L02: Electrical Activity Of The Heart Flashcards
Describe the pathway of the electrical activity of the heart to create mechanical activity (muscle contraction)
1) SAN fires AP whihc propogates to the left and right atrium
2) then to AVN
3) then to bundle of his and purkinje fibres
4) electrical conduction from ventricular endocardium, myocardium and then to epicardium which is the other layer muscle of the heart
What are the 3 main cell types in the heart
SAN and AVN cells
Cardiac myocytes
Purkinje fibres
Why is the SAN referred to as a cardiac pacemaker
It has no resting membrane potential which allows them to spontaneously generate AP.
If an SAN does not have a resting membrane potential what does it have instead
Pacemaker potential
How does the pacemaker potential allow depolarisation
It reaches the threshold value quicker
Describe the movement of ions that give the pacemaker potential
Inward movement of sodium
Inward movement of calcium
Outward movement of potassium
Overall the inward movement of sodium and calcium exceed potassium excretion
Describe the movement of ions in the SAN at a depolarisation phase
Calcium influx via voltage gated calcium channels
Describe the movement of ions at the repolarisation phase
Voltage gated calcium channels close
Voltage gated potassium channels open for potassium efflux
Which nervous system innervates the SAN
Autonomic nervous system
What are the 2 branches of the autonomic nervous system
Sympathetic
Parasympathetic
Which nerve carries the parasympathetic neurones to the SAN
Vagus nerve
Which nerve carries the sympathetic neurones to the SAN
Cardiac sympathetic nerve
How do we alter the heart rate of the heart
By altering the slope of the pacemaker potential
What is the overall effect of the parasympathetic NS to the heart rate
Decrease heart rate
What is the overall effect of the sympathetic NS on heart rate
Increase heart rate
What neurotransmitter does the parasympathetic nervous system release on post ganglionic neurones
Ach
What receptor does ach act on in the heart
M2 receptors
What type of neurotransmitter does the post ganglionic sympathetic neurones release
NA/ adrenaline
Which receptors does noradrenaline or adrenaline act on in the heart
Beta 1 receptors (we have 1 heart)
How does the parasympathetic NS decrease the heart rate
1) ach released from the vagus nerve acts on M2 receptors on the SAN cells.
2) potassium influx in the ‘pacemaker potential’ increase
3) this reduces the slope of the pacemaker potential so it takes longer to reach threshold and initiate a depolarisation
What is the effect of the parasympathetic ns on the pacemaker potential called
Negative chronotropic effect
How does the sympathetic NS cause an increased heart rate
1) NA/A is released from the cardiac sympathetic nerve to the beta-1 receptors on the SAN cells
2) this increases the sodium and calcium influx by opening the channels more (increased conductance)
3) this increases the slope of the pacemaker potential so the threshold value is met quicker for depolarisation to occur
What is the effect of the sympathetic nervous system on the pacemaker potential called
Positive chronotropic effect
List the pacemaker cells in order of the fastest to slowest
SAN
AVN
Purkinje fibres
Which pacemaker cells drives the heart rate
SAN
Why do we have a AVN delay
To allow the blood in the atrium to fill the ventricles before depolarisation occurs
What type of a resting potential does cardiac myocytes have
Stable resting potential
What initiates cardiac myocytes (in the walls of the heart) to fire action potential
Cardiac myocytes receive the voltage of depolarisation from the SAN,AVN and purkinje fibres. This causes VOLTAGE gated sodium channels to open and cause depolarisation
What are the phases of ventricular myocyte AP called
Phase 0: rapid depolarisation Phase 1: partial rapid repolarisation Phase 2: plateau Phase 3 : terminal repolarisation Phase 4: stable resting potential
What causes the rapid depolarisation (phase 0) to occur
Fast Voltage gated sodium channels open for sodium influx
What causes the partial rapid repolarisation (phase 1)
Sodium channels are inactivated
Fast voltage gated potassium channels open for potassium influx
What causes the plateau (phase 3)
Calcium influx and potassium efflux is balanced
There is no/little change in AP
What causes the terminal repolarisation (phase 3) to occur
Opening of delayed rectifier potassium channel so potassium efflux occurs more
What is the refractory period
The period of time from the initial depolarisation to the point of a second depolarisation(AP) to be stimulated
What are the 2 types of refractory period
Absolute
Relative
What is the absolute refractory period
No second AP can occur because all the voltage gated sodium channels are closed
What is the relative refractory period
Second AP can occur as voltage gated sodium channels begin to open/recover
What does voltage gated sodium channels need to recover
Time
More negative potential (to get to resting membrane potential quicker)
If there is a shorter duration so action potential will there be a slower or faster recovery of sodium channels
Faster
What is a conduction velocity
The speed at which an impulse propagates down
How are many cardiac myocytes coupled
Electrically coupled by GAP junctions
What causes the movement of ions between GAP junctions
A large voltage gradient
What is conduction velocity determined by
Charge gradient between cells
Action potential amplitude
What is the difference between SAN and cardiac myocytes action potential
SAN:
1) pacemaker potential: sodium influx by HCN channels, calcium influx and potassium efflux
2) depolarisation: calcium influx by voltage gated calcium channels
3) repolarisation: potassium efflux by voltage gated potassium channels
Cardiac myocytes:
Has 4 phases:
Phase 0 (depolarisation): sodium influx by voltage gated sodium channel
Phase 1 (rapid partial repolarisation): potassium efflux by voltage gated potassium channel
Phase 2 (plateau): calcium influx= potassium efflux
Phase 3 (terminal repolarisation): potassium efflux by delayed rectifier potassium channels
Phase 4 (resting potential).
What is the difference between a positive and negative chronotropic effect
Postive chronotropic effect: via the SNS which increases the sodium and calcium influx to give a steep gradient of the pacemaker potential
Negative chronotropic effect: via the PNS which increases potassium efflux to give a less steep pacemaker potential gradient
