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The downward slope - Action potentials in cardiac muscle

6 important questions on The downward slope - Action potentials in cardiac muscle

What are the different phases in the action potential of the cardiac muscle?

phase 0: rapid depolarisation- influx of Na+ ions
phase 1: initial repolarisation- K+ switch on and off- allows some K+ to enter exit cell.
phase 2: plateau- normal refractory period- Efflux of K+ and influx of Ca2+ through slow opening channels.
The plateau phase is different for different cells of the heart e.g. the cells of the SAN do not contain a plateau phase.
phase 3: slow repolarisation
phase 4: return to normal resting potential- Ca2+ close and intracellular Ca2+ pumped out of cell. K+ ion channels open. Efflux of K+ leads to return of the membrane potential.

What is the important of the ca2+ that enter through the t-tubules?

Without this extra influx of ca2+, the contraction of the cardiac muscle would be weak

What is the importance of the concentration of the Ca2+ ?

The concentration of the ca2+ will determine the strength of the contraction of the cardiac muscle cell.
The concentration is dependent on the concentration of ca2+ in the extracellular fluid- because the t-tubules open into the extracellular spaces that surround the cardiomyocyte.

How is Ca2+ pumped out of the cell?

- SERCA ATPase and associated PLB protein (on SR)
- PMCA and NCX (on cell membrane)

How is the action potential affected after an MI?

- Remodelling of the gap junctions- dysregulated cadherins in the infarcted tissue
- T-tubules number decrease following an MI

How is the action potential affected in a patient who has congenital heart failure?

The intracellular concentration of ca2+ decreases- does not peak as high as it should. Additionally, it takes longer for the membrane potential to return to normal.

The question on the page originate from the summary of the following study material:

Cases for The downhill slope

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