cardiac action potentials and conduction pathway Flashcards
The heart is myogenic. What does this mean?
The heart generates electrical activity and contraction without external stimulation.
What are pacemaker potentials
action potentials generated by SAN
What do the firing rate of the action potentials of the SAN equate to?
the heart rate
What does the rhythm of the action potentials fired by the SAN equate to?
the heart rhythm
What are the 3 phases of pacemaker potentials?
Phase 4: resting membrane potential
Phase 0: upstroke, depolarisation
Phase 3: downstroke, repolarisation
How are I-f channels activated?
by hyperpolaristaion
as the cell gets more negative inside, the more I-f channels open
Hyperpolarisation-activated Na+ channels
When I-f channels are open, what do they allow in to the cell?
Na+
How is threshold reached (Phase 4)
The more I-f channels open More Na+ enters the cell Cell becomes more positive Cell starts to depolarise Cell soon reaches threshold and phase 0 will commence
What occurs during phase 0
Activation of Voltage-Gated Ca2+ Channels not Na+ channels.
Ca2+ influx
Producing depolarisation
Over time theses VGCCs start to switch off
Now we move on to phase 3
What occurs during phase 3?
Voltage-Gated K+ channels opens
There is K+ efflux (K+ moving out of cell)
Cell becomes more negative
This repolarisation
However as the cell becomes more negative, I-f channels start to activate (during hyperpolarisation)
What is meant by the SAN having an unstable/non-equilibrium resting membrane potential (RMP) and what causes this?
The RMP constantly wants to depolarise and reach a threshold for depolarisation (Phase 0), this is caused by expression of If channels
What allows for fast condutcion of electrical activity throughout the heart?
Intercalated discs between cardiac cells
What is the journey of electrical activity throughout the heart?
Initiation of a pacemaker potential at the SAN ->
Conduction across both atria to atrial-ventricle node (AVN) ->
AVN slows down electrical conduction to allow time for ventricles to fill ->
Action potential sent to the Bundle of His -> Action potential sent to purkinje fibres (between Bundle of His and Purkinje fibres there is a fast conduction pathway which allows both ventricles to be stimulated and contracted together)
What does the P wave represent on an ECG?
Artrial depolarisation
What does the QRS complex represent on an ECG?
Ventricle depolarisation
What does the ST interval represent on an ECG?
Interval between ventricle depolarisation and repolarisation
What does the T wave on an ECG represent?
ventricle repolarisation
What information does an ECG provide?
An ECG provides information on electrical conduction through the heart and does not provide information on contraction
Where does ventricular excitation and contraction begin and spread to?
begins at apex and spreads to base
What is the order of atrial/ventricular action potential phases?
Phase 4 (Stable Resting Potential) -> Phase 0 (Depolarisation) -> Phase 1 (Brief Repolarisation) -> Phase 2 (Plateau Phase) -> Phase 3 (Repolarisation)
Why is the Plateau phase of the Atrial/Ventricular action potential important? x3
- The length of the plateau phase means that the ventricular cells are refractory, i.e. they cannot fire any more Action Potentials. 2. Therefore, there is no twitching as one action potential produces one contraction.
- Therefore, there is proper ejection of blood from the heart
Describe electrical-contraction coupling through the heart
- electrical activity generated in SA node (non-contractile tissue) spreads out into the atria to produce contraction → blood is going to be ejected from the atria to the ventricles
- electrical activity then enters the AVN which delays conduction before stimulating ventricles. this allows the ventricles to fill properly during atrial contraction
- excitation conducts rapidly through bundle of His into purkinje fibres
- excitation through purkinje fibres is conducted throughout the ventricular contractile tissue producing contraction
- ventricular excitation and contraction begins at the apex and spreads up to base of the heart.
What happens during phase 0 of the atrial/ventricular action potential?
voltage gated Na+ channels open
Na+ influx
