CVS: Electrical And Molecular Mechanisms

Question 1

How is the resting membrane potential of cardiac cells is

generated?

Answer 1

The concentration of potassium ions inside and outside of the cell.

Question 2

Describe the steps in a ventricular (cardiac) action potential.

Answer 2

1. Opening of V-gated Na channels (and then subsequent inactivation of these channels) create the upstroke
2. Transient outward potassium ions cause a sudden repolarisation
3. Opening of V-gated calcium channels (and some potassium channels) causes repolarisation.

(Note: no hyperpolarisation)

Question 3

Describe the steps in a pacemaker (cardiac) action potential

Answer 3

1. Funny current generated by influx of sodium through HCN channels
2. Opening of V-gated calcium channels creates more rapid depolarisation
3. Opening of V-gated potassium channels causes repolarisation
4. Never reaches a Plato

Question 4

What are HCN channels and how do they function?

Answer 4

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)

Question 5

How does the initial excitation travel through the heart?

Answer 5

Gap junctions between myocytes.

Question 6

How are SA myocytes specialised?

Answer 6

Spontaneously active with little contractile motility, they never sit at rest!

Question 7

Why does the SA node set the hearts rhythm?

Answer 7

It is the fastest to depolarise

Question 8

What is bradycardia?

Answer 8

AP fires too slowly

Question 9

What is asystole?

Answer 9

AP fails

Question 10

What is tachycardia?

Answer 10

AP fires too quickly

Question 11

What is fibrillation?

Answer 11

Electrical activity becomes random

Question 12

What is the normal range of potassium?

Answer 12

3.5-5.5mmol/L

Question 13

What effects does hyperkalemia have on the heart?

Answer 13

Membrane potential depolarises a bit which inactivated the V-gated sodium channels which in turn slows the upstroke.

Question 14

What effects does hypokalemia have on the heart?

Answer 14

Lengthens AP

Question 15

Name 2 treatments of hyperkalaemia.

Answer 15

Insulin+glucose

Calcium gluconate

Question 16

What can lengthened AP lead to?

Answer 16

Early after depolarisations (EADs) which can lead to oscillations in the membrane and ultimately VF

Question 17

Describe the processes of excitation - contraction coupling in ventricular myocardial cells.

Answer 17

1. Neighboring cells depolarise causing L-type calcium channels to open in T-tubule syste
2. Localised calcium entry opens CICR channels in SR
3. This binds to troponin C and the contraction cascade begins.

Question 18

Where are smooth muscle cells located within blood vessels?

Answer 18

Tunica media

Question 19

What receptor is involved in the contraction of peripheral blood vessels?

Answer 19

Alpha 1

Question 20

How is contraction of peripheral blood vessels initiated?

Answer 20

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.