4.2 - Initiation and Coordination of Contraction

Question 1

The heart contains specialized cells known as ____

Answer 1

autorhythmic cells (pacemaker cells)

Question 2

Where are the autorhythmic cells found in the heart?

Answer 2

• sinoatrial node (SA node)

- Right atrium, near superior vena cava

Question 3

What is the function of pacemaker cells?

Answer 3

generate APs without input from the nervous system

Question 4

What is the pacemaker potential?

Answer 4

• starting point of -60mV

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

Question 5

Why do they have unstable membrane potentials?

Answer 5

Because they have different membrane channels than other excitable cells

Question 6

What are funny channels? What are they permeable to?

Answer 6

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

- permeable to K and Na

Question 7

When membrane potential is -ve ______

Answer 7

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

Question 8

What happens when funny channels close?

Answer 8

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

Question 9

What happens at the end of depol. ?

Answer 9

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

Question 10

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

Answer 10

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

Question 11

What modulates the rate of pacemaker potentials?

Answer 11

The autonomic division

• sympathetic: stressful situations
• parasympathethic: rest and digest

Question 12

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

Answer 12

• 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

Question 13

Explain how the release of acetylcholine leads to heart rate decreasing

Answer 13

• 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

Question 14

How does electrical communication take place in the heart

Answer 14

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

Question 15

Explain how the events of conduction take place

Answer 15

• 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

Question 16

why is the signal delayed by the AV node?

Answer 16

to ensure that atria have finished contracting

Question 17

What is an ectopic beat?

Answer 17

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

Question 18

What is arrythmia and when can it develop?

Answer 18

• 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

Question 19

List and explain the types of arrythmia and their symptoms:

Answer 19

• 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