Term
| What are the four phases of the ventricular action potential? |
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Definition
Phase 0: depolarization Phase 1 = partial repolarization Phase 2 = plateau Phase 3 = repolarization Phase 4 = Resting membrane potential |
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Term
| What occurs during phase 0 of the ventricular AP? |
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Definition
Phase 0:
Fast voltage-gated Na+ channels open --> Na+ influx |
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Term
| What occurs during phase 1 of the ventricular AP? |
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Definition
Phase 1:
Partial repolarization due to closing of Na+ channels and opening of K+ channels |
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Term
| What occurs during phase 2 of the ventricular AP? |
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Definition
Plateau phase Slow voltage-gated Ca++ channels (L-type) open -> Ca++ influx Voltage-gated K+ channels close -> reduce K+ efflux, however ungated K+ channels still open Inward Ca++ current is balanced by outward K+ current Ca++ influx -> contraction + release Ca++ from SR |
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Term
| What occurs during phase 3 of the ventricular AP? |
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Definition
Repolarization Ca++ channels close -> stop Ca++ influx Voltage-gated K+ open -> K+ efflux |
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Term
| What occurs during phase 4 of the ventricular AP? |
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Definition
| Phase 4: Stable resting membrane potential due to high K+ permeability |
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Term
| What are the different phases of the SA node AP? |
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Definition
Phase 0 = Upstroke phase Phase 1 & 2 = NOT present Phase 3 = repolarization Phase 4 = spontaneous depolarization or pacemaker potential |
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Term
| What occurs during phase 0 of the SA node AP? |
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Definition
Depolarization spike Phase 0: Slow voltage-gated Ca++ channels (T-type) open -> Ca++ influx |
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Term
| What occurs during phase 3 of the SA node AP? |
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Definition
Repolarization Phase 3: Voltage-gated K+ open -> K+ efflux |
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Term
| What occurs during phase 4 of the SA node AP? |
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Definition
Spontaneous depolarization "Specific" Na+ channels open -> inward Na+ current If -> increase excitability -> once reach threshold action potential is generated Other mechanisms: Decrease K+ conductance Rate of phase 4 depolarization set the rate of the heart |
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Term
| Explain transmission of the cardiac impulse from the SA node to the atria |
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Definition
SA node is pacemaker Rapid velocity in most of the artia 0.3 m/sec More rapid in specific area (e.g. internodal pathways) 1 m/sec |
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Term
| Explain transmission of the cardiac impulse from the atria to the ventricles |
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Definition
From Atria to ventricles AV nodal and bundle delay = 0.13 second Very rapid transmission in purkinje fibers (1.5 - 4 m/sec) Direction of cardiac impulse transmission *AV node -> AV bundle -> right and left bundle branches -> purkinje fibers -> ventricular fibers (from endocardium to the epicardium) Signal conduction in AV bundle is one-way direction |
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Term
| Explain the AV nodal and bundle delay |
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Definition
In transmission of the cardiac impulse from the SA node all the way to the ventricles there is an AV nodal and bundle delay of 0.13 second Significance: allows ventricular filling, the atria contract before the ventricles so the ventricles will be full when they contract. doesn't make sense to contract atria and ventricles at the same exact time Mechanism: small number of gap junctions Fibrous tissues between atria and ventricle acts as insulator - slows down transmission |
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Term
| Explain the differing velocities of cardiac impulse transmission through the heart |
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Definition
DON'T HAVE TO MEMORIZE #'s From SA to AV node = 0.03 sec AV node and bundle delay = 0.13 sec From AV bundle to end of purkinje fibers = 0.03 sec From endocardium (end of purkinje fibers) to epicardium (ventricular muscle fibers) = 0.03 sec * Understand that the time of cardiac transmission is pretty constant except for the AV node and bundle delay Fastest conducting fiber = Purkinje fiber (1.5 - 4 m/sec) - ventricles are bigger so need to conduct faster to keep up Slowest conducting fiber = AV node (0.01 - 0.05 m/sec) - slow to cause the AV nodal and bundal delay |
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Term
| What are the intrinsic firing rates of various parts of the heart? |
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Definition
Normal pacemaker and intrinsic firing rate of various part of the conductive system: SA node - 100-110 per minute AV node - 40-60 per minute Bundle of His - 40 per minute Purkinje fibers - 15-20 per minute SA node is the normal pacemaker of the heart because it has the highest intrinsic firing rate |
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Term
| Explain the normal sinus rhythm |
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Definition
1- Action potential originate in SA node 2- Regular SA node impulse at 60-100 impulses/min 3- Correct sequence and timing for the heart activation |
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Term
| What are the effects of parasympathetic stimulation of the heart? |
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Definition
1) decrease frequency of SA action potential -> decrease heart rate (negative chronotropic effect) 2) decrease AV conduction velocity (negative dromotropic effect) |
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Term
| What are the mechanisms of parasympathetic effects of the heart? |
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Definition
Ach released at vagal nerve ending -> stimulation of muscarinic (M2) receptors 1- decrease Na+ channels If current via Gi pathway 2- increase membrane permeability for K+ which leads to K+ efflux -> hyperpolarization of sinus nodal fibers -> more difficult to fire action potential |
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Term
| What are the effects of sympathetic stimulation on the heart? |
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Definition
1) increase frequency of SA action potential -> increase heart rate (positive chronotropic effect) 2) increase AV conduction velocity (positive dromotropic effect) 3) increase strength of cardiac contractility (positive inotropic) |
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Term
| What are the mechanisms of sympathetic stimulation on the heart? |
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Definition
Norepinephrine at sympathetic nerve ending -> stimulation of beta 1 receptors increase Na+ channels If current (also increase Ca2+ influx was proposed) via Gs protein pathway -> more positive resting membrane potential -> easy to fire action potential |
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