3.5 Cardiac Electrophysiology & Cardiac Cycle Flashcards
What are the four components of the conduction system of the heart?
- Sinoatrial node
- Atrioventricular node
- Bundle of His (incl. left and right bundles)
- Purkinje Fibres
Are the cells that create action potentials in the heart nerve cells? If not, what are they?
No. They are modified muscle cells.
Where is the sinoatrial node located?
At the border of the superior vena cava and the right atrium
Where is the atrioventricular node located?
At the border between the right atrium and the right ventricle (this makes sense: atrioventricular)
Which of the two branches of the his bundle is larger? Why?
The left bundle branch is larger. It requires more muscles because the muscle has to work harder.
Where are purkinje fibres located?
The ends the firbes of the bundle of his
What type of cells do the purkinje fibres deliver excitation to?
Cardiomyocytes
Is the conduction system of the conduction system able to generate action potential without being stimulated? If so, what structure allows this?
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.
How many action potentials can the sinoatrial node generate per minute?
60-100 APs/min
Why doesn’t the AV node generate its own action action potentials?
- 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
How is information transported from the SA node to the AV node?
Via intra-atrial muscle bundles
What is the name given to the bundle of muscles that runs between the left and right atria?
The Bachman’s bundle
Where do the muscles of the bundle of his run through?
The inter-ventricular septum
Outline the process of depolarisation and hyperpolarisation of the cardiac conduction system
- 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
Why does polarisation have to occur from the apex of the heart upward?
- All blood vessels exit upward
- Blood must be squeezed from the bottom of the heart upward toward the top
Are the action potentials of SA node cells and cardiomyocytes of the atria and ventricles similar?
- NO. THEY ARE NOT.
- Cardiomyocytes need more calcium. This is an on-switch for the cardiomyofilaments that are necessary for muscle contraction
What part of our autonomic nervous system is most influencing the heart when we are at rest? What chemical is associated with this effect?
- Parasympathetic Nervous System
- Acetylcholine (Ach)
In terms of ions, how does parasympathetic stimulation slow down the heart beat?
It increases potassium currents, thus slowing depolarisation (more loss of positive, takes longer time to become positive)
How does sympathetic stimulation increase the heart rate (in terms of ions)?
Increases calcium and sodium ion currents, causing faster depolarisation (more positive, takes less time to become positive)
What are the four phases of diastole?
- Isovolumic relaxation
- Rapid inflow into ventricles
- Diastasis (reduced inflow into ventricles)
- Atrial contraction (tops up ventricles)
What are the three phases of systole?
- Isovolum ic contraction
- Rapid ventricular ejection
- Reduced ventricular ejection
Recall the equation for calculating cardiac output
Stroke volume x Heart rate
What is preload?
The end-diastolic pressure/volume. The amount of blood that has filled up the heart prior to contraction.
What is afterload?
The pressure in the aorta (or resistance in circulation)
How might high blood pressure influence afterload? Why could this be harmful?
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.
