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PHSL232 > Lecture 3 - Excitation > Flashcards

Lecture 3 - Excitation Flashcards

1
Q

What causes RMP?

A

An uneven distribution of ions across the cell membrane

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2
Q

What is the ion permeability through the cell membrane?

A

High K+ permeability
Low Na+ permeability (10%)
Very low Ca+ permeability

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3
Q

What is the structure of the conduction system in the heart?

A

Sinoatrial node (SA)
Origin of action potential (AP)
Atrioventricular node (AV)
Bundel of His
Purkinje fibers
Cardiomyocytes

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4
Q

Where is the SA node located?

A

Right atrium

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5
Q

Why does the AV node go through atrial muscle?

A

Because AP cannot go through valves

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6
Q

What is the sinoatrial (SA) node?

A

The origin point of action potentials in the heart.
The SA node sends out an AP which is then conducted through the atria.

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7
Q

What is the the AV node?

A

Once the atria are fully depolarised, the signal reaches the AV node.
The AV node is the only location in which an AP can travel from the atria to the ventricles.

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8
Q

What allows for the delay between depolarisation and contraction of the atria and the ventricles?

A

Slow conduction through AV node

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9
Q

What is the Bundel of His?

A

The Bundel of His has branches through the ventricular septum. These branches rapidly conduct the AP down to the apex of the heart.

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10
Q

Once AP is at the apex of the heart, where does it go?

A

The AP is sent through the Purkinje fibres and consequent cardiomyocytes from the bottom of the ventricle to the top

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11
Q

Why is depolarisation in the ventricles down up?

A

This down-up depolarisation means that the ventricles will contract first at its base, aiding them to be able to push all the blood out of the heart.

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12
Q

What is the structure of cardiomyocytes?

A

Interwoven
Branch at either end
Intercalated disks
Gap-junctions

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13
Q

How does Cardiomyocyte Structure support Conduction of AP?

A

Cell-to-cell conduction
(charge movement)
Electrical coupling
Functional syncytium
all-or-none contraction

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14
Q

What are conduction of AP rates in the atria?

A

SA node generates AP’s at ~100 min-1
Conducted through atrium at ~0.5 m s-1
Conducted slowly through AV node at ~0.05 m s-1
Delay permits full depolarisation and contraction of the atria before depolarisation and contraction of the ventricles

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15
Q

What are conduction of AP rates in the ventricles?

A

Rapidly ~5.0 m s-1 through
Bundle of His
Bundle branches
Purkinje fibres
Ventricular myocardium spreads at ~0.5 m/s
Allows synchronous depolarisation and contraction of
all ventricular regions

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16
Q

The speed of conduction of the action potential spreads slowest in the:
A. SA Node
B. Atria
C. AV Node
D. Bundle of His

A

C. AV Node

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17
Q

What is the difference in AP in pacemaker cells vs ventricular cells?

A

Origin of AP in sinoatrial node (SA)
(‘Leaders’)
AP in ventricular cell (cardiomyocyte)
(‘Followers’) - Sitting at RMP until signal comes

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18
Q

What are the 3 phases of AP at the SA node (pacemaker cells)?

A

Pacemaker or pre potential (Phase 4)
Upstroke (Phase 0)
Repolarisation (Phase 3)

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19
Q

Why is membrane potential not completely flat in pacemaker cells?

A

Due to funny sodium channels which are active at RMP, letting slow influx of Na+ - this is reduction.

20
Q

What happens during Pacemaker or pre potential (Phase 4)?

A

“Rest” membrane potential is -60/-70 mV
Unstable due to funny Na+ channels: slow influx of Na+
Late phase T-type Ca2+ channels (TTCC): influx of Ca2+
Threshold reached at -50/-40 mV

21
Q

What happens during Upstroke (Phase 0) in pacemaker cells?

A

L-type or Voltage operated Ca2+ channels (LTCC): Ca2+ influx
The threshold for these calcium channels to open is around -50/-40mV. The influx of calcium then rapidly depolarises the cell

22
Q

What happens during Repolarisation (Phase 3) in pacemaker cells?

A

slow K+ efflux

23
Q

What are the phases of AP propagation in ventricular cells?

A

Stable rest membrane potential: -90 mV (Phase 4)
Upstroke (Phase 0)
Early repolarisation (Phase 1)
Cardiac Ca2+ Plateau (Phase 2)
Late repolarisation (Phase 3)

24
Q

What happens during phase 4 in ventricular cells?

A

At phase 4 we have a stable rest membrane potential at -90mV and we have the flux of potassium

25
Q

What happens during phase 0 in ventricular cells?

A

Upstroke (Phase 0)
Fast depolarisation, fast Na+ channels: influx of Na+
Threshold at -65 mV
This fast depolarisation overshoots which causes the beginning of the next phase (phase 1)

26
Q

What happens during phase 1 in ventricular cells?

A

Early repolarisation (Phase 1)
Fast Na+ channels close, Na+ influx stops
Small K+ efflux

27
Q

What happens during phase 2 in ventricular cells?

A

Cardiac Ca2+ Plateau (Phase 2)
Sustained depolarisation
L-Type or Voltage operated Ca2+ channels (LTCC): Ca2+ influx
Counterbalances K+ efflux

28
Q

What happens during phase 3 in ventricular cells?

A

Late repolarisation (Phase 3)
Inactivation Ca2+ channels
Activation of K+ efflux
Fast repolarisation

29
Q

Describe APs in the heart

A

In the heart, AP vary throughout its conduction system. These AP depend on the cells function and localisation.

30
Q

In ventricular cells, slow depolarisation occurs through funny Na+ channels BECAUSE the resting membrane potential of ventricular cells is stable at around -90mV

A

D - only SA node (pacemaker cell) has funny sodium channels

31
Q

What is ECG?

A

A recording of potential changes at the skin surface that result from depolarisation and repolarisation of heart muscle

32
Q

How do we record ECG?

A

To record an ECG we place recording electrodes on the left arm (LA), right arm (RA), and the left leg (LL)
- measures direction and relative size of potential changes

33
Q

What is a bipolar lead?

A

Combination of 2 electrodes is called a bipolar lead

34
Q

What does Lead I measure?

A

Lead I: potential difference LA and RA
(LA is positive, RA is negative)

35
Q

What does Lead II measure?

A

Lead II: potential difference LL and RA
(LL is positive, RA is negative)

36
Q

What does Lead III measure?

A

Lead III: potential difference LL and LA
(LL is positive, LA is negative)

37
Q

What is Einthoven triangle?

A

Standard Limb Leads (I, II, III)

38
Q

What causes deflections in ECG?

A

Depolarisation towards + electrode = +ve deflection
Repolarisation away from + electrode = +ve deflection
Depolarisation away from + electrode = -ve deflection
Repolarisation towards + electrode = -ve deflection

39
Q

What type of deflection occurs during P wave in limb lead II?

A

SA cells depolarise
Right and left atria depolarise towards AV node
Depolarisation towards + electrode
Positive deflection (P wave)

40
Q

What type of deflection occurs during Q wave in limb lead II?

A

In ventricular septum, overall
direction of depolarisation is slightly away
Small negative deflection (Q wave)

41
Q

What type of deflection occurs during R wave in limb lead II?

A

Ventricular depolarisation from ‘inside to outside’
towards detecting electrode
Large positive deflection (R wave)

42
Q

What type of deflection occurs during S wave in limb lead II?

A

Late ventricular depolarisation from ‘inside to outside’ away from detecting electrode
Small negative deflection (S wave)

43
Q

What type of deflection occurs during T wave in limb lead II?

A

Repolarisation ventricles from ‘outside to inside’
away from the detecting electrode
Positive deflection (T wave)

44
Q

What are augmented leads?

A

Same electrodes as standard limb leads
(RA, LA and LL)
1 positive electrode + 2 negative electrodes
Three new leads generated (unipolar)
Augmented limb leads (aVR, aVL, aVF)
Term “aV” refers to augmented voltage measured
Now six (hexa) leads!

45
Q

Describe the Hexaxial Reference System

A

Action potentials at different locations in the heart at different times
The pattern of the ECG depends on which lead is used to measure
The main deflection is positive for all leads (except aVR)
All leads frontal plane of body

PHSL232 (31 decks)

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