Cardiac Action Potential

Front 1

How does the cardiac action potential differ from that of skeletal?

Back 1

It differs in several ways.
1. Some cardiac cells are auto-excitable
2. All cardiac muscle cells are connected electrically by gap junctions, and contract as a single unit
3. Cardiac action potential has an extended absolute refractory period, which is the period of time following a contraction during which the muscle cannot contract again.

Front 2

Define cardiac refractory period.

Back 2

The cardiac refractory period is the period of time following a contraction during which the muscle cannot contract again.

Front 3

Approximately what percentage of cardiac fibers are auto-rhythmic?

Back 3

About 1%. They possess the unique ability to depolarize spontaneously and command the pace of the heart.
The autorhythmic cells initiate the cardiac action potential.

Front 4

The cardiac action potential involves 5 phases. True/false

Back 4

True. Phases are staged as 0-4.

Front 5

What is phase 0 of the cardiac action potential?

Back 5

This is the rapid depolarization phase. The auto-rhythmic cells of the heart have an unstable resting potential that is continuously depolarizing, and constantly drifting toward threshold levels. These spontaneously changing membrane potentials are actually pacemaker potentials, that initiate contractions of the heart.

Front 6

What happens exactly during the rapid depolarization phase>

Back 6

During this phase, sodium ion channels on the cell membranes open up, and the influx of positive sodium ions helps to further depolarize the cell.

Front 7

What is phase 1 of the cardiac action potential?

Back 7

Phase 1 starts as the sodium ion channels begin to inactivate. The result is a slowing of sodium ions, as the cell continues to lose potassium and chloride ions.

Front 8

Describe phase 2 of the cardiac action potential.

Back 8

Phase 2 is the plateau phase. When the threshold of approximately -40mv is attained, the calcium ion channels open up.

Front 9

What is meant by plateau in phase 2 of the cardiac action potential?

Back 9

During the plateau segment of phase 2, calcium ions rush into the cell and initiate the rising phase of the action potential, effectively reversing the membrane potential.

Front 10

Define phase 3 of the cardiac action potential.

Back 10

Phase 3 is represented by the rapid repolarization of the cardiac action potential. here, the potassium channels open, and potassium ions flow from the cell and the cell re-polarizes.

Front 11

What occurs in the last phase(4) of the cardiac action potential?

Back 11

At phase 4, the cell has returned to its resting membrane potential. It will soon be stimulated to reinitiate spontaneous depolarization.