Unit 2 Lecture 3: Cardiac Action Potentials

Question 1

The resting membrane potential of the pacemaker potential is -60mV. What is the resting membrane potential of the Atria Ventricular Cardiac Action Potential Cycle?

Answer 1

-90mV

Cardiac action potential is the stuff the contractions occuring in the cardiac muscle cells of atria and ventricles themselves

Question 2

Where does the cardiac action potential receive it’s signal from in order to start depolarizing, considering that they are non-authrhythmic cells

Answer 2

PACEMAKER CELLS; send signals that cause depolarization and those signals are eventually sent to the ventricles (& atria) which will signal the movement of Na+ into the cell and then Ca2+ to be released

Pacemaker = heartbeat; Cardiac APs = Contractility

Question 3

Explain the Cardiac Action Potential Cycle

Answer 3

1. Na+ in the extracellular space moves into the cell depolarizing it quickly (K+ moves out to keep it negative)
2. Threshold is reached at -70mV
3. Continues until the action potential reaches +30mV and at this point Na+ permeability decreases while L-type channels open up increasing permeability of Ca2+ into the cell
4. Plateau is maintained at full repolarization where Ca2+ continues to move in & K+ permeability increases so K+ moves out simultaneously

Predominantly controlled by K+ and K+ leaky channels

Question 4

How do the cardiac muscle cells pass on Na+ from one cell to another?

Answer 4

Cardiac muscle cells are connected to each other by gap junctions which allows Na+ from one cardiac muscle cell to move into the other and then continue to other muscle cells

Question 5

How do the cardiac muscle cells know when an action potential is taking place?

Answer 5

• The cardiac muscle cells contain sensitive sodium channels
• When sodium moves through the gap junctions this triggers the sensitive sodium channels to open and allow more sodium in

Question 6

How long does an action potential take place in the atria and ventricles? Explain why it is that specific number?

Answer 6

In the atria it takes 150ms and in the ventricles it takes 250ms

• It takes quite some time in cardiac muscle cells because Ca2+ channels (L-type) take a while to open up which makes the AP longer than need be.
• Benefit: More time for Ca2+ to be picked up and for the action potential to fully run it’s course throughout the heart

Skeletal muscle APs are 5ms because release of Ca2+ is instantaneous and L-type channel is voltage gated so it mechanically interacts with RyR (Ryanodine receptor)

Question 7

Explain the three steps of excitation-contraction coupling?

Answer 7

1. Membrane depolarization ==> AP down T-tubule
2. Entry of Ca2+ into cell through L-type channels
3. Ca2+ binds to Ca2+-dependant Ca2+ channels (RyR receptors)

Question 8

Explain how Ca2+ binding to RyR works and what it does?

Answer 8

• Ca2+ entry via L-type channels along the T-tubules into the cell attaches to RyR which induces the Sarcoplasmic reticulum to release more calcium into the cytoplasm of the muscle cell
• RyR is found within cardiac muscle cells surrounding the SR

Question 9

Purpose of the refractory period?

Answer 9

It indicates when the next AP can occur and this is important because we need enough time to induce calcium release and calcium pick up before the next AP

250ms refractory period in the cardiac muscle cell

Question 10

Explain what is happening in the image?

Answer 10

1. Gap channels (Na+) on the cardiac muscle cells allow for influx of Na+ into the cell
2. Voltage-gated L-type channels detect the change in voltage caused by Na+ and allows for Ca2+ influx into the T-tubule
3. Binds to RyR which then causes more Ca2+ release from SR which can allow for binding of troponin, rolling over of tropomyosin and then binding of actin and myosin head
4. Contraction of cardiac muscle cell

Question 11

During contraction, how can we allow for the cardiac muscle to stop contracting? What do we need to limit?

Answer 11

We need to limit the influx of Ca2+ which as a matter of fact is immediately picked up as soon as it’s released by the SR and it is sent back into the SR

Question 12

How are skeletal muscle and cardiac muscle different other than by name?

Answer 12

• Skeletal muscle does not have gap junctions
• L-type channels and RyR are mechanically interacting in skeletal muscle while in cardiac muscle the L-type channels pushes the calcium onto RyR

Question 13

What is the primary purpose of the plateau phase of the cardiac action potential?

Answer 13

• To cause Ca2+ induced-Ca2+ release
• Excitation contraction coupling