Topic 23 - Electrical activity of the heart, electromechanical coupling

Question 1

Words to include in electrical activity of the heart

Answer 1

• Excitable tissues
  
  • Working muscle fibers
    
    • Elongated AP
      
      • Plateu
    • Contractile components of myocardium
    • Syncitium
      
      • Gap junctions
    • Endocardium
    • Epicardium
    • Apex
  • Pacemaker cells
    
    • Resting membrane potential
    • Automatic depolarization
    • Sinoatrial node (SA node)
    • Atrioventricular node (AV node)
    • Ca²⁺ influx (ø Na⁺)
    • Round pacemaker cells
    • Elongated pacemaker cells
  • Conductive system
    
    • Synchronized contraction
    • Subendocardial conduction
    • Nomotropic excitation
    • Heterotropic excitation
    • Anulus fibrosus
      
      • Electrical resistance
    • Subepicardium contraction
    • Sinoatrial node
    • Atrioventricular node
    • Left posterior bundle
    • Right bundle
    • Bachmann’s bundle
    • His bundle
    • Tawara bundles
    • Purkinje fibers
• Myocytes
  
  • Myocardium
• Neural factors influencing heart rate
  
  • Chronotrop
  • Dromotrop
  • Bathmotrop
  • Ionotrop
• Sympathetic
• Parasympathetic
• Roud cells of SA node
  
  • Sympathetic effect
    
    • β1-receptor
    • G-protein mediated IC cAMP (↑)
    • MDP (Max Diastolic Depolarization)
    • SDD (Spontaneous Diastolic Depolarization)
    • Threshold (↓)
    • Heart rate (↑)
    • Norepinephrine
      
      • Neural signal
    • Epinephrine
      
      • Endocrinological signal
  • Parasympatheic effect
    
    • Acetylcholine
    • Muscarinic acetylcholine receptor
    • cAMP (↑)
    • MDP (Max Diastolic Depolarization)
    • SDD (Spontaneous Diastolic Depolarization)
    • Threshold potential (↑)
    • Hyperpolarization
    • Heart rate (↓)
    • Metabotropic effect

Question 2

Words to include in action potential of working fibers

Answer 2

• Membrane potential
• Resting membrane potential (RMP)
  
  • -90 mV
• Action potential (AP)
• Threshold potential
• Voltage sensitive fast sodium channels
• Depolarization (0 phase)
  
  • Na⁺ influx
• Overshoot (1 phase)
• Plateu (2 phase)
  
  • Ca²⁺ influx
  • K⁺ efflux
• Full repolarization (3 phase)
  
  • K⁺ efflux
    
    • Electrochemical gradient
  • Ca²⁺ channels close
• Refractory phase
  
  • Absolute refractory phase
  • Relative refractory phase
  • Supernormal refractory phase
    
    • Premature contraction
• Mechanogram

Question 3

Words to include in electromechanical coupling

Answer 3

• T-type rynoid sensitive Ca²⁺ channel
• L-type DHP sensitive Ca²⁺ channel
• Na⁺/Ca²⁺ antiporter
• T-tubulus
• IC space
• EC space
• ATP dependent, Ca²⁺ “in” pump
• Sarcoplasmic reticulum (SR)
• Mitochondria
• Diad
  
  • Structural unit
• Electrical stimulus
  
  • Excitation
• Mechanical stimulus
  
  • Contraction
• Conformational change
• IC Ca²⁺
• Sarcomere
• Acetylcholine
• Ca²⁺ signal
  
  • Elimination
    
    • Relaxation

Question 4

Electrical activity of the heart

Give the excitable tissues

Answer 4

• There are 3 excitable tissues:
  
  1. Working muscle fibers
  2. Pacemaker cells
  3. Conductive system
• Called myocytes
  
  • Location: myocardium
• Additional elements:
  
  1. Anulus fibrosus (ø conducting)​
  2. Aschoff-Tawara node (delays atrial signal)

Question 5

Electrical activity of the heart

Working muscle fibers

Answer 5

• Generate elongated AP / plateu
  
  • ​Prevents the heart from early secondary contraction
• ​Contractile components of myocardium
• Constiture a syncitium
  
  • ​The muscle cells are connected via gap junctions that ensure an instant conduction of electrical activities from one cell to another

Question 6

Electrical activity of the heart

Action potential of working fibers

Answer 6

• Membrane potential
  
  • ​Electrical differences measured between the outer and inner side of the membrane (sarcolemma)
• Resting membrane potential (RMP)
  
  • Electrical differences measured between the outer and inner side of the membrane in resting state
  • Ø pacemaker cells
• Action potential (AP)
  
  • Stimuli → ion channels of the membrane open and the ion exchange between the two sides lead to electrical changes
• Reaching the threshold potential: the voltage sensitive fast sodium channels open and a sudden influx of the Na⁺ from the EC occur
• Enters 0. phase of the AP
• Depolarization (0. phase)
• Overshoot (1. phase)
• Plateu (2. phase)
  
  • Role: prevents a premature generation of a new AP
• Full repolarization (3. phase)

Question 7

Electrical activity of the heart

Difference between AP and mechanogram of cardiac and skeletal mucles

Answer 7

• Ø plateu phase in skeletal AP
• The skeletal AP lasts for millisec in contrast to the 200 msec AP of the heart
• The cardiac mechanogram is almost parallel with the AP
• The skeletal mechanogram develops only after the AP has vanished

Question 8

Electrical activity of the heart

Refractory phases of working heart muscle fibers

Answer 8

1. Absolute refractory phase
   
   • No stimulus can elicit a new AP before the end of phase 2 (plateu)
2. Relative refractory phase
   
   • A stimulus given after the end of the plateau phase but before reaching the threshold potential elicits an answer in function of the strength of the stimulus
   • Strong stimuli can elicit the formation of a new AP
3. Supernormal phase
   
   • A slight stimulus can elicit a new AP → produce a premature new contraction

Question 9

Electrical activity of the heart

Pacemaker cells

Answer 9

• Ø resting membrane potential, but turns into automatic depolarization
• Specialized heart muscle cells
• Location:
  
  • Sinoatrial node (SA node)
  • Atrioventricular node (AV node)
• The pacemaker AP is slower and lower than that of cardio myocytes
  
  • Reason: depends solely on calcium influx (not sodium)
• Round pacemaker cells
  
  • Site of the generation of the excitation
• Elongated pacemaker cells
  
  • Conduct and synchronize the excitation generated in the round pacemaker cells

Question 10

Electrical activity of the heart

Conductive system

Answer 10

• Provides rapid spreading of stimuli
  
  • Providing synchronized contraction between atria and ventricles
• Small animals: Subendocardial conduction
  
  • ​Conductive fibers do not go deeply into working muscle
• Large animals: Subepicardium conduction
  
  • Deeply into ventricle
• If the signal comes from the SA node: nomotropic excitation
• If the signal comes from the AV node: heterotropic excitation
  
  • Delays the conductivity
• Anulus fibrosus
  
  • Electric resistance
• His bundle → Tawara bundles → Purkinje fibers
  
  • Responsible for fast conduction

Question 11

Electrical activity of the heart

Neural factors influencing heart rate

Answer 11

• Chronotrop
  
  • ​Frequncy, time of contraction
• Dromotrop
  
  • ​Speed of conduction
• Bathmotrop
  
  • ​Threshold
• Ionotrop
  
  • ​Force of contraction
• By stimulating the round cells of the SA node:
  
  1. Sympathetic effect:
     
     • Stimulation of β1-receptor
       
       • Stimulates G-protein mediated IC cAMP ↑
       • MDP (max diastolic depolarization) ↑
       • SDD (spontaneous diastolic depolarization) ↑
       • Threshold ↓
       • Heart rate ↑
     • Same effect can be triggered by:
       
       • Norepinephrine (neural signal)
       • Epinephrine (endocrinological signal)
  2. parasympathetic effect:
     
     • ACh stimulate muscarinic acetylcholine receptors on round cells
       
       • cAMP ↓
       • MDP ↓
       • SDD slope ↓
       • Threshold potential ↑
       • Hyperpolarization
       • Heart rate ↓
     • Metabotropic effect

Question 12

Electromechanical coupling

Define electromechanical coupling

Answer 12

Connection between electric stimulus (excitation) and mechanical signal (contraction)

Question 13

Electromechanical coupling

Structural unit

Answer 13

• Diad
  
  • ​T-tubulus + 1 sarcoplasmic reticulum
• Skeletal muscle: triad

Question 14

Electromechanical coupling

Compartments

Answer 14

• T-type rynoid sensitive Ca²⁺ channel
• L-type DHP sensitive Ca²⁺ channel
• Na⁺/Ca²⁺ antiporter
• T-tubulus
• IC space
• EC space
• ATP dependent, Ca²⁺ “in” pump
• Sarcoplasmic reticulum (SR)
• Mitochondria

Question 15

Electromechanical coupling

Steps

Answer 15

1. AP spreads onto the cell
2. AP reaches the T-tubules and activates the tubular L-type Ca²⁺ channel (voltage-gated, DHP-sensitive channels)
3. Conformation changes of L-type channels opens the T-type channels on the SR (rianodin-sensitive channels)
4. A large amount of Ca gets into the sarcoplasma from SR
5. Elevating sarcoplasmatic level of Ca²⁺ opens the Ca²⁺- dependent Ca²⁺ channels on the SR
6. Elevating sarcoplasmatic level of Ca²⁺ also opens the Ca²⁺-dependent Ca²⁺ channels on the cell membrane (sarcolemma)
7. Result: huge amount of Intra Cytoplasmic Ca²⁺ is around the sarcomeres → contraction

Question 16

Electromechanical coupling

Elimination of calcium signal

Answer 16

• After the contraction:
  
  1. Na⁺/Ca²⁺ antiporter into EC space
  2. ATP-dependent Ca²⁺ transporter into SR
  • Result: IC Ca²⁺ concentration ↓ → relaxation