Electrical Conductivity of Heart Flashcards
What is an action potential (excitation signal) created by in the heart?
The sinoatrial (SA) node
What is the SA node?
A specialised clump of myocardial conducting cells located in the superior and posterior walls of the right atrium
What is the pacemaker of the heart?
The SA node
How is the impulse then spread to the AV node and myocardial contractile cells?
Via internodal pathways
What are the internodal pathways?
3 bands (anterior, middle, posterior) that lead directly from SA node to AV node
How do the cells of the AV node then transmit the action potential?
Transmit AP more slowly and delays impulse
As impulse spreads across both atria, what happens?
Atria contract –> atrial systole Causes blood to move from atria to ventricles
What is purpose of delay caused by AV node?
Ensures atria have enough time to fully eject blood into ventricles before ventricular systole
After the AV node, where does the impulse then pass?
Into the atrioventricular bundle (bundle of His). The impulse spreads down to the ventricles along the bundle of His to the apex of the heart.
What is the bundle of His?
A continuation of the specialised tissue of the AV node and serves to transmit the electrical impulse from the AV node to the Purkinje fibres of the ventricles
At the apex of the heart, what does the bundle of His divide into?
The right bundle branch (RBB) and left bundle branch (LBB)
What is purpose of split into RBB and LBB?
Supplies left and right ventricles
What does RBB do?
Conducts the impulse to the Purkinje fibres of the right ventricle
What does the LBB do?
Conduct impulse to the Purkinje fibres of the left ventricle
What/where are Purkinje fibres?
Additional myocardial conductive fibres that spread impulse from AV to the myocardial contractile cells in the ventricles. Network of specialised cells that are abundant with glycogen and have extensive gap junctions.
What is purpose of Purkinje fibres?
This rapid conduction allows coordinated ventricular contraction (ventricular systole) and blood is moved from the right and left ventricles to the pulmonary artery and aorta respectively
What are the non-pacemaker action potential cells?
Atrial myocytes, ventricular myocytes, Purkinje cells
What are the phases of the AP in non-pacemaker cells?
- Phase 4 2. Phase 0 3. Phase 1 4. Phase 2 5. Phase 3 6. Phase 4
What is initial phase 4 in non-pacemaker?
Resting membrane potential –> stable around -90 mV
Why is the resting membrane potential very negative in phase 4 in non-pacemaker?
Potassium channels are open (K+ conductance and K+ currents are high) K+ ions leaving cell and making membrane potential more -ve inside At same time fast Na+ channels and L-type slow Ca2+ channels are closed
How does phase 4 progress to phase 0 in non-pacemaker?
Wave of depolarisation goes into contractile cell and membrane potential becomes more positive
What is rapid depolarisation in phase 0 due to in non-pacemaker?
Voltage-gated fast Na+ channels open so Na+ enters cell –> membrane potential becomes more positive K+ channels close –> less K+ leaves cell Moves membrane potential away from equilibrium potential for potassium (which is negative) and towards equilibrium potential for sodium (which is positive)
What are the cells rapidly depolarised to in phase 0 in non-pacemaker?
A threshold voltage of about -70 mV
What is phase 1 in non-pacemaker?
Initial repolarisation
How does phase 0 progress to phase 1 in non-pacemaker?
Na+ channels close and membrane starts to repolarise as K+ leaves through opening of special transient outward K+ channel
Is phase 1 long or short in non-pacemaker?
Very brief Short-lived, hyper polarising outward K+ current
Briefly describe Na, K and Ca in phases of AP in non-pacemaker cells?
4 –> Resting potential 0 –> Increase in permeability to Na 1 –> Decrease in permeability to Na 2 –> Decrease in permeability to K (fast K+ channels close) and increase in permeability to Ca 3 –> Increase in permeability to K (slow K+ channels open) and decrease in permeability to Ca 4 –> Resting potential
What is phase 2 in non-pacemaker?
Plateau phase
How does phase 1 progress to phase 2?
Flattening due to: - Decrease in K+ permeability - Increase in Ca2+ permeability (slow) Voltage gated L-type calcium
Why is there a plateau phase in AP in non-pacemaker?
Repolarisation is delayed. Prolongs AP duration. Distinguishes cardiac action potentials from much shorter APs found in nerves and skeletal muscle
What is phase 3 in AP in non-pacemaker?
Rapid repolarisation
How does phase 2 progress to phase 3 in AP in non-pacemaker?
Calcium channels close Activation of K+ channels (K+ exits rapidly which repolarises cell back to resting potential)
What is depolarisation?
Cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell
What is repolarisation?
Change in membrane potential that returns it to a negative value just after the depolarisation phase of an action potential which has changed the membrane potential to a positive value
Where are cardiac pacemaker cells mostly found?
In the SA node
Why are pacemaker cells called auto rhythmic?
They beat naturally