Exam 2 – Cardio Ch 10 Flashcards
What is the pathway of the heartbeat?
Begins in the sinoatrial (SA) node
Internodal pathway to atrioventricular (AV) node
Impulse delayed in AV node
AV bundle takes impulse into ventricles
Left and right bundle branches of Purkinje fibers take impulses to all parts of ventricles
What is the importance of the delay in AV nodes?
Allows atria to contract before ventricles
What are conduction fibers?
Modified cardiomyocytes
Do conduction fibers contract?
Yes, just not as much
What is the function of the conduction system?
Conduct depolarization quickly to all parts of myocardium
What is the sinus node?
Specialized cardiac muscle connected to atrial muscle
What does the sinus node act as?
Pacemaker
Why does sinus node act as a pacemaker?
Because membrane leaks Na+ into cell and resting membrane potential is -55 to -60mV
SELF-EXCITATORY
What happens when membrane potential reaches -40mV in sinus node?
Slow Ca++ channels open causing action potential
After 100-150 msec, Ca++ channels close
What happens when Ca++ channels close in sinus node action potential?
K+ channels open more thus returning membrane potential to -55mV
LOOK AT RHYTHMICAL DISCHARGE OF SINUS NODAL FIBER CHART
LOOK AT RHYTHMICAL DISCHARGE OF SINUS NODAL FIBER CHART
What do the internodal pathways transmit?
Cardiac impulses throughout atria
What are the 3 internodal pathways?
Anterior
Middle
Posterior
What does the anterior interatrial band of the internodal pathway carry?
Impulses to left atrium
reaches AV node in 0.03 seconds
What does the AV node delay?
Cardiac impulses
slows down to make sure ventricles are behind atria at a good speed
Delay allows time for atria to empty its blood into ventricles before ventricular contraction begins
Where is the delay located?
AV node
What is the delay in the AV node?
0.09 sec
What is the delay in the AV bundle?
0.04 sec
Define the atrial conductive system
Organized so that the cardiac impulse does not travel from the atria into ventricles too rapidly
What is the Purkinje system?
Fibers lead from AV node through AV bundle into ventricles
What speed conduction does the purkinje system have?
Fast due to many gap junctions at intercalated discs
Fast conduction allows almost instantaneous transmission of the cardiac impulse throughout entire remainder of cardiac impulse
How do the gap junctions in the purkinje system contribute to fast conduction?
Rapid transmission caused by high level of permeability of gap junctions at intercalated discs between successive cells that make up purkinje fibers
What is the direction of conduction in AV bundles?
One way conduction through bundles
What is the only conducting path between atria and ventricles?
AV node-AV bundle
divides into left and right bundles
What is the importance of one way conduction in AV bundles?
Inability of action potentials to travel backwards from ventricles to atria
Prevents re-entry of cardiac impulses
Forward conduction
What is the transmission time between AV bundles and last of ventricular fibers?
0.06 seconds
QRS time
What is the normal rate of discharge in sinus node?
70-80/min
What is the normal rate of discharge in AV node?
40-60/min
What is the normal rate of discharge in Purkinje fibers?
15-40/min
Why is the sinus node the pacemaker of the heart?
Because of its faster discharge rate
Sets tone of the heartbeat for AV nodes and Purkinje fibers
Why are the sinus nodes self excitatory?
Conduction system is very leaky and builds up a potential
What is the most important function of the AV node?
It can take over the beating of the heart if the sinus node fails
Keeps animal alive
Why can the AV node and Purkinje fibers not self-excite?
The sinus node discharges so too fast
What is an ectopic pacemaker?
Portion of heart with a more rapid discharge than sinus node
also occurs when transmission from sinus node through AV node is blocked (AV block)
What does an ectopic pacemaker cause?
Abnormal sequence of contraction of different parts of heart and can cause significant debility of heat pumping
What becomes pacemaker of heart during sudden onset of AV block when sinus node discharge does not get through?
Purkinje system
What is Stokes-Adams syndrome?
Delay in pickup of heat beat
New pacemaker is AV node or penetrating part of AV bundle
When no contractions are occurring while waiting on another pacemaker to take over
What happens if the heart doesn’t beat for 4 seconds?
Pass out
Could lead to death
What are the parasympathetic effects on heart rate?
Parasympathetic (vagal) nerves, which release acetylcholine at their endings, innervate SA node and AV junctional fiber proximal to AV node causes hyperpolarization
What causes hyperpolarization due to parasympathetic effects?
Increased K+ permeability in response to acetylcholine
This causes decreased transmission of impulses which may temporarily stop heart rate
Ventricular escape occurs
What is acetylcholine affects on the heart?
Decreased rate of rhythm of sinus node
Decreased excitability of AV junctional fibes between atrial musculature and AV node, which slows transmission of cardiac impulse into ventricles
What are the sympathetic effects on heart rate?
Releases norephinephrine at sympathetic endings
Causes increased sinus node discharge
Increased rate of conduction of impulse
Increases force of contraction in atria and ventricles
