Session 4 - Electrical Mechanisms In The Heart

Question 1

What ion controls the resting membrane potential

Answer 1

Potassium

Question 2

What is an equilibrium potential

Answer 2

When the concentration and chemical gradients are equal

Question 3

Why does the resting membrane potential not equal the equilibrium potential for potassium

Answer 3

As the membrane is not solely permeable to potassium

Question 4

What is the equilibrium potential for potassium

Answer 4

-95 mV

Question 5

What are the intracellular and extracellular potassium concentrations

Answer 5

Intra - 140 mM

Extra - 4mM

Question 6

True or false: only a small movement of ions its needed to cause depolarisation

Answer 6

True

Question 7

What are the intracellular and extracellular sodium concentrations

Answer 7

Intra - 10mM

Extra - 123mM

Question 8

How long are the action potentials for SAN and ventricle myocytes

Answer 8

Around 250 ms

Question 9

What causes the upstroke in ventricular action potentials

Answer 9

Opening of voltage gated sodium channels causing an influx of sodium channels until they inactivate

Question 10

What occurs after the steep upstroke in the action potential graph of ventricle cells

Answer 10

The membrane potential goes down for a small amount of time due to a transient outward potassium current - this causes an initial fast repolarisation

Question 11

What causes the plateau in the action potential graph of ventricle cells

Answer 11

There is the opening of L type voltage gated calcium channels giving an influx of calcium ions which balance the efflux of potassium as some potassium Channels are open

Question 12

What causes the repolarisation in the action potential graph of ventricular myocytes

Answer 12

The efflux of potassium ions through voltage gated potassium channels (and other potassium channels)

Question 13

True or false: ventricular myocytes have a shorter resting potential

Answer 13

False - they have a longer resting potential

Question 14

What causes the initial slope to threshold in the action potential graph of the SAN myocytes

Answer 14

HCN channels (slow Na channels) allow an influx of sodium channels which depolarises the cell to threshold - this is the funny current

Question 15

What causes the activation of more HCN channels

Answer 15

A more negative membrane potential

Question 16

What does HCN stand for

Answer 16

Hyperpolarisation-activated Cyclic Nucleotide-gated channels

Question 17

What causes the upstroke on the action potential graphs for the SAN myocytes

Answer 17

Opening of voltage Gated Calcium channels (L type)

Question 18

Why is the upstroke of the action potential graph for SAN myocytes due to calcium not sodium

Answer 18

The membrane potential is not negative enough to re-activate the sodium channels which have inactivated during the slow depolarisation to threshold

Question 19

What causes the repolarisation of the action potential graph of SAN myocytes

Answer 19

The opening of voltage gated potassium channels

Question 20

Why does the SAN set the rhythm for contraction in the heart

Answer 20

They are the fastest to depolarise

Question 21

Why does a delay occur at the AVN

Answer 21

They are slower to depolarise - this gives time for the atria to contract before the ventricles

Question 22

What happens if action potential fire too slowly

Answer 22

Bradycardia

Question 23

What happens if action potentials fail

Answer 23

Asystole

Question 24

What happens if action potentials fire too quickly

Answer 24

Tachycardia

Question 25

What happens if the electrical activity become random

Answer 25

Fibrillation

Question 26

What is the normal range of plasma potassium

Answer 26

3.5 - 5.5 mM

Question 27

What is hyperkalaemia

Answer 27

High potassium concentration in the blood

Question 28

What is hypokalaemia

Answer 28

Low potassium concentration in the blood

Question 29

Why are cardiac myocytes so sensitive to changes in potassium concentration

Answer 29

- they have a resting membrane potential close to the equilibrium potential for potassium therefore, potassium permeability dominates their RMP - the heart has many different types of potassium channels which can behave differently in different concentration of potassium

Question 30

What effect does hyperkalaemia have on the action potentials of ventricular myocytes

Answer 30

When the plasma potassium concentration increases the cell is more depolarised this inactivates Na channels so their not available for the upstroke so their a slower upstroke and a narrower action potential

Question 31

What are the risks with hyperkalaemia

Answer 31

- the heart can stop (asystole) | - may get an initial increase in excitability

Question 32

What do the risks of hyperkalemia depend on

Answer 32

How quickly it develops and its severity

Question 33

What is the treatment for hyperkalemia

Answer 33

- calcium glauconate (makes the heart less excitable) | - insulin + glucose (promotes potassium moving into the cells)

Question 34

What is the effect of hypokalaemia on ventricular myocytes

Answer 34

Lengthens the action potential | Delays depolarisation

Question 35

What are the problems of hypokalaemia

Answer 35

Longer action potentials can lead to early depolarisations, leading to oscillation in membrane potential which results in ventricular fibrillation

Question 36

What proportion of calcium enters cells across the sacrolemma and from being released from the SR

Answer 36

25% from the sacrolemma | 75% from the SR

Question 37

Outline excitation-coupling contraction

Answer 37

Following depolarisation down the t tubules L type Calcium Channels open allowing the increase of Ca. Ca enters causes calcium induced calcium release as they bind to Ryanodine receptors on the SR.

Question 38

What happens to the calcium once it enters the cell to cause contraction

Answer 38

- binds to troponin C - this causes a conformational change moving tropomyosin from the myosin binding site on actin - allows for the sliding filament model

Question 39

How is the calcium concentration returned to normal

Answer 39

- most calcium is pumped back into the SR through the SERCA | - calcium exits across the cell membrane through the NCX and PMCA

Question 40

In smooth muscle cells: what happens to the calcium that moves into the cells

Answer 40

- Binds to calmodulin which can then activate MLCK. | - the MLCK phosphorylates the myosin light chain to allow interaction with actin

Question 41

What does MLCK stand for

Answer 41

Myosin light chain kinase

Question 42

What else can cause the release of calcium in smooth muscle cells other than the opening of calcium channels

Answer 42

Adrenaline or noradrenaline binds to alpha 1 receptors causing the release of calcium from the SR

Question 43

What does MLCP stand for

Answer 43

Myosin Light Chain Phosphatase

Question 44

What does MLCP do

Answer 44

Takes off a phosphate from myosin head to inactivate it

Question 45

What does phosphorylation of MLCK cause

Answer 45

It inhibits the action of MLCK so inhibits phosphorylation of the myosin light chain (myosin head) inhibiting contraction

Question 46

What can phosphorylate MLCK

Answer 46

PKA