Electrical + Molecular Mechanisms

Question 1

How is the resting potential set?

Answer 1

Myocytes are permeable to K+
K+ moves down concentration gradient out of cell
Movement makes inside of cell negative

Question 2

What happens in a ventricular action potential?

Answer 2

1. Opening of V-gated Na+ channels
2. Transient outward K+ current
3. Opening of V-gated Ca2+ channels
4. Ca2+ channles inactivate
5. V-gated K+ channels open

Question 3

Draw a ventricular action potential

Answer 3

Google answer

Question 4

Summarise a cardiac action potential

Answer 4

RMP due to background K+ channels

Upstroke = opening of V-gated Na+ channels - influx of Na+

Initial repolarisation = tranisent outward K+ channels

Plateau = opening of V-gated Ca2+ channels - influx of Ca2+ (balanced with K+ efflux)

Repolarisation = opening of V-gated K+ channels - efflux of K+

Question 5

What happens in the SAN action potential?

Answer 5

1. Pacemaker potential - influx of sodium
2. Opening of V-gated Ca2+ channels - depolarisation
3. Opening of V-gated K+ channels - repolarisation

Question 6

Draw a SAN action potential

Answer 6

Google answer

Question 7

What is the pacemaker potential?

Answer 7

Initial slope to threshold - funny current (If)

Activated at potentials <50mV

More negative = more it activates

Question 8

What are HCN channels?

Answer 8

Hyperpolarisation-activated Cyclic Nucleotide-gated channels

Allow influx of Na+ which depolarise cell

Question 9

What happens when a single action potential spreads through heart?

Answer 9

Heart contracts

Question 10

What happens if action potential fire too slowly?

Answer 10

Bradycardia

Question 11

What happens if action potentials fail?

Answer 11

Asystole

Question 12

What happens if action potentials fire too quickly?

Answer 12

Tachycardia

Question 13

What happens if electrical activity is random?

Answer 13

Fibrillation

Question 14

What is hyperkalaemia?

Answer 14

High plasma K+

> 5.5mmol/L

Question 15

What is hypokalaemia?

Answer 15

Low plasma K+

<3.5mmol/L

Question 16

What is the effect of hyperkalamia?

Answer 16

Membrane potential depolarises a bit
This inactivates some V-gated Na+ channels

Slows upstroke

Question 17

What are the risks of hyperkalaemia?

Answer 17

Initial increase in excitability

Asystole

Question 18

How do you treat hyperkalaemia?

Answer 18

Calcium gluconate

Insulin + glucose

Question 19

What is the effect of hypokalaemia?

Answer 19

Lengthens action potential

Delays repolarisation

Question 20

What are the risks of hypokalaemia?

Answer 20

Longer action potentials can lead to early after depolarisations
Oscillations in membrane potential
VF

Question 21

What is excitation-contraction coupling?

Answer 21

Depolariation opens L-type Ca2+ channels in T-tubule system

Localised Ca2+ entry opens CICR (calcium induced calcium release) channels in the SR

Link between both channels

• 25% across sarcolemma
• 75% released from SR

Question 22

How is cardiac myocyte contraction regulated?

Answer 22

Ca2+ binds to troponin C

Conformational change shifts tropomyosin to reveal myosin binding site on actin filament

Question 23

How do cardiac myocytes relax?

Answer 23

Must return intracellular Ca2+ to resting levels

• pumped back to SR (SERCA)
• exit across cell membrane (Ca2+ATPase + Na/Ca exchanger)