Lecture 9 - The excitable heart Flashcards
Electrical cells of the heart
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
Contractile cells of the heart
Makes up 99% of cells in the heart
Striated appearance (associated with the high amount of actin and myosin)
Performs the contractile function
Actin and myosin in electrical and contractile cells
Electrical cells have less actin/myosin filaments than contractile cells
Percentage of electrical cells and percentage of contractile cells
1% is electrical, 99% are contractile cells
Sinoatrial node (SAN)
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
Action potential propagate in conduction/electrical cells and contractile cells
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
Intercalated disks and gap junctions ___________
Connect cardiac cells
Pass the electrical signal through ICDs, ICDs connect most cells of the heart
Gap junctions in cardiac cells
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
Gap junctions and their role in spreading the impulse
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
The conduction pathway of the heart
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
What is the order of the conduction pathway?
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
Where does the conduction pathway start and where does the conduction pathway end?
SA node is the start
Ends with the Purkinje fibres
Why is the SA node known as the ‘pacemaker’?
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.
What is the function of the AV node? Why does it pause the conduction signal?
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.
Why does the conduction signal travel down to the bottom of the heart and then back up the ventricular walls?
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.
If the left atrium is contracting, what else is happening?
The right atrium is also contracting
The two pumps ______
work together as one
Excitation and the conduction pathway
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
Electrocardiodiagram (ECG)
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
What are depolarisation and repolarisation? How do they relate to contraction and relaxation?
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
Conduction pathway in terms of ECG features
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
What electrical events are generating the P wave? QRS complex? And the T wave?
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
What mechanical events (contraction/relaxation) are occurring during each part of the ECG?
P wave - atria contraction (atria depolarisation)
QRS complex - ventricle contraction (ventricle depolarisation)
T wave - ventricular relaxation (ventricle repolarisation)
Lubb sound comes from
The sound of the AV valves shutting to prevent the flow of blood back into the atria
Dupp sound comes from
The sound of the semilunar valves shutting
Putting it all together - cardiac cycle with the mechanical and electrical events put together
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
P wave is produced by
Atrial depolarisation
QRS complex is produced by
Ventricular depolarisation
atrial repolarisation occurs too
T wave is produced by
Ventricular repolarisation
R-R interval
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
Lubb description
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
Dupp description
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
What are the lubb-dupp sounds a result of?
Result of vibrations generated in the walls of the ventricles and major arteries by valve closure
Sternal angle
Useful landmark which indicates the level at which the second rib articulates anteriorly.
Important vertical landmarks
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
R-R interval and heart rate
R-R interval decreases as the heart rate increases
Sounds, QRS complex and T wave
QRS complex always proceeds the first sound
T wave always before second sound
Why is it important to maintain relatively constant MAP?
TO maintain adequate perfusion to vital organs
