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What are the four important characteristics of the heart?
Automaticity, excitability, conductivity, and contractility.
What is the cardiac conduction system composed of?
SA node, AV node, AV bundle (Bundle of His), right and left bundle branches, and Purkinje fibers.
What is the function of the SA node?
It acts as the heart's pacemaker, initiating each heartbeat and setting the pace of the heart.
What is the resting membrane potential (RMP) of the SA node?
Approximately -55 mV.
Why is the SA node able to generate action potentials spontaneously?
Its membrane is naturally leaky to Na+, which decreases the membrane potential until it reaches the threshold.
What happens during the prepotential phase in the SA node?
A gradual depolarization due to sodium influx.
What causes the depolarization in the SA node action potential?
The opening of voltage-gated calcium channels.
What triggers repolarization in the SA node?
Potassium efflux and decreased calcium permeability.
Why does the AV node slow impulse conduction?
To allow complete atrial depolarization and contraction before ventricular depolarization.
What is the AV nodal delay?
A slowdown in conduction through the AV node to about 0.05 m/s.
What separates the atria and ventricles electrically?
A non-conductive fibrous barrier, except at the AV bundle.
What happens after the impulse leaves the AV node?
It travels through the Bundle of His, right and left bundle branches, and Purkinje fibers to the ventricular muscles.
How do Purkinje fibers contribute to the conduction system?
They ensure rapid propagation of action potentials to the ventricular muscle.
What is the resting membrane potential of ventricular muscle?
-90 mV.
What are the five phases of the ventricular muscle action potential?
Phase 0: Rapid depolarization due to Na+ influx.
Phase 1: Brief repolarization as Na+ channels inactivate.
Phase 2: Plateau phase due to sustained Ca+ influx.
Phase 3: Repolarization as K+ efflux increases.
Phase 4: Return to resting membrane potential.
What is unique about the plateau phase in cardiac muscle?
It prolongs depolarization, allowing sustained contraction.
How is excitation-contraction coupling initiated in cardiac muscle?
Calcium influx during the plateau phase triggers additional calcium release from the sarcoplasmic reticulum.
What role does calcium play in muscle contraction?
It binds to troponin, shifting tropomyosin to expose actin binding sites for crossbridge cycling.
How is calcium removed to relax the muscle?
By Ca+-ATPase pumps in the sarcoplasmic reticulum and cell membrane, and Na+-Ca+ exchangers.
What is the absolute refractory period (ARP) in cardiac muscle?
The period during which the muscle cannot be re-excited, lasting nearly the entire action potential.
Why is the ARP important in cardiac function?
It prevents tetanic contractions and allows sufficient relaxation for ventricular filling.
How does parasympathetic stimulation affect the heart rate?
It releases acetylcholine, increasing K+^++ efflux and decreasing the slope of the prepotential phase, slowing the heart rate.
How does sympathetic stimulation affect the heart rate?
It releases noradrenaline, increasing Ca+ influx and the slope of the prepotential phase, speeding up the heart rate.
What is the effect of ANS stimulation on AV nodal delay?
Parasympathetic activity prolongs it, while sympathetic activity reduces it.
What is the key difference between pacemaker and non-pacemaker cells?
Pacemaker cells can spontaneously generate action potentials, while non-pacemaker cells require external stimulation.
Do atrial and ventricular muscle cells have a prepotential phase?
No, they rely on electrical activity propagated from the SA node.
What intrinsic rate does the SA node set in the absence of external influences?
Approximately 100 beats per minute.
What happens to the heart rate at rest in healthy adults?
It is reduced to about 70 beats per minute due to parasympathetic dominance.
Why is synchronization between atrial and ventricular contraction important?
It ensures efficient blood flow and prevents backflow.
What ensures one-way conduction from the atria to the ventricles?
The AV node and Bundle of His.
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