What are the different types of action potentials seen in the heart? Which is the slowest?
Ventricular, atrial and nodal. Nodal is the slowest.
How does the nodal action potential differ from the ones in atria and ventricles?
The nodal action potential does not have the slight depolarization phase with a plateau. It constantly decreases back to its pacemaker potential.
What is responsible for the diastolic resting potential seen in this ventricular action potential diagram? How are these channels different from those in nerves?
Non-voltaged gated, self-rectifying potassium channels. When these are plugged up, the aid in depolarization of the cell. When not plugged up, they keep the cell around -90mV.
What is responsible for the rapid depolarization seen in this ventricular action potential diagram?
Opening of Na+ VGCs.
What is responsible for the brief repolarization seen in this ventricular action potential diagram?
Na+ VGCs close by themselves. Then, another K+ VGC briefly opens, allowing K+ to leave the cell and repolarize briefly.
What factors contribute to the plateau seen in this ventricular action potential diagram?
A small Na+ inward flux, opening of slow L-type Ca2+ channels with inward flux, rectifying K+ channels slowing K+ exit from cell and opening of 2 rectifying K+ channels allowing K+ to leave the cell.
What factors contribute to the repolarization seen in this ventricular action potential diagram?
The Ca2+ VGCs close and all K+ rectifying channels become unplugged to restore the diastolic membrane potential.
What is the main factor responsible for the absolute refractory period seen below?
The second gate of the Na+ VGC only reopens once the cell repolarizes. Thus, without a functional Na+ VGC, the cell cannot have another action potential.
What is responsible for action potentials produced during the relative refractory period?
As the cell repolarizes, Na+ VGCs reset and can be stimulated to open by a large enough stimulus.
When is it easier for the cell to produce another action potential?
The supranormal period. At this point, most fast Na channels have been reset, rectifying channel have not all opened up yet and the resting potential is closer to the threshold potential so you need a smaller stimulus to trigger an action potential.
What are the key differences between nodal and ventricular action potentials?
Duration (nodal is longer), resting potential (not constant in nodal), depolarization (slower in nodal) and no plateau in nodal.
What is happening to the ion permeabilities of this nodal cell?
At first, K+ permeability decreases and aids in cell depolarization. Then the "funny" Na+ channel opens on REPOLARIZATION and brings the cell back towards threshold. Finally, a Ca2+ channel opens and contributes to depolarization that opens the other Ca2+ L channel that causes the nodal action potential.
How does the ANS control heart rate?
Nodal action potentials control heart rate and most control takes place during phase 4. The ANS controls heart rate by varying Na+, K+ and Ca++ conductances in the cell.
You're having a rough day and your stress levels are high. Your wife gives you a neck massage when you get home to lower your heart rate. How might this physiologically lower your heart rate?
The neck rub may stimulate the parasympathetic nervous system's vagus nerve. This in turn depolarizes the threshold and releases ACh which causes the following changes in channel permeabilities: decrease in Na+, decrease in Ca++ and increase in K+. This all results in hyperpolarization and requires more work to cause an action potential and decreases the heart rate.
What are the main effects on heart rate by ACh?
It depolarizes the action potential threshold, hyperpolarizes the pacemaker potential and decreases the pacemaker potential slope. These all work to decrease heart rate.
What are the main effects on heart rate by norepinephrine?
It promotes depolarization by increasing Na+ and Ca++ permeabilities and decreasing K+ permeability. This makes it easier to repeat action potentials and increases the heart rate.
How do cardiac muscle cells differ from skeletal muscle cells as far as where repolarization takes place first after a depolarization?
In skeletal muscle the area that depolarized first repolarizes first. In cardiac muscle the area that depolarizes last repolarizes first because the muscle cell on the surface of the heart usually repolarize faster than the muscle cells inside the heart.
What creates the peak in an ECG?
The ECG measures the total extra cellular potential of a muscle strip. When the strip is half depolarized, the total potential (E2 - E1) will be at its greatest because the negative E1 will add to E2 due to the subtracting function.
What results in a positive and negative deflection in an ECG?
A positive deflection occurs when the lead axis and depolarization vector point in the sam direction. A negative deflection occurs when they point in opposite directions.
What does an ECG tell you when the voltage = 0?
It tells you that the depolarization vector in that muscle strip is perpendicular to your lead axis.
Why is the voltage projection less for the same muscle strip when you place leads obliquely instead of parallel?
When you drop the perpendicular vector on the oblique lead, the value projects less.
Where do you place your leads in an ECG?
How do you figure out the mean cardiac vector?
Head to tail addition and measuring the mean vector. The number of cells polarizing and their orientation with respect to each other will determine how large or small the mean cardiac vector will be. Dropping lines parallel to the lead axis gives you the voltage reading on your ECG.
When is your mean cardiac vector equal to zero?
Either when all cells are depolarized or when all cells are repolarized.