L12 Spread of Excitation Across the Heart Flashcards
In the heart, where is the electrical impulse initiated?
Pacemaker cells of the sinoatrial (SA) node.
How is the action potential (generated by SA node) conducted across the atria?
Via gap junctions of intercalated discs of atrial myocytes
The action potential generated at the SA node crosses the atria and stimulates the atrioventricular node. How is the action potential carried from the AV node?
Bundle of His, then Purkinje system
What is the resting membrane potential of the sinoatrial pacemaker cells?
-60mV
What is the membrane potential threshold for triggering an action potential in the SA pacemaker cells?
-40mV
What happens when the pacemaker cell membrane potential reaches -40mV?
Reaching the -40mv threshold triggers the opening of L-type (long-lasting) voltage gated calcium ion channels.
These L-type voltage gated calcium ion channels depolarise the the cells slowly to about +10mV.
What happens after the pacemaker cell membrane potential is depolarised to +10mV?
Repolaristation is mediated by the:
- opening of potassium channels
- activation of the sodium-potassium pumps
- activation of the sodium-calcium exchange system
What are the three phases of the cardiac myocyte action potential?
- Depolarisation phase
- Plateau phase
- Repolarisation phase
What happens in the depolarisation phase of the cardiomyocyte action potential?
Cardiac myocytes have a resting membrane potential of -90mV, but when the voltage gated sodium ion channels open the cell rapidly depolarises to about +20mV. Once opened, the sodium channels become inactivated for a short period of time.
True or false: Pacemaker cells depolarise rapidly, while cardiomyocytes depolarise slowly.
False.
Pacemaker cells depolarise relatively slowly to about +10mV. This is facilitated by L-type voltage gated calcium ion channels.
Cardiomyocytes depolarise rapidly to about +20mV, facilitated by voltage gated sodium ion channels.
What happens in the plateau phase of cardiomyocyte action potential?
The sodium current triggers the opening of voltage-gated slow calcium channels (bound within the membrane), which allow a small influx of calcium into the cells.
This small influx triggers more calcium to be released from the sarcoplasmic reticulum.
This process is accompanied by 250ms decreased permeability to potassium.
What happens during the repolarisation phase of cardiomyocyte action potential?
Potassium channels open and the sodium-potassium pump is activated to repolarise the cell.
Where is the AV node located?
Just anterior to the opening of the coronary sinus
What structure forms the point of electrical contact between the atria and the ventricles?
The AV node
What structure extends into the ventricular septum where it divides into a left bundle and a right bundle, which continue to the apex of the heart?
The bundle of His
