USMLE Step 1 Physiology Review 55 07 Myocardial Action (1_3)

USMLE Step 1 Physiology Review 55 07 Myocardial Action (1_3)

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Play USMLE Audio MP3 55 07 Myocardial Action (1_3) Below

Begin 55 07 Myocardial Action (1_3) Transcription

What is the key concept in myocardial action?

  • The rush of Sodium into the cell triggers the influx of Calcium and the efflux of Potassium+

What controls the Sodium+, Potassium+, and Calcium2+ gates?

  • These are all voltage triggered gates

How many phases are there in the cardiac action potential?

  • Five

What happened to the membrane potential in phase zero?

  • The cell membrane rapidly depolarizes

What is the approximate value of the resting membrane potential prior to depolarization.

  • -85mV

What happens to Sodium in phase zero?

  • Sodium rushes in.

Student doctor, please pause the tape and summarize the information discussed thus far on myocardial action.

  • The key concept in myocardial action is the rush of Sodium into the cell triggers the influx of Calcium and the efflux of Potassium. The Sodium, Potassium, and Calcium gates are all voltage triggered gates. There are five phase in the cardiac action potential. In phase zero, the cell membrane rapidly depolarizes. The approximate value of the resting membrane potential prior to depolarization is -85mV. In phase zero, Sodium rushes in.

Now back to more question on myocardial action.

What happens to the membrane potential in phase one?

  • There is partial repolarization.

What happens to the Sodium channels and therefore the flow of Sodium in phase one?

  • The voltage gated Sodium channels are inactivated and the Sodium influx slows down

What happens to the Potassium channels and therefore the flow of Potassium in phase one?

  • The voltage gated Potassium channels are beginning to open and Potassium

is beginning to rush out.

What happens to the action potential in fast fibers in phase two?

  • It plateaus.

What happens to Calcium channels and there the flow of Calcium in phase two?

  • The voltage gated Calcium channels opens and there is a Calcium influx

Since there is an influx of positively charged ions, why does the action potential plateau?

  • The Potassium efflux balances the Calcium influx.

What does the Calcium influx trigger?

  • Myocyte contraction.

Student doctor please pause the tape and summarize the information discussed on myocardial action since the last summary.

  • The first question was about what happens to the membrane potential in phase one. In phase one, there is partial repolarization of the membrane.  The voltage gated Sodium channels are inactivated and the Sodium influx slows down. In addition, the voltage gated Potassium channels are beginning to open and Potassium is beginning to rush out. In phase two, the action potential in fast fibers plateaus. The voltage gated Calcium channels opens and there is a Calcium influx in phase two. Even with the influx of positively charged ions, the action potential plateaus because the Calcium influx is balanced by the Potassium efflux.  The Calcium influx triggers myocyte contraction.

What happens to the membrane potential in phase three?

  • There is a rapid repolarization.

What happens to the Potassium channels and therefore the flow of Potassium in phase three?

  • The voltage gated slow Potassium channels open and there is a massive efflux of Potassium.

So big deal, there is an efflux of Potassium in phase two as well, why is there a rapid repolarization in phase three whereas in phase two there is a plateau?

  • Because, now in phase three, the Potassium efflux that is no longer balanced by a Calcium influx.

Why? (Please pause the tape)

  • The voltage gated Calcium channels close stopping the in-rush of Calcium so that the efflux of Potassium is no longer balanced and the membrane is rapidly repolarized back to -85mV

What happens to the membrane potential in phase four?

  • The membrane is held at resting potential.

Student doctor please pause the tape and summarize the information discussed since the last summary on the myocardial action.

The first question was about what happens to the membrane potential in phase three. In phase three, there is a rapid repolarization. The voltage gated slow Potassium channels open and there is a massive efflux of Potassium. There is a rapid repolarization in phase three whereas in phase two there is a plateau because now, in phase three, the Potassium efflux that is no longer balanced by a Calcium influx because the voltage gated Calcium channels close stopping the in-rush of Calcium, so that the efflux of Potassium is no longer balanced and the membrane is rapidly repolarized back to -85mV. During phase four, the membrane is held at resting potential.

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