Electrical activity of the heart

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  • Created by: SamDavies
  • Created on: 05-05-19 23:05
What does myogenic mean?
Cardiac cells are called myogenic because they can generate action potentials which spread throughout the myocardium, causing it to contract as a single unit
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What happens during phase 0 of ventricular action potentials?
Depolarisation of a nearby cell causes leakage of Na+ and Ca2+ into cardiomyocytes via gap junctions. This slightly increases membrane potential which opens VgNaC, causing Na+ influx. This increases membrane potential from -70mv to +20mv
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What happens during phase 1 of ventricular action potentials?
VgNaC close and VgKC open causing an efflux of K+. This starts to decrease the membrane potential (partial repolarisation) but only to +5mV
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What happens during phase 2 of ventricular action potentials?
At +5mV, VgCaC open, causing Ca2+ influx. K+ efflux is simultaneously occurring so this causes a plateau
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What happens during phase 3 of ventricular action potentials?
VgCaC close. With only VgKC open, K+ ions continue to efflux, decreasing the membrane potential rapidly (repolarisation)
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What happens during phase 4 of ventricular action potentials?
VgKC close, bringing the cell back to its resting membrane potential
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What happens during phase 4 of pacemaker action potentials?
At ~60mV, Na channels allow a slow inward current of Na+ ions (called 'funny currents'). This increases the membrane potential slightly
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What happens during phase 0 of pacemaker action potentials?
When the membrane potential due to the funny currents hits -40mV, threshold potential is reached and VgCaC open, causing Ca2+ influx. Membrane potential increases to +10mV
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What happens during phase 3 of pacemaker action potentials?
VgCaC shut and VgKC open, causing K+ efflux. VgKC shut at -60mV as there is still Na+ ions entering. This means it will precede to threshold again and again (unstable)
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What is cardiac excitation-contraction coupling?
The process whereby action potentials trigger myocytes to contract
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How does cardiac excitation-contraction coupling work?
Ca2+ binds to troponin C which is part of a complex found on myosin heads. This exposes a site on actin which was previously covered by tropomyosin. This causes ATP hydrolysis and conformation changes so that myosin and actin can now bind and contrac
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What does the P wave of an ECG signify?
Atrial depolarisation
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What does the QRS complex of an ECG signify?
Ventricular depolarisation
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What does the T wave of an ECG signify?
Ventricular repolarisation
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What does the PR interval of an ECG signify?
Delay of AV node to allow filling of ventricles. Normal is 0.12 - 0.2 seconds. Abnormalities can indicate heart block
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What does the ST segment of an ECG signify?
The beginning of ventricular repolarisation - it should lie flat
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How to calculate heart rate on an ECG trace?
Count number of complete R wave to R wave in a 6 second rhythm ***** and multiply by 10
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What is heart block?
There is an abnormality in the transmission of electrical signals from the atria to the ventricles (also called atrioventricular block)
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What is first degree heart block?
All electrical signals reach the ventricles but the signal is slowed through the AV node (longer PR interval). Rarely causes symptoms and there is usually no need for treatment
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What is second degree heart block?
Not all electrical signals reach the ventricles from the atria, so some QRS complexes do not follow P waves.
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What is third degree heart block?
None of the electrical signals reach the ventricles from the atria, so the atria and the ventricles beat independently of each other, i.e. there is non-synchronised
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What is bundle branch block?
A condition where there is a delay or blockage along the electrical impulse pathway. This can occur on the right or left side of the venticles, making the impulse travel slightly longer as it is rerouted to the endpoint
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What do chronotropic drugs do?
Increase heart rate, e.g. digoxin (negative chronotropes like beta blockers decrease heart rate)
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What is preload?
Volume of blood entering the ventricles - can be used interchangeably with end-diastolic volume (about 120ml)
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What is afterload?
The resistance in the arteries as the blood leaves the left ventricle
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What happens to the stroke volume in a heart failure patient?
The stroke volume will be lower than normal and to a certain point no matter how much the end-diastolic volume increases, the stroke volume will plateau
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Other cards in this set

Card 2

Front

What happens during phase 0 of ventricular action potentials?

Back

Depolarisation of a nearby cell causes leakage of Na+ and Ca2+ into cardiomyocytes via gap junctions. This slightly increases membrane potential which opens VgNaC, causing Na+ influx. This increases membrane potential from -70mv to +20mv

Card 3

Front

What happens during phase 1 of ventricular action potentials?

Back

Preview of the front of card 3

Card 4

Front

What happens during phase 2 of ventricular action potentials?

Back

Preview of the front of card 4

Card 5

Front

What happens during phase 3 of ventricular action potentials?

Back

Preview of the front of card 5
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