Cardiac Action Potentials

Question 1

Compare and contrast the cardiomyocytes and pacemaker cells.

Answer 1

• Pacemaker cells present only in few areas of heart (mostly SAN) / cardiomyocytes - majority of heart cells
• Pacemaker cells - no sarcomeres - no role in contractile nature of heart

Question 2

Describe electrical activity of the heart.

Answer 2

• Electrical activity generated in SAN depolarises the atrial cardiomyocytes leading to contraction
• Depolarises AVN
• Depolarises the Bundle of His
• Passes down the Purkinje fibres, which spreads to the apex and back around to depolarise the walls of the ventricles

Question 3

Describe the characteristic action potentials of pacemaker cells.

Answer 3

• Stage 4 - ‘funny current’ - slow depolarisation from -60mV to -40mV
• Stage 0 - reaches threshold of -40mV - rapid depolarisation
• Stage 1 - rapid repolarisation to -60mV

Question 4

What does it mean for the pacemaker cells to have automaticity?

Answer 4

No extrinsic stimuli required

Question 5

Describe the three main channels that govern the membrane potential of pacemaker cells.

Answer 5

• HCN channels - open when membrane potential lower than -40mV. Form funny current - allow for sodium influx - slow depolarisation
• Close when threshold met. VGCCs open - rapid calcium influx and fast depolarisation to +20mV(at which they close)
• Repolarisation by voltage gated potassium channels
• Once below -40mV, HCN channels open

Question 6

Describe the three ways the parasympathetic nervous system can reduce heart rate.

Answer 6

• Longer funny current - longer time to reach threshold potential (by inhibition of HCN channel) - EXAMPLE: Ivabradine
• Raising the threshold potential to activate VGCCs - EXAMPLE: Verapamil
• Decreasing maximum membrane potential cell can achieve - longer time to repolarise - increased expression of potassium channels

Question 7

Describe the ways sympathetic stimulation can increase heart rate.

Answer 7

• Opposing effects of PNS on HCN channels - increased speed of funny current - achieves threshold potential quicker
• Increased calcium conduction through VGCCs - increased CICR

Question 8

Describe cardiac action potential propagation.

Answer 8

• Pacemaker cells and cardiomyocytes linked by gap junctions
• Calcium passes through gap junctions into neighbouring cells - localised depolarisation
• Opening of voltage gated sodium channels - rapid depolarisation

Question 9

What does it mean for the cardiomyocytes to form a syncytium?

Answer 9

• Form a single network of cells - connected by gap junctions
• Depolarisation of one cell will cause rapid depolarisation of neighbouring cells

Question 10

Describe cardiomyocyte action potentials.

Answer 10

• Depolarisation of membrane - sodium influx
• Initial repolarisation - potassium efflux
• Plateau phase - calcium influx
• Rapid repolarisation - potassium efflux
• Return to RMP (-90mV)

Question 11

Cardiac action potentials have a larger timescale than neuronal action potentials. Suggest why.

Answer 11

• Presence of plateau phase

Question 12

Describe depolarisation in cardiomyocyte action potentials.

Answer 12

• Opening of voltage-gated sodium channels
• Localised depolarisation
• Opening of neighbouring sodium channels
• Fast depolarisation due to rapid opening

Question 13

Describe initial repolarisation of cardiomyocyte action potentials.

Answer 13

• Depolarisation reaches around +20mV
• Voltage gated sodium channels inactivated state (no stimulation will open channel - until membrane potential repolarised back to -80mV - absolute refractory period - prevents tetany)
• Opening of fast potassium channels - potassium efflux

Question 14

Describe the plateau phase.

Answer 14

• Initial repolarisation brief - fast potassium channels close
• VGCCs and delayed rectifier potassium channels open - membrane potential plateaus
• This is where muscle contraction occurs - calcium influx leads to myofibril contraction