RA15 CVS Electrophysiology

Question 1

Sequence of activation of the heart

Answer 1

SA node -> AV node -> His bundle -> left/right bundle branch -> Purkinje fibers

Question 2

Pacemaker of the heart

Answer 2

SA node

Question 3

Conduction speed of:
- SA node
- AV node
- His bundle
- Purkinje fibers

Answer 3

SA node: fast
AV node: slow (AV node delay)
His bundle: fast
Purkinje fibers: fast

Question 4

Action potential speed of:
- SA node
- AV node
- Atrial muscle
- Ventricular muscle

Answer 4

SA node and AV node: slow action potential

Atrial and ventricular muscle: fast action potential

Question 5

Negative resting membrane potential is determined primarily by (…) ions

Answer 5

Negative resting membrane potential is determined primarily by K+ ions

Question 6

Depolarisation: net movement of (…) charge (…) the cell

Repolarisation: net movement of (…) charge (…) the cell

Answer 6

Depolarisation: net movement of positive charge into the cell

Repolarisation: net movement of positive charge out of the cell

Question 7

Phases of fast action potential

Answer 7

0: Upstroke (depolarisation)
- Na+ channels open: influx of Na+

1: Initial repolarisation
- Transient outward K+ channels open: efflux of K+

2: Plateu
- L-type Ca2+ channels open: influx of Ca2+
- I(Ks) channels open: efflux of K+
- Influx of Ca2+ and efflux of K+ balance out

3: Repolarisation
- I(Ks), I(Kr)/hERG, and I(K1) channels open: increased efflux of K+

4: Resting membrane potential
- I(K1) channels open
- Balanced inward/outward currents

Question 8

Properties of slow action potential

Answer 8

1. Automaticity: SA node spontaneously generates an action potential
2. Unstable resting membrane potential
3. No sustained plateau

Question 9

Phases of slow action potential

Answer 9

4: Slow depolarisation
- HCN Na+ channels open: influx of Na+ - I(f) current
- Activated by repolarisation of membrane below the If threshold (about −40/−50 mV)

0: Fast depolarisation
- T-type and L-type Ca2+ channels open: influx of Ca2+

3: Repolarisation
- K+ channels open: efflux of K+

Question 10

Latent pacemakers

Answer 10

• AV node
• Bundle of His
• Purkinje fibers

Question 11

How is current conducted from nodal tissue to contractile cells?

Answer 11

Through gap junctions within intercalated disks

Question 12

Duration of:
- P wave
- AV node delay
- PR interval
- QRS complex
- 1 cycle of depolarisation

Answer 12

P wave (SA node to AV node): 60msec

AV node delay (AV node to His bundle): 100 msec

PR interval (SA node to His bundle): 160msec

QRS complex (His bundle to last myocyte in left ventricle): 80msec

1 cycle (AV node to last myocyste in LV): 240 msec

Question 13

Sympathetic stimulation of the heart

Answer 13

• Norepiphephrine -> β receptor
• Activates G protein
• G(αs) -> adenyl cyclase -> cAMP -> PKA
• PKA -> phosphorylates L-type Ca channels -> increased no. of channels and duration of opening -> increased activator Ca2+ -> increased heart rate
• PKA -> phosphorylates HCN funny Na channels -> increased rate of SA node depolarisation (phase 4) -> more action potentials per unit time -> increased heart rate