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CARDIOLOGY PHYSIOLOGY > Origin and spread of cardiac excitation > Flashcards

Origin and spread of cardiac excitation Flashcards

1
Q

Where is the SA node located?

A

Junction of SVC with the RA.

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2
Q

Where is the AV node located?

A

Right posterior portion of the interatrial septum.

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3
Q

What do the atrial fibres contain?

A

Purkinje-type fibres.

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4
Q

What do the atrial fibres do?

A

Connect the SA node to the AV node.

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5
Q

What are the three bundles of atrial fibres?

A
  1. Anterior internodal tract of bachman.
  2. Middle internodal tract of Wenckebach.
  3. Posterior internodal tract of Thorel.
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6
Q

What is the AV node continuous with?

A

Bundle of His.

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7
Q

Describe the bundle of His?

A

Has two branches:
1. Left bundle branch at top of the interventricular septum.
2. Right bundle branch.

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8
Q

What does the left bundle branch of His divide into?

A
  1. Anterior fascicle.
  2. Posterior fascicle.
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9
Q

Where do the branches and fascicles run?

A

Subendocardially down either side of the septum and come into contact with the Purkinje system.

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10
Q

What is the conduction system composed of?

A
  1. Modified cardiac muscles - fewer striations and indistinct boundaries.
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11
Q

Describe P cells?

A

Small round cells with few organelles that connected with each other by gap junctions.

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12
Q

What separates the atrial muscle to the ventricular muscle?

A

Fibrous tissue ring.

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13
Q

Where does the SA node develop from?

A

Structures on the right side of the embryo.

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14
Q

Where does the AV node develop from?

A

Structures on the left side of the embryo.

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15
Q

What innervates the SA node?

A

Right vagus nerve.

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16
Q

What innervates the AV node?

A

Left vagus nerve.

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17
Q

Where does each sympathetic fibre come from?

A

Stellate ganglion.

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18
Q

What are vagal fibres?

A

Endocardial.

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19
Q

What are noradrenergic fibres?

A

Epicardial.

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20
Q

How does acetylcholine act on the heart?

A

Presynaptically to reduce norepinephrine release from the sympathetic nerves.

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21
Q

How does neuropeptide Y act on the heart?

A

Released from noradrenergic endings and may inhibit the release of AcH.

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22
Q

What is the resting membrane potential of myocardial fibres?

A

-90mV.

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23
Q

What separates the individual fibres?

A

Membranes.

24
Q

How does depolarisation spread between the individual fibres?

A

Spreads radially through the membranes due to the presence of gap junctions.

25
Q

What are the phases of action potentials?

A
  1. Phase 0 - Rapid depolarisation.
  2. Phase 1 - Initial rapid repolarisation.
  3. Phase 2 - Plateau.
  4. Phase 3 - Slow repolarisation process.
  5. Phase 4 - Return to resting membrane potential.
26
Q

What causes phase 0?

A

Sodium influx through rapidly opening sodium channels.

27
Q

What causes phase 1?

A

Inactivation of sodium channels.

28
Q

What causes phase 2?

A

Calcium influx through more slowly opening calcium channels.

29
Q

What causes phase 3?

A

Net potassium efflux through multiple types of potassium channels.

30
Q

What does the pacemaker potential do?

A

Trigger the next impulse.

31
Q

What happens at the peak of each impulse?

A

Potassium efflux begins and as potassium inside the cell decreases a channel (H channel) that passes both sodium and potassium is activated.

32
Q

What happens when the h channel current increases?

A

Depolarisation starts - forming the first part of the pacemaker potential. Calcium channels then open.

33
Q

What are the types of calcium channels in the heart?

A
  1. Long lasting - L.
  2. Transient - T.
34
Q

What happens when the T calcium channels open?

A

Completes the pre=potential.

35
Q

What happens when the L calcium channels open

A

These channels open causing the current to produce the impulse.

36
Q

What else occurs during the prepotential?

A

Calcium sparks - local calcium release from the sarcoplasmic reticulation.

37
Q

What are the action potentials in the SA and AV nodes due to?

38
Q

What other differences do you see in the AV and SA node compared to other myocytes?

A
  1. No sharp rapid depolarising spike before the plateau.
  2. Pre-potential is only prominent in SA and AV nodes.
39
Q

What are latent pacemakers?

A

These are cells prsent in other portions of the conduction system that can take over when the SA and AV nodes are depressed or conduction is blocked.

40
Q

What fibres do not have pre-potential ability?

A
  1. Atrial muscle fibres.
  2. Ventricular muscle fibres.
41
Q

When do atrial and ventricular muscle fibres discharge?

A

Spontaneously only when injured or abnormal.

42
Q

What happens when cholinergic vagal fibres to nodal tissue are stimulated?

A
  1. Membrane becomes hyperpolarised - slope of prepotentials is decreased.
  2. Acetylcholine released at the nerve ends increases the potassium conductance of nodal tissue.
43
Q

What mediates the process of stimulation in cholinergic vagal fibres?

A

M2 Muscarinic receptors.

44
Q

How are the muscarinic receptors involved?

A
  1. These open a special set of potassium channels. These will eventually slow the affect of hyperpolarisation.
  2. Decrease cAMP in the cells which will slow the opening of the calcium channels.
45
Q

What will result when M2 muscarinic receptors slow the opening of calcium channels?

A

Decrease firing rate.

46
Q

What can potentially stop spontaneous discharge for some time?

A

Strong vagal stimulation.

47
Q

What does stimulation of sympathetic cardiac nerves do?

A

Increases the depolarising effect of hyperolarisation current and the rate of spontaneous discharge increases.

48
Q

How does sympathetic cardiac nerves work?

A
  1. Norepinephrine is secreted by the sympathetic endings.
  2. Norepinephrine binds to B1 receptors.
  3. Increase in intracellular cAMP.
  4. cAMP will cause opening of long calcium channels.
  5. Increase in calcium current.
  6. Increase in speed of depolarisation phase of the impulse.
49
Q

What influences the rate of discharge from SA and AV node?

A
  1. Temperature - increase in temperature increases frequency.
  2. Drugs.
50
Q

What is the speed of conduction at the SA node?

51
Q

How fast is atrial depolarisation?

A

0.1 second.

52
Q

What is the speed of conduction at the AV node?

53
Q

What shortens AV delay?

A

Stimulation of sympathetic nerves to the hearted.

54
Q

What lengthens AV delay?

A

Stimulation of the vagus nerve.

55
Q

How fast is conduction in the purkinje system?

A

0.08-1 second - 4m/s.

56
Q

Where does depolarization start in ventricular muscle?

A
  1. Left side of interventricular septum.
  2. Moves to the right across the mid portion of the septum.
  3. Spreads down the septum to the apex of the heart.
  4. Returns along the ventricular walls to the AV groove - goes from endocardial to epicardial surface.
  5. Last parts to be depolarised: posterobasal portion of LV, pulmonary conus, uppermost portion of the septum.

CARDIOLOGY PHYSIOLOGY (4 decks)

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