Flashcards in rhythmical excitation of heart ( chapter 10) Deck (21)
how much sooner do atria contract than ventricles?
about 1/6 of a second
SA node location
superior posterolateral wall of right atrium. Its an ellipsoid shape
Resting membrane potential of SA node
-55--60 mv in comparison to -85--90 mv of ventricular muslce. This is cause the SA fibers naturally leak sodium and calcium ions into the cell which makes the cell more possitive.
why is the resting potential -55 mv?
Anytime the potential is less negative than -55 mv the fast sodium channels become blocked. The gates of these channels close and remain so. Therefore only the slow sodium-calcium channels can open ( activate) and cause the action potential.
Self exiting nodal fibers
due to high sodium concentration in extracellular fluid outside of nodal fibers, the sodium leaks into the nodal fibers and causes the resting potential to gradually become less negative between heart beats. Once it reaches ~ -40 mv it becomes activated and fires ( so the leakiness of sodium and calcium is what allows the nodal fibers to self fire)
Why does the leakiness to sodium and calcium not cause the sinus nodal fibers to remain depolarized all the time?
sodium-calcium channels inactivate 100-150 milliseconds after opening
then the potassium channels open releasing alot of possitive charge.
excess negative inside the fiber
anterior interatrial band
passes through the anterior walls of the atria to the left atra
Anterior, middle, posterior internodal pathways
bands that curve through the anterior, lateral, and posterior atrial walls and terminate at the AV node. They have similar fibers to purkinje fibers of the ventricles
what is the total delay time of the AV node?
.13 second plus .03 from travel from the SA node so a total of .16 second.
what causes the slow conduction through the AV node?
less gap junctions
Start at AV node and travel through the ventricles. They transmit the action potential 1.5-4 meters per second which is 6 times the norm of a ventricular fiber.
what causes the purkinje fibers fast conduction?
high number of gap junctions in intercalated discs.
what causes the one way conduction pathway?
the fibrous barrier between the atria and ventricles
why is the SA node the pacemaker?
its rhythmical discharge is faster than any other part of the heart
when pacemaker is any other than SA node
conduction fales to pass into ventricles. This will result in a ectopic pacemaker in the purkinje system that will beat at a different rate ( usually like 15-40 beats/ min)
stokes adam syndrome
during an AV block when the purkinje system is trying to establish a new pacemaker the ventricles dont pump blood for 5 to 20 seconds. The person faints after 4-5 seconds and blood stops going to the brain. ( can be lethal)
releases acetocholine twhich decreases the rate of sinus node and decreases excitability of AV junctional fibers between atrial muscle and AV node
Acetylcholine in parasympathetic stimulation
Causes potassium to leak out which leaves the fiber in a hyperpolarized state which makes the tissue less excitable. When the SA node is hyperpolarized it reaches levels like -65 to -75 compared to -55 to -6.0 mv. This greatly slows the rate of rythmicity of these fibers.