Lecture 7: Cardiac Muscle Tissue

Question 1

Describe the “First Half” of blood’s journey through the heart.

Answer 1

• Deoxygenated Blood enters the heart through the Superior Vena Cava (for head and upper extremities), or through the Inferior Vena Cava for lower extremities.
• The blood is funneled into the Right Atrium, and pushed through the Tricuspid Valve.
• After passing the tricuspid valve, the blood enters the Right Ventricle until it gets pushed up the Pulmonary Valve into the Pulmonary Artery
• The pulmonary artery carries the blood off into the lungs

Question 2

Describe the “Second Half” of blood’s journey through the heart.

Answer 2

• Oxygenated blood reenters the heart through the Pulmonary Vein.
• The vein delivers the blood to the Left Atrium, where it is pushed through the Mitral Valve
• After passing through Mitral Valve, Blood sits in the Left Ventricle.
• From the Left Ventricle, the blood passes through Aortic Valve into the Aorta. From there, the Aorta delivers the oxygenated blood back into both the upper and lower extremities.

Question 3

Describe the features and characteristics of Cardiac Muscle Tissue. (Be able to compare it against skeletal muscle tissue)

Answer 3

• Sarcomeric arrangement (therefore, striated)
• Mononucleated
• Central nuclei
• Syncytium
• Intercalated discs
• Cells may branch

Question 4

How is the action potential different in cardiac muscle fiber?

Answer 4

• Averages about 105 mV, and can rise from about -85 to + 20 mV
• The potential stays fully depolarized for about 0.2 seconds. Unlike skeletal muscle tissue, which is much faster, this is a very long time span for a cell.
• The cardiac muscle cell undergoes sudden repolarization following the depolarization plateau.

Question 5

How do skeletal muscle fibers compare to cardiac muscle fiber with regard to the T-Tubules and sarcoplasmic reticulum?

Answer 5

• In cardiac muscle, T-tubules are found along the z-line rather than at the ends of the thick filaments.
• There is only one cisterna per T-tubule instead of two
• T-tubules form Diads with the sarcoplasmic reticulum instead of triads
• The sarcoplasmic reticulum isn’t as extensive or prominent in cardiac muscle
• Cardiac muscle cells form a synctium

Question 6

There are two types of cardiac action potentials: Fast and Slow.

Describe the Fast one.

Answer 6

• Found in atria, ventricles and conduction system
• Very rapidly conducting but non-contractile in Purkinje fibers
• Rapidly conducting and contractile in atrial and ventricular fibers
• High amplitude (100 mV)
• These conduct well, but don’t contract well.
• Fast action potentials are due to changes in conductance of potassium, sodium, and calcium ions.
• Conductance pattern is mostly due to voltage dependent gates.

Things that result in a faster conduction velocity:
Greater AP amplitude, More rapid rate of rise of phase 0, and Larger cell diameter

Question 7

There are two types of cardiac action potentials: Fast and Slow.

Describe the Slow one.

Answer 7

• Found in SA and AV nodal tissues
• Conducts slowly
• Automatically depolarizes during resting phase, and more rapidly in the SA node than in the AV node
• Low amplitude (60 mV)
• These conduct slowly but rapidly contract
• No fast sodium ion gates
• Upstroke (negative to positive) of action potential is due to calcium (therefore it proceeds slowly).
• Resting phase potential 4 is close to -60 mV rather than -90 mV characteristic of fast action potentials.
• Change in potential (amplitude) is less than that for fast action potentials.
• SA and AV nodal tissue will spontaneously depolarize slowly to reach threshold during phase 4 (resting phase).

Question 8

Describe each of the phases of a cardiac muscle action potential (See Lecture 7 - Slide 21 for further diagram)

Answer 8

-Phase 4: Resting potential
-Phase 0: Rapid depolarization
-Phase 1: Initial, incomplete repolarization
-Phase 2: Plateau or slow decline of membrane potential
-Phase 3: Repolarization
Why it’s out of order, I have no idea.
So it should go from 4 to 0 to 1 to 2 to 3 back to 4.

Question 9

Compare the characteristics between:

1. Fast-type contractile myocytes
2. Fast-type non-contractile myocytes
3. Slow-type non-contractile myocytes

Answer 9

1. Large diameter, High amplitude, Rapid onset of action potential
2. Very large diameter, Very rapid upstroke
3. Small diameter, Low amplitude, Slow rate of depolarization

Question 10

Explain the role of calcium, sodium, and potassium ions in the creation of the cardiac muscle action potential plateau (26)

Answer 10

In skeletal muscle, the sodium channels close rapidly.

• In cardiac muscle the sodium channels also close rapidly, but the calcium channels open slowly and stay open for a longer period of time.
• In cardiac muscle there is also a delay in the opening of the potassium channels.
• The large concentration of both calcium ions and potassium ions is responsible for the plateau.

Question 11

Describe the automaticity of the heart (29)

Answer 11

Some cardiac tissues will gradually depolarize during phase 4, eventually reaching threshold.

• • These tissues include the SA and AV nodes and the Purkinje fibers.
• • The SA node usually depolarizes more rapidly than the others and reaches threshold first and, by default, becomes the normal “pacemaker” of the heart’s rhythmicity.
• Rate of depolarization determines the rhythmicity of the cell.
• Gradual depolarization during phase 4 is due to:
• • Special sodium channels which open following phase 3
• SA = Sinoatrial; SA node has the fastest intrinsic pulsation rate.

Question 12

Describe the resting membrane potential of a resting node fiber

Answer 12

• 55 to -60 mV (Threshold ≈ -40 mv) (That’s a postive 55)
• Fast sodium channels are already inactivated (blocked).
• • Inactivation gates close when membrane potential is less negative than -55 mV.
• Therefore, only slow sodium-calcium channels can open.
• • Therefore, atrial nodal action potential is slower to develop.
• • Therefore, repolarization is also slower.
• There is a slow leak of sodium ions back into the cells.
• Membrane potential becomes more positive.
• At -40 mV, sodium-calcium channels become activated.
• Sodium-calcium channels are inactivated within 100-150 msec after opening.
• Large numbers of potassium channels open at the time the sodium calcium channels become inactivated.
• Nodal cells become repolarized.
• Potassium channels remain open for a few tenths of a second.

Question 13

What’s the resting potential of a ventricular fiber?

Answer 13

-85 to -90 mV

Question 14

What are the exchanging roles and relationships between Ca, K, and Na? (See slide 34 and memorize the fuck out of it)

Answer 14

In a sinus nodal fiber:

1. Sodium influx at phase 4
2. Calcium influx at phase 0
3. Potassium Efflux at phase 3.

In a Ventricular Muscle Fiber:
This doesn’t have as obvious of phases, so this’ll be harder…but:
- Sodium Influx when it shoots up
- Fast Potassium efflux at the peak
- Calcium influx for the first phase of repolarization (at this point potassium is at equilibrium in and out.
- Delayed Potassium efflux during the downward peak.
Christ i suck at this.

Question 15

Describe the SA node’s role as a pacemaker

Answer 15

• Action potentials originating in the SA node generate a “sinus” rhythm.
• Action potentials that originate anywhere else are said to be from an ectopic focus or pacemaker

Question 16

Describe the mechanism of calcium release during the contraction of a cardiomyocyte with regard to DHP and ryanodine channels and compare with calcium release in skeletal muscle fibers (38).

Answer 16

First off, there’s fewer calcium channels in cardio muscle than skeletal. Just felt like clarifying that.

• When an action potential travels along the sarcolemma of the cardiac myocytes, it enters the T-tubules (which are part of the sarcolemma). - Calcium enters from the extracellular fluid through the voltage-dependent calcium channels (dihydropyridine receptor channels) of the T tubules.
• The elevated level of cytoplasmic calcium triggers more calcium to enter from the cisternae of the longitudinal sarcoplasmic tubules through channels called ryanodine receptors.
• The elevated cytoplasmic calcium binds to troponin and results in myofilament contraction.

Question 17

Describe the role of the two calcium transporters involved in cardiac muscle relaxation (41)

Answer 17

• SERCA
• Sarcoplasmic reticulum calcium ATPase:
• Stimulated by phosphorylation via an integral SR protein called phospholambian which, when phosphorylated, reduces its ability to inhibit the SERCA pump.
• Returns calcium to the sarcoplasmic reticulum during diastole:
• This allows for an even greater calcium release on the next beat as well as fast clearance of calcium from the sarcoplasm
• Sodium-calcium exchanger in the sarcolemma:
• Transports calcium out of the cell.

Question 18

Memorize. The. Fuck out of slide 43.

And 47.

Answer 18

sob

…don’t forget to memorize the PQRST waves.

Question 19

Describe the first 3 steps in the cardiac cycle

Answer 19

1. Atria as primer pumps:
   - About 80% of blood flows from the atria to the ventricles before the atria contract.
   - Atria, therefore, can add an additional 20% by contraction.
2. Ventricular systole:
   - AV valves are closed during systole.
3. End of ventricular systole:
   - AV valves open at the end of systole because of increased pressures in the atria.

Question 20

Describe the middle 4 steps of the cardiac cycle

Answer 20

4. First third of diastole:
   - Rapid filling
5. Middle third of diastole (diastasis):
   - Small amount of blood flows into the ventricles representing blood that continues to flow into atria during diastole.
6. Last third of diastole:
   - Atria contract to push last 20% of blood into the ventricles.
7. Isometric (isovolumic) contraction:
   - Ventricles contract, but semilunar valves do not open for 0.02 to 0.03 seconds.

Question 21

Describe the final 2 steps of the cardiac cycle

I should clarify that I have no idea if these are in a correct order or not.

Answer 21

1. Period of rapid ejection:
   - Occurs when left ventricular pressure is a little above 80 mm Hg and right ventricular pressure is slightly above 8 mm Hg.
   - Semilunar valves open.
   - About 70% of the blood is ejected.
   - Occurs during the first third of ejection.
2. Period of slow ejection:
   - Remaining 30% of blood is ejected from the ventricles.
   - Occurs during the last two-thirds of ejection.

Question 22

Describe the Frank Starling Law

Answer 22

• The greater the heart muscle is stretched during filling, the greater the force of contraction and the greater the quantity of blood pumped into the aorta.
• The stretching of the cardiac muscle brings the actin and myosin filaments to a more nearly optimal degree of overlap for force generation
• This produces a greater force of contraction; basically saying if a Lot of Blood goes into the Heart, A Lot of blood leaves the heart. A contraction only occurs (I think) if the optimal amount of blood enters and leaves.

Question 23

What is EDV?

Answer 23

• This is how much blood is in the heart at the end of the diastole. Can be increased to more during exercise or stress.
• EDV (end diastolic volume) = 110-120 ml
• Can be increased to about 150-180 ml

Question 24

What is SV

Answer 24

SV (stroke volume) = 70 ml

• Amount of blood pumped out of the heart each time the ventricle contracts.

Question 25

What is ESV?

Answer 25

• ESV (end systolic volume) = 40-50 ml
• Can be as little as 10-20 ml
• What is left in the ventricle at the end of a stroke.

Question 26

What is ejection fraction and how does one find it?

Answer 26

• Ejection fraction = SV/EDV = 70/110 ≈ 64%

- Percent of blood able to be ejected from the ventricle per pump…i think.

Question 27

How does one increase their stroke volume?

Answer 27

• Increasing EDV: Gives more blood sitting in the ventricle, so more blood is likely to be pumped out.
• Decreasing ESV: This technically occurs both at once. But yeah.

Question 28

Study the diagram on Slide 53, Lecture 7

And slide 54. In fact all of the diagrams through 60. That’s a little too complicated to make a flashcard for.

Answer 28

Do it.

Question 29

Describe the flow vs. velocity of blood in the proximal aorta, the distal aorta, and arteries.

Answer 29

• Blood in proximal aorta:
• Mean velocity = 40 cm/s
• Flow is phasic
• Velocity ranges from 120 cm/s (systole) to negative value before aortic valves close in diastole.
• Blood in distal aorta and arteries:
• Velocity is greater in systole than diastole.
• Forward flow is continuous because of elastance of vessel walls during diastole.

Question 30

Describe the forces altering blood flow

Answer 30

• The rate of blood flow to each tissue is usually precisely controlled in relation to tissue need.
• Active tissues may need 20 to 30 times as much blood flow than at rest. Tissues get different amounts of blood at different times depending on what they need. The heart cannot feed the entire tissues at once.
• Cardiac output cannot exceed 4-7 X greater than at rest.
• Microvessels of each tissue monitor tissue needs.
• Needs of tissues act directly on local blood vessels.
• Nervous control and hormones also help control tissue blood flow.

Question 31

Review the diagrams from slides 64-68. They don’t look too hard, but they’re still pretty ntk. Like everything else in this damn lecture.

Answer 31

Do it.