • The resting potential is due to a high resting conductance to potassium.  As potassium ions diffuse out, the cell interior within one nanometer of the membrane becomes electronegative.  The resting potential approaches the equilibrium potential of potassium.  The resting membrane potential is slightly less than the equilibrium potential of potassium because of a slight sodium leak across the membrane.  The cord conductance equation is an average of equilibrium potentials weighted by ionic conductances.  You can use it to calculate the exact values for membrane potentials.  The sodium potassium pump maintains the steady state concentrations of ions within the cell, in spite of sodium and potassium leaks.  The concentration differences of various ions across the membrane and the membranes relative permeability to these ions determine the magnitude of the membrane potential.

 

 

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