52 Questions
What is the ionic basis of automaticity in cardiac impulse formation?
Calcium channel activation
What is the mechanism of overdrive suppression in cardiac conduction?
Inhibition of latent pacemakers by faster pacemakers
What is the characteristic feature of unidirectional block and reentry in altered impulse conduction?
Anterograde conduction only
What is the physiological basis of antiarrhythmic therapy primarily targeting in the cardiac conduction system?
Altered impulse conduction
Which ion is the main responsible for spontaneous depolarization in pacemaker cells?
Calcium
What is responsible for the rapid upstroke in nonpacemaker cells?
Sodium entry through fast sodium channel
What maintains low intracellular calcium concentration?
Sodium-calcium exchanger
What determines the resting potential of the cell?
Na+/K+ ATPase pump and K+ leak channels
Which type of channels are targets of calcium channel blockers?
L-type calcium channels
What is responsible for K+ exiting the cell to repolarize it?
Potassium channels
What maintains ion gradients and resting membrane potential?
Na+/K+ pump
Where does calcium entry occur during phase 2 of the action potential?
Sarcolemma
What membrane proteins are contained in the sarcolemma?
Voltage-gated sodium, calcium, and potassium channels
What is present on the SR membrane?
Calcium release channels
What is responsible for gradual depolarization during phase 4 of the action potential in cells with natural automaticity?
Calcium influx
What is the role of the Na+/K+ pump in maintaining the cell's ionic balance?
Pumping three sodium ions in for every two potassium ions out
What is the intrinsic rate of the SA node?
60-100 bpm
Which cells involve a spontaneous phase 4 depolarization and slow inward leak of current until the cell spontaneously depolarizes?
Pacemaker cells
What maintains resting membrane potential by pumping 3 Na+ out and returning 2 K+ in?
Na+/K+ ATPase pump
Which part of the heart is prone to inflammation and calcified debris that can interfere with impulse conduction?
Bundle of His
What are arrhythmias that are categorized as supraventricular or ventricular based on their origin called?
Tachycardias
What results from reduced automaticity or conduction block?
Bradycardias
What is the intrinsic rate of the AV node?
45-50 bpm
What is the specialized conducting system that includes the SA node, AV nodal region, ventricular conducting system, bundle of His, and Purkinje fibers called?
The specialized conducting system
What is characterized by rapid depolarization and sustained depolarization before repolarization in nonpacemaker cells?
Fast response
What is involved in phases of rapid depolarization, transient early repolarization, plateau phase, membrane repolarization, and resting membrane potential in nonpacemaker cells?
Action potentials
What facilitates action potentials in various cardiac regions and cells, capable of bidirectional movement?
Gap junctions
What is the term for arrhythmias that vary in severity from benign palpitations to life-threatening conditions?
Arrhythmias
What can lead to overdrive suppression in cardiac impulse formation?
Fastest pacemaker cells inhibiting other automatic cells
How do electrotonic interactions affect pacemaker cells and myocardial cells?
Affect their membrane potentials and automaticity
What influences the rate of pacemaker cell discharge?
Resting membrane potential and the slope of phase 4 depolarization
What can lead to spontaneous conversion of fast response cells to slow pacemaker response?
Changes in cardiac pacing, such as in severe coronary artery disease
How many phases are cardiac action potentials divided into?
5
What does the Q-T interval on the EKG reflect?
Effect of various drugs
What conditions can cause dysrhythmias?
Myocarditis and viral infections
What do cardiac arrhythmias relate to?
Alterations in the specialized cells of the heart's conduction system
Where may arrhythmias arise from altered impulse formation?
At the SA node or other sites, including specialized conduction pathways or regions of cardiac muscle
What is the effect of hyperpolarizing current on pacemaker cells?
Inhibition of other automatic cells
What is the main factor influencing the slope of phase 4 depolarization in pacemaker cells?
Resting membrane potential
How does severe coronary artery disease affect cardiac pacing?
Can lead to spontaneous conversion of fast response cells to slow pacemaker response
What is the main effect of calcium channel blockers on heart rate and conduction speeds in the SA and AV nodes?
Decrease heart rate and conduction speeds
What is the role of the absolute refractory period in the cardiac action potential?
Prevents initiation of an action potential
What is the characteristic of the relative refractory period in the cardiac action potential?
Requires a smaller than normal stimulus
What is the vulnerable zone for dysrhythmias to develop?
Supernormal excitatory period
What is responsible for the action potential upstroke of pacemaker cells?
Smaller calcium current
What determines the firing rate of automatic cells in the specialized conduction pathway?
Slope of phase 4 spontaneous depolarization
What sets the heart rate and prevents spontaneous firing of other potential pacemaker sites?
SA node
Which cells act as backup pacemakers if the SA node slows or fails to fire?
AV node and bundle of His
What changes the firing rate of the cell in the pacemaker current (If)?
Magnitude of the maximum diastolic potential
What is the impact of alterations in the pacemaker current (If) on the firing rate of the cell?
Changes the firing rate
What determines the intrinsic rates of firing in automatic cells?
Slope of phase 4 spontaneous depolarization
What acts as backup pacemakers if the SA node fails to fire?
AV node and bundle of His
Study Notes
Cardiac Impulse Formation and Arrhythmias
- Overdrive suppression occurs when the fastest pacemaker cells inhibit other automatic cells from firing, due to ion distribution and hyperpolarizing current.
- Electrotonic interactions occur between pacemaker cells and myocardial cells through gap junctions, affecting their membrane potentials and automaticity.
- The rate of pacemaker cell discharge varies with the resting membrane potential and the slope of phase 4 depolarization, influenced by catecholamines and acetylcholine.
- Changes in cardiac pacing, such as in severe coronary artery disease, can lead to spontaneous conversion of fast response cells to slow pacemaker response, potentially causing serious arrhythmias.
- Cardiac action potentials are divided into 5 phases, with each phase corresponding to specific changes in ion channels and contributing to the EKG waveform.
- The Q-T interval on the EKG is affected by various drugs and can predispose to dysrhythmias.
- Dysrhythmias can be caused by conditions such as myocarditis, viral infections, hypertrophy of myocardial cells, and other inflammatory conditions.
- Cardiac arrhythmias are related to alterations in the specialized cells of the heart's conduction system, affecting automaticity, excitability, conductivity, or refractoriness.
- Arrhythmias may arise from altered impulse formation at the SA node or other sites, including specialized conduction pathways or regions of cardiac muscle.
Cardiac and Vascular Function: Role of Calcium Channel Blockers
- Arterioles and their impact on vascular tone play a crucial role in cardiac and vascular function
- The involvement of contractile cells and pacemaker cells in the excitation-contraction coupling process is vital for heart function
- Calcium channel blockers have a significant impact on heart rate acceleration and increased conduction speeds in the SA and AV nodes
- Refractory periods in the cardiac action potential (AP) and the EKG are essential for ventricular emptying and refilling before the next contraction
- The absolute refractory period does not allow initiation of an AP, while the relative refractory period requires a larger than normal stimulus
- The supernormal excitatory period is a vulnerable zone for dysrhythmias to develop
- Refractory periods are caused by unique two-part gating of fast Na+ channels
- The action potential upstroke of pacemaker cells relies on a smaller calcium current and has a less rapid rate of rise compared to non-pacemaker cells
- Automatic cells in the specialized conduction pathway have different intrinsic rates of firing, determined by the slope of phase 4 spontaneous depolarization, maximum negative diastolic potential, and threshold potential
- The SA node, with the fastest rate of depolarization, sets the heart rate and prevents spontaneous firing of other potential pacemaker sites
- Latent pacemakers, such as the AV node and the bundle of His, act as backup pacemakers if the SA node slows or fails to fire
- Alterations in the pacemaker current (If), magnitude of the maximum diastolic potential (MDP), and threshold potential change the firing rate of the cell
Test your knowledge of cardiac impulse formation and arrhythmias, as well as the role of calcium channel blockers in cardiac and vascular function. Explore topics such as overdrive suppression, electrotonic interactions, cardiac pacing, refractory periods, and the impact of calcium channel blockers on heart rate and conduction speeds.
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