Lecture 3 Flashcards
If a cell has a resting membrane potential, it means that it is what?
polarised
What is the resting membrane potential for a cell compared to the outside of the cell?
it is negatively charged
Does a cell have a high or low K+ permeability? What does this mean?
it has a high K+ permeability which means that K+ can easily flow out to keep the resting membrane potential negative
Does a cell have a high or low Na+ permeability? What does this mean?
it has a low Na+ permeability which means that it can’t enter easily to keep the resting membrane potential negative
Does a cell have a high or low Ca2+ permeability? What does this mean?
it has a low Ca2+ permeability which means that it can’t enter easily to keep the resting membrane potential negative
If a depolarisation reaches threshold, what does this cause?
an action potential
What is a conduction?
a depolarisation wave
At resting membrane potential, the inside of the cell is negatively charged compared to the outside of the cell. What happens when the cell depolarise?
this changes so the inside of the cell is positively charged compared to the outside of the cell
Where does the conduction system start in the heart?
at the pacemaker cells in the sinoatrial node
Where is the sinoatrial node located?
it is located at the top of the right atria (atria on the left of the diagram)
From the sinoatrial node, the depolarisation propagates down the atria to where?
the atrioventricular node
What is the purpose of the AV node?
to conduct the siganl from the atria to the ventricles
Where is the AV node located?
in the right atria, up against the boundary of the atria and the ventricles
At the border between the atria and the ventricles, you have the fibrocartilaginous tissue (valves). What do these act as and what does this mean?
these act as an insulator which means that an electrical signal cannot pass through this structure and go from the atria to the ventricles without going through the AV node
Once the signal moves through the AV node, we go through a structure known as the what? Where is this located?
bundle of His
adjacent to the annulus of the tricuspid valve, distal to the atrioventricular node, and slightly proximal to the right bundle branch and left bundle branch
After going through the bundle of His, where does the signal go?
It splits into the left and right bundle branches and travels down the septum to the heart to the apex
At the apex of the heart, where does the signal go?
It wraps up around the heart through the purkinje fibres and then into individual cardiomyocytes
How is the structure of the cardiomyocyte specialised for the conduction of action potentials?
Each cell is interwoven and branch at either end. There are intercalated discs with gap junctions to allow rapid ion flow between cels so that all the cells can interact and contract together in a functional syncytium
At what rate per minute is the SA node generating action potentials?
100 APs per minute
At what speed are action potentials conducted through the atrium?
0.5 ms-1
At what speed are action potentials conducted through the AV node?
0.05 ms-1
Why are action potentials conducted more slowly through the AV node than the SA node?
This delay allows full depolarisation and contraction of the atria before depolarisation and contraction of the ventricles
Why does the heart contract from the apex, upwards?
in order to push the blood upwards to the arteries to leave the heart
At what speed does the action potential spread through the ventricular myocardium?
0.5 ms-1
At what speed does the action potential spread through the bundle of His, bundle branches, Purkinje fibres?
5 ms-1
Why is the spread of action potential through the ventricular myocardium slower than in the bundle of His, bundle branches, Purkinje fibres?
because it allows synchronous depolarisation and contraction of all ventricular regions
The speed of conduction of the action potential spreads slowest in the what?
AV node
What are the two types of cells that conduct action potentials in the heart? Where are these located generally?
pacemaker cells (in the SA node)
follower cells (such as the cardiomyocytes in the apex of the ventricles, although it is the same diagram for the atria)
How many phases are there of the pacemaker cells in the SA node?
3
What are the names of the 3 phases in the pacemaker cells in the SA node?
4, 0, 3
Of the three phases of the pacemaker cells, what one reflects the “resting” membrane potential? Why is this in inverted commas?
This is phase 4
This is the pre potential phase 4 and the “resting” membrane potential fluctuates between -60mV and -70mV (so it is not really resting which is why is in inverted commas)
Why is phase 4 o a pacemaker cell not really resting?
It is unstable because of funny Na+ channels.
What is the effect of funny Na+ channels in the pacemaker cells?
they cause a slow influx of Na+ which means the MP increases which causes the unstable RMP
What channels open at the end of phase 4 in the pacemaker cells? What does this cause and what is the effect of it?
there are late phase T-type Ca2+ channels that open which cause an influx of Ca2+ ions to raise the membrane potential to a threshold at -50mV/-40mV
What is the phase after phase 4 in the pacemaker cells?
Phase 0
What happens during phase 0 in the pacemaker cells?
Due to the threshold reached from the influx of Ca2+ ions in phase 4, L-type/voltage operated Ca2+ channels open which stay open for a long time which causes a huge influx of Ca2+ and so there is a huge depolarisation to above 0mV
What is the phase after phase 0 in the pacemaker cells?
3
What happens during phase 3 in the pacemaker cells?
The Ca2+ channels are inactivated when the cell is depolarised but the funny Na+ cells are still open a bit so the MP stops increasing. K+ leaves the cell as they are very permeable so the MP decreases and repolarisation occurs. Na channels are reactivated and the process starts again
How many phases are there of the follower cells in the ventricle?
5
What are the names of the 5 phases in the follower cells in the ventricle?
0, 1, 2, 3, 4
Of the 5 phases of the ventricular cells, what one reflects the resting membrane potential? What is the resting membrane potential and what is causing this?
This is stage 4
this is a stable resting membrane potential of -90mV and it is kept low because K+ keeps leaving and there is no Na+ of Ca2+ leaking into the cell
An action potential comes from the pacemaker cells through the _________ __________ and reaches the ________ cell. This allows ______ to enter the cell
gap junctions
ventricular
Na+
What happens when Na+ enters the ventricular cells?
the cell is going to get a little bit more positive because of the ions flowing through the gap junctions
Once the ventricular cell reaches _______ mV, we reach a threshold. What happens here? What phase is this?
-65mV
there is a rapid depolarisation which is phase 0
What causes the rapid depolarisation during phase 0 in the ventricular cells?
The rapid opening of the Na+ channels
What happens during phase 1 in the ventricular cell?
This is when the Na+ channels inactivate because the cell is now depolarised. This means that there is no more positive charge entering the cell but as K+ is still leaving the cell, the cell starts to repolarise
What happens when the cell membrane potential begins to decrease after rapid depolarisation in the ventricular cells? What phase is this?
Due to the small drop in resting membrane potential (see slide 46), the call is now at a resting membrane potential which activates the L-type or voltage-gated Ca2+ channels. This causes a Ca2+ entry into the cell to balance the K+ efflux. There is no net charge movement.
This is phase 2
What happens after phase 2 in the ventricular cells?
this is phase 3
there is inactivation of the calcium channels so no more positive charge is entering the cell but K+ is still leaving so the cell repolarises back to RMP
What channels allow slow Na+ influx in the pacemaker cells?
funny Na+ channels
What channels cause an influx of calcium in the pacemaker cells?
Late phase T-type Ca2+ channels (TTCC)
What channels cause a fast depolarisation in the pacemaker cells due to the entry of Ca2+ ions?
L-type or voltage operated Ca2+ channels (LTCC)
What causes a fast depolarisation in the follower cells?
Na+ influx
What causes a depolarisation plateau in the follower cells?
L-type or voltage operated Ca2+ channels (LTCC) which cause a Ca2+ influx
What does different action potentials in conduction system depend on?
function and location
In ventricular cells, slow depolarisation occurs through funny Na+ channels BECAUSE the resting membrane potential of ventricular cells is stable at around -90mV
false, true
There can be different action potentials at different __________ in the heart at different _________
locations
times
Define an ECG
This is a recording of potential changes t the skin surface that result from depolarisation and repolarisation of heart muscle
How does the recording on the ECG differ from the action potentials of different cells?
Why is this?
The amplitude of the recording of an individual cell is about 100mV but by the time it gets to your body it is more like 1mV because the action potential has had to propagate from the heart through all the other tissues in the body. If there is an electrode on the wrist or foot, the action potential has had to propagate all the way down there and some of it will be lost
Whereabouts do we put the electrodes on the body to record an ECG?
On the left arm, right arm and left leg
What do the three electrodes forming an ECG allow us to measure?
the size, and direction of action potential propagation
Each pair of electrodes is called a what?
lead
One end of the electrode pair is going to be _________ and the other end is going to be _________ . This means the lead is _______
positive
negative
bipolar
Where does lead | go?
It shows the potential difference between the left and right arm: the left arm has the positive electrode and the right arm has the negative electrode
Where does lead || go?
It shows the potential difference between the left leg and the right arm: the left leg is positive and the right arm is negative
Where does lead ||| go?
It shows the potential difference between the left leg and the left arm: the left leg is positive and the left arm is negative
Describe how limb lead || works and how it shows things on the ECG trace
Imagine you have a camera on your left hip/left thigh looking up diagonally through your chest towards your right arm. You are looking at the view down lead ||. Anytime there is a depolarisation towards the camera (from right arm down towards left leg), it shows as a positive deflection on the ECG and anytime there is a depolarisation away from the camera, is shows a negative deflection on the trace. Anytime there is a repolarisation away from the camera, there was a positive deflection on the trace and anytime there was a repolarisation towards the camera, there was a negative deflection on the trace.
Which lead shows the typical ECG trace?
lead ||
Depolarisation towards camera =
positive deflection on trace
Depolarisation away from camera =
negative deflection on trace
Repolarisation away from trace =
positive deflection on trace
Repolarisation towards trace =
negative deflection on trace
What happens to cause the P wave on the lead || ECG?
The SA node is depolarising and there is an action potential and depolarisation through the atria. This is pointing towards the camera. This gives us the positive deflection of the P wave
What happens to cause the QR wave on the lead || ECG?
We are moving through the AV node, going down the bundle of His and left and right bundle branches. The action potential isn’t moving towards or away from the camera. It is therefore hard to see on the ECG, there is no large up or downward deflection so there is only a small drop on the trace
What happens to cause the R wave on the lead || ECG?
There is ventricular depolarisation towards the camera so very large positive deflection on the ECG
What happens to cause the S wave on the lead || ECG?
The depolarisation wraps up around the heart so we have a depolarisation moving away from the camera so there is a small negative deflection on the ECG
What happens to cause the T wave on the lead || ECG?
The ventricles now repolarise away from the camera and so there is a positive deflection on the ECG
What are augmented limb leads?
We can create another three viewpoints without having to stick anymore electrodes on the body. They are created by keeping a single positive point but then combining two negative electrodes.
Is atrial repolarisation shown on an ECG?
this occurs at about the same time as the ventricles depolarise and so the atrial repolarisation is hidden by the depolarisation
What are the names of the three augmented leads?
augmented lead right (aVR)
augmented lead left (aVL)
augmented lead front (aVF)
Depending on which lead you look at (augmented or not), the ECG will look really __________
different
Which augmented lead is the mirror image to a lead || ECG?
aVR
What can we use these ECG measurements for?
to find the exact placement of our heart
As well as non-augmented and augmented leads, there are 6 other electrodes you can stick on your chest. What are they called?
V1 V2 V3 V4 V5 V6
