Cardiac Action Potential Flashcards
What do the pacemaker cells do
Set the heart rate generated in SA node
How does the heart generate the initial impulse if the SAN stops working
The AVN takes over this is known as the ectopic pacemaker
Why is there a bottleneck in depolarising the AV node and bundle of his
This slight delay allows the ventricles to fill up
What happens in the first stage (funny current) of an action potential in cardiac pacemaker cells
Starts at stage 4 where the pacemaker cell is at -60mv, HCN (type of Na/K Channel) channels are open which allow slow influx of Na+ which will slowly depolarise until -40mv is reached, the current produced by influx of Na+ is known as funny current
Unlike other myocytes and neurones this baseline is higher to help with the automaticity of pacemaker depolarisation
What happens in the second stage (depolarisation) of an action potential in cardiac pacemaker cells
This is also known as stage 0, When the membrane potential reaches -40mv rapid depolarisation occurs as HCN channels close and voltage gated calcium channels open, the rapid influx of Ca2+ causes fast depolarisation to +20mv at which the close
What happens in the third stage (repolarisation) of an action potential in cardiac pacemaker cells
When the membrane potential reaches +20mv the calcium channels close and the voltage gated K+ channels open which enables an eflux of K+ out of the cell depolarising to -40mv where the HCN channels re-open
What are the 3 main mechanisms how the parasympathetic nervous system decreases heart rate
- Lengthens funny current so takes longer for the pacemaker cell to reach threshold potential via the inhibition of HCN channels. Ivabradine is a drug which is used to treat heart failure by blocking HCN channel so the heart rate can slow allowing more time for ventricular filling
- Increasing the threshold potential for activation of voltage gated calcium channels. This is achieved through inhibition of the voltage gated calcium channels. Verapamil is an anti-arrhythmic drug that blocks the actions of voltage gated calcium channels. In doing so it reduces the influx of calcium into the cell, which also has the effect of reducing the contractile force of the heart
- Thirdly by decreasing the maximum membrane potential that the cell can achieve, thus taking longer to reach the repolarisation potential. This is achieved through increased expression of potassium channels, which leads to a more hyperpolarised membrane potential
How does the sympathetic nervous system increase heart rate
The speed of the funny current is increased so threshold potential is reached faster
The other mechanism is by increasing calcium conduction through voltage gated calcium channels. This increased calcium conductance has the added effect of increasing the degree of calcium induced calcium release (CICR) from sarcoplasmic reticulum, thus increasing intracellular calcium levels to a greater extent, leading to more forceful contraction of the myocardial sarcomeres
How do action potentials travel from pacemaker cells to cardiomyocytes
During the depolarisation phase of the action potential, calcium enters into the pacemaker cell.
Pacemaker cells and cardiomyocytes are linked via gap junctions. When calcium enters the cell, some of this can diffuse through the gap junctions into neighbouring cardiomyocytes, leading to localised depolarisation of that membrane
The subsequent depolarisation leads to the opening of voltage gated sodium channels on the membranes of cardiomyocytes, leading to rapid depolarisation
Note that cardiomyocytes are also linked via gap junctions to each other, so depolarisation of one cardiomyocyte leads to the depolarisation of the neighbouring cardiomyocyte in the same way, through the diffusion of sodium
What is a synctium
The cardiomyocytes that make up the walls of the heart form a synctium
Why are cardiomyocytes said to be anisotropic (a substance which has a different property in one direction compared with another)
Because of the presence of gap junctions in their long endings, the conduction velocity along the long axis is much greater than it is along the short axis
How do the atrial and ventricular synctium ensure the atria contract just before the ventricles
There is an atrial syncytium and a ventricular syncytium. The atria and ventricles being separated by a fibrous tissue through which electrical conductance cannot pass. The electrical signal is instead passed through the tight penetrating section of the atrioventricular bundle, allowing the atria to contract just before the ventricles.
What are the stages of cardiomyocyte action potential
What occurs during depolarisation of a cardiomyocyte
Ca2+ travelling through the gap junctions from neighbouring cells causes the membrane potential to increase which opens the voltage gated sodium channels, this leads to an influx of Na+ which depolarises the membrane.
What occurs during the initial repolarisation of a cardiomyocyte
Once the depolarisation reaches around +20mV, voltage gated sodium channels close to an inactivated state. During this state, no amount of stimulation will open the channel. This state persists until the membrane potential is repolarised back to -80mV. This state of inactivation is known as the absolute refractory period, during which another action potential cannot be generated. This refractory period prevents cardiac muscle from undergoing tetany (involuntary contractions), which would prove fatal.
The K+ channels open which leak K+ leading to repolarisation
What occurs during the plateau phase of the cardiomyocyte
K+ delayed rectifier channels are open, K+ fast channels close and L- type Ca2+ channels are open (this is where contraction occurs) so the efflux of K+ and influx of Ca2+ creates a plateau
What occurs during the repolarisation phase of the cardiomyocyte
The L- type Ca2+ channels close and the and slow K+ channels open leading to the efflux of K+ ions so the membrane repolarises back to the baseline so the voltage gated Na+ channels become sensitive again
What occurs during the resting membrane potential of cardiomyocytes
The Na+/K+ ATPase exchanges 2 K+ ions in and 3 Na+ ions out which goes against the concentration gradient to reach a membrane potential to -80mv
If the concentration of Ca2+ is too high following an action potential the Na+/Ca2+ exchanger swaps extracellular Na+ for intracellular Ca2+
What is the absolute and relative refractory periods in cardiomyocytes
The absolute refractory period is the period between depolarisation and the end of the plateau and lasts around 180ms. During this time, voltage gated sodium channels are in the inactive state and no amount of stimulation can trigger an action potential
What follows is called the relative refractory period. As the membrane repolarises, voltage gated sodium channels slowly begin to change configuration to the voltage-sensitive state. During this period, an action potential can be generated, but because the number of sodium channels in the voltage-sensitive state is relatively low, the amount of stimulation required to trigger an action potential during this phase is much greater
How does calcium entry during the plateau phase initiate contraction in cardiomyocytes
Calcium induced calcium release
Calcium entry into the cell from the extracellular space occurs at T-tubules and activates ryanodine receptor Ca2+ release channels, which causes the release of large amounts of calcium from sarcoplasmic reticulum
This calcium then interacts with actin and myosin filaments to cause contraction
Calcium is then removed from the intracellular space through both sarcoplasmic reticulum Ca2+-ATPase channels and through the sodium-calcium exchanger in the cell membrane
What do waves on the ecg show
The P-wave represents depolarisation of cardiomyocytes within the atria
The flat line represented by the PR-segment represents atrial contraction. Remember that during this point, the cardiac action potential has plateaued – there is very little net charge difference across the membrane and so this does not show up as a change in the electrical conductance on an ECG
The R wave represents ventricular depolarisation. The relative thickness of the ventricles (particularly the left ventricle) with respect to the atria make ventricular depolarisation appear as a greater peak on the ECG
The ST interval represents ventricular contraction – again appearing as a flat line because there is little net charge difference across the membrane as calcium enters the cell
