CVS: Electrical And Molecular Mechanisms Flashcards
How is the resting membrane potential of cardiac cells is
generated?
The concentration of potassium ions inside and outside of the cell.
Describe the steps in a ventricular (cardiac) action potential.
- Opening of V-gated Na channels (and then subsequent inactivation of these channels) create the upstroke
- Transient outward potassium ions cause a sudden repolarisation
- Opening of V-gated calcium channels (and some potassium channels) causes repolarisation.
(Note: no hyperpolarisation)
Describe the steps in a pacemaker (cardiac) action potential
- Funny current generated by influx of sodium through HCN channels
- Opening of V-gated calcium channels creates more rapid depolarisation
- Opening of V-gated potassium channels causes repolarisation
- Never reaches a Plato
What are HCN channels and how do they function?
Hyperpolarisation-activated, Cyclic Nucleotide-gated channels
The more negative the cell becomes the more this type of channel is activated. (Have to be more negative than -50mV)
How does the initial excitation travel through the heart?
Gap junctions between myocytes.
How are SA myocytes specialised?
Spontaneously active with little contractile motility, they never sit at rest!
Why does the SA node set the hearts rhythm?
It is the fastest to depolarise
What is bradycardia?
AP fires too slowly
What is asystole?
AP fails
What is tachycardia?
AP fires too quickly
What is fibrillation?
Electrical activity becomes random
What is the normal range of potassium?
3.5-5.5mmol/L
What effects does hyperkalemia have on the heart?
Membrane potential depolarises a bit which inactivated the V-gated sodium channels which in turn slows the upstroke.
What effects does hypokalemia have on the heart?
Lengthens AP
Name 2 treatments of hyperkalaemia.
Insulin+glucose
Calcium gluconate
What can lengthened AP lead to?
Early after depolarisations (EADs) which can lead to oscillations in the membrane and ultimately VF
Describe the processes of excitation - contraction coupling in ventricular myocardial cells.
- Neighboring cells depolarise causing L-type calcium channels to open in T-tubule syste
- Localised calcium entry opens CICR channels in SR
- This binds to troponin C and the contraction cascade begins.
Where are smooth muscle cells located within blood vessels?
Tunica media
What receptor is involved in the contraction of peripheral blood vessels?
Alpha 1
How is contraction of peripheral blood vessels initiated?
Contraction is initiated by phosphorylation of myosin
light chains by myosin-light chain kinase. An increase in intracellular Ca2+ causes formation of a Ca2+ - calmodulin complex which activates myosin –light chain kinase.
