L02: Electrical Activity Of The Heart

Question 1

Describe the pathway of the electrical activity of the heart to create mechanical activity (muscle contraction)

Answer 1

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

Question 2

What are the 3 main cell types in the heart

Answer 2

SAN and AVN cells
Cardiac myocytes
Purkinje fibres

Question 3

Why is the SAN referred to as a cardiac pacemaker

Answer 3

It has no resting membrane potential which allows them to spontaneously generate AP.

Question 4

If an SAN does not have a resting membrane potential what does it have instead

Answer 4

Pacemaker potential

Question 5

How does the pacemaker potential allow depolarisation

Answer 5

It reaches the threshold value quicker

Question 6

Describe the movement of ions that give the pacemaker potential

Answer 6

Inward movement of sodium
Inward movement of calcium
Outward movement of potassium

Overall the inward movement of sodium and calcium exceed potassium excretion

Question 7

Describe the movement of ions in the SAN at a depolarisation phase

Answer 7

Calcium influx via voltage gated calcium channels

Question 8

Describe the movement of ions at the repolarisation phase

Answer 8

Voltage gated calcium channels close

Voltage gated potassium channels open for potassium efflux

Question 9

Which nervous system innervates the SAN

Answer 9

Autonomic nervous system

Question 10

What are the 2 branches of the autonomic nervous system

Answer 10

Sympathetic

Parasympathetic

Question 11

Which nerve carries the parasympathetic neurones to the SAN

Answer 11

Vagus nerve

Question 12

Which nerve carries the sympathetic neurones to the SAN

Answer 12

Cardiac sympathetic nerve

Question 13

How do we alter the heart rate of the heart

Answer 13

By altering the slope of the pacemaker potential

Question 14

What is the overall effect of the parasympathetic NS to the heart rate

Answer 14

Decrease heart rate

Question 15

What is the overall effect of the sympathetic NS on heart rate

Answer 15

Increase heart rate

Question 16

What neurotransmitter does the parasympathetic nervous system release on post ganglionic neurones

Answer 16

Ach

Question 17

What receptor does ach act on in the heart

Answer 17

M2 receptors

Question 18

What type of neurotransmitter does the post ganglionic sympathetic neurones release

Answer 18

NA/ adrenaline

Question 19

Which receptors does noradrenaline or adrenaline act on in the heart

Answer 19

Beta 1 receptors (we have 1 heart)

Question 20

How does the parasympathetic NS decrease the heart rate

Answer 20

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

Question 21

What is the effect of the parasympathetic ns on the pacemaker potential called

Answer 21

Negative chronotropic effect

Question 22

How does the sympathetic NS cause an increased heart rate

Answer 22

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

Question 23

What is the effect of the sympathetic nervous system on the pacemaker potential called

Answer 23

Positive chronotropic effect

Question 24

List the pacemaker cells in order of the fastest to slowest

Answer 24

SAN
AVN
Purkinje fibres

Question 25

Which pacemaker cells drives the heart rate

Answer 25

SAN

Question 26

Why do we have a AVN delay

Answer 26

To allow the blood in the atrium to fill the ventricles before depolarisation occurs

Question 27

What type of a resting potential does cardiac myocytes have

Answer 27

Stable resting potential

Question 28

What initiates cardiac myocytes (in the walls of the heart) to fire action potential

Answer 28

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

Question 29

What are the phases of ventricular myocyte AP called

Answer 29

``` Phase 0: rapid depolarisation Phase 1: partial rapid repolarisation Phase 2: plateau Phase 3 : terminal repolarisation Phase 4: stable resting potential ```

Question 30

What causes the rapid depolarisation (phase 0) to occur

Answer 30

Fast Voltage gated sodium channels open for sodium influx

Question 31

What causes the partial rapid repolarisation (phase 1)

Answer 31

Sodium channels are inactivated | Fast voltage gated potassium channels open for potassium influx

Question 32

What causes the plateau (phase 3)

Answer 32

Calcium influx and potassium efflux is balanced | There is no/little change in AP

Question 33

What causes the terminal repolarisation (phase 3) to occur

Answer 33

Opening of delayed rectifier potassium channel so potassium efflux occurs more

Question 34

What is the refractory period

Answer 34

The period of time from the initial depolarisation to the point of a second depolarisation(AP) to be stimulated

Question 35

What are the 2 types of refractory period

Answer 35

Absolute | Relative

Question 36

What is the absolute refractory period

Answer 36

No second AP can occur because all the voltage gated sodium channels are closed

Question 37

What is the relative refractory period

Answer 37

Second AP can occur as voltage gated sodium channels begin to open/recover

Question 38

What does voltage gated sodium channels need to recover

Answer 38

Time | More negative potential (to get to resting membrane potential quicker)

Question 39

If there is a shorter duration so action potential will there be a slower or faster recovery of sodium channels

Answer 39

Faster

Question 40

What is a conduction velocity

Answer 40

The speed at which an impulse propagates down

Question 41

How are many cardiac myocytes coupled

Answer 41

Electrically coupled by GAP junctions

Question 42

What causes the movement of ions between GAP junctions

Answer 42

A large voltage gradient

Question 43

What is conduction velocity determined by

Answer 43

Charge gradient between cells | Action potential amplitude

Question 44

What is the difference between SAN and cardiac myocytes action potential

Answer 44

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).

Question 45

What is the difference between a positive and negative chronotropic effect

Answer 45

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