Session 4: Cellular and Molecular Events in the Heart and Blood Vessels Flashcards
Explain the ventricular (cardiac) action potential.
The cardiac action potential is longer than a nerve action potential.
It is usually at around -85 to -90 mV at resting membrane potential. There is the an opening of voltage gated sodium channels done by neighbouring cells which depolarise the cardiomyocyte quickly. As threshold is reached an action potential is propagated. There is then a quick attempt at repolarisation of a transient outward K+ current. However, and this is what is special for cardiomyocytes, there is an opening of voltage gated Ca2+ channels which with the K+ channels open as well makes the action potential balance at a plateau for a while. The calcium channel then inactivate and more voltage gated K+ channels open.
More briefly explain cardiac action potential.
Na+ influx = depolarisation
Ca2+ influx and K+ efflux = plateau
Ca2+ inactivation and K+ efflux = repolarisation
How is the resting membrane potential achieved?
By background K+ channels.
What type of voltage gated calcium channels are the ones that are opened at the plateau?
L-type
SA node action potential differs from cardiac action potential. How?
It can spontaneously depolarise and does not need a nerve impulse. There is an initial slope to the threshold which is called a funny current where there is an influx of Na+.
This influx is activated when the membrane potential becomes more negative than -50mV. This means that the depolarisation starts due to hyperpolarisation!
The channels are hyperpolarisation-activated cyclic nucleotide-gated channels or also called HCN channels.
The more negative the membrane potential is the more it activates.
As the membrane potential becomes more positive, Ca2+ channels will open. As more and more open a more steep curve will form and action potential will be reached.
There is then an inactivation of the Ca2+ channels and opening of voltage-gated K+ channels.
There is no plateau in this form of action potential.
Which part of the heart is fastest to depolarise? Which is next?
SA node is the fastest.
AV node is number two.
What is the term for an action potential that fire too slowly?
Bradycardia
What is the term for an action potential the fail?
Asystole
What is the term for an action potential that fire too quickly?
Tachycardia
What is the term for when the electrical activity becomes random?
Fibrillation
What is the concentration for hyperkalaemia?
[K+] at > 5.5 mmol/L
What is the concentration for hypokalaemia?
[K+] at < 3.5 mmol/L
Why are cardiac myocytes so sensitive to changes in [K+]?
Because K+ permeability is the dominant permeability in the resting membrane potential.
Because the heart has many different types of K+ channels that behave in different ways.
What are the effects of hyperkalaemia?
[K+] increase means that the resting membrane potential will become less negative. It depolarises the myocytes and slows down the upstroke for the action potential, this is because there is already some inactivation of Na+ channels. There is a less steep increase to the action potential and the action potential will not be as long.
What are risks with hyperkalaemia?
- The heart can stop (asystole) as there is an inactivation of Na+ channels
- It may initially increase excitability
- Depends on the extent and how quickly it develops
