02/03/16

Question 1

Sequential Excitation of Chambers of the Heart

Answer 1

• Particular cardiac cells located in the sinoatrial (SA) node have the ability to initiate an action potential
  
  • Spreads through the muscle cells in the upper part of the heart (right/left atria)
  • Via intercalated discs
  • Results in contraction of the two chambers
• Normal sinus rhthym; SA node initiates action potentials at 60-100/min
  
  • Conducted in the atrioventricular (AV) node
    
    • Coordinates the sequential contraction of the upper (atrial) and lower (ventricular) chambers of the heart
• If SA node fails to provide action potential, AV node can independtly generate action potentials at 40-60 bpm
• Subsequent transmission of the action potential to the ventricles
  
  • Via specialized heart cells (cardiomyocytes) in the right and left bundle branches and Purkinje fibers that spread throughout the ventricles
  • These conduction cells contain increased numbers of sodium ion channels and mitochondria while containing fewer cardiac muscle fibers
• If both the SA and AV nodes fail to provide an action potential, the Purkinje fibers or ventricular myocytes can also produce a coordinated contraction at rates slower than the AV node.

Question 2

Ion Channels and Pumps in the Heart

Answer 2

Question 3

Ionic components of the action potential of the heart

Answer 3

0. Rising phase: Na+ channels are activated, rapid depolarization, Na+ infux
1. Cells start to repolarize: outward flow of K+ ions, Na+ channels inactivate
2. Ca2+ channels open (L-type) and ions flow inwards; K+ current is still flowing out (Ca2+ influx, K+ efflux)
   
   • Activation of Ca2+ channels are slower and remain open longer compared to Na+
   • K+ efflux: delayed rectifier K+ channels
   • Plateau is caused by no net current and membrane potential does not change
3. Continued K+ efflux, Ca2+ removed by Na+/Ca2+ exchanger
4. Resting phase: hyper polarization

Question 4

Pacemaker potential during diastole

Answer 4

• Phase 4:
  
  • Diastolic repolarization requires HCN channels (hyperpolarization-activated, cyclic nucleotide gated ion channels)
• Pacemaker cells are capable of initiating self-depolarization during phase 4

Question 5

Action Potentials in Cardiac Tissues

Answer 5

• SA node→atria→AV node→ventricles
• AV node is the “gate keeper”
  
  • As electrical impulses reach AV node, a delay in conduction allows atria time to contract and fully empty their contents before ventricular contraction
• Purkinje fiber activation:
  
  • Large, fast inward current
  • Rapid conduction
  • A lot of Na+ channels
• P: atrial contraction
• PR: atria→AV node
• PQR: ventricular depolarization
• QRS: depolarization of ventricles
• QT: ventricular action potential
• T: repolarization

Question 6

Sodium Channel Gating

Answer 6

• Closed→change in membrane potential→open→inactivated→(hyperpolarizes-recovery)→closed
• Membrane responsiveness

Question 7

Vmax and Effective Refractory Period

Answer 7

• Vmax or dV/dt
  
  • Rate of rise of the action potential n mV/sec
  • Slope
• deltaV is the time from close to open
  
  • (-)deltaV is time from inactive to closed (recovery time)
• ERP: Effective Refractory Period
  
  • Period from beginning of action potential until cell can fire another action potential
  • Membrane responsiveness–enough Na+ channels to fire
  • Modified by arrhythmia drugs

Question 8

Arrhythmia Definitions

• Bradycardia
• Tachycardia
• Flutter
• Fibrillation

Answer 8

• Bradycardia: Resting heart rate below 60 bpm
• Tachycardia: Steady heart rate about 100 bpm
• Flutter: Regular contractions above 200 per minute
• Fibrillation: Irregular, asynchronous contractions about 200 per minutes
  
  • Atrial Fibrillation
  • Ventricular Fibrillation

Question 9

Normal ventricular excitation vs. Ventricular Fibrillation

Answer 9

• Ventricular Fibrillation: AKA cardia arrest
  
  • Heart no longer pumps but cells are still firing
  • No coordination of contraction
  • Rapid stimulation of ventricles

Question 10

Cellular pathologies that cause arrhythmias

Answer 10

1. Abnormal autonomic regulation
2. Tissue malformation
3. Tissue damage and remodeling

Question 11

Cellular pathologies that cause arrhythmias: Abnormal Autonomic Regulation

Answer 11

• Excessive sympathetic tone:
  
  • Sinus tachycardia: rapid heart rate, but normal sinus rhythm
• Excessive parasympathetic tone:
  
  • Sinus bradycardia: slow heart rate, but normal sinus rhythm
  • AV block: atrial action potentials without ventricular action potentials

Question 12

Cellular pathologies that cause arrhythmias: Tissue Malformation

Answer 12

• Abnormal conduction pathways
  
  • Example: Wolff-Parkinson-White Syndrome
    
    • Abnormal tissue pathway