Cardiac Muscle - Nordgren Flashcards
What are the three ways that cardiac action potentials differ from skeletal muscle action potentials?
- Self-generating
- Conducted directly from cell to cell
- Long duration (long refractory period)
What are the three most important ions in the determination of cardiac transmembrane potential?
- Sodium (Na+) - interstitial fluid
- Calcium (Ca2+) - interstitial fluid
- Potassium (K+) - intracellular fluid
What changes electrical potential in cardiac muscle?
- Current flowing (movement of ions) through the cell membrane
- rate of change is proportional to NET current
What is the electrochemical basis of membrane potentials?
-
Concentrations differences
- e.g. high [K+] inside cell –> K+ leaves
-
Electrical potentials
- e.g. Negative inside cell –> K+ enters
What is the normal equilibrium potential value of K+?
-90mV
What is the normal equilibrium potential value of Na+?
+60 mV
How do the three states of ion channels (open, closed, inactive) determine membrane ion permeability?
Membrane’s permeability = the NET STATUS of the ion channels.
(e.g. “high permeability to Na+ –> many of the Na+ ion channels are open)
How do Activation Gates respond to membrane depolarization?
open QUICKLY
How do Inactivation Gates respond to membrane depolarization?
close SLOWLY
In general, what do action potentials in myocardial Contractile Cells look like?
- APs similar to those of neurons and skeletal muscle
- “Fast Response” or “Depolarization Party”
- 4-0-1-2-3
- Resting membrane potential (4) = -90mV
In general, what do action potentials in myocardial Pacemaker (Autorhythmic) Cells look like?
- Generate APs spontaneously due to unstable membrane potential
- “Slow Response”
- Slower/unstable initial depolarization
- 4-0-3-4
- no plateau
What are the five phases of myocardial Contractile Cells?
- 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+
What are the four phases of myocardial Pacemaker Cells?
- 4 = slow/unstable depolarization
- funny channels open
- large Na+ influx
- small K+ efflux
- transient Ca2+ channel opens
- small Ca+ influx
- funny channels open
- 0 = rapid depolarization
- long-lasting Ca2+ channel opens
- huge Ca2+ influx
- long-lasting Ca2+ channel opens
- 3 = rapid repolarization
- K+ channel opens
- large K+ efflux
- K+ channel opens
What are the main differences in AP between the contractile and the pacemaker cells?
- 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
Why/How does the AV node slow transmission of the action potential signal?
- 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.
What does the T-wave on an electrocardiogram represent?
Ventricular repolarization
What does the P-wave on an electrocardiogram represent?
Atrial depolarization
What does the R-wave on an electrocardiogram represent?
Ventricular depolarization
What does the PR interval on an electrocardiogram represent?
Conduction time through atria & AV node
What does the QT interval on an electrocardiogram represent?
Total duration of ventricular systole (contraction)
What does the ST segment on an electrocardiogram represent?
plateau phase of ventricular APs
What major cardiac event is missing from an electrocardiogram? Why?
- Atrial Repolarization
- masked by large R-wave
- larger tissue –> large depolarization wave
How does acetylcholine slow the heart rate?
- Increases permeability of resting membrane to K+
- Decreases diastolic current through funny channels (Na+influx/K+efflux)
- prolongs time to depolarization
How does norepinephrine increase the heart rate?
- Increase diastolic inward currents through funny channels
- shortens time to depolarization
Why is calcium necessary for muscular contraction?
Calcium displaces tropomyosin, and binds to troponin to iniate contraction.
Ca2+ allows actin & myosin to interact, causing contraction.
What is the difference between preload and afterload?
- Preload:
-
passive/resting tension placed on cardiac muscle cells before contraction
- fxn of the volume and pressure at end of diastole
-
passive/resting tension placed on cardiac muscle cells before contraction
- Afterload:
-
active tension placed on cardiac muscle cells during contraction
- fxn of resistance the left ventricle must overcome to circulate blood
-
active tension placed on cardiac muscle cells during contraction
Summarize the Law of LaPlace? What does it mean/imply?
- 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)
What is Isometric Contraction?
- “Fixed Length”
- ends of muscle held rigidly
- AV valves close –> SL valves open
What is Isotonic Contraction?
- “Fixed Tension”
- Activation of unrestrained muscle (not fixed length)
- causes it to shorten without force because it has nothing to develop force against
What is the Inotropic state?
Altered cardiac activity due to an agent or substance
(e.g. ACh, NE)
What are the refractory periods of the cardiac cell electrical cycle?
- 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
What is the normal pathway of action potential (electrical) conduction through the heart?
- SA Node (right atrium)
- Left atrium (Bachmann’s Bundle)
- AV Node
- Bundle of His (Atrioventricular Bundle)
- Right/Left Ventricles (Bundle Branches)
- Purkinje fibers
How do cardiac parasympathetic nerves alter the heart rate and conduction of cardiac action potentials?
Parasympathetic (ACh/Vagus Nerve) –> Slow HR
“Negative Chronotropic Effect”
(hyperpolarize RMP)
How do cardiac sympathetic nerves alter the heart rate and conduction of cardiac action potentials?
Sympathetic (NE) –> Increase HR
“Positive Chronotropic Effect”
(shortens time to depolarization)
What are the sub cellular structures responsible for cardiac muscle cell contraction?
- 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
Briefly describe the excitation-contraction process?
(Hint: Ca2+)
- 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
