Origin and Conductance of Cardiac Impulse

Question 1

What is autorythmicity?

Answer 1

When the heart is capable of beating rhythmically in the absence of external stimuli.

Question 2

Where does excitation of the heart normally originate?

Answer 2

In the pacemaker cells in the sino atrial node (SA node)

Question 3

What are pacemaker cells?

Answer 3

Cluster of specialised cells that initiate the heart beat

Question 4

Where is the SA node locates?

Answer 4

in the upper RIGHT ATRIUM close to where the Superior Vena Cava enters the right atrium

Question 5

What does the SA node do?

Answer 5

Drives the pace for the entire heart

Question 6

What is sinus rhythm?

Answer 6

A heart controlled by the sino-atrial node ie normal rhythm

Question 7

Do pacemaker cells have a stable resting potential?

Answer 7

No, they exhibit spontaneous pacemaker potential

Question 8

What is the function of the spontaneous pacemaker potential?

Answer 8

It allows the membrane potential to reach a threshold to generate an action potential in the SA nodal cells.

Question 9

What is the role of potassium in the spontaneous membrane potential?

Answer 9

Potassium leaving th cell will hyperpolarise the cell. Potassium remaining in the cell will depolarise the cell.

Question 10

What other molecule affects pacemaker potential?

Answer 10

transient Ca++ influx

Question 11

What happens when the threshold is reached?

Answer 11

• Once threshold is reached action potential will be fired
• Activation of long lasting calcium channels
• Calcium influx to pacemaker cells
• Causing depolarisation

Question 12

How does repolarisation occur?

Answer 12

• Activation of potassium channels making calcium leave the cell causing repolarization.
• Closing of calcium channels occurs when potassium channels open.

Question 13

What is the bundle of His and what is its function?

Answer 13

Specific conductance pathway across the septum. It divides into two pathways to get to each ventricle.

Question 14

What are the Purkinje fibres?

Answer 14

End point of the electrical signal that cause contraction of the ventricles

Question 15

what is cell to cell conductance?

Answer 15

Action potentials can spread from one cell to another via gap junctions located in intercalated disks which contain a specialised function.

Question 16

What is the Av node?

Answer 16

• small bundle of specialised cardiac cells

- They are small in diameter and have slow conduction velocity

Question 17

Where is the AV node located?

Answer 17

at the base of the right atrium; just above the junction of atria and ventricles

Question 18

The AV node is the ONLY point of electrical contact between atria and ventricles
True or false

Answer 18

True

Question 19

How do action potentials spread?

Answer 19

• cell to cell conductance across the atria

- cell to cell conduction from SA node to AV node however there are also some internodal pathways

Question 20

What is the main function of the AV node?

Answer 20

Delay the conductance of the action potential. Delay is important as atrium needs to have time to contract before the ventricles contract.

Question 21

The action potential in contractile cardiac muscle cells DIFFERS considerably from the action potential in PACEMAKER cells
true/false?

Answer 21

True

Question 22

Describe the phases of ventricular muscle action potential

Answer 22

Phase 0- fast Na+ influx
Phase 1- closure of Na+ channels and transient K+ efflux
Phase 2- Mainly Ca++ influx
Phase 3- closure of Ca++ channels and K+ efflux (repolarisation stage)
Phase 4- resting membrane potentials (-90)

Question 23

What is the Plateau phase of action potential?

Answer 23

• The action potential in contractile cardiac muscle cells DIFFERS considerably from the action potential in PACEMAKER cells
• unique to contractile cardiac muscle cells
• mainly due to influx of Ca++ through L type Ca++ channels

Question 24

What mainly influences the heart rate?

Answer 24

Autonomic nervous system

Question 25

What supplies the parasympathetic signals to the heart? And what does it supply?

Answer 25

Vagus nerve, the AV AND SA nodes

Question 26

How does the parasympathetic affect the heart rate?

Answer 26

• exerts a continuous slowing effect on the SA node by slowing the spontaneous firing of the node
• ie produces a normal resting heart rate
• increases AV nodal delay

Question 27

What is the neurotransmitter involved in parasympathetic supply of the heart?

Answer 27

Acetyl choline acting through muscarinic M2 receptors

Question 28

Give an example of a drug that can reverse the effects of the parasympathetic supply of the heart

Answer 28

ATROPINE is a competitive inhibitor of acetylcholine - used in extreme bradycardia to speed-up the heart

Question 29

what are the effects of vagal stimulation on pacemaker potentials?

Answer 29

• time to reach the threshold increases
• slope of pacemaker potential decreases
• negative chronotropic effect?

Question 30

What is negative chronotropic effect?

Answer 30

Something that slows the heart rate

Question 31

What do the cardiac sympathetic nerves supply?

Answer 31

SA node, AV node and myocardium

Question 32

What are the effects of the sympathetic nerves on the heart?

Answer 32

• increases heart rate
• decreases AV nodal delay
• increases the force of contraction

Question 33

What is the neurotransmitter involved in sympathetic supply of the heart?

Answer 33

noradrenaline acting through B1 adrenoceptors

Question 34

what are the effects of sympathetic stimulation on pacemaker potentials?

Answer 34

• increases pacemaker potential slope
• causes pacemaker potential to reach threshold quicker
• positive chronotropic effect

Question 35

What is Positive chronotropic effect

Answer 35

-speeds up heart rate

Question 36

What is an ECG?

Answer 36

The ECG is a record of depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface