Cardiac Action Potential (Nursing)

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.