L11 - Cardiac physiology

• What type of muscle is cardiac muscle?

  Striated and branched
• What are intercalated discs in cardiac muscle?

  They connect muscle cells with desmosomes
• What is the role of gap junctions in cardiac muscle?

  They transmit electrical currents
• What is a functional syncytium in the heart?

  Chambers act as single coordinated units
• What are the two types of cardiac muscle cells?

  Contractile and conducting cells
• What is the function of contractile cardiac muscle cells?

  They generate force of contraction
• What is the role of conducting cardiac muscle cells?

  They set the rate of contraction
• What does the intrinsic conduction system do?

  Sets the basic rhythm of the heart
• How does the heart depolarize and contract?

  Without nervous system stimulation
• What modifies the basic rhythm of the heart?

  Extrinsic innervation from the ANS
• What are pacemaker potentials?

  Unstable resting potentials leading to action potentials
• Where are the fastest conducting cells located?

  In the sinoatrial (SA) node
• How is the action potential conducted throughout the heart?

  Via gap junctions and the conducting system
• What is the sequence of the intrinsic conduction system?

  1. Sinoatrial (SA) node
  2. Atrioventricular (AV) node
  3. Atrioventricular (AV) bundle (bundle of His)
  4. Purkinje fibers
• What is the function of the atrioventricular (AV) node?

  It connects atrium and ventricle electrically
• What does the AV node do before ventricles contract?

  Delays firing slightly
• What is the role of the AV bundle (bundle of His)?

  Conducts impulses into ventricles from AV node
• What do Purkinje fibers do?

  Distribute impulses throughout ventricles
• How do ventricles contract?

  Beginning at apex and moving towards atria
• How does the autonomic nervous system modify heart rate?

  Through cardiac centers in the medulla oblongata
• What does the cardioacceleratory center do?

  Increases heart rate and contractile force
• What is the function of the cardioinhibitory center?

  Slows heart rate via the vagus nerve
• How does the sympathetic branch adjust heart rate?

  Releases norepinephrine and epinephrine
• What receptors do norepinephrine and epinephrine bind to in the heart?

  Adrenergic receptors (β1 type)
• What effect does norepinephrine have on depolarization?

  Increases rate of depolarization to threshold
• How does the parasympathetic branch adjust heart rate?

  Releases acetylcholine that binds to receptors
• What receptors does acetylcholine bind to in the heart?

  Muscarinic cholinergic receptors (M2 type)
• What effect does acetylcholine have on depolarization?

  Decreases rate of depolarization to threshold
• What is the resting membrane potential of contractile cardiac muscle cells?

  Stable with no pacemaker potential
• What happens during rapid depolarization of contractile cardiac muscle cells?

  Requires outside stimulation
• What characterizes the action potential of contractile cardiac muscle cells?

  Prolonged relative to skeletal muscle
• Why is the refractory period longer in cardiac muscle?

  Ensures heart can refill with blood
• What does an electrocardiogram (ECG) measure?

  Sum of membrane potential changes in heart
• What are the three deflections in a typical ECG?

  P wave, QRS complex, T wave
• What does the P wave represent in an ECG?

  Atrial depolarization
• What does the QRS complex represent in an ECG?

  Ventricular depolarization
• What does the T wave represent in an ECG?

  Ventricular repolarization
• What are the mechanical events of the cardiac cycle?

  • Systole: contractile phase
  • Diastole: relaxation phase
  • Atrial systole followed by ventricular systole
  • Mechanical events follow electrical events seen on ECG
• What happens during ventricular filling?

  AV valves open, blood flows into ventricles
• What percentage of total ventricular volume flows passively into ventricles?

  ~80%