Lecture 9 - The excitable heart

Question 1

Electrical cells of the heart

Answer 1

These cells are conduction cells, their job is not to participate in contraction but instead they are specialised to perform their job of moving electrical signal quickly throughout the heart and spread the impulse as quickly as possible

Almost no actin and myosin since their job doesn’t involve contraction, the actin and myosin in contractile cells slows down the electrical impulse

‘pale’ striated appearance, low actin and myosin

Includes - purkinje cells, AV nodal cells

Make up 1% of cells in the heart

Question 2

Contractile cells of the heart

Answer 2

Makes up 99% of cells in the heart

Striated appearance (associated with the high amount of actin and myosin)

Performs the contractile function

Question 3

Actin and myosin in electrical and contractile cells

Answer 3

Electrical cells have less actin/myosin filaments than contractile cells

Question 4

Percentage of electrical cells and percentage of contractile cells

Answer 4

1% is electrical, 99% are contractile cells

Question 5

Sinoatrial node (SAN)

Answer 5

The sinoatrial node (SA node) is a specialized myocardial structure that initiates the electrical impulses to stimulate contraction, and is found in the atrial wall at the junction of superior caval vein and the right atrium

Question 6

Action potential propagate in conduction/electrical cells and contractile cells

Answer 6

Depolarisation states at the sinoatrial node and this signal spreads to neighbouring cells. In a contractile cell there is increased cytosolic Ca2+ level, cross bridge attachment and contraction

Question 7

Intercalated disks and gap junctions ___________

Answer 7

Connect cardiac cells

Pass the electrical signal through ICDs, ICDs connect most cells of the heart

Question 8

Gap junctions in cardiac cells

Answer 8

ICDs that connect cardiac cells contain gap junctions …
Pores with low resistance to ionic current - pores are resistance to various types of substances moving in between except they are very low resistance to ions and their ions are what make up our electrical signal, because the ICDs have these gap junctions they are able to move the electrical signal from cell to cell to cell through these pores
Allow current flow between adjacent cells

Question 9

Gap junctions and their role in spreading the impulse

Answer 9

Along the conduction pathway which is fast - lots of gap junctions between electrical cells, very little resistance to electrical signal moving through the cell through the gap junctions from one cell to another
Between electrical and contractile cells which is fast -there are gap junctions between electrical/condctions and contractile cells which allows the movement of signal from the electrical cells to the contractile cells to get them to contract
Between contractile cells which is slow - 99% of the cells in the heart are contractile cells therefore the electrical cells can’t directly communicate so you can pass this signal between contractile cells but it happens slowly

Increased speed of impulse throughout the heart due to the gap junctions
Millions of cardiac cells to behave as one - a functional syncytium

Question 10

The conduction pathway of the heart

Answer 10

Starts at the sinoatrial node which is also known as the pace maker. It is a bundle of cell that sits right on top of the right atrium. It is in charge of triggering the electrical events needed to make the heart beat. Cells in the SA node are highly specialised so that they let out a continuous and spontaneous release of electrical energy (don’t need brain input). The SA node sends the electrical impulse in 3 different directions - first directly into the right atrium, second it sends some of the electrical energy across the interatrial bundle and over into the left atrium which issuing to cause contraction which is why the two atrium contract, third path is sending the electrical impulses through the internodal bundles which lead down to the atrioventricular node and this AV node collects the electrical signal and then puts a pause on it/holds it for a little and it does this because without it the ventricles would start contracting at the same time as the atria and the blood would be going at every direction at once, so once the atria are done the signal gets passed on to the ventricles to contract. It does this through the AV bundle which splits into a right and left half to correspond with the ventricles this structure brings the signal down the septal wall of the heart down to the bottom of the heart and it makes its way back up the wall of the heart through the purkinje fibres, these fibres reach into the walls of the ventricle and they send the electric signal deep into the ventricular walls and ultimately triggers the contraction of the ventricles themselves

Question 11

What is the order of the conduction pathway?

Answer 11

SA (sinoatrial) node - internal bundles - Atrioventricular node (AV node) - AV bundle and bundle branches - Purkinje fibers

AV node pauses the signal and then sends it off

Question 12

Where does the conduction pathway start and where does the conduction pathway end?

Answer 12

SA node is the start

Ends with the Purkinje fibres

Question 13

Why is the SA node known as the ‘pacemaker’?

Answer 13

The sinoatrial (SA) node or sinus node is the heart’s natural pacemaker. It’s a small mass of specialized cells in the top of the right atrium (upper chamber of the heart). It produces the electrical impulses that cause your heart to beat.

Question 14

What is the function of the AV node? Why does it pause the conduction signal?

Answer 14

The AV nodes is primarily an electrical gatekeeper between the atria and ventricles and introduces a delay between atrial and ventricular excitation, allowing for efficient ventricular filling.

Question 15

Why does the conduction signal travel down to the bottom of the heart and then back up the ventricular walls?

Answer 15

Bring the signal down to the bottom of the septal wall and then we bring it up the ventricular wall causing the walls of the ventricles to contract from the bottom up to the top, which will take the blood up to pulmonary artery and aorta - it is like pushing out toothpaste from the end of the tube, it is the most effective.

Question 16

If the left atrium is contracting, what else is happening?

Answer 16

The right atrium is also contracting

Question 17

The two pumps ______

Answer 17

work together as one

Question 18

Excitation and the conduction pathway

Answer 18

Quiescence (inactivity) ends when excitation spreads from the SA node (starts depolarising) - electrical impulse originates in the SA node

The atria are fully depolarised and contract

Atria repolarise (signal is leaving the atria and it is moving away and the atria is moving back to the way it was - relax) and relax, while AV node send excitation to ventricles (AV node sends the signal on down the AV bundles, down the septal wall of the heart and it is even starting to creep up the walls of the ventricle

Ventricles fully depolarised and contract - this is when we would see the isovolumetric contract phase and this is when ejection would start to happen

Ventricles begin to repolarise and relax

Ventricles fully repolarised and relaxed and the heart is back to quiescence - back and ready for the next cycle through the conduction pathway

Question 19

Electrocardiodiagram (ECG)

Answer 19

Measures the difference in electric fields from one wire to another - measures the change in electrical state of some chamber of the heart, any time there is a repolarisation or a depolarisation event it is seen on an ECG as it is an electrical change event

A single lead detects a difference between electrodes

Question 20

What are depolarisation and repolarisation? How do they relate to contraction and relaxation?

Answer 20

Depolarisation = electrical signals arriving at some point in the heart and causing it to contract  
Repolarisation = electrical signal is leaving an area of the heart, this area of the heart is going back to the baseline by relaxing

Question 21

Conduction pathway in terms of ECG features

Answer 21

1-Atrial depolarisation initiated by the SA node, causes the P wave
2- With atrial depolarisation complete the impulse is delayed at the AV node (little flat section just before QRS complex)
3- Ventricular depolarisation begins at apex, causing the QRS complex. Atrial repolarisation occurs
4- Ventricular depolarisation is complete, little flat section before T wave
5- Ventricular repolarisation begins at apex, causing the T wave
6 - Ventricular repolarisation is complete, little flat section after T wave

Question 22

What electrical events are generating the P wave? QRS complex? And the T wave?

Answer 22

During the P wave, atrial depolarisation occurs

QRS complex is causes by ventricular depolarisation (atrial repolarisation is occurring at the same time)

T wave is caused by ventricular repolarisation

Question 23

What mechanical events (contraction/relaxation) are occurring during each part of the ECG?

Answer 23

P wave - atria contraction (atria depolarisation)

QRS complex - ventricle contraction (ventricle depolarisation)

T wave - ventricular relaxation (ventricle repolarisation)

Question 24

Lubb sound comes from

Answer 24

The sound of the AV valves shutting to prevent the flow of blood back into the atria

Question 25

Dupp sound comes from

Answer 25

The sound of the semilunar valves shutting

Question 26

Putting it all together - cardiac cycle with the mechanical and electrical events put together

Answer 26

First event is that the SA node fires, electrical signals that comes out of the SA node and out into the right atrium, out across the interatrial bundles to the left atrium so both the atria are getting their electrical signal. Depolarisation of the two atria is caught here in the P wave. Directly after the P wave, there is a bump in the pressure of the atria (increase in pressure) and this is because we have our depolarisation through the atria and they start contracting and building that pressure and at the exact same time you see a bump in the ventricle pressure back you are packing some of that pressure down into the ventricles. There is also a slight bump in ventricular volume which shows the amount of blood that was in the atria that just got pushed into the ventricles. Now the atria repolarise and the ventricles start to depolarise which is associated with the QRS complex and as the ventricles start contracting, we also need the AV valves to close and so you get the ‘lubb’ sound. We have now moved into the isovolumetric ventricular contraction phase, this is the phase where the semilunar valves are closed and there is no where fore the blood to go so volume remains constant and then we start contracting the walls of the ventricles, so they are squeezing that blood which causes ventricular pressure to shoot straight up very fast and this phase is short because we build pressure very quickly and eventually the ventricular pressure exceeds the pressure in the aorta and when this happens we then move into the next phase and the semilunar valves open and now we are pushing blood out into the aorta, so at this point we have moved into the ejection phase. And we can see this through ventricular volume since the volume of the blood decreases significantly because we are pushing it out into the aorta and the pressure in the ventricles continues to rise for a while because they are still squeezing the blood trying to push as much as possible out and at the same time there is also a big spike in the pressure in the aorta because we just pushed a bunch of blood and pressure out of the ventricles and into the aorta itself, so there is a simultaneous continued increase in pressure of the ventricle and aorta. Finally, the ventricles start to run out to blood and run out of pressure and eventually the pressure in the ventricles reaches a plateau and then starts to fall and eventually it falls so far that it falls below the pressure in the aorta and this causes the semilunar valves to snap back shut so bow we are back in a situation where the ventricles are cut off fin everything else, can’t receive blood from atrium and can’t eject blood to the aorta so now we are in the isovolumetric ventricular relaxation phase so there is nothing left to do except to repolarise the ventricles associated with the T wave and this is now the opposite phase of our contraction and now we are seeing constant volume and pressure falling due to the relaxation of the walls. The pressure keeps on falling to a point and at this point the AV valves open up which. Ow allows for a path for blood to move from the veins to the atria and into the ventricles and the ventricles can now start refilling with blood (passive filling stage), start adding blood and adding blood until you reach a plateau and at this point you are ready to start the cycle again with another SA nose fire and getting the atria going again

Question 27

P wave is produced by

Answer 27

Atrial depolarisation

Question 28

QRS complex is produced by

Answer 28

Ventricular depolarisation

atrial repolarisation occurs too

Question 29

T wave is produced by

Answer 29

Ventricular repolarisation

Question 30

R-R interval

Answer 30

Is the time between the peaks of two consecutive QRS complexes and is the time between heart beats

Note - heart rate can be determined by dividing 60 seconds by the R-R interval

Question 31

Lubb description

Answer 31

The first heart sound is caused by the closure if the AV valves, signals the onset of systole. It is a soft, low pitched and relatively long sound

Question 32

Dupp description

Answer 32

The second heart sound caused by the closure of the aortic and pulmonary valves, signals the onset of diastole. It is a sharper, higher pitched and shorter sound

Question 33

What are the lubb-dupp sounds a result of?

Answer 33

Result of vibrations generated in the walls of the ventricles and major arteries by valve closure

Question 34

Sternal angle

Answer 34

Useful landmark which indicates the level at which the second rib articulates anteriorly.

Question 35

Important vertical landmarks

Answer 35

The midaxillary line running through the centre of the axilla (armpit) on the lateral surface of the thorax and the midclavicular line running vertically through the mid point of the clavicle

Question 36

R-R interval and heart rate

Answer 36

R-R interval decreases as the heart rate increases

Question 37

Sounds, QRS complex and T wave

Answer 37

QRS complex always proceeds the first sound

T wave always before second sound

Question 38

Why is it important to maintain relatively constant MAP?

Answer 38

TO maintain adequate perfusion to vital organs