2.01 - The Cardiac Action Potential Flashcards
What are the cardiac specialisation supporting action potential spread?
Intercalated Discs and the Impulse conduction system
What two structures form the intercalated disc? And describe them
Desmosomes: complexes of cell adhesion protein
Gap Junctions: made up from connexins. Form the electrical syncytium. Change in potential in one cell will spread throughout the myocardium
What are the components of the impulse conduction system?
Sinoatrial node (SA)
Atrioventricular Node (AV)
Bundle Of His
Purkinje Fibres
What is the resting membrane potential of a ventricular myocyte?
-90mV
What are the phases of a ventricular myocyte action potential? Draw it…
- Fast Depolarisation
- Early (partial) repolarisation (fast)
- Plateau
- Final repolarisation (slow)
- Restoration of RMP
Describe Phase 0
Threshold at -60 to -70mV
Activation of fast inward (INa) channels.
Suppression of Inward (IK1) channels
Overshoot determined by [Na+]
Describe Phase 1 of the ventricular myocyte action potential
Early (partial) depolarisation
Inactivation of fast inward sodium channels (INa)
Activation of outward K channels
Describe phase 2 of the ventricular myocytes action potential
Plateau (Ca2+ shoulder)
Activation of calcium channels –> influx of calcium
Describe phase 3 of the ventricular myocyte action potential
Inactivation of inward calcium channels
Activation of outward potassium channel known as delayed rectifier
Reactivation of inward sodium and calcium channel
Describe phase 4 of the cardiac myocyte action potential
Activation of fast inward potassium channel known as the inward rectifier
This determines resting membrane potential
What are and describe the refractory periods in ventricular myocytes?
Aboslute Refractory Period: very few sodium channle can be reactivated above Vm>-50mV
Absolute refractory period (ERP): only after this, myocytes nearby can be activated as current spread is too small for activation
Relative refractory period (RRP): Follow ARP, during this time no full AP can be generated
Describe the speed of action potential propagation in the heart
Depolarisation spreads passively through the myoctes but is sped up by gap junctions between cells (the larger the junciton the faster)
Which part of the conduction system of the heart is fastest?
Purkinje fibres. 4m/s
Compared to myocytes (1m/s), SAN (0.05m/s)
What are the phases of an action potential in sinoatrial and atrioventricular nodes?
Draw it
0: Depolarisation
3: Repolarisation
4: Variable resting membrane potential
Describe phase 0 of the nodal action potential
Mostly a calcium spike. Not much sodium involved
Activation of L-type Calcium channels
Describe phase 3 of a nodal action potential
Inactivation of L-type calcium channels
Activation of delayed rectifier K channel
Describe phase 4 of the nodal action potential
early deactivation of the delayed rectifier (turning off an outward is seen as a net inward current)
Followed by a set of opening of channels that generate an inward current - predominantly sodium
Describe the pacemaker currents in nodal membrane potentials
The pacemaker potential of a nodal cell is due to the decay of the resting membrane potential until a calcium spike
It arises from a number of channels. Particularly If (funny) channel. This channel results in a mixed cation current but largely an inward sodium current leading to gradual loss of resting membrane potential and therefore depolarisation
Coded for by HCN genes (HCN4 in humans)
Describe the hierarchy of the pacemaker cells
SA node: 60-100bpm. natural rate around 100bpm, but this is reduced due to parasympathetic activation
AV Node: 40-60bpm. Slower due to a smaller If current –> slower depolarisation –> decreasred heart rate. Will take over pacing if not paced in time by SA node. Decay rate not as steep as AVN –> longer to reach threshold –> rate drops
Purkinje cells: 20-40bpm. Even slower If than AV node plus the RMP is much more hyperpolarised (-90mV)
Cells with the highest AP rate set the heart rate as all other conduction system cells are depolarised by them and therefore rendered functionally inactive
Describe the spread of excitation in the heart and the timing of excitation in the various parts of the heart
Atria are excited within 80-90ms (right before the left
The first excitation of the septum seen after a 140ms delay (AV delay)
Ventricles then excitied within 50-60ms with the right slightly earlier than the left
Ventricles excite faster than atria because there are larger fibres generating larger currents –> faster depolarisation
List some pharmacological agents that are used to modify the cardiac action potential and the channels that they target
