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CV Module > Phys-Cardiac Excitation & Contraction > Flashcards

Phys-Cardiac Excitation & Contraction Flashcards

1
Q

Where is the action potential of cardiac contraction initiated?

A

The SA Node.

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2
Q

What are the two types of cells that make up the Heart?

A
  1. Contractile cells

2. Conducting cells

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3
Q

What are the contractile cells?

A

Constitute the majority of atrial and ventricular tissues and are the WORKING cells of the heart.

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4
Q

How do CONTRACILE cells do their work?

A

Action potentials in contractile cells lead to contraction and generation of force or pressure.

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5
Q

What tissues are constructed of conducting cells?

A

1.SA Node
2.Atrial internodal tracts
3.AV Node
4Bundle of His
5.Purkinje system

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6
Q

Do conducting cells contribute to the generation of force?

A

No, they function to spread the action potentials over the entire myocardium.

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7
Q

What is a specialized feature of the conducting tissues?

A

The ability to spontaneously generate action potentials except the SA node.

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8
Q

What is the sequence of the Action potential through the Myocardium?

A
  1. SA node
  2. Atrial Internodal tracts and atria
  3. AV Node
  4. Bundle of His
  5. Purkinje system
  6. Ventricles
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9
Q

How does the Action potential spread from the SA node to the Atria?

A

Via the atrial Atrial internodal tracts, simultaneously conducted to the AV node.

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10
Q

What is important about the AV node?

A

Conduction through the AV node is considerably slower than in the other cardiac tissues.

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11
Q

What is the purpose for the AV node having slower conduction velocities than cardiac tissues?

A

This ensures the ventricles have enough time to fill with blood before they are activated to contract.

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12
Q

What are the consequences of increased velocity of AV node conduction?

A
  1. Decreased ventricular filling
  2. Decreased Stroke volume
  3. Decreased Cardiac output
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13
Q

Where is conduction of the Action potential fastest in the myocardium?

A

Through the His-Purkinje system

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14
Q

What does the term normal sinus rhythm mean?

A

It means that the pattern and timing of the electrical activation of the heart are normal.

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15
Q

What three criteria must be met in order for ones rhythm to be considered NORMAL SINUS RHYTHM?

A
  1. The Action potential must originate at the SA node
  2. The SA node pulses must occure regularly at a rate of 60-100 impulses per minute.
  3. The activation of the myocardium must occur in the correct sequence and with the correct timing and delays
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16
Q

What ion determines the resting membrane potential of a cardiac cell?

A

K+

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17
Q

What is the role of Na+-K+ ATPase

A

Primarily to maintain Na+ and K+ concentration gradients across the cell membrane

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18
Q

What is an inward current?

A

Brings positive charge into the cell and depolarizes the membrane potential

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19
Q

What is an outward current?

A

Takes positive charges out of the cell and hyperpolarizes the membrane potential

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20
Q

What is the Threshold?

A

Is the membrane potential at which the action potential is inevitable.

21
Q

What happens at Threshold potential?

A

At threshold potential, net inward current becomes larger than net outward current and the resulting depolarization become self sustaining and gives rise to the upstroke of the action potential

22
Q

What features (characteristics) do action potentials of the Ventricles, Atria, and Purkinje system share?

A
  1. Long duration
  2. Stable resting membrane potential
  3. Plateau (No plateau in SA nodal AP)
23
Q

What is complementary to these tissues Long Duration of their action potentials?

A

A Long refractory period.

24
Q

What is the cause of PHASE 0 (rapid depolarization)?

A

Caused by a transient increase in Na+ conductance (Gna), produced by depolarization-induced opening of activation gates in the Na channels.

25
Q

Why doesn’t the membrane potential reach the +65mv equilibrium potential of Na+?

A

Because the inactivation gates on the Na+ channels close in response to depolarization (slower than the activation gates)

26
Q

What is phase 1?

A

Initial repolarization

27
Q

What are the two mechanisms by which phase 1 (initial repolarization) is achieved?

A
  1. Inactivation gates on the Na+ channels close in response to depolarization, decreasinsing Gna and ceasing the inward current.
  2. The outward current of K+
28
Q

What is the cause of Phase 2 (The Plateau)?

A

There is an inward and outward current equilibrium achieved by an increase in Ca2+ conductance which results in an inward Ca2+ current. To balance the inward Ca2+ current there is an outward current of K+

29
Q

What is another name for the inward calcium current?

A

Slow inward current (slower kinetics of Ca2+ channels)

30
Q

What type of Ca2+ channels open during the the plateau?

A

L-Type channels (inhibited by Calcium channel blockers)

31
Q

What does the entry of Ca2+ into the cell result in?

A

Initiates the release of more Ca2+ from intracellular stores for excitation-contraction coupling. Called Ca2+ induced Ca2+ release.

32
Q

What occurs during Phase 3 (repolarization)?

A
  1. Decrease in Ca2+ conductance so reduced inward current for Ca2+
  2. Increased K+ conductance resulting in a large outward current of K+
33
Q

What is phase 4?

A

Resting membrane potential. Inward and outward currents are equal.

34
Q

What channels is responsible for the K+ conductance in phase (IV) 4?

A

The Ik1 channel

35
Q

What balances the outward K+ current in phase 4?

A

Inward Na+ and Ca+ current

36
Q

Why is the driving force on K+ low in phase 4?

A

Because the resting membrane potential is close to the K+ equilibrium potential (outward K+ current is very small)

37
Q

What are the three differences between SA node fibers and those of AVP that optimizes the SA as the pacemaker.

A
  1. Automaticity-can spontaneously generate action potentials without neural input.
  2. Has an unstable resting potential
  3. Has no sustained plateau
38
Q

What is Phase 0 of the SA node

A

Upstroke of the action potential not as sharp as the upstroke in other types of cardiac tissue.

39
Q

What is the cause for Phase 0 (depolarization) in SA node tissue?

A

Increase in Ca2+ conductance (Gca) and an inward Ca2+ current.

40
Q

In the Sa Naode whic channels carry the CA2+ into the cells during phase 0?

A

T-type Ca2+ channels. These channels not inhibited by drugs such as verapamil

41
Q

What two (2) phases are absent in SA nodal action potential?

A

Phases 1 and 2

42
Q

What is phase 3 of SA nodal action potential generation?

A

Increase in K+ conductance resulting in an outward current of K+ which re-polarizes the membrane potential

43
Q

What is phase 4 of the SA nodal action potential?

A

The longest phase, also accounts for the automaticity of SA nodal cells

44
Q

What is the most negative value of the membrane potential during phase 4 of SA nodal action potential?

A

-65mv

45
Q

How is the slow depolarization of the membrane potential of SA node cells accomplished?

A

The If channels let in Na+ until it reaches threshold when T-type Ca2+ channels are opened for the upstroke.

46
Q

What sets the heart rate?

A

The rate of Phase (IV) 4 depolarization sets the heart rate

47
Q

What is the rule when it comes to pacemakers?

A

The pacemaker with the fastest rate of phase 4 depolarization controls the heart rate

48
Q

Which cells have the shortest action potential duration and thus the shortest refractory period?

A

SA nodal cells

49
Q

What are the circumstances under which a latent pacemaker takes over and becomes the pacemaker of the heart?

A
  1. If the SA node firing rate decreases (e.g due to Vagal stimulation) or stops completely
  2. The intristic rate of firing of one of the latent pacemakers becomes faster than the SA node
  3. If the conduction of action potentials from the SA node to the rest of the heart is blocked because of disease in conducting pathways.

CV Module (24 decks)

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