3.5 Cardiac Electrophysiology & Cardiac Cycle

Question 1

What are the four components of the conduction system of the heart?

Answer 1

• Sinoatrial node
• Atrioventricular node
• Bundle of His (incl. left and right bundles)
• Purkinje Fibres

Question 2

Are the cells that create action potentials in the heart nerve cells? If not, what are they?

Answer 2

No. They are modified muscle cells.

Question 3

Where is the sinoatrial node located?

Answer 3

At the border of the superior vena cava and the right atrium

Question 4

Where is the atrioventricular node located?

Answer 4

At the border between the right atrium and the right ventricle (this makes sense: atrioventricular)

Question 5

Which of the two branches of the his bundle is larger? Why?

Answer 5

The left bundle branch is larger. It requires more muscles because the muscle has to work harder.

Question 6

Where are purkinje fibres located?

Answer 6

The ends the firbes of the bundle of his

Question 7

What type of cells do the purkinje fibres deliver excitation to?

Answer 7

Cardiomyocytes

Question 8

Is the conduction system of the conduction system able to generate action potential without being stimulated? If so, what structure allows this?

Answer 8

Yes, it can. If we cut off all autonomic nervous input to the heart, it would still beat. The main structure that is responsible for this is the sinoatrial node.

Question 9

How many action potentials can the sinoatrial node generate per minute?

Answer 9

60-100 APs/min

Question 10

Why doesn’t the AV node generate its own action action potentials?

Answer 10

• Because it is slower than the SA node
• If the SA node is damaged, ischemic, thrombose etc, then the AV node can take over, but it’s slower

Question 11

How is information transported from the SA node to the AV node?

Answer 11

Via intra-atrial muscle bundles

Question 12

What is the name given to the bundle of muscles that runs between the left and right atria?

Answer 12

The Bachman’s bundle

Question 13

Where do the muscles of the bundle of his run through?

Answer 13

The inter-ventricular septum

Question 14

Outline the process of depolarisation and hyperpolarisation of the cardiac conduction system

Answer 14

• Membrane is negative
• Sodium ions enter (depolarisation)
• Transient calcium channels open (depolarisation)
• L-type calcium channels open (depolarisation)
• Threshold is reached
• Hyperpolarisation in positive direction
• K+ is released from the cell, making it more negative
• Repeat

Question 15

Why does polarisation have to occur from the apex of the heart upward?

Answer 15

• All blood vessels exit upward
• Blood must be squeezed from the bottom of the heart upward toward the top

Question 16

Are the action potentials of SA node cells and cardiomyocytes of the atria and ventricles similar?

Answer 16

• NO. THEY ARE NOT.
• Cardiomyocytes need more calcium. This is an on-switch for the cardiomyofilaments that are necessary for muscle contraction

Question 17

What part of our autonomic nervous system is most influencing the heart when we are at rest? What chemical is associated with this effect?

Answer 17

• Parasympathetic Nervous System
• Acetylcholine (Ach)

Question 18

In terms of ions, how does parasympathetic stimulation slow down the heart beat?

Answer 18

It increases potassium currents, thus slowing depolarisation (more loss of positive, takes longer time to become positive)

Question 19

How does sympathetic stimulation increase the heart rate (in terms of ions)?

Answer 19

Increases calcium and sodium ion currents, causing faster depolarisation (more positive, takes less time to become positive)

Question 20

What are the four phases of diastole?

Answer 20

• Isovolumic relaxation
• Rapid inflow into ventricles
• Diastasis (reduced inflow into ventricles)
• Atrial contraction (tops up ventricles)

Question 21

What are the three phases of systole?

Answer 21

• Isovolum ic contraction
• Rapid ventricular ejection
• Reduced ventricular ejection

Question 22

Recall the equation for calculating cardiac output

Answer 22

Stroke volume x Heart rate

Question 23

What is preload?

Answer 23

The end-diastolic pressure/volume. The amount of blood that has filled up the heart prior to contraction.

Question 24

What is afterload?

Answer 24

The pressure in the aorta (or resistance in circulation)

Question 25

How might high blood pressure influence afterload? Why could this be harmful?

Answer 25

High blood pressure means that the arterial pressure is higher, meaning the left ventricle must reach a higher pressure before it can open the aortic valve. This means the heart must work harder with each beat.