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68 Cards in this Set
- Front
- Back
Why is the SA node said to be the pacemaker for the heart?
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Although other cells in the heart undergo spontaneous depolarization, the SA node cells have the fastest intrinsic rate.
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How do the cells in the SA node compare to other cardiac cells?
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They have a reduced quantity of contractile proteins and are smaller than contractile cells or Purkinje cells
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What is the conduction velocity in the atria?
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1 meter/sec
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Which specialized conduction pathways in the atrium directly connect the SA node with the AV node and contribute to the action potential through Bachmann's bundle?
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Atrial muscle fibers
Atrial internodal tracts |
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How are action potentials rapidly conducted from the right atrium to the left atrium?
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Via Bachmann's bundle
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Why is the conduction velocity in the AV node much slower than that in the SA node?
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The cells are small and have fewer gap junctions connecting the cells
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What is the conduction speed in the AV node?
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0.01 - 0.05 meters/sec
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What are the 2 functions of the AV node?
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1. Serves as the only electrical connection between the atrial and ventricle muscle fibers
2. Delays the spread of excitation from the atria to the ventricles |
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List the order of propagation of action potentials in the heart.
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1. SA node
2. AV node 3. Bundle of his 4. R and L bundle branches 5. Purkinje fibers |
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As in nerve and skeletal muscle, the resting membrane is very permeable to ____ions and relatively impermeable to _____ions.
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Permeable to K+ ions
Impermeable to Na+ ions |
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What type of channels open during Phase 0 of an action potential in cardiac muscle cells?
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Fast Na+ voltage-gated channels
(rapid depolarization) |
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During phase 0 of the cardiac action potential, inactivation of fast Na+ channels close in response to a peak depolarization value of about ________mV.
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+20 mV
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What type of ionic flow is responsible for phase 1 of the cardiac action potential?
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(fast Na+ channels have closed)
K+ channels open and K+ flows out of cell (initial repolarization) |
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Which phase of the cardiac action potential represents a membrane potential near 0 mV?
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Phase 2 (plateau).
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Describe the flow of ions responsible for phase 2 of the cardiac action potential.
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Influx of Ca++ through L-type Ca++ channels
Efflux of K+ ions |
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Nifedipine, dilitiazem, and verapamil can block which type of channels?
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L-type Ca++ channels.
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When do L-type Ca++ channels open during the cardiac action potential?
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At the end of the initial repolarization
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Describe the flow of ions during phase 3 of the cardiac action potential.
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The efflux of K+ begins to overcome the influx of Ca++, so repolarization begins to occur. The Ca++ current continues to diminish until it reaches zero, so repolarization rate increases.
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What is the resting membrane potential of a cardiac contractile cell?
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About -85 mV.
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Describe the conductance and driving forces of K+, Na+, and Ca++ relative to each other during Phase 4 of the cardiac action potential
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(current = conductance x driving force)
K+ --> Conductance is high, driving force is low Na+, Ca++ --> Conductance is low, driving force is high |
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How are the excess Na+ ions that entered the cell during phase 0 eliminated during phase 4 of the cardiac action potential?
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Na+/K+-ATPase
(3 Na+ ions exit, 2 K+ ions enter) |
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How are the excess Ca++ that entered the cell during the plateau phase of the cardiac action potential eliminated?
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1. Na+-Ca++ exchanger (3 Na+ for 1 Ca2+)
2. True Ca2+ pumps in the cell membrane 3. Ca2+ion ATPase (SERCA_ pump (2 H+ ions for 2 Ca2+ ions for each ATP hydrolyzed) |
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How do action potentials in the SA node differ from action potentials in cardiac contractile cells?
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1. Exhibits automaticity-- action potentials are generated spontaneously
2. Does not have a stable resting membrane potential 3. Does not exhibit a plateau fase |
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Describe the flow of ions during phase 0 of a pacemaker action potential.
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Influx of Ca2+ through T-type Ca2+ channels
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What is the difference between the Ca2+ channels in the cardiac contractile cells and those in the SA node?
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Contractile cells = L-type Ca2+ channels (blocked by nifedipine, diltiazem, and verapamil).
Conduction cells = T-type Ca2+ channels (not blocked by drugs) |
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Describe the flow of ions during phase 3 of the pacemaker potential.
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Efflux of K+ ions
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Compare the speeds of depolarization in contractile and pacemaker potentials.
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Contractile cells = rapid depolarization
Pacemaker cells = slow depolarization |
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Describe phase 4 of the pacemaker potential.
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Spontaneous depolarization produced by the opening of Na+ channels ("If current).
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The "If" current describes the flow of which ions in which type of action potential?
How is "If" turned on? |
Flow of Na+ ions during the slow, spontaneous depolarization of the pacemaker potential.
Turned on by the repolarization of the previous action potential |
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Of the latent pacemakers, which are the slowest?
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Purkinje fibers
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What is the conduction velocity in the Purkinje fibers?
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2 - 4 meters/sec.
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Which cardiac fibers have the highest conduction velocity?
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Purkinje fibers
(2 - 4 m/sec) |
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The conduction velocity of an action potential depends on what 3 factors?
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1. Size of the inward current during the initial upstroke
2. Rate of rise of the upstroke (dV/dT0 3. Cable properties of the cardiac muscle (internal resistance) |
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Why can no action potential be initiated during the absolute refractory period, no matter how large a stimulus is applied?
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Because the Na+ channels are closed
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During a cardiac action potential, Na+ channels begin to recover during which period?
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Effective refractory period
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Which period lasts from the end of the absolute refractory period until the membrane potential reaches approximately -70 mV?
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Relative refractory period (RRP)
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During which period have the Na+ channels recovered, making it possible to generate another action potential with a greater than normal stimulus?
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Relative refractory period (RRP).
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Describe the parameters of the supranormal period (SNP).
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From the end of the RRP until the membrane is fully repolarized at about -85 mV.
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What is the clinical importance of the supranormal period of the cardiac action potential?
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A lethal arrhythmia can be initiated at if another action potential from some other site is generated during this time.
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What are the 3 possible mechanisms by which the rate of firing of pacemaker potentials could be altered?
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1. Steepness of phase 4 depolarization
2. Maximum diastolic potential to which the cell repolarizes. 3. Threshold potential |
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What would happen to the firing rate if the maximum diastolic potential to which the cell repolarizes become more negative?
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The cell would take longer to reach threshold and the rate of firing would DECREASE
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In the SA node, norepinephrine activates which receptors?
How is the flow of ions affected? |
B1 adrenergic receptors
Increaed influx of Na+ ("If") results in a steeper phase 4 depolarization. |
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What are the two effects of NE on the heart?
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1. Increases inward Na+ current (If), increasing steepness of depolarization.
2. Increases conduction velocity of action potentials through the AV node *Positive chronotropid AND dromotropic effect |
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ACh binds to which muscle receptors in the heart?
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Muscarinic cholinergic (M2) receptors.
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List the 3 effects that ACh has on cardiac tissue.
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1. Decreases inward Na+ current (If), slowing rate of spontaneous depolarization
2. Increases K+ conductance, hyperpolarizing the cell. 3. Decrease the influx of Ca2+, increasing the threshold potential. |
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What causes triggered activity?
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Afterdepolarizations
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When do early and delayed afterdepolarizations occur?
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Early --> late phase 2 to middle phase 3
Delayed --> late phase 3 to early phase 4 |
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Which interval of the ECG correlates with the conduction time through the AV node?
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PR interval
(time from the onset of atrial activation to the onset of ventricular activation) |
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How long is the normal PR interval?
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0.16 sec
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The P wave and QRS complex have similar durations -- what is the normal time span?
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0.06 - 0.10 sec
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Which portion of the ECG represents the isoelectric time period?
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ST segment.
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Describe the duration of the ST segment in terms of depolarization and repolarization.
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Lasts from the end of the ventricular depolarization to the beginning of ventricular repolarization.
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Shifts downward or a downward sloping ST segment are indicative of what type of injury?
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Myocardial infarction
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Elevations above the baseline of the ST segment in an ECG are usually indicative of what type of injury?
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Acute myocardial injury
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What is the normal value of the QT interval (in seconds)?
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0.4 seconds
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How can heart rate be determined from an ECG?
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By measuring the R-R interval, one cardiac cycle can be determined.
heart rate = 1/ cycle length |
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Fast-response action potentials are recorded from what type of fibers?
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Atrial and ventricular myocardial fibers and from ventricular specialized conducting (Purkinje) fibers.
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Slow-response action potentials are recorded from what type of cells?
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SA and AV nodal cells and from abnormal myocardial cells that have been partially depolarized
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Which type of afterdepolarizations are more likely to occur when the basic cycle length of the initiating beats is very long and when the cardiac action potentials are abnormally prolonged?
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Early afterdepolarizations
(during phase 3) |
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Which type of afterdepolarization is more likely to occur when the basic cycle length of initiating beats is short and when the cardiac cells are overloaded with Ca2+?
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Delayed afterdepolarizations
(late in phase 3 or in phase 4) |
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What is the effect of tetrodotoxin on cardiac action potentials?
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Blocks the fast Na+ channels in the action potentials recorded in a purkinje fiber
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What is the effect of Isoproterenol on T-type and L-type Ca2+ channels?
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Isoproterenol (beta adrenergic agonist)
*Significantly increases the L-type Ca2+ current, but has little affect the T-type channel |
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What is the threshold for the pacemaker potential?
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-55 mV
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What is the effect of Ca2+ channel antagonists on cardiac afterload?
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Decrease cardiac afterload
(heart contracts at lower pressure) |
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What type of pathology is characterized by an inverted P wave and normal QRS complexes and T waves?
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Premature atrial depolarization
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What type of pathology is characterized by bizarre QRS complexes and T waves and is followed by an compensatory pause?
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Premature ventricular depolarization
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What type of pathology is characterized by a long P-R interval (> 0.2 sec)?
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First-degree AV block
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What type of pathology is characterized by a 2:1 ratio of P waves to QRS complexes?
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Second-degree AV block
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