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Year 1: Cardiovascular System > 7. Electrical Properties of the Heart > Flashcards

7. Electrical Properties of the Heart Flashcards

1
Q

What is meant when the ion is in equilibrium?

A

The point where the electrical gradient is equal to the concentration gradient

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2
Q

Describe the movement of in a particular ion in equilibrium?

A

Ions can move back and forth randomly but there is no net movement of ions

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3
Q

How can the resting membrane potential be predicted?

A

Using the Nernst equation with potassium. The Goldman-Hodgkin-Katz

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4
Q

How is resting membrane potential established?

A

Through the movement of potassium through channels. It does NOT have anything to do with the sodium-potassium pump.

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5
Q

How will the membrane potential change? What dictates the value of the membrane potential?

A

Depends on the relative permeabilities to different ions

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6
Q

How long does a nerve action potential last?

A

2ms

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7
Q

How long does an action potential in the heart last?

A

200-400ms

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8
Q

What is the upstroke caused by?

A

This is caused by the opening of sodium channels

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9
Q

What does the influx of sodium mean?

A

The cell drives towards the equilibrium potential of sodium

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10
Q

After the upstroke what happens?

A

The sodium channels inactivate, so the membrane potential starts to recover

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11
Q

As the sodium channels inactivate what occurs?

A

There is brief increase in the permeability to potassium which repolarises the membrane.

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12
Q

What term is used to describe the sodium channels not opening for a long period of time?

A

Absolute refractory period

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13
Q

Why will cardiac muscle not tetanise?

A

It has a long absolute refractory period

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14
Q

What is the point of increasing the permeability to calcium?

A

The influx of calcium balances the efflux of potassium?

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15
Q

What channels does calcium move through?

A

L type Calcium channels (L = long lasting)

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16
Q

When does total repolorisation occur?

A

The eventual inactivation of the L-type calcium channels and the opening of a another subtype of potassium channel

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17
Q

What type of contraction is required to produce an effective pump?

A

Long slow contraction?

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18
Q

Define absolute refractory period

A

Time during which no action potential can be initiated regardless of stimulus intensity

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19
Q

Define relative refractory period

A

Period after absolute refractory period where an action potential can only be elicited with stimulus strength larger than normal

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20
Q

Define full recovery time

A

The time at which a normal action potential can be elicited with normal stimulus

21
Q

How is the internal electrical activity of the heart modulated?

A

It is modulated by sympathetic and parasympathetic nerves

22
Q

What are the phases of the action potential?

A 
Phase 0 = upstroke
Phase 1 = early repolarisation
Phase 2 = plateau
Phase 3 = repolarization
Phase 4 = resting membrane potential (diastole)
23
Q

What is Phase 1 caused by?

A

Early repolarisation is caused by the inactivation of sodium channels and the transient outward potassium current starting

24
Q

What is phase 2?

A

The plateau stage caused by the calcium influx which keeps the membrane potential more positive

25
Q

What does the influx of calcium cause?

A

The influx of calcium triggers calcium to be released from intracellular stores used for contraction

26
Q

What can be used for antihypertensive therapy?

A 
Calcium permeability inhibiting drugs: 
Nifedipine
Nitrendipine
Nisoldipine
These work by blocking the L-type calcium channels in the smooth muscle cells preventing contraction
27
Q

Which current is responsible for fully repolarising the cell?

A

IK1 switches off during depolarisation but as the membrane gradually becomes more repolarised the IK1 channel switches on

28
Q

Describe the IK1 current?

A

This IK1 current is large and flows during diastole

29
Q

What does IK1 do?

A

Stabilises the resting membrane potential and reduces the risk of arhythmia

30
Q

Where are there no IK1 channels?

A

SA node cells

31
Q

How much Na influx are there in the SA node cells?

A

very little Na influx

32
Q

How is the upstroke produced in SA node cells?

A

It is produced by Ca influx

33
Q

What Ca channels are there in SA node cells?

A

T-type Ca channels. These channels activate at a more negative potential than L-type Ca channels

34
Q

What does sympathetic stimulation of the heart cause?

A

A steeper pacemaker which means it reaches the threshold potential more quickly therefore increasing heart rate

35
Q

What does parasympathetic stimulation of the heart cause?

A

A decrease in the gradient of the pacemaker potential which means the it takes longer for the membrane potential to be reached thus decreasing heart rate

36
Q

What are the four basic components of the conduction system of the heart?

A
  1. Sinoatrial node
  2. Inter-nodal Fibre Bundles
  3. Atrioventricular Node
  4. Ventricular bundles
37
Q

What is the sinoatrial node?

A

Small mass of specialised cells situated in the superior aspect of the right ventricle

38
Q

Where the sinoatrial node located?

A

On the anterolateral margin between the orifice of the superior vena cava and atrium

39
Q

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

A

They conduct the action potential to the AV node at a greater velocity than ordinary atrial muscle

40
Q

What is the function of the AV node?

A

Electrically connects the conduction systems between atrial and ventricular chambers

41
Q

How long of a delay does the AV node produce?

A

0.1 seconds

42
Q

Where does the bundle of His descend from?

A

The AV node

43
Q

How many branches does the bundle of His divide into?

A

Two bundle branches

44
Q

What makes up the bundle branches?

A

Purkinje fibres

45
Q

Where do the terminal Purkinje fibres extend?

A

Beneath the endocardium and penetrate approximately one-third of the distance into the myocardium

46
Q

What is essential for impulse propagation?

A

Low resistance

47
Q

How is low resistance established?

A

Gap junctions

48
Q

Describe the excitation sequence

A
  • Initially the SA node fires and the action potential propagates across the atria
  • The depolarisation moves towards the electrode so you get a small upward deflection
  • The depolarisation then moves away from the electrode giving a downstroke
  • It moves towards the electrode as it moves down the bundle branches
  • It moves away from the electrode as it goes up the Purkinje fibres

Year 1: Cardiovascular System (21 decks)

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