Term
| What is cardiac excitability? |
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Definition
| The ability to stimulate cardiac fibers during refractory period. |
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Term
| Why is there a greater increase in the amplitude of the fast response the later in RRP that excitation occurs? |
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Definition
| B/c the later in RRP the more Na channels are available to be open. |
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Term
| What is the difference between the fast and slow response? |
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Definition
| Recovery of full excitability is much slower in the slow response and even when the fiber is completely repolarized it still may be difficult to evoke a response. |
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Term
| What does cycle length refer too? |
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Definition
| The time between successive action potentials? |
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Term
| How does cycle length alter the duration of action potentials in cardiac cells? |
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Definition
Decrease in cycle length, decreases the duration of action potential
Increase in cycle length increases the duration of action potential |
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Term
| What are changes in cycle length associated with? |
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Definition
| Initiation or termination of certain arrythmias. |
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Term
| Describe the route of electrical conduction through the heart. |
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Definition
-SA node acts as the pacemaker and initiates the spread of action potentials through the atria
-Excitation reaches the AV node where conduction is slowed
-Excitation spreads rapidly throughout ventricles via Purkinje fibers so that ventricular myocytes conduct in coordinated fashion
-Autonomic nervous system controls various aspects of cardiac function, such as heart rate and contraction strength |
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Term
| T/F Contractions require intact innervation. |
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Definition
| False-a perfused heart can beat outside of the body |
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Term
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Definition
| Ability of the heart to initiate its own beat |
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Term
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Definition
| regularity of pacemaking activity |
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Term
| What types of cells does the SA node contain? |
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Definition
| Both pacemaker cells and cells for conducting impulses. |
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Term
| How can pacemaker frequency be varied? |
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Definition
Changing-
-Rate of depolarization during phase 4
-maximal negativity during phase 4
-threshold potential |
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Term
| How does the sympathetic nervous system increase heart rate? |
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Definition
| By increasing rate of diastolic depolarization. |
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Term
| How does the parasympathetic system slow heart rate? |
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Definition
| By hyperpolarizing the pacemaker cell membrane and reducing the rate of diastolic depolarization |
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Term
| What takes over if the SA node is destroyed? |
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Definition
| Latent or ectopic pacemakers |
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Term
| Explain the mechanism that prevents secondary pacemaker cells from firing at their intrinsic rate? |
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Definition
| Secondary pacemaker cells become hyperpolarized when they fire at a rate faster than their intrinsic rate. This means it takes longer for them to repolarize to threshold potential and there firing is therefore controled by the faster SA node pacemaker cells |
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Term
| What can cause latent pacemaker cells to take over? |
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Definition
-If there is a decrease or cessation of SA node firing.
-If the intrinsic rate of fring of latent pacemaker is faster than that of SA node
-If there is a block in conduction of action potentials from SA node |
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Term
| What is an impulse that is initiated by a latent pacemaker cell known as? |
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Definition
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Term
| What is an escape rhythm? |
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Definition
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Term
| If latent pacemaker cells cause problems when they fire, why not just have the pacemaker cells of the SA node? |
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Definition
| Latent pacemaker cells act as a backup and keep the heart beating should the pacemaker cells of the SA node fail. |
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Term
| Where does the cardiac impulse spread to from the SA node? |
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Definition
| Cardiac impulse begins in the SA node spreading throughout the right atrium. Then to the left atrium and eventually reaches the AV node. |
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Term
| What is the normal entry of cardiac impulse into the ventricles? |
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Definition
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Term
| What is the bundle of His? |
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Definition
| The bundle of His is the upper portion of specialized conducting system for the ventricles |
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Term
| What is the purposed of slowing the conduction velocity in the AV node? |
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Definition
| Causes a delay in the conduction of impulese from the atria to ventricles allowing times for the ventricles to fill during atrial contraction |
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Term
| What does the bundle of His divide into? |
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Definition
| Bundle branches that divide into Purkinje fibers--which are a network of conducting fibers |
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Term
| What is the intrinsic firing rate of Purkinje fibers? |
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Definition
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Term
| T/F Purkinje cells have a long refractory period. |
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Definition
| True. This prevents premature excitations fo atria from causing premature contractions of ventricles. |
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Term
| What might cause a conduction block? |
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Definition
| Ischemia, fibrosis, inflammation, certain drugs. Conduction blocks can be unidirectional or bidirectional. |
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Term
| What effect might conduction blocks have on latent pacemaker cells? |
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Definition
| If conduction from the SA node is blocked then latent pacemaker cells may fire b/c the over drive suppression that inhibits them has been removed. |
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Term
| What is one way to treat an AV block? |
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Definition
| Implantation of a permanent pacemaker. |
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Term
| What type of heart beat abnormalities can a unidirectional block lead to? |
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Definition
| Arrhythmias and fibrillations |
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Term
| If the heart has a unidirectional block, how does conduction impulse pass through it? |
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Definition
| Cardiac impulses willrepeatedly re-excite myocardial regions through which it has already passed |
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Term
| If cardiac excitation occurs in a circular pattern in a bidirectional block what is this known as? |
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Definition
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Term
| During what phases of action potential might abnormal afterpolarizations occur? |
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Definition
Early afterpolarization occurs during phase 3 during influx of calcium and efflux of K
Delayed afterpolarization- occurs at end of phase 3 or early in phase 4- develops in states of high intracellular calcium |
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Term
| Which abnormal afterpolarizations are typically seen when the heart rate is slow? When the heart rate is fast? |
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Definition
Slow heart beat tends to be early afterdepolarizations
Fast heart rate tends to be delayed after polarization |
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