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Let’s start our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.

Play USMLE Audio MP3 55 08 Myocardial Action (2_3) Below

Begin 55 08 Myocardial Action (2_3) Transcription

What three processes or mechanisms contribute to ion redistribution across the membrane?

•  There are leak currents, there is a Potassium/Sodium pump and there is a Sodium/Calcium exchanger

With regard to leak currents, what is leaking in?

• Both Sodium and Calcium are leaking in.

As Calcium and Sodium leak in they move down their concentration gradient, what mechanisms create the gradient?

•  Both the pump and the exchanger create the gradient.

What is leaking out?

• Potassium

Why is Potassium leaking out?

• Because the concentration of Potassium is greater inside the cell than it is outside the cell.

What mechanism is creating the concentration gradient?

• The Sodium/Potassium pump.

Student doctor, please pause the tape and summarize the information discussed since the last summary on Myocardial action

•  The first question was about what three processes or mechanisms contribute to ion redistribution across the membrane. The following three processes contribute to ion redistribution across the membrane. There are leak currents, there is a Potassium/Sodium pump and there is a Sodium/Calcium exchanger. Both Sodium and Calcium are leaking in. As Calcium and Sodium leak in they move down their concentration gradient. Both the pump and the exchanger create the gradient for Calcium and Sodium. Potassium is leaking out because the concentration of Potassium is greater inside the cell than it is outside the cell. The Sodium/Potassium pump is creating the Potassium concentration gradient.

Okay you have the Sodium/Potassium pump and the Calcium exchanger which one uses the energy from the concentration gradient of Sodium?

• The Calcium exchanger uses the energy from the concentration gradient of Sodium letting some Sodium in the cell in exchange for taking Calcium out

With regards to Sodium the actions of the Sodium/Potassium pump and the Calcium exchanger oppose one another, although not perfectly, what is the net effect with regard to the Sodium concentration?

• Since the Sodium/Potassium pump pumps more Sodium out than the Calcium exchanger lets in, the net effect is more Sodium outside the cell

How about Potassium, is there more Potassium inside the cell or outside the cell due to the actions of the Sodium/Potassium pump and Calcium exchanger?

• There is more Potassium inside the cell actively transported by the Sodium/Potassium pump.

Where is the concentration of Calcium the greatest?

• There is more Calcium outside the cell transported by the Calcium exchanger

The myocyte contracts during what phase?

• Phase 2

Which ion actually causes the contraction?

• Calcium

What is the effect of the Potassium efflux?

• It repolarizes the membrane.

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

• The question was about which mechanism uses the concentration gradient of Sodium as a driving force. The Calcium exchanger uses the energy from the concentration gradient of Sodium, letting some Sodium in the cell in exchange for taking Calcium out of the cell. Since the Sodium/Potassium pump pumps more Sodium out than the Calcium exchanger lets in, the net effect is more Sodium outside the cell. There is more Potassium inside the cell actively transported by the Sodium/Potassium pump. There is more Calcium outside the cell transported by the Calcium exchanger. The myocyte contracts during Phase 2. The Calcium actually causes the contraction. The Potassium efflux repolarizes the membrane.

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