Electrical activity of the heart Flashcards
What forms electrical synapses?
What does this allow?How?
Desmosomes (cell-cell connections)
Allowing:
- FAST spread of electrical activity from one cell to the next
- Coordinated contraction of the atria and the ventricles
As there are NO GAPS between the cells
Describe the pathway of electrical conduction through the heart
1) Initiation at the SINO-ATRIAL NODE
2) Conduction to the atria and atrioventricular node
3) Passes through the ATRIOVENTRICULAR RING
4) Passage through the bundle of His in the AV ring
5) Purkinje system distribution at the apex of the heart to the ventricular muscle cells
What is the main pacemaker of the heart?
The SINO-ATRIAL node
When can the AV node become the dominant pacemaker of the heart?
When there is a problem with the SA node
What is the atrioventricular ring of the heart?
The STRUCTURAL part
Where MYOCYTES are attached to
Where is the SA node found?
In the POSTERIOR aspect of the heart
At the junction between the superior vena cava and right atrium
Where does the electrical activity from the SA node travel to?
To the:
- RIGHT and LEFT atrium (left atrial through the Brachmann’s bundle)
- Atrioventricular node (via the atrial myocytes)
What is the speed of conduction of the atrial myocardium?
1.00m/s
How does the electrical conduction travel from the sinoatrial node to the left atrium?
What does the passage of this electrical activity through here cause?
Brachmann’s bundle - the INTERATRIAL tract from the right atrium to left atrium
Causes contraction of both of the atria
Where is the atrioventricular node found?
In the posterior part of the heart
RIGHT side interatrial septum
What is the septum?
Fibrous structure which spilts the right and left sides of the heart
What is the conductance speed from the AN to the N?
0.005m/s (slower than the atrial myocardium)
What is the AV delay?
What is is required for?
Delay of impulses from the AV node
Required to:
- Allow the atrial contraction to finish
- Maximise expulsion of blood from the atria –> ventricles
What is AV refractiveness?
What is it required for?
A delay before electrical activity can pass through the AV node again
Required to:
- Prevent excess ventricular contraction
What is AV refractiveness?
What is it required for?
A delay before electrical activity can pass through the AV node again
Required to:
- Prevent excess ventricular contraction
When does the AV refractiveness increase?
Why?
At HIGH heart rate
To give more time for the atria to contract and expel blood into the ventricles
As a higher heart rate required higher blood flow in order to provide more oxygen
What is the contraction speed through the bundle of his?
Purkinje fibres?
Ventricular muscle?
What are the different conduction velocities important for?
1m/s
4m/s
1m/s
Different velocities are important for the normal timing of the contraction of the different parts of the heart
Maximise the spread of the electrical signal and the contraction of the heart/pumping of blood
Where do the Bundle of His and the Purknje fibres lie in the heart?
In the septum
How does the heart contract?
Why?
In a twisting/spiraling manner
To evoke a torsion and expel blood into the vessels
Why does spiral muscle contraction occur?
Due to the timing and way the ventricular muscles contract
What are the 2 types of cardiac action potentials?
Why are they different to?
1) Nodal
2) Contractile (myocyte)
Different to the action potentials seen in nerves
Which parts of the heart have ‘nodal’ cardiac action potentials?
- SA node
- AV node
Which parts of the heart have ‘contractile’ cardiac action potentials?
- Atrial muscle
- Purkinje fibres
- Ventricular muscle
What do the action potentials from the nodes do?
INITIATE contraction of the heart
What is the difference between ‘nodal’ and ‘contractile’ action potentials in the heart?
Nodal APs are AUTORHYTHMIC:
- Can fire APs without receiving any other signals
Contractile APs are NON-AUTORHYHMIC:
- Need signals from the pacemaker cells to produce an action potential
What can modify the nodal AP signals?
Inputs from the sympathetic/parasympathetic nervous system
What is the shape of the nodal AP?
Similar to normal AP:
- Slow increase
- Reach a peak
- Slow decrease
What is the shape of a contractile AP?
- Very SHARP increase
- Very TINY but SHARP decrease to a flat line
- Flat line for a little
- Gradual decrease
What is the resting membrane potential of pacemaker cells?
Not constant
When do pacemaker cells depolarise?
-60mV
depolarise slowly
What properties do the pacemaker cells show?
Automaticity
Rhythmicity
What can happen if the nodes are not functioning?
The Purkinje fibres can sometimes act as pacemakers
Switch from contractile to nodal properties
What is the dominant node?
The node that fires the APs more frequently
Should be the SA node
Describe the pacemaker pre-potential
Slow influx of Na+ - causes slow increase in membrane potential
What is the pacemaker pre-potential mediated by?
Low K+ efflux
High Na+ (cation) influx
What is the pacemaker pre-potential regulated by?
- Innervation
- Temperature
- Other pacemakers (in disease conditions)
What is the If current?
‘Funny current’:
- Inward current through a hyper-polarization induced CATION selective channel
When is the If current inhibited?
During REPOLARISATION
When is the If current activated?
At the END of REPOLARISATION
What is the threshold of the pacemaker action potential?
What happens at this threshold?
-40mV to -50mV
At the threshold - fast Ca2+ influx to mediate DEPOLARISATION
What causes the repolarisation of the membrane?
K+ EFFLUX
How are pacemaker cells controlled?
How is this done?
By fibres regulating the PREPOTENTIAL:
1) VAGAL fibres (ACh)
- PARAsympathetic NS
2) SYMPATHETIC fibres (NAdr)
Achieved by:
- Regulating the If current using cAMP
AND/OR
- Calcium clock oscillations
How do the vagal fibres regulate the prepotential of the pacemaker cells?
What effect does it have on the heart rate?
- Ach released
- HYPERPOLARISATION of the membrane (more -ve)
1) LONGER the prepotential slope
- SLOWER the heart rate
2) DECREASED prepotential slope (more -ve)
- Increase time to reach AP
How do the sympatheic fibres regulate the prepotential of the pacemaker cells?
What effect does it have on the heart rate?
INCREASE the prepotential slope
INCREASE the firing rate (reach AP faster)
INCREASE heart rate
How do vagal/sympathetic fibres regulate the If current?
Regulate cAMP:
- Ach (from vagal fibres) INHIBITS cAMP by DECREASING the activation of M2
- NAdr (from sympathetic fibres) ACTIVATES cAMP by INCREASING the activation of B1 and B2
What are calcium clock oscillations?
Rhythmic alterations of SR Ca2+ release
Describe the AP of cardiac muscle
1) CONSTANT resting membrane potential (unlike pacemaker cells)
2) MASSIVE INFLUX of Na+ into the cell
- Huge, sharp increase in membrane potential
- FAST activation of voltage gated Na+ channels
3) LONG PLATEAU phase
- Elongation of depolarisation
4) FAST DEPOLARISATION and OVERSHOOT
- K+ channels open
- Rapid outflow
What causes the elongation of depolarisation in the AP of cardiac muscle?
Entry of Ca2+ through SLOW calcium channels
What is the consequence of the elongation of depolarisation in the AP of cardiac muscle?
Unable to undergo SUMMATION
Refractory period
What is the difference between the ‘absolute’ and ‘relative’ refractory periods in the AP of cardiac muscle?
Absolute:
- CANNOT fire anymore AP
Relative:
- POSSIBLE to fire AP but must have a very large AP for this to occur
- RARE in the heart
Which channels are involved in the AP of cardiac myocytes?
Nav 1.5
Cav 1.2
HCN4
K+
What is the NAV 1.5 channel?
Na+ voltage gated channel in cardiac myoctyes
What inhibits the NAV 1.5 channel?
TTX and local anaesthetics
What is the Cav 1.2 channel?
Ca2+ VG channel
What is the HCN4 channels?
If channel (pacemaker current)
