USI Summer Session Chapter 18 A&P - The Heart part 3

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Cardiac Muscle Contraction

Depolarization

Rhythmic and spontaneous

opens voltage-gated fast Na+ channels in the sarcolemma

Reversal of membrane potential from -90 mV to +30 mV

Depolarization wave

T tubules causes the SR to release Ca2+

opens slow Ca2+ channels in the sarcolemma
Ca2+ surge prolongs the depolarization phase (plateau)

Repolarization

results from inactivation of Ca2+ channels and opening of voltage-gated K+ channels

Intrinsic cardiac conduction system

A network of noncontractile (autorhythmic) cells that initiate and distribute impulses to coordinate the depolarization and contraction of the heart

Autorhythmic Cells

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

pacemaker

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

bundle branches

Two pathways in the interventricular septum that carry the impulses toward the apex of the heart

Purkinje fibers

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

Homeostatic Imbalances

Defects in the intrinsic conduction system may result in Arrhythmias or Fibrillation

Arrhythmias

irregular heart rhythms

Fibrillation

rapid, irregular contractions; useless for pumping blood

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