lecture 9 - electrical properties of the heart

Question 1

Where is the relative potassium potential in the pacemaker cells compared to myocardial cells?

Answer 1

Less negative

Question 2

What is the relative resting membrane potential of pacemaker cells compared to myocardial cells?

Answer 2

Less negative (closer to zero)

Question 3

Why does the resting membrane potential of the SA node pacemaker cells need to be unstable and less negative than cardiomyocytes?

Answer 3

To be able to generate spontaneous and rhythmic depolarisations - Action Potentials

Question 4

What are the 3 phases of the SA node action potential?

Answer 4

• Phase 4: Pre-potential/pacemaker potential
• Phase 0: upstroke
• Phase 3: repolarisation

Question 5

What happens in the Phase 4/ the pre-potential phase of an SA node action potential?

Answer 5

Potential slowly declines spontaneously (becomes less negative)

Question 6

What causes the decay in pacemaker potential over time at the beginning of an SA node AP?

Answer 6

Inward Na+ current (if (f=funny) current) into cell depolarises it.

Question 7

What happens in the Phase 0/the upstroke phase of an SA node action potential?

Answer 7

When depolarisation reaches a certain voltage, Calcium channels open and depolarise triggering the action potential.

Question 8

What happens in the Phase 3/late repolarisation phase of an SA node action potential?

Answer 8

K+ exits the cell to restore the negative membrane potential .

Question 9

What is the natural rate of discharge of the SA node?

Answer 9

100 per minute

Question 10

What nerve provides parasympathetic innervation of the SA node?

Answer 10

Vagus nerve

Question 11

What nerve/s provides sympathetic innervation of the SA node?

Answer 11

cardiac nerves

Question 12

What is the effect of sympathetic stimulation on heart rate?

Answer 12

Increases heart rate

Question 13

What is the effect of parasympathetic stimulation on heart rate?

Answer 13

Decreases heart rate

Question 14

How does changing the slope of the pacemaker potential change heart rate?

Answer 14

Increasing the slope of the potential makes APs closer together and increases heart rate, and vice versa

Question 15

How does sympathetic stimulation change the slope of the pacemaker potential in the SA node?

Answer 15

Increases the slope

Question 16

How does parasympathetic stimulation change the slope of the pacemaker potential in the SA node?

Answer 16

Decreases the slope (and hyper polarises the SA node)

Question 17

What receptor is used for sympathetic activation of the SA node by noradrenaline?

Answer 17

B1- adrenergic receptors

Question 18

What is the effect on the pacemaker potential of sympathetic signalling at the SA node?

Answer 18

Increased slope of the pacemaker potential by increasing Na and Ca permaeabilities, allowing the membrane to more quickly depolarise to reach threshold and fire.

Question 19

What is the intracellular signalling pathway for noradrenaline binding to B1-adrenergic receptors in the SA node?

Answer 19

1.) binding of NA leads to increase in intracellular concentration of cAMP
2.) cAMP increases funny Na+ current into the cell - depolarises it
3.) cAMP also activates protein Kinase A to activate Ca2+ channels to increase Ca2+ current into the cell - depolarises it
4.) Protein Kinase A activates K+ channels to shorten depolarisation - increases the rate of pumping out K+ at end
5.) The result is a shortened duration of AP/ incr. HR

Question 20

What is the pathway of intracellular signalling for parasymapthetic fibres in the SA node?

Answer 20

1.) ACh binds to muscarinic receptors,
2.) cAMP levels drop
3.) Sympathetic stimulation drops
4.) Na+, Ca2+, have reduced activation - reduced depolarisation of the cells
5.) K+ has greater permeability, hyperpolarising the membrane to make it harder to reach threshold.
ULTIMATELY = Reduced slope of the pacemaker potential

Question 21

What neurotransmitter is used in parasympathetic activation of the SA node?

Answer 21

ACh

Question 22

What are the 2 ‘lower order pacemakers’ of the heart?

Answer 22

AV node, conducting system (Purkinje fibres)

Question 23

What are the components of the heart’s excitation-conduction system?

Answer 23

1.) Sinoatrial node
2.) Atrioventricular node
3.) Bundle of His
4.) Left/Right bundle branches
5.) Purkinje fibres

Question 24

What structures join cardiomyocytes at each end?

Answer 24

Intercalated discs

Question 25

What junctions are found in cardiomyocytes?

Answer 25

gap junctions

Question 26

What is the purpose of the delay created by the AV node?

Answer 26

Permits full depolarisation and contraction of the atria before the ventricles are depolarised.

Question 27

What are the 5 phases of the cardiac action potential?

Answer 27

• Phase 4: resting membrane potential
• Phase 0: upstroke
• Phase 1: early repolarisation
• Phase 2: Plateau phase
• Phase 3: Late repolarisation

Question 28

What occurs during the upstroke phase of a cardiac muscle action potential?

Answer 28

Stimulus from adjacent cell causes depolarisation and an increase in permeability to sodium ions, causing a massive depolarisation and then hyper polarisation, during which more Na+ channels oopen

Question 29

What occurs during the early repolarsiation phase of a cardiac muscle action potential?

Answer 29

Na+ channels self inactivate as the cell hyper polarises, decreasing the Na permeability. K+ ions begin to flow out of the cell aiding in the repolarisation

Question 30

What occurs during the plateau phase of a cardiac muscle action potential?

Answer 30

There is a sustained depolarisation due to the flow of Ca2+ into the cell, which resists the repolarisation occuring from the outflow of k+, briefly stabilising the potential - a ‘plateau’

Question 31

What occurs during the late repolarisation phase of a cardiac muscle action potential?

Answer 31

Potassium permeability increases and the myocardial cells are fully repolarised to RMP

Question 32

Why can’t cardiac contractile force summate/produce tetanic contractions?

Answer 32

Electrical and mechanical activity of cardiomyocytes overlaps in time so can’t be summed together - there is a long refractory period.

Question 33

What is the name of the period where another action potential cannot be stimulated in a cardiomyocyte?

Answer 33

Absolute Refractory Period