Origin and conduction of cardiac impulse

Question 1

Where does excitation of the heart originate from?

Answer 1

Pacemaker cells in the SA node

Question 2

What is autorhythmicity?

Answer 2

The heart can beat without an external stimuli

Question 3

What is sinus rhythm?

Answer 3

A heart driven by the SA node (normal)

Question 4

What potential do pacemaker cells exhibit?

Answer 4

Spotaneous Pacemaker potential

Question 5

What generates an action potential?

Answer 5

The spontaneous pacemaker potential takes the membrane potential to threshold which genereates the action potential

Question 6

What is pacemaker potential? and what is it due to?

Answer 6

1. slow depolarisation
2. Due to:
   decrease in K+ efflux
   Na+ and K+ influx
   transient Ca+ influx

Question 7

What occurs during the rising phase of action potential?

Answer 7

Ca++ influx - depolarisation

Question 8

What causes the falling phase of action potential?

Answer 8

1. inactivation of L type Ca++ channels (depolarisation)

2. activation of K+ channels resulting in K+ efflux

Question 9

Route taken for excitation to spread to ventricles?

Answer 9

SA node -> AV node -> Bundle of His -> Purkinje Fibres

Question 10

How does excitation spread through cell to cell conduction?

Answer 10

Gap junctions

Question 11

Why is conduction delayed in the AV node?

Answer 11

To allow atrial systole to preceed ventricular systole

Question 12

What is special about the AV node?

Answer 12

Only point of electrical contact between the atria and ventricles

Question 13

Ventricular Myocyte Action potential: Phase 0?

Answer 13

fast Na+ influx - depolarisation

Question 14

Ventricular Myocyte Action potential: Phase 1?

Answer 14

closure of Na+ channels and transient K+ efflux

Question 15

Ventricular Myocyte Action potential: Phase 2?

Answer 15

Ca+ through L-type Ca++ channels

Question 16

Ventricular Myocyte Action potential: Phase 3?

Answer 16

closure of Ca++ channels - repolarisation

Question 17

Ventricular Myocyte Action potential: Phase 4?

Answer 17

resting membrane potential

Question 18

What is normal resting heart rate?

Answer 18

60-100BPM

Question 19

What is a bradycardic heart rate?

Answer 19

under 60 BPM

Question 20

What is a tachycardic heart rate?

Answer 20

Over 100 BPM

Question 21

What does the vagus nerve do to the SA node under resting conditions?

Answer 21

Exerts a continuous parasympathetic influence. Vagal tone dominates under resting conditions and slows the intrinsic heart rate

Question 22

What is a negative chronotropic effect on the heart and what causes it?

Answer 22

1. slowing of the heart rate

2. stimulation of parasympathetic nerves

Question 23

What is a positive chronotropic effect on the heart and what causes it?

Answer 23

1. increase heart rate

2. stimulation of sympathetic nerves

Question 24

What is a sympathetic neurotransmitter and what is its effect on the pacemaker potential slope?

Answer 24

1. noradrenaline - acting through b1-adrenoreceptors

2. increase

Question 25

What is a parasympathetic neurotransmitter and what is its effect on the pacemaker potential slope?

Answer 25

1. acetyl-choline - acting through muscarinic M2 receptors

2. decrease