Generating the cardiac rhythm

Question 1

What is heart muscle able to do without the input of nerves?

Answer 1

Depolarise and contract rhythmically

Question 2

What are the three types of cardiac action potential?

Answer 2

SA and AV nodes
Atrial muscle
Purkinje fibres and ventricular muscle

Question 3

In what three ways do cardiac action potentials differ from each other and also from action potentials in muscles or nerves?

Answer 3

Duration
Shape
Ionic basis of depolarisation

Question 4

Where is the sino atrial node located?

Answer 4

Wall of right atrium where superior vena cava enters

Question 5

Does the sino atrial action potential have a resting membrane potential?

Answer 5

No

Question 6

How many action potentials does the SA node have per minute?

Answer 6

100

Question 7

Is this rate of 100 AP/min changing?

Answer 7

It constantly changes as the SA node is a pacemaker

100 beats per minute is the intrinsic rate

Question 8

Describe what is happening in the rising limb of the SA node action potential

Answer 8

Inward movement of Na+
Decreased conductance of K+
Inward movement of Ca+

Question 9

What is the cardiac equivalent of a resting membrane potential?

Answer 9

Pacemaker potential

Question 10

Describe what is happening when the action potential fires

Answer 10

Inward movement of Ca+ depolarises the cell
Decrease of conductance of K+ continues
Cell repolarised by outward movement of K+

Question 11

If the intrinsic rate of the SA node is 100 beats per minute, why is resting heart rate closer to 70 beats per minutes?

Answer 11

Vagal tone

Parasympathetic vagal nerve innervates the atria

Question 12

Where do sympathetic nerves that innervate the heart exit the spinal cord?

Answer 12

T1-L2

Question 13

How is heart rate controlled?

Answer 13

Autonomic nervous system

By altering pacemaker potential

Question 14

How does increased parasympathetic vagal activity lower the heart rate?

Answer 14

Hyperpolarising cells
Opens K+ channels
Reduces gradient of pacemaker potential
Takes longer to reach threshold

Question 15

What does negative chronotropic mean?

Answer 15

Slows heart rate

Question 16

How is heart rate neuronally increased at SA node?

Answer 16

Sympathetic nervous system
Noradrenaline
Beta1 receptors
Increased Na+ and Ca+ conductance

Question 17

How is heart rate hormonally increased at the SA node?

Answer 17

Adrenaline acting on beta1 receptors

Increased Na+ and Ca+ conductance

Question 18

What does functional synctium mean?

Answer 18

All cells are structurally and electrically coupled together and so their behaviour reflects that of one muscle cell

Question 19

What allows current to flow between adjacent cardiac cells?

Answer 19

Gap junctions

Question 20

Why is the SA node the pacemaker as opposed to the AV node?

Answer 20

It is faster and so dictates pace

Question 21

How quickly does conduction move through the atria?

Answer 21

0.5m/s

Question 22

How long does it take an impulse to travel from the SA node to the AV node?

Answer 22

0.09s

Question 23

What is the pacemaker rate of the AV node?

Answer 23

40 beats per minute

Question 24

What is the conduction rate at the AV node?

Answer 24

0.05m/s

Question 25

What does AV delay ensure?

Answer 25

Atrial depolarisation, contraction and ejection before the ventricles depolarise

Question 26

Where does the current flow after the AV node?

Answer 26

Through the bundle of His and the the Purkinje fibres

Question 27

At what speed does current flow through the bundle of His?

Answer 27

2m/s

Question 28

At what speed does current flow through the Purkinje fibres?

Answer 28

4m/s

Question 29

Is AV delay constant?

Answer 29

No it changes in response to heart rate, e.g. with exercise

Question 30

What is an ectopic beat?

Answer 30

Random ventricular contraction with no stimulation

Question 31

In what order does the ventricle depolarise?

Answer 31

From the endocardium to the epicardium,

Question 32

What is the resting potential of a ventricular myocyte?

Answer 32

-80 to -90

Question 33

What are the phases of ventricular myocyte action potentials?

Answer 33

``` 0 rapid deoplarisation 1 partial repolarisation 2 plateau 3 repolarisation 4 resting potential ```

Question 34

What is responsible for 0?

Answer 34

Inward movement of Na+ | Through fast voltage gated channels

Question 35

What is responsible for 1?

Answer 35

Inactivation of Na+ channels

Question 36

What is responsible for 2?

Answer 36

Inward Ca+ movement and outward movement of K+ | Momentarily in equilibrium

Question 37

What is responsible for 3?

Answer 37

Outward movement of K+

Question 38

How long is the absolute refractory period of a ventricular myocyte?

Answer 38

200ms

Question 39

What is different between skeletal muscle and ventricular muscle twitch and the delay after an action potential?

Answer 39

Ventricular muscle contracts while the action potential is firing Skeletal muscle contracts after the action potential has occurred

Question 40

Is tetanic contraction possible in cardiac muscle?

Answer 40

No