Exam #2: Electrophysiology of the Heart Flashcards Preview

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Flashcards in Exam #2: Electrophysiology of the Heart Deck (62):
1

What are intercalated discs? What is their function?

- Intercalated discs are interdigitations of sarcolemma between cardiac myocytes
- Contain gap junctions that increase the speed of signal transduction between myocytes

2

What are pacemaker cells?

Intrinsic rhythm generators in the heart, including nodal cells of:
- SA node
- AV node
- Purkinje fibers

3

How does the resting membrane potential differ from the equilibrium potential?

Resting membrane potential is the summation of the equilibrium potentials for each ion

4

Describe the intracellular vs. extracellular concentration of Na+, K+. Ca++, & Cl-.

Na+= high ECF
K+= high ICF
Ca++=v. high ECF
Cl-= high ECF

5

Describe the electrical driving forces on the basis of charge. How does this compare to the chemical driving force?

Electrical driving force:
- Cations want to move in (attracted by the negative resting membrane potential)
- Anions want to remain out of the cell (repulsed by the negative resting membrane potential)

*Chemical driving force is dependent on the concentration gradient for each particular ion.

6

What is resistance. Write Ohm's Law.

Resistance is the force that impedes the flow of ions.

I= Vm-Veq/R

7

What maintains the Na+ & K+ concentration gradients?

Na+/K+ ATPase Pump

8

What ion is the cell most permeable to at rest? What is the effect of this?

- The cell membrane is most permeable to K+ at rest
- Resting membrane potential is closest to the equilibrium potential for K+

9

Describe the general sequence the cardiac action potential in terms of cellular permeability & ion channels.

1) Fast Na+ channels open, leading to Na+ entry & rapid depolarization
2) Slow Ca++ channels open leading to the plateau phase caused by Ca++ influx
3) Change in K+ permeability throughout the action potential leads to K+ efflux & causes repolarization

10

What is the resting membrane potential in a cardiac myocyte?

-90mV

11

What is phase 0 of the AP in a ventricular myocyte?

Depolarization due to opening of Fast Na+ channels

12

What is phase 1 of the AP in a ventricular myocyte?

Early repolarization when Fast Na+ channels inactivate (close) & some K+ channels open

13

What is phase 2 of the AP in a ventricular myocyte?

- Plateau phase where the membrane potential is approximately 0
- Due to slow Ca++ channels with Ca++ influx, BALANCED by K+ efflux

14

What is phase 3 of the AP in a ventricular myocyte?

Rapid repolarization as Ca++ channels are closing, & K+ channels open

15

What is phase 4 of the AP in a ventricular myocyte?

Resting membrane potential; only K+ channels are open

16

What is responsible for the absolute (effective/normal) refractory period in the ventricles? What is its duration?

- 0.25-0.3 sec
- Na+ inactivation gate is closed & Na + pore is closed

17

What is responsible for the absolute (effective/normal) refractory period in the atria? What is its duration?

- 0.15 sec
- Na+ inactivation gate is closed & Na + pore is closed

18

What is the relative refractory period? What is responsible for the relative refractory period?

More difficult to excite, but no impossible
- Na+ channel inactivation ( Na+ pore is closed but the inactivation gate open)

19

How does the effective (absolute) refractory period in cardiac muscle compare to skeletal muscle?

Longer effective refractory period in cardiac muscle; thus, it cannot tetanize

20

What happens when another action potential is stimulated during the relative refractory period in the cardiac myocyte?

Interruption of normal ion flow that may result in ventricular fibrillation

21

How does the nodal action potential compare to the ventricular myocyte action potential?

- Missing phases 1 &2 i.e. no rapid influx of Na+ b/c no fast Na+ channels

22

What happens during phase 3 in nodal cell?

Repolarization as K+ channels open

23

What happens in phase 4 in the nodal cell?

Slow leak of Na+, called the "funny current" slowly changes the membrane potential until threshold is reached

24

What is the effect of NE on pacemaker cells?

- Increases Ca++ permeability
- More positive charge enters the cell & cell reaches threshold faster

25

What is the effect of ACh on pacemaker cells?

Increases permeability of K+

26

What is the function of the SA node?

Primary pacemaker

27

What are the principal time-dependent & voltage dependent currents of the SA node?

Ica, Ik, & If

28

What is the effect of Beta-adrenergic stimulation on the SA node?

- Increased conduction velocity
- Increased pacemaker rate

29

What is the cholinergic effect on the SA node?

- Decreased pacemaker rate
- Decreased conduction velocity

30

What is the function of the atrial muscle?

Expel blood from the atria

31

What are the principal time-dependent & voltage dependent currents of the atria?

Ina, Ica, Ik

32

What is the effect of beta-adrenergic stimulation of the atria?

Increased strength of contraction

33

What is the effect of cholinergic stimulation on the atria?

Little effect

34

What is the function of the AV node?

Secondary pacemaker

35

What are the principal time-dependent & voltage dependent currents of the AV node?

Ica, Ik, If

36

What is the effect of beta-adrenergic stimulation of the AV node?

- Increased conduction velocity
- Increased pacemaker rate

37

What is the effect of cholinergic stimulation of the AV node?

- Decreased pacemaker rate
- Decreased conduction velocity

38

What is the function of the Purkinje fibers?

- Rapid conduction of action potential
- Tertiary pacemaker

39

What are the principal time-dependent & voltage dependent currents of the Purkinje fibers?

Ina, Ica, Ik, If

40

What is the effect of beta-adrenergic stimulation of the Purkinje fibers?

Increased pacemaker rate

41

What is the effect of cholinergic stimulation of the Purkinje fibers?

Decreased pacemaker rate

42

What is the function of the ventricular muscle?

Expel blood from ventricles

43

What are the principal time-dependent & voltage dependent currents of the ventricular muscle?

Ina, Ica, Ik

44

What is the effect of beta-adrenergic stimulation of the ventricular muscle?

Increased contractility

45

What is the effect of cholinergic stimulation of the ventricular muscle?

Little effect

46

Describe the sequence of depolarization in the heart.

1) SA node
2) Atria
3) AV node
4) Septum
5) Apex
6) Ventricular free walls

47

What is the velocity of signal conduction through the purkinje fibers?

4 m/s

48

What is the velocity of signal conduction through the cardiac myocytes?

0.3-0.5 m/sec

*Roughly a 1/10 of the purkinje fibers

49

What is the SA node?

Normal impulse generator of the heart (70-80 bpm)

50

How does impulse from the SA node reach the left atria?

Internodal tracts

51

How does the resting membrane potential of the SA node compare to the cardiac myocytes?

Less negative (-60mV vs. -90mV)

52

What is the function of the AV node?

Delay of impulse
- Allows for atrial contents to fill the ventricle

53

How does the AV node slow conduction?

Expression of fewer gap junctions

54

What are the branches of the left bundle branch?

1) Septal fascicle
2) Left posterior fascicle
3) Left anterior fascicle

55

What is the function of the purkinje fibers?

Very rapid transmission of the electrical impulse that allows for the synchronous contraction of ventricular myocytes

56

Outline the steps of excitation-contraction coupling in the heart.

1) Action potential spreads down T-tubule & there is an inward Ca++ movement through L-Type Ca++ channels (Dihydropyridine receptors)
2) Ca++ induced Ca++: Ryanodine receptor on SR activated to release MORE Ca++
3) Increased ICF Ca++ binds Troponin C & displaces tropomyosin from myosin-binding sites on actin
4) Cross-bridging occurs

Relaxation occurs when Ca++ is reaccumulated in the SR via Ca++ ATPase (SERCA) & out of the cell via the Ca++/Na+ exchanger

57

How do the concentration & electrical gradients differ for Cl-?

Cl- concentration is high on the outside of the cell; however, resting membrane potential of the cell is negative. Thus,
- Concentration gradient favors entry into the cell
- Electrical gradient favors efflux from the cell

58

What are the effects of increased extracellular potassium (hyperkalemia)?

1) Slows HR
2) Dilates heart
3) Can block conduction through AV-bundle

59

What is the effect of hypercalcemia on the heart?

Increases cardiac contraction (positive ionotrope)

60

What is the effect of hypocalcemia on the heart?

Decreased cardiac contraction (negative ionotrope)

61

What are the effects of K+ channel blockers on the heart?

1) Increased AP duration & absolute refractory period
2) Prolongation of QT interval on ECG

62

What are the effects of Ca++ channel blockers on the heart?

Slows the rate of conduction at the SA & AV nodes by delaying Ca++ entry

E.g. verapamil

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