Heart Automaticity

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Heart Automaticity

• Cardiac muscle depolarizes and contracts on its
  own, with no outside influences
• Independent but coordinated activity due to
• Gap junctions
• Intrinsic cardiac conduction system
• Noncontractile cardiac cells specialized to
  initiate and distribute impulses through the heart

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Autorhythmic Cells

• Unstable resting potential continuously
  depolarize, drifting slowly toward the threshold
  for firing
• Pacemaker potentials initiate action potentials
  that spread throughout the heart to trigger its
  rhythmic contractions

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How do pacemaker cells automatically depolarize?

• Less potassium moving out of cell
• Sodium continues to move into cell
• Therefore gradually raise the charge difference
  across cell membrane, making it more positive
• When the membrane potential reaches a threshold,
  then Calcium channels open
• Rising phase of AP due to Calcium (NOT sodium
  like other cell types)

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Sequence of Excitation in the Heart

• 1. Sinoatrial node
• 2. Atroventricular node
• 3. Atrioventricular bundle
• 4. Right and left bundle branches
• 5. Ventricular walls via Purkinje fibers

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Sinoatrial Node

• Located in the right atrial wall
• Generates action potentials about 75 times a
  minute
• Sets pace for heart rate
• pacemaker
• Sinus rhthym

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Atrioventricular Node

• Located in inferior portion of interatrial
  septum, above tricuspid valve
• Impulse is delayed momentarily at AV node to
  allow atria to complete contraction before the
  ventricles contract

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AV Bundle (Bundle of His)

• Located at superior part of the interventricular
  septum
• Only electrical connection between the atria and
  the ventricles

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Right and Left Bundle Branches

• Follow interventricular septum toward the apex

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Purkinje Fibers

• Penetrate into the apex of the heart, then turn
  superiorly into the ventricular walls

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Modifying the Basic Rhythm

• Via the central nervous system
• Sympathetic cardioacceleratory center
• the accelerator
• Parasympathetic cardioinhibitory center
• the brakes

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Electrocardiography (EKG)

• Monitors electrical currents traveling through
  the heart
• Three distinguishable waves
• P wave atrial depolarization
• QRS wave ventricular depolarization
• T wave ventricular repolarization