Cardiac Action Potential (Nursing)

by Jasmine Clark

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    00:01 So now that we've talked about the noncontractile pacemaker cells of the heart, let's take a look at the contractile muscle fibers that make up the bulk of the heart and are actually responsible for the pumping action itself.

    00:18 These are going to be slightly different than the skeletal muscle fibers and it's going to be a slightly different type of contraction because the cardiac muscle action potentials have a plateau.

    00:33 So let's look a little more closely at the action potential itself.

    00:38 In the cardiac muscle fibers, depolarization opens fast voltage-gated sodium channels allowing for sodium to enter the cell.

    00:50 Then by positive feedback, this influx of sodium is going to cause a rise in the action potential so that goes from -90 mV all the way up to +30 mV.

    01:05 This is what's known as a rapid depolarization.

    01:10 Next, depolarization by sodium is also going to open the slow calcium channels.

    01:19 At +30 mV, the sodium channels are gonna close but the slow calcium channels are going to remain open thus prolonging the depolarization.

    01:32 This appears as a sort of plateau or flattening out of the depolarization.

    01:40 After about 200 milliseconds, these slow calcium channels are now going to close themselves and subsequently, the voltage-gated potassium channels are going to open.

    01:54 This causes a rapid efflux of potassium out of the cell, thus repolarizing the cell to its resting membrane potential.

    02:06 Calcium is pumped both back into the sarcoplasmic reticulum and out of the cell into the extracellular space.

    02:16 So how is this different from what happens in our skeletal muscles? Well first, the action potential in our skeletal muscles only lasts about 1 to 2 milliseconds while the action potential in our cardiac muscle cells last 200 milliseconds.

    02:35 Also, contraction in our skeletal muscle cells is going to last about 15 to 100 milliseconds whereas in a cardiac contraction, it also lasts about 200 milliseconds.

    02:47 so the action potential is about as long as the contraction itself.

    02:55 Because of this longer action potential and contraction, we get a sustained contraction that ensures an efficient ejection of the blood from the ventricles.

    03:09 Also, because of the longer refractory period, we do not have the potential for tetanus or tetanic contraction in our cardiac muscle cells.

    About the Lecture

    The lecture Cardiac Action Potential (Nursing) by Jasmine Clark is from the course Cardiovascular System: Heart – Physiology (Nursing).

    Author of lecture Cardiac Action Potential (Nursing)

     Jasmine Clark

    Jasmine Clark

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