electrical activation of the heart

Question 1

key stages

Answer 1

• Electrical stimulus in individual cardiac myocyte
• Excitation- contraction coupling (how electrical stimulus leads to muscle cell contraction)
• Wave propagation – spreading of electrical stimulus to neighbouring cells
• Rapid spread throughout the heart allowing coordinated muscle contraction
Ions can move directly through gap junctions

Question 2

resting membrane potential

Answer 2

• Negative intracellular potential (-90mV)

1. Unbalanced ion concentrations and active membrane pumps (Na and Ca moves out and K moves in) result in excess external positive charge
2. Selective permeability allows K to diffuse out of the cell causing negative polarisation inside the cell
3. Chemical and electrical forces reach a point of equilibrium – the equilibrium potential

Question 3

action potential

Answer 3

• Phase 4: RMP – selective K+ permeability only, other channels are closed
• Phase 0: depolarisation – when a threshold potential is reached, Na channels open and Na rushes in, this depolarises the membrane. At +30 mV the Na channels close abruptly
• Phase 1: partial repolarisation – transient outward K+ channels open and then close
• Phase 2: plateau – competing but balanced currents maintain a plateau phase (inward Ca, outward K)
• Phase 3: repolarization – Ca channels close and rectifying K channels activate thus repolarising the membrane

Question 4

comparison to skeletal muscle

Answer 4

contraction of cardiac muscle lasts longer than skeletal muscle
up to 15 times longer
due to slow calcium channels
decreased permeability of membrane to potassium after action potential

Question 5

sinus node

Answer 5

• Normally determines the rate the heart beats
• Resting membrane potential of -55 to -60 mV
• Related to slow Na+ inflow
• Gradually drifts towards threshold for discharge
• Fast Na+ channels closed (inactivating gate closed)
• Action potential driven by slow Ca2+ channels

Question 6

AV node

Answer 6

transmits cardiac impulse between atria and ventricles
delays impulse - allows atria to empty blood into ventricles, few gap junctions, AV fibres are smaller than atrial fibres