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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.