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My LSS- CVS > CVS 7: Electrical properties of the heart > Flashcards

CVS 7: Electrical properties of the heart Flashcards

1
Q

If you had 2 chamber separated with an impermeable membrane containing different concentrations of a solution what would be the p.d.?

A

No potential difference between the chambers.

Because of the impermeable barrier even though you have a concentration gradient.

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2
Q

If you had two chambers containing different concs of K+ separated by a permeable membrane what would happen at first?

A
  • The membrane is more permeable to K+ than any other ions
  • K+ ions diffuse down their Conc gradient carrying their +ve charge
  • +ve charges build up on one side and en electrical gradient builds up
  • The electrical gradient opposes the movement of K+
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3
Q

What happens at equilibrium according the potassium hypothesis?

A
  • Electrical gradient = K+ gradient

- Ions move back and forth randomly through the channel but there’s no net movement of ions

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4
Q

Which equation helps you to predict the resting membrane potential? What is the resting membrane potential of a cardiomyocyte?

A

The Nernst equation

-80mV

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5
Q

What causes the membrane potential to change?

A

It changes based on the relative permeabilities to different ions

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6
Q

Which equation is used to give us a better understanding of membrane potential? and why is it better?

A

Goldman- Hodgkin- Katz equation

Takes into account of relative permeabilities of the membrane to different ions

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7
Q

How long do nerve and cardiac APs tend to last? Why is there this difference?

A

Nerve: 2ms
Cardiac: 200-400ms
(cardiac is much longer because long, slow contraction is needed to produce an effective pump)

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8
Q

Draw and annotate a diagram of a cardiac action potential, showing the different refractory periods as well

A 
x= (ms) 0-300
y= membrane potential (mv) -100- =50
- flat line until 0 which is RP at -80mV
- Straight line up at 0 to +30mV 
- a quick notch afterwards 
- plateau
- quick drop back to -80mv by 280ms
- ARP up to 180ms 
- RRP from 180-220ms
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9
Q

What is the absolute refractory period?

A

sodium channels are shut and cannot be opened for a long time.

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10
Q

What is characteristic of the ARP in cardiac tissue? What is the consequence of this?

A
  • Cardiac cells have a long absolute refractory period

- so that you can’t restimulate the muscle for a long time and cardiac muscle will not tetanize

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11
Q

What is the relative refractory period?

A

Period after ARP where an AP can be elicited but only by a STRONG stimulus.

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12
Q

What are the different phases of the cardiac action potential?

A 
Phase 0= upstroke
Phase 1= early repolarisation
Phase 2= plateau
Phase 3= repolarisation
Phase 4= resting membrane potential (diastole)
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13
Q

Phase 0/4: What determines the resting membrane potential?

A

K+ flowing out of the cells

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14
Q

Phase0: What determines he upstroke?

A

opening of Na+ and an increase in membrane permeability to sodium

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15
Q

Phase 1: What happens in this phase?

A

Early depolarisation caused by shutting of sodium channels

- transient outward K+ current starts

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16
Q

Phase 2: Why does the membrane potential plateau?

A

There is an increase in permeability to Ca2+ .

The influx of Ca2+ balances the efflux of K+

17
Q

Phase 2: What sort of channel is used for the influx of Ca2+

A

L- type Calcium Channels (l= long- lasting)

18
Q

Phase 2: Why is there an influx of Ca2+? What is it’s purpose?

A
  • Ca2+ needed to trigger Ca2+ release from intracellular stores (CICR)
  • Ca2+ needed for contraction
19
Q

Phase 2: How is this phase targeted in anti-hypertensive therapy? Give examples of drugs used as well

A

Calcium Channel Antagonists are used which inhibit L-type calcium channels:

  • Nifedipine
  • Nitrendipine
  • Nisoldipine
20
Q

Phase 3: What happens during this phase? Which current is activated?

A

REPOLARISATION

  • K+ currents are activated and K+ moves out > inward flow of Ca2+
  • IK1 current is activated which fully repolarises the cell
  • Ik1 is large and flows during diastole
21
Q

Phase 3: What does IK1 reduce the risk of and how?

A

IK1 reduces the risk of arrhythmia because it requires a large stimulus to excite the cells

22
Q

Why do different parts of the heart have different action potential shapes?

A
  • They have different ionic currents flowing

- This is because they have different expression of ion channels

23
Q

Why are the electric properties of the heart describe to be INTRINSIC?

A
  • has its own independent generation and propagation system

- Myogenic= can beat independently even after being separated from its nerve supply

24
Q

What modulates cardiac activity?

A

Sympathetic and Parasympathetic activity from the Autonomic Nervous system which controls the intrinsic beating of the heart

25
Q

What are the differences between SAN and ventricular cells?

A
  • No IK1 channels in the SAN
  • Ca2+ influx not Na+ influx creates the upstroke
  • more unstable membrane potential
  • T- type Ca2+ channels not L-type Ca2+ channels because they activates at more -ve potentials
  • presence of pacemaker current (the upward slope)
26
Q

What is the consequence of SAN cells not having IK1 channels

A

IK1 is involved in stabilising the membrane potential

- no IK1= unstable membrane potential

27
Q

What happens when you have sympathetic stimulation of the SAN?

A

(By ADRENALINE)

  • pacemaker potential is steeper
  • Threshold potential is attained more quickly
  • heart rate increases
28
Q

What happens when you have parasympathetic stimulation of the SAN?

A

(By ACETYLCHOLINE)

  • pacemaker potential is less steep
  • takes longer to attain threshold potential
  • heart rate decreases
29
Q

Where is the SAN located?

A

Below the epicardial surface a the boundary between the right atrium and superior vena cava

30
Q

What are the four stages of the conduction system?

A
  1. SAN
  2. Inter-nodal fibre bundle (stimulates the atria)
  3. AVN
    4) Ventricular bundles (left and right branches and Purkinje fibres)
31
Q

What helps propagate the impulse?

A

neighbouring cells excite easily because of LOW MEMBRANE RESISTANCE between cells (gap junctions and intercalated discs)

32
Q

What is the function of the inter-nodal fibre bundles?

A

Conduct the AP to the AVN faster than through atrial muscle

33
Q

What is the function of the AVN?

A
  • To connect the conduction systems between atrial and ventricular chambers
  • To produce a short delay
34
Q

What is the function of the Purkinje fibres?

A

Conducts AP to the base so that excitation of the ventricles proceed from the apex to the base

35
Q

On an ECG what does and upward and downwards deflection mean?

A

Upwards: wave of DEpolarisation towards the +ve electrode
Downwards: Wave of DEpolarisation away from the +ve electrode

36
Q

What effect does a wave of REpolarisation have on the eCG?

A

It is opposite to the effects of a wave of Depolarisation

37
Q

Describe what PQRSTU on an ECG are?

A 
P= atrial depolarisation
QRS= ventricular depolarisation
T= Ventricular repolarisation

My LSS- CVS (21 decks)

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