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Cardiovascular system > Cardiac impulses > Flashcards

Cardiac impulses Flashcards

1
Q

Where are electrical impulses in the heart formed?

A

The pacemaker cells of the sino-atrial node

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2
Q

Where is the sino-atrial node located?

A

Inferior to the Superior Vena Cava in the right atrium

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3
Q

What causes the pacemaker potentials in the sino-atrial nodes?

A

The funny current
Ca2+ influx
Decreased K+ influx

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4
Q

What is the funny current?

A

Current due to slow Na+ influx through HCN channels (Hyperpolarisation-activated Cyclic Nucleotide-gated channels)

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5
Q

What does a pacemaker action potential look like?

A
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6
Q

What causes the rising phase of the sino-atrial action potential (Phase 0 - Depolarisation)?

A

When the threshold is reached at around -40mV, the action potential is triggered
This occurs due to opening of L-type Ca2+ voltage-gated channels
This causes Ca2+ influx which depolarises the membrane

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7
Q

What causes the falling phase of the sino-atrial action potential (Phase 3 - Depolarisation)?

A

When the maximum potential is reached, L-type Ca2+ channels become inactivated
K+ ion channels open, causing influx

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8
Q

Describe the spread of excitation throughout the heart

A

Sino-atrial node begins excitation
Excitation spreads across the atria causing contraction
This reaches the Atrio-Ventricular node
This delays the impulse for up to 0.2ms
The impulse travels down the common Bundle of His
This branches to the left and right Bundle of His
This travels down to the apex of the heart
It then travels back up the purkinje fibres to the ventricle walls
This causes ventricular contraction

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9
Q

How is excitation spread between myocytes?

A

Via communicating (gap) junctions

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10
Q

What are the 5 phases of myocyte action potentials?

A

Phase 0 - Rising phase
Phase 1 - Small, fast fall
Phase 2 - Plateau
Phase 3 - Falling phase
Phase 4 - Resting potential

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11
Q

What occurs during phase 0 of myocyte action potential?

A

Rising phase is caused by fast Na+ influx, reaching a potential difference of around +20mV

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12
Q

What occurs during phase 1 of myocyte action potential?

A

Small, falling phase is caused by closure of the Na+ channels and opening of K+ channels causing efflux

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13
Q

What occurs during phase 2 of myocyte action potential?

A

The plateau phase is caused by opening of L-type Ca2+ channels, causing Ca2+ influx, while K+ continuous to move out

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14
Q

What occurs during phase 3 of myocyte action potential?

A

The large, fast fall is caused by closure of the L-type Ca2+ channels, while K+ channels remain open

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15
Q

What occurs during phase 4 of myocyte action potential?

A

Return to resting membrane potential of around -90mV as K+ channels then close

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16
Q

What are the components of an ECG wave?

A

P wave
PR interval
QRS complex
ST interval
T wave
TP interval

17
Q

What causes the P wave on an ECG?

A

Atrial depolarisation

18
Q

What causes the PR interval on an ECG?

A

AV nodal delay

19
Q

What causes the QRS complex on an ECG?

A

Ventricular depolarisation (Masking atrial depolarisation)

20
Q

What causes the ST interval on an ECG?

A

Ventricular systole

21
Q

What causes the T wave on an ECG?

A

Ventricular depolarisation

22
Q

What causes the TP interval on an ECG?

A

Diastole

23
Q

What nerve exerts a continuous influence on the Sino-Atrial Node?

A

Vagus nerve - It dominates under normal resting conditions

24
Q

What effect does continuous vagal influence have on heart rate?

A

It decreases the heart rate from around 100bpm to around 70bpm

25
Q

What effect does sympathetic innervation have on heart rate?

A

It increases the slope of the funny current in pacemaker cells by increasing Ca2+ and Na+ influx, therefore increasing heart rate

26
Q

What effect does parasympathetic innervation have on heart rate?

A

It decreases Na+ and Ca2+ influx, so decreases the slope of the pacemaker current, therefore decreasing heart rate
It also increases AV node delay

27
Q

How does Atropine work?

A

It is a competitive inhibitor of acetylcholine receptors
This increases the slope of pacemaker potential and decreases AV node delay, therefore increasing heart rate

28
Q

How does cardiac muscle contraction occur?

A

Ca2+ moves into the cell during phase 2 of myocyte action potential
This causes Ca2+ induced Ca2+ release from the sarcoplasmic reticulum
Ca2+ binds to troponin on the actin filament
This causes a conformational change of tropomyosin, which uncovered the myosin binding site
Myosin heads then bind to these and ATP allows them to bend, causing actin to slide over the myosin filament, causing contraction

29
Q

What is pre-load?

A

The force on myocytes during pre-filling

30
Q

What is after-load?

A

The force of contraction required to eject blood

31
Q

What is the End Diastolic Volume?

A

The volume of blood present before contraction

32
Q

What is the End Systolic Volume?

A

The volume of blood present after contraction

Cardiovascular system (45 decks)

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