USMLE Step 1 Physiology Review 52 14 Action Potentials (2 of 2)

USMLE Step 1 Physiology Review 52 14 Action Potentials (2 of 2)

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Play USMLE Audio MP3 52 14 Action Potentials (2 of 2) Below

Begin 52 14 Action Potentials (2 of 2) Transcription

Okay, now let’s learn about generation, activation, inactivation and refractory periods.

During an action potential, as membrane potential becomes more positive, what happens to the conductance of sodium?

  •  It increases.

Would this kind of connection between the changing membrane potential and the increased sodium conductance indicate a positive feedback response or a negative feedback response?

  • Positive feedback response.

Okay, let’s say there was a depolarization.  Then what happens?  Please pause the tape.

  • The depolarization triggers sodium channels to open, which lets more sodium to enter the cell, which depolarizes the cell more, which causes more sodium channels to open and so on.

What kind of depolarization do you need to trigger a positive feedback response?

  • A suprathreshold depolarization; about ten millivolts.

In a sentence, state the relationship between threshold, membrane potential and action potential.  Please pause the tape.

  • Threshold is the membrane potential level where action potentials are initiated. 

A subthreshold depolarization may cause a local response but does not trigger the positive feedback response.  Why?

  • You may want to pause the tape here.  Insufficient sodium channels are opened by the weaker depolarization to trigger the positive feedback response.

Student Doctor, please pause the tape and summarize the information about the positive feedback response and supra and subthreshold depolarizations.

  •  During an action potential as membrane potential becomes more positive, the conductance of sodium increases.  This kind of connection indicates a positive feedback response.  The depolarization triggers sodium channels to open, which lets more sodium into the cell, which depolarizes the membrane more, which causes more sodium channels to open and so on.  You need a suprathreshold depolarization, about ten millivolts, to trigger a positive feedback response.  Threshold is the membrane potential level where action potentials are initiated.  A subthreshold depolarization may cause a local response but does not trigger the positive feedback response because insufficient sodium channels are opened by the weaker depolarization.

Okay, now for some more questions.

As the action potential progresses, does the peak in potassium conductance precede or follow the peak in sodium conductance?

  • The peak in potassium conductance follows the peak in sodium conductance.

As activation continues, is the conductance of the membrane to sodium less than or greater than membrane conductance for potassium?

  • Greater than.

By about how much?

  • Conductance of sodium is about fifty times greater than the conductance of potassium.

Because of the greater sodium conductance relative to the potassium conductance, the membrane moves toward the sodium equilibrium potential, and this is responsible for what phase of the action potential?

  • The rising phase of the action potential.

What happens spontaneously to the sodium conductance channels within fractions of a millisecond after they open?

  • They close.

Is the inactivated state of the sodium channels opened or closed?

  • Closed.

Patch clamping is useful for studying what?

  • Single ionic channels.

What other membrane potential does the action potential approach?

  • The action potential approaches but never reaches the equilibrium potential for sodium.

Student Doctor, please pause the tape and summarize the information about action potentials discussed since the last summary.  First, we compared sodium and potassium conductances.

  • As the action potential progresses the peak in potassium conductance follows the peak in sodium conductance.  As activation continues, the conductance of sodium is about fifty times greater than the conductance of potassium.  Because of the greater sodium conductance relative to the potassium conductance, the membrane potential moves toward the sodium equilibrium potential, and this is responsible for the rising phase of the action potential.  The sodium conductance channels spontaneously close within fractions of a millisecond after they open.  The inactivated state of the sodium channels are closed.  Patch clamping is useful for studying single ionic channels.  The action potential approaches but never reaches the equilibrium potential for sodium. 

Okay, now for some questions about refractory periods.

With regard to action potentials, what is the difference between absolute and relative refractory periods?  Please pause the tape.

  • During an absolute refractory period the membrane cannot be stimulated to produce an action potential, but action potentials may occur during relative refractory periods.

During absolute refractory periods, what happens to the membrane that precludes action potentials?

  • Sodium conductance is totally inactivated. 

Compared to non-refractory period action potentials, what level of depolarization is required to initiate a spike during a relative refractory period?  Please pause the tape.

  •  To initiate an action potential spike during relative refractory periods, a significantly larger than normal depolarization is needed.

During the relative refractory period there is a prolonged increase in potassium conductance.  How would this affect the threshold for spike initiation?  Pause the tape.

  • The increased potassium conductance opposes depolarization and raises the threshold.

What is the term for increased threshold for spike initiation?

  • Accommodation.

During relative refractory periods, some sodium channels are still recovering from being activated.  How does this affect the action potential produced?

  • The amplitude of the action potential is less than normal.

Student Doctor, please pause the tape and summarize the information discussed this far on refractory periods and accommodation.

  • Refractory periods can be classified as absolute or relative.  During an absolute refractory period the membrane cannot be stimulated to produce an action potential, but action potentials may occur during relative refractory periods.  During absolute refractory periods the complete inactivation of sodium conductance precludes action potentials.  To initiate an action potential spike during relative refractory periods a significantly larger than normal depolarization is needed.  During the relative refractory period there is a prolonged increase in potassium conductance that opposes the depolarization and raises the threshold of action potential initiation.  Accommodation is the term for increased threshold for spike initiation.  During relative refractory periods, some sodium channels are still recovering from being activated and so the amplitude of the action potential is less than normal.

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52-13 Action Potentials (1 of 2)

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