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Rhythmic and spontaneous
opens voltage-gated fast Na+ channels in the sarcolemma
Reversal of membrane potential from -90 mV to +30 mV
T tubules causes the SR to release Ca2+
opens slow Ca2+ channels in the sarcolemma
Ca2+ surge prolongs the depolarization phase (plateau)
Intrinsic cardiac conduction system
A network of noncontractile (autorhythmic) cells that initiate and distribute impulses to coordinate the depolarization and contraction of the heart
Have unstable resting potentials (pacemaker potentials or prepotentials) due to open slow Na+ channels
At threshold, Ca2+ channels open
Explosive Ca2+ influx produces the rising phase of the action potential
Repolarization results from inactivation of Ca2+ channels and opening of voltage-gated K+ channels
Sinoatrial (SA) node
Generates impulses about 75 times/minute (sinus rhythm)
Depolarizes faster than any other part of the myocardium
Atrioventricular (AV) node
Smaller diameter fibers; fewer gap junctions
Delays impulses approximately 0.1 second
Depolarizes 50 times per minute in absence of SA node input
Atrioventricular (AV) bundle
Also called Bundle of His
Only electrical connection between the atria and ventricles
Two pathways in the interventricular septum that carry the impulses toward the apex of the heart
Complete the pathway into the apex and ventricular walls
AV bundle and Purkinje fibers depolarize only 30 times per minute in absence of AV node input
Defects in the intrinsic conduction system may result in Arrhythmias or Fibrillation
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