Cardiac Muscle - Nordgren

Question 1

What are the three ways that cardiac action potentials differ from skeletal muscle action potentials?

Answer 1

1. Self-generating
2. Conducted directly from cell to cell
3. Long duration (long refractory period)

Question 2

What are the three most important ions in the determination of cardiac transmembrane potential?

Answer 2

1. Sodium (Na⁺) - interstitial fluid
2. Calcium (Ca²⁺) - interstitial fluid
3. Potassium (K⁺) - intracellular fluid

Question 3

What changes electrical potential in cardiac muscle?

Answer 3

• Current flowing (movement of ions) through the cell membrane
  
  • rate of change is proportional to NET current

Question 4

What is the electrochemical basis of membrane potentials?

Answer 4

• Concentrations differences
  
  • e.g. high [K⁺] inside cell –> K⁺ leaves
• Electrical potentials
  
  • e.g. Negative inside cell –> K⁺ enters

Question 5

What is the normal equilibrium potential value of K⁺?

Answer 5

-90mV

Question 6

What is the normal equilibrium potential value of Na⁺?

Answer 6

+60 mV

Question 7

How do the three states of ion channels (open, closed, inactive) determine membrane ion permeability?

Answer 7

Membrane’s permeability = the NET STATUS of the ion channels.

(e.g. “high permeability to Na⁺ –> many of the Na⁺ ion channels are open)

Question 8

How do Activation Gates respond to membrane depolarization?

Answer 8

open QUICKLY

Question 9

How do Inactivation Gates respond to membrane depolarization?

Answer 9

close SLOWLY

Question 10

In general, what do action potentials in myocardial Contractile Cells look like?

Answer 10

• APs similar to those of neurons and skeletal muscle
  
  • “Fast Response” or “Depolarization Party”
  • 4-0-1-2-3
  • Resting membrane potential (4) = -90mV

Question 11

In general, what do action potentials in myocardial Pacemaker (Autorhythmic) Cells look like?

Answer 11

• Generate APs spontaneously due to unstable membrane potential
• “Slow Response”
• Slower/unstable initial depolarization
• 4-0-3-4
  
  • no plateau

Question 12

What are the five phases of myocardial Contractile Cells?

Answer 12

• 4 = RMP around -90mV
  
  • Na⁺ channels open
  • K⁺ channels (inward rectifier) close
• 0 = rapid depolarization
  
  • Na⁺ influx
• 1 = initial repolarization
  
  • Na⁺ channels close
  • Fast K⁺ channels (transient outward) open causing K⁺ influx
• 2 = plateau (almost net 0 movement)
  
  • K⁺ channels close
  • Ca²⁺ channels (slow inward) open
• 3 = rapid repolarization
  
  • ​Slow K⁺ channels (delayed rectifier) open causing K⁺ efflux
  • Ca²⁺ channels close
• 4 = return to RMP
  
  • K⁺ channels (delayed rectifier) close
  • K⁺ channels (inward rectifier) open causing influx of K⁺

Question 13

What are the four phases of myocardial Pacemaker Cells?

Answer 13

• 4 = slow/unstable depolarization
  
  • funny channels open
    
    • large Na⁺ influx
    • small K⁺ efflux
  • transient Ca²⁺ channel opens
    
    • small Ca⁺ influx
• 0 = rapid depolarization
  
  • long-lasting Ca²⁺ channel opens
    
    • huge Ca²⁺ influx
• 3 = rapid repolarization
  
  • K⁺ channel opens
    
    • large K⁺ efflux

Question 14

What are the main differences in AP between the contractile and the pacemaker cells?

Answer 14

• Phase 4:
  
  • RMP vs. unstable pacemaker potential
• Phase 0:
  
  • mediated by Na⁺ in contractile cells
  • mediated by Ca²⁺ in pacemaker cells
• Phase 1 & 2:
  
  • absent in pacemaker cells
• Phase 3:
  
  • no difference

Question 15

Why/How does the AV node slow transmission of the action potential signal?

Answer 15

• Determined by # of gap jxns in the intercalated discs
  
  • fewer gap jxns = slower
• AV node has fewer gap junctions that the SA node and atrial myocardium.

Question 16

What does the T-wave on an electrocardiogram represent?

Answer 16

Ventricular repolarization

Question 17

What does the P-wave on an electrocardiogram represent?

Answer 17

Atrial depolarization

Question 18

What does the R-wave on an electrocardiogram represent?

Answer 18

Ventricular depolarization

Question 19

What does the PR interval on an electrocardiogram represent?

Answer 19

Conduction time through atria & AV node

Question 20

What does the QT interval on an electrocardiogram represent?

Answer 20

Total duration of ventricular systole (contraction)

Question 21

What does the ST segment on an electrocardiogram represent?

Answer 21

plateau phase of ventricular APs

Question 22

What major cardiac event is missing from an electrocardiogram? Why?

Answer 22

• Atrial Repolarization
  
  • masked by large R-wave
  • larger tissue –> large depolarization wave

Question 23

How does acetylcholine slow the heart rate?

Answer 23

• Increases permeability of resting membrane to K⁺
• Decreases diastolic current through funny channels (Na⁺influx/K⁺efflux)
  
  • prolongs time to depolarization

Question 24

How does norepinephrine increase the heart rate?

Answer 24

• Increase diastolic inward currents through funny channels
  
  • shortens time to depolarization

Question 25

Why is calcium necessary for muscular contraction?

Answer 25

Calcium displaces tropomyosin, and binds to troponin to iniate contraction. Ca²⁺ allows actin & myosin to interact, causing contraction.

Question 26

What is the difference between preload and afterload?

Answer 26

* Preload: * *passive*/resting tension placed on cardiac muscle cells *before* contraction * fxn of the volume and pressure at end of diastole * Afterload: * *active* tension placed on cardiac muscle cells *during* contraction * fxn of resistance the left ventricle must overcome to circulate blood

Question 27

Summarize the Law of LaPlace? What does it mean/imply?

Answer 27

* The *total* ventricular wall tension (T) depends on both intraventricular pressure (P) and the internal ventricular radius (r). * T = P x r * Implication: * easier for muscle cells to produce adequate internal pressure at end of ejection (small radius) than beginning of ejection (large radius)

Question 28

What is Isometric Contraction?

Answer 28

* "Fixed Length" * ends of muscle held rigidly * AV valves close --\> SL valves open

Question 29

What is Isotonic Contraction?

Answer 29

* "Fixed Tension" * Activation of unrestrained muscle (not fixed length) * causes it to shorten without force because it has nothing to develop force against

Question 30

What is the Inotropic state?

Answer 30

Altered cardiac activity due to an agent or substance (e.g. ACh, NE)

Question 31

What are the refractory periods of the cardiac cell electrical cycle?

Answer 31

* Contractile cells * Phase 0 - Phase 3 * From rapid depol --\> plateau --\> rapid repol * Pacemaker cells * Phase 0 + Phase 3 * From rapid depol --\> rapid repol * _{Cells are in absolute refractory state during most of the AP} * _{cannot be stimulated for another firing} * _{precludes summated or tetanic contractions}

Question 32

What is the normal pathway of action potential (electrical) conduction through the heart?

Answer 32

1. SA Node (right atrium) 2. Left atrium (Bachmann's Bundle) 3. AV Node 4. Bundle of His (Atrioventricular Bundle) 5. Right/Left Ventricles (Bundle Branches) 6. Purkinje fibers

Question 33

How do cardiac parasympathetic nerves alter the heart rate and conduction of cardiac action potentials?

Answer 33

Parasympathetic (ACh/Vagus Nerve) --\> Slow HR "Negative Chronotropic Effect" (hyperpolarize RMP)

Question 34

How do cardiac sympathetic nerves alter the heart rate and conduction of cardiac action potentials?

Answer 34

Sympathetic (NE) --\> Increase HR "Positive Chronotropic Effect" (shortens time to depolarization)

Question 35

What are the sub cellular structures responsible for cardiac muscle cell contraction?

Answer 35

* Intercalated Disks * end of two adjacent cells * Fascia Adherens * anchoring site of actin * Macula Adherens (Desmosome) * bind intermediate filaments * **Gap Junctions** * channels formed of *connexin* * facilitate transmission of electrical impulse

Question 36

Briefly describe the excitation-contraction process? (Hint: Ca²⁺)

Answer 36

* AP enters * VG-Ca²⁺ channels open * Ca²⁺ influx * Ryanodine receptor channels release Ca²⁺ * Ca²⁺ SPARK * Ca²⁺ displaces tropomyosin & binds troponin * Actin/Myosin change conformation which initiates contraction * Ca²⁺ unbinds from troponin --\> Relaxation * Ca²⁺ pumped back out (exchanged with Na⁺) * Na⁺ pumped out, K⁺ in via Na/K ATPase