lecture 9 - electrical properties of the heart Flashcards
Where is the relative potassium potential in the pacemaker cells compared to myocardial cells?
Less negative
What is the relative resting membrane potential of pacemaker cells compared to myocardial cells?
Less negative (closer to zero)
Why does the resting membrane potential of the SA node pacemaker cells need to be unstable and less negative than cardiomyocytes?
To be able to generate spontaneous and rhythmic depolarisations - Action Potentials
What are the 3 phases of the SA node action potential?
- Phase 4: Pre-potential/pacemaker potential
- Phase 0: upstroke
- Phase 3: repolarisation
What happens in the Phase 4/ the pre-potential phase of an SA node action potential?
Potential slowly declines spontaneously (becomes less negative)
What causes the decay in pacemaker potential over time at the beginning of an SA node AP?
Inward Na+ current (if (f=funny) current) into cell depolarises it.
What happens in the Phase 0/the upstroke phase of an SA node action potential?
When depolarisation reaches a certain voltage, Calcium channels open and depolarise triggering the action potential.
What happens in the Phase 3/late repolarisation phase of an SA node action potential?
K+ exits the cell to restore the negative membrane potential .
What is the natural rate of discharge of the SA node?
100 per minute
What nerve provides parasympathetic innervation of the SA node?
Vagus nerve
What nerve/s provides sympathetic innervation of the SA node?
cardiac nerves
What is the effect of sympathetic stimulation on heart rate?
Increases heart rate
What is the effect of parasympathetic stimulation on heart rate?
Decreases heart rate
How does changing the slope of the pacemaker potential change heart rate?
Increasing the slope of the potential makes APs closer together and increases heart rate, and vice versa
How does sympathetic stimulation change the slope of the pacemaker potential in the SA node?
Increases the slope
How does parasympathetic stimulation change the slope of the pacemaker potential in the SA node?
Decreases the slope (and hyper polarises the SA node)
What receptor is used for sympathetic activation of the SA node by noradrenaline?
B1- adrenergic receptors
What is the effect on the pacemaker potential of sympathetic signalling at the SA node?
Increased slope of the pacemaker potential by increasing Na and Ca permaeabilities, allowing the membrane to more quickly depolarise to reach threshold and fire.
What is the intracellular signalling pathway for noradrenaline binding to B1-adrenergic receptors in the SA node?
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
What is the pathway of intracellular signalling for parasymapthetic fibres in the SA node?
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
What neurotransmitter is used in parasympathetic activation of the SA node?
ACh
What are the 2 ‘lower order pacemakers’ of the heart?
AV node, conducting system (Purkinje fibres)
What are the components of the heart’s excitation-conduction system?
1.) Sinoatrial node
2.) Atrioventricular node
3.) Bundle of His
4.) Left/Right bundle branches
5.) Purkinje fibres
What structures join cardiomyocytes at each end?
Intercalated discs
What junctions are found in cardiomyocytes?
gap junctions
What is the purpose of the delay created by the AV node?
Permits full depolarisation and contraction of the atria before the ventricles are depolarised.
What are the 5 phases of the cardiac action potential?
- Phase 4: resting membrane potential
- Phase 0: upstroke
- Phase 1: early repolarisation
- Phase 2: Plateau phase
- Phase 3: Late repolarisation
What occurs during the upstroke phase of a cardiac muscle action potential?
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
What occurs during the early repolarsiation phase of a cardiac muscle action potential?
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
What occurs during the plateau phase of a cardiac muscle action potential?
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’
What occurs during the late repolarisation phase of a cardiac muscle action potential?
Potassium permeability increases and the myocardial cells are fully repolarised to RMP
Why can’t cardiac contractile force summate/produce tetanic contractions?
Electrical and mechanical activity of cardiomyocytes overlaps in time so can’t be summed together - there is a long refractory period.
What is the name of the period where another action potential cannot be stimulated in a cardiomyocyte?
Absolute Refractory Period
