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SF3- Exam 1 > Physiology of Cardiac Muscle > Flashcards

Physiology of Cardiac Muscle Flashcards

1
Q

What are the major characteristics of cardiac muscle?

A
  • Mononuclear
  • Striations faintly visible
  • INTERCALATED DISK
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2
Q

Which three cell junctions make u intercalated discs?

A

Gap Junctions

Desmosomes

Adhering junctions

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3
Q

What level of heart excitiation is being represented in the picture?

A

SA node generates impulses; atrial excitation begins

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4
Q

What level of heart excitiation is being represented in the picture?

A

Impluse delayed at AV node

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5
Q

What level of heart excitation is being represented in the picture?

A

Impulse passes to heart apex; ventricular excitation begins

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6
Q

What level of heart excitation is being represented in the picture?

A

Ventricular excitation complete

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7
Q

What is the sequence of heart excitation?

A
  • SA node generates impulses at about 75 times/min
  • AV node delays the by 0.1 second
  • Impulse passes from atria to ventricles via the AV bundle of His
  • AV bundle splits into two pathways in the interventricular septum
    • Bundle branches carry the impuse toward the apex of the heart
    • Purkinje fibers carry the impulse to the heart apex and ventricular walls
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8
Q

Why is it important that the AV node delays the impulse by approximately 0.1 seconds?

A

It ensures that the atria have ejected their blood into the ventricles first before the ventricles contract.

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9
Q

Pacemaker cells use _________ influx for rising phase of the action potential.

A

Calcium (as opposed to sodium)

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10
Q

Pacemaker cells have unstable resting potentials called _____________.

A

Pacemaker potentials

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11
Q

Where are pacemaker cells normally found?

A

In the SA node

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12
Q

The main ions found outside the cell at rest are ______________ and ___________, whereas inside the cell it is mainly _____________.

A

Sodium and chloride; potassium

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13
Q

What are the 5 phases of the ventricular myocyte membrane potential?

A

Phase 0- Depolarization

Phase 1- Transient

Phase 2- Plateau

Phase 3- Repolarization

Phase 4- Resting Potential

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14
Q

What occurs in phase 4 of the ventricular myocyte membrane potential?

A
  • Phase 4 occurs when the cell is at rest, in a period known as diastole.
  • Voltage ~~ -90mV
  • Membrane is most permeable to K+

NOTE: Pacemaker cells are never at rest

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15
Q

What occurs in phase 0 of the ventricular myocyte membrane potential?

A
  • Depolarization
  • In non-pacemaker cells (i.e. ventricular cells), this is produced predominantly by the activation of Na+ channels, which increases the membrane conductance (flow) of Na+.
  • These channels are activated when an action potential arrives from a neighbouring cell, through gap junctions.

NOTE: In pacemaker cells (e.g. sinoatrial node cells), however, the increase in membrane voltage is mainly due to activation of L-type calcium channels.

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16
Q

What occurs in phase 1 of the ventricular myocyte membrane potential?

A
  • Transient
  • Phase begins with the rapid inactivation of the Na+ channels by the inactivation gate
    • Reduces the movement of sodium into the cell.
  • At the same time potassium channels open and close rapidly, allowing for a brief flow of potassium ions out of the cell, making the membrane potential slightly more negative.
    • This is referred to as a ‘notch’ on the action potential waveform.

NOTE: There is no obvious phase 1 present in pacemaker cells.

17
Q

What occurs in phase 2 of the ventricular myocyte membrane potential?

A
  • Plateau
  • The membrane potential remaining almost constant, as the membrane very, very slowly begins to repolarize.
  • Phase is important in preventing irregular heartbeat

NOTE: There is no plateau phase in pacemaker cells

18
Q

The cardiac action potential is also known as _________.

A

dV/dt

19
Q

What does the membrane potential remain relatively constant in the plateau phase?

A
  • During this phase delayed rectifier potassium channels allow potassium to leave the cell while L-type calcium channels (activated by the flow of sodium during phase 0), allow the movement of calcium into the cell.
  • This calcium, binds to and opens more calcium channels (called ryanodine receptors) located on the sarcoplasmic reticulum within the cell, allowing the flow of calcium out of the SR.
  • This calcium influx also activates chloride channels, which allow Cl− to enter the cell. Together the movement of both Ca2+ and Cl− oppose the voltage change caused by K+.
  • The increased calcium concentration also increases the activity of the sodium-calcium exchanger, and the increase in sodium entering the cell increases activity of the sodium-potassium pump. The movement of all of these ions results in the membrane potential remaining relatively constant.
20
Q

What occurs in phase 3 of the ventricular myocyte membrane potential?

A
  • Repolarization
  • The L-type Ca2+ channels close, while the slow delayed rectifier (IKs) K+ channels remain open as more potassium leak channels open.
21
Q

In a healthy sinoatrial node, the pacemaker potential is the main determinant of the___________

A

heart rate.

22
Q

What are the steps to the pacemaker potential?

A
  1. Decrease in potassium permeability accompanied by slow sodium entry
  2. Fast calcium channels open
    * Increase in calcium permeability
  3. Decrease in calcium permeability and an increase in potassium permeability
  4. Slow depolarization
  5. Fast calcium channels open
  6. Action potential
  7. Threshold
23
Q

The rate of automaticity of pacemaker cells is determined by:

A
  • Rate of diastolic depolarization
  • Magnitude of maximum diastolic potential
  • threshold potential level
24
Q

How can the automaticity of pacemaker cells be explained?

A

These cells begin leaking sodium into the cell, as soon as they return to their resting state

25
Q

Spontaneous depolarization of the pacemaker cell leads to myogenic conduction through ___________.

A

Intercalated discs

26
Q

The heart is stimulated by sympathetic cardio-acceleratory center. Which neurotransmitter is secreted by this center?

A

Norepinephrine

27
Q

The heart is inhibited by the parasympathetic cardio-inhibitory center. Which neurotransmitter is secreted by this center?

A

Acetylcholine

28
Q

What factors are affects by the different extrinsic innervations of the heart?

A

Firing rate

Force of contraction

Velocity of contraction

29
Q

What are the steps to excitation- conraction-relaxation coupling in cardiac muscle?

A
  1. Depolarization
  2. ICa, L
    * L- type Calcium channels
  3. ICaICaR
    * Ryanodine receptor 2 (RyR2) (calcium-induced-calcium-release)
  4. Troponin C activation
  5. Tropomyosin reveal binding site on actin
  6. Myosin head ATPase also activated by Ca2+ and Mg2+:myosin head corked perpendicular to actin active site
  7. Myosin cross bridge attach to actin binding site (ADP)
  8. Mg-ATP split into ADP + P: rowing inward movement of actin filaments.
  9. Power stroke (spring release) by bending myosin head towards its arm
  10. ATP binding to myosin head causes detachment of myosin from actin
  11. In presence of Ca2+ and Mg2+, myosin ATPase splits ATP again for another cycle
  12. Ca2+ removal
  13. Bridges detach without ATP splitting
  14. Relaxation
30
Q

Cardiac muscle preload

A

Cardiac muscle preload is normally less than that required to bring fiber length to optimal length. Thus cardiac muscle operates at preloads below optimal length. An increase in preload can grade contractile force and/or extent of shortening

31
Q

The ___________ refers to the stretch of the ventricle just before the onset of contraction due to the blood in it

A

preload

32
Q

Force-Velocity Relationship

A

Force and velocity are inversely related - with no load (afterload), force is negligible and velocity is maximal: in an isometric contraction, where no external shortening occurs, force is maximal and velocity is zero.

  1. A certain weight preloads the muscle (blood in ventricles), stretching the elastic elements
  2. When the muscle is stretched in this way, it must first undergo internal shortening to remove the slack in the series elastic element before tension develops (isometric phase of contraction)
  3. When the developed tension equals the load (afterload), the weight is lifted without further stretch of the elastic elements (isotonic phase of contraction)
33
Q

Length-Force Relationship

A
  • When sarcomeres are stretched beyond their optimal length, the force of contraction decreases due to less overlap of the thick and thin filaments, which means less cycling of cross-bridges
  • When the ventricles fill with blood, this causes the muscle fibres to stretch, causing overlap of filaments, therefore causing cycling of cross-bridges and this stimulates contraction: too much blood in the ventricles will cause the muscle fibres to over-stretch and therefore less cycling occurs, meaning the contraction becomes weaker
34
Q

The intensity of the active state can be increased by :

A
  • Ligand- 2nd messenger paths
  • Increase in excitation frequency: Treppe

SF3- Exam 1 (26 decks)

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