4.2 - Initiation and Coordination of Contraction Flashcards

1
Q

The heart contains specialized cells known as ____

A

autorhythmic cells (pacemaker cells)

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2
Q

Where are the autorhythmic cells found in the heart?

A
  • sinoatrial node (SA node)

- Right atrium, near superior vena cava

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3
Q

What is the function of pacemaker cells?

A

generate APs without input from the nervous system

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4
Q

What is the pacemaker potential?

A
  • starting point of -60mV

- unstable membrane potential slowly drifts upwards from pacemaker potential until reaches threshold and initiates AP

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5
Q

Why do they have unstable membrane potentials?

A

Because they have different membrane channels than other excitable cells

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6
Q

What are funny channels? What are they permeable to?

A
  • specialized If channels (I=current, f=funny channels)

- permeable to K and Na

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7
Q

When membrane potential is -ve ______

A

Na+ influx > K+ efflux = net influx of +ve charge = slow depolarization of membrane

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8
Q

What happens when funny channels close?

A

Ca2+ channels open = continued depol. = threshold reached = many more Ca2+ channels open, rapid influx of Ca2+ = steep depol. phase of AP

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9
Q

What happens at the end of depol. ?

A

Ca2+ channels close, K+ channels open slowly, efflux of K+ causes repol.

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10
Q

What is the major difference between AP and PP in pacemaker cells?

A

Influx:
A.P = only Ca2+
P.P = both Na+ and Ca2+

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11
Q

What modulates the rate of pacemaker potentials?

A

The autonomic division

  • sympathetic: stressful situations
  • parasympathethic: rest and digest
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12
Q

Explain how the release of norepinephrine and epinephrine leads to heart rate increasing

A
  • Norepinephrine (sympathetic neurons) and epinephrine (adrenal medulla) bind to b1 adrenergic receptors
  • release of cAMP through signalling pathway binds to open funny channels
  • channels stay open longer, increasing permeability to Na+ and Ca2+
  • increased depol. rate increases rate of action potentials, thus increasing heart rate
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13
Q

Explain how the release of acetylcholine leads to heart rate decreasing

A
  • ACh released from parasympathetic neurons, binds to muscarinic receptors
  • increases K+ permeability, hyperpolarizing the cell
  • pacemaker potential starts at more negative value, taking longer to reach threshold potential, decreases heart rate
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14
Q

How does electrical communication take place in the heart

A
  • autorhythmic/pacemaker cells initiate the electrical excitation of the heart
  • depol. spreads to neighboring cardiac cells via gap junctions in intercalated discs
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15
Q

Explain how the events of conduction take place

A
  • action potentials are fired at the SA node, spreads to adjacent cells
  • rapid spread through cells of internodal pathway, slow spread through contractile cells of atrium due to cytoplasmic resistance
  • signal passed through AV node ONLY at AV junction: fibrous connective tissue acts as insulator preventing electrical signals from atrium to the ventricle
  • AV node ONLY pathway for action potential
  • signal delay at AV node
  • signal carried to bottom of heart through bundle of His
  • Bundle of His divides into left and right branches: purkinje fibres transmit signals rapidly to ensure all contractile cells at apex contract together
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16
Q

why is the signal delayed by the AV node?

A

to ensure that atria have finished contracting

17
Q

What is an ectopic beat?

A

When the heart beat is generated by a pacemaker cell out of the SA node

18
Q

What is arrythmia and when can it develop?

A
  • abnormal heart rhythm
  • when another part of the heart takes over as pacemaker
  • when the SA node develops an abnormal rate or rhythm
  • when the normal conduction pathway is disrupted
19
Q

List and explain the types of arrythmia and their symptoms:

A
  • Bradycardia: slow heart beat = <60bpm
    symptoms: fatigue, dizziness, light-headedness, fainting
  • Tachycardia/tachyarrythmia: rapid heart beat = >100bpm
    symptoms: palpitations, dizzinezz, light-headedness, fainting (not enough time for heart to pump out enough blood due to rapid pumping)
  • Ventricular fibrillation (V fib): disorganized contraction
    cardiac emergency, ventricle quivers, heart cannot pump blood, heart collapse and sudden death unless defibrillator is used

atrial fibrillation: atria quiver
- blood not pumped out completely when heart beats
- allows blood to pool and clot
- can lead to stroke if blood clot lodges in brain
to reduce risk of stroke, people put on warfarin/aspirin