Generating the Cardiac Rhythm

Question 1

Which nerve(s) are the source of sympathetic innervation to the heart?

Answer 1

Spinal nerves T1-T4.

Question 2

Which nerve(s) are the source of parasympathetic innervation to the heart?

Answer 2

The vagus nerve.

Question 3

What is the heart rate of a heart that receives no external innervation?

Answer 3

100bpm.

Question 4

What is myogenic rhythmicity / autorhythmicity?

Answer 4

The ability of cardiac muscles to depolarise and contract rhythmically without external innervation.

Question 5

What are the three types of action potentials in the heart?

Answer 5

1 - Those arising from SA and AV nodes.

2 - Those arising in atrial muscle.

3 - Those arising in purkinje fibres and ventricular muscle.

Question 6

Which structure is known as the pacemaker of the heart?

Answer 6

The sinoatrial node.

Question 7

Are the cells that constitute the sinoatrial node contractile or non-contractile?

Answer 7

Non-contractile.

Question 8

What electrical event must occur between action potentials in the sinoatrial node?

Answer 8

The pacemaker potential.

Question 9

Which ions are responsible for the pacemaker potential in the SA node and how are they involved?

Answer 9

• Inward movement of Na+ (NOT voltage gated channels) increases voltage.
• Inward movement of Ca2+ through T-type channels (low voltage gated) increases voltage.
• A decrease in conductance of K+ decreases outward movement so increases voltage.

Question 10

Which ions are responsible for the action potential in the SA node and how are they involved?

Answer 10

For the initial increase in voltage:

1 - Inward movement of Ca2+ through L-type channels (high voltage gated) for initial upwards slope.

2 - A further decrease in K+ conductance decreases outward movement so contributes to initial upwards slope.

For the repolarisation:

3 - Outward movement of K+.

Question 11

How does parasympathetic activity decrease heart rate?

What this effect on the heart rate known as?

Answer 11

• By opening K+ channels, thereby hyperpolarising the cell and increasing K+ conductance, reducing the slope of the pacemaker potential.
• Known as a negative chronotropic effect.

Question 12

How does sympathetic activity increase heart rate?

What is this effect on the heart rate known as?

Answer 12

• By increasing Na+ and Ca+ conductance, thereby increasing the slope of the pacemaker potential.
• Known as a positive chronotropic effect.

Question 13

Why are heart muscle cells collectively known as a functional syncytium?

Answer 13

Because the contraction of cardiac muscle is coordinated along its entire length, as the current flows to adjoining cells through gap junctions at intercalated discs.

Question 14

What is the rate of conduction through the atria?

Answer 14

0.5m/s.

Question 15

Describe the profile of electrical activity of atrial tissue.

Answer 15

• Stable resting membrane potential of -80mV.
• Very steep depolarisation; almost vertical to +10mV.
• Initially steep repolarisation, then plateau.
• Repolarisation steepens until -80mV resting potential (no hyperpolarisation).

Question 16

Where does the electrical current pass after depolarising the atria?

Answer 16

The atrioventricular node.

Question 17

Describe the profile of electrical activity of the sinoatrial and atrioventricular nodes.

Answer 17

• A very shallow pacemaker potential, beginning at -60mV.
• A smooth increase at the beginning of the action potential (-40mV) to reach +10mV.
• An equally smooth decrease to -60mV, where the pacemaker potential begins again.

Question 18

Why does the atrioventricular node have pacemaker capability if it is overridden by the SA node?

Answer 18

In order to take over pacemaker responsibility should the sinoatrial node cease to function.

Question 19

What is the rate of conduction through the atrioventricular node?

Answer 19

0.05 m/s

Question 20

What can affect the conduction rate through the atrioventricular node?

Answer 20

The sympathetic nervous system only (can act to increase the rate of conduction).

Question 21

Why is the rate of conduction so much slower through the atrioventricular node than in the atria?

Answer 21

To ensure atrial depolarisation, contraction and ejection before depolarisation of the ventricles.

Question 22

Where does the electrical current pass after depolarising the atrioventricular node?

Answer 22

The Bundle of his.

Question 23

What is the rate of conduction through the Bundle of His?

Answer 23

2m/s.

Question 24

Where does the electrical current pass after depolarising the Bundle of His?

Answer 24

Left and right bundle branches.

Question 25

What is the rate of conduction through the left and right bundle branches?

Answer 25

2m/s.

Question 26

Where does the electrical current pass after depolarising the left and right bundle branches?

Answer 26

Purkinje fibres.

Question 27

What is the rate of conduction through Purkinje fibres?

Answer 27

4m/s.

Question 28

What is the pacemaker rate of the atrioventricular node?

Answer 28

40 bpm.

Question 29

List the areas of the heart that are supplied with parasympathetic innervation from the vagus nerve.

Answer 29

1 - The SA node.

2 - The AV node.

Question 30

List the areas of the heart that are supplied with sympathetic innervation.

Answer 30

1 - The SA node.

2 - The AV node.

3 - The left and right bundle branches.

4 - Purkinje fibres.

Question 31

How long does the atrial action potential last?

Answer 31

200ms.

Question 32

How long does the ventricular action potential last?

Answer 32

250-300ms.

Question 33

How long does the sinoatrial node action potential last?

Answer 33

200ms.

Question 34

How long does the atrioventricular action potential last?

Answer 34

200ms.

Question 35

Describe the profile of electrical activity of ventricular tissue.

Answer 35

• Stable resting membrane potential of -80mV.
• Very steep depolarisation; almost vertical to +25mV.
• Initially steep repolarisation (partial repolarisation), then plateau.
• Repolarisation steepens again until -80mV resting potential (no hyperpolarisation).

Question 36

What causes the plateau during repolarisation for ventricular tissue?

Answer 36

Inward Ca2+ movement via L-type Ca2+ channels AND outward movement of K+ after inactivation of Na+ channels.

Question 37

Why is it important that cardiac myocytes have a long absolute refractory period?

Answer 37

To prevent further contractions from being generated, as this would result in summation / tetanic contractions, and therefore an increase in muscle tension.