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Questions and Answers
What is the lowest membrane potential value for pacemaker cells in the sinoatrial node?
What is the lowest membrane potential value for pacemaker cells in the sinoatrial node?
What triggers the opening of funny sodium channels in pacemaker cells?
What triggers the opening of funny sodium channels in pacemaker cells?
What role do T-calcium channels play in the action potential of a pacemaker cell?
What role do T-calcium channels play in the action potential of a pacemaker cell?
At what membrane potential does the fast depolarization occur due to the opening of L-calcium channels?
At what membrane potential does the fast depolarization occur due to the opening of L-calcium channels?
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What happens to the membrane potential during repolarization in pacemaker cells?
What happens to the membrane potential during repolarization in pacemaker cells?
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How does the pacemaker potential begin again after repolarization?
How does the pacemaker potential begin again after repolarization?
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How do the firing rates of pacemaker cells in the atrioventricular node compare to those in the sinoatrial node?
How do the firing rates of pacemaker cells in the atrioventricular node compare to those in the sinoatrial node?
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What is the main reason the Purkinje fibers have a longer time to reach threshold compared to the sinoatrial node?
What is the main reason the Purkinje fibers have a longer time to reach threshold compared to the sinoatrial node?
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What initiates the fast depolarization phase in working myocardial cells?
What initiates the fast depolarization phase in working myocardial cells?
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During which phase do L-Ca+ channels open, contributing to a plateau in membrane potential?
During which phase do L-Ca+ channels open, contributing to a plateau in membrane potential?
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What occurs at the end of the absolute refractory period (ARP)?
What occurs at the end of the absolute refractory period (ARP)?
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What is the primary effect of the effective refractory phase on myocardial action?
What is the primary effect of the effective refractory phase on myocardial action?
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What is the consequence of ectopic regions of impulse generation in cardiac muscle?
What is the consequence of ectopic regions of impulse generation in cardiac muscle?
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How does the inactivation gate of voltage-gated sodium channels respond when membrane potential exceeds -40 mV?
How does the inactivation gate of voltage-gated sodium channels respond when membrane potential exceeds -40 mV?
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What type of electrical feedback mechanism allows defibrillation to restore normal heart rhythm?
What type of electrical feedback mechanism allows defibrillation to restore normal heart rhythm?
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During which phase does the membrane potential remain relatively unchanged, leading to a plateau?
During which phase does the membrane potential remain relatively unchanged, leading to a plateau?
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What characterizes the absolute refractory period in working myocardial cells?
What characterizes the absolute refractory period in working myocardial cells?
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What allows the cardiac muscle to contract rhythmically during the relative refractory period?
What allows the cardiac muscle to contract rhythmically during the relative refractory period?
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Study Notes
Action Potential of Pacemaker Cells
- Pacemaker cells, notably in the sinoatrial node, lack a true resting membrane potential.
- Membrane potential fluctuates between -55 to -60 mV, with a relatively positive lowest value due to high sodium ion permeability.
- Hyperpolarization below -40 mV triggers the opening of funny voltage-gated sodium channels.
- At -55 to -60 mV, sodium influx initiates the pacemaker potential (slow diastolic depolarization).
- At -50 mV, transient calcium (T-type) channels open, allowing calcium influx to further depolarize the membrane.
- Once the threshold of -40 mV is reached, long-lasting calcium (L-type) channels open, resulting in fast depolarization.
- Repolarization occurs when L-type calcium channels close and voltage-gated potassium channels open, allowing potassium efflux.
- The cycle repeats when membrane potential drops below -40 mV, leading to the next pacemaker potential.
Atrioventricular Node and Bundle Impulses
- Atrioventricular node offers a slower pacemaking rate compared to the sinoatrial node (-70 mV).
- Purkinje fibers have an even lower resting potential (-80 to -85 mV) and take longer to reach threshold.
Action Potential of Working Myocardial Cells
- Working myocardial cells maintain a resting membrane potential of -85 to -90 mV.
- Pacemaker cells typically provide the initial impulse needed for depolarization.
- External stimuli can also evoke action potentials in these cells.
Phases of Action Potential
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Fast Depolarization (Phase 0):
- Triggered by pacemaker impulse; voltage-gated Na+ channels open, leading to rapid sodium influx.
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Initial Fast Repolarization (Phase 1):
- Occurs at +35 to +40 mV; voltage-gated Na+ channels close, K+ channels open, with minimal chloride influx assisting repolarization.
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Slow Repolarization/Plateau (Phase 2):
- L-Ca2+ channels open for calcium influx while K+ efflux occurs, maintaining potential stability for around 100 ms.
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End Fast Repolarization (Phase 3):
- L-Ca2+ channels close and additional K+ channels open, restoring resting membrane potential without hyperpolarization.
Changes in Excitability During Action Potential
- Voltage-gated sodium channels feature activation and inactivation gates.
- Excitable states shift from:
- Resting Membrane Potential: Activation gate closed, inactivation gate open (100% excitability).
- Absolute Refractory Period (ARP): Both gates open initially, inactivation gate then closes, leading to 0 excitability until -40 mV.
- Relative Refractory Phase (< -40 mV): Activation gates closed, inactivation gates open, allowing excitability to rise back to 100%.
Effective Refractory Phase and Fibrillation
- Effective refractory phase encompasses ARP and part of the relative refractory phase, during which contraction cannot occur.
- Ectopic regions in muscle cells can lead to unsynchronized contractions, resulting in:
- Atrial Fibrillation: Affects the atria.
- Ventricular Fibrillation: Affects the ventricles.
- Defibrillation involves delivering an electrical current to synchronize cardiac contraction.
Impulse Circulation and Re-Entry Excitation
- The absolute refractory period prevents backward impulse transmission through the cardiac muscle.
- Late repolarization or directional blocks can disrupt normal impulse propagation, leading to potential arrhythmias.
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Description
Explore the unique characteristics of action potentials in pacemaker cells, focusing on the changes in ion permeability and membrane potential. This quiz covers the pacemaker activity in the sinoatrial node, discussing the role of funny sodium channels and the dynamics of ion exchange during different phases.
