Action potentials in the heart

Question 1

AP in the SAN - 1st step (depolarisation)

Answer 1

• AP is generated
• At -60mV it leads to the opening of HCN channels(hyperpolarisation gated cyclic nucleoside chanels)
• Opening of HCN alows Na+ ions to move in to the cell, causing depolarisation fom -60mV to -40mV

Question 2

AP in the SAN - 2nd stage (2nd depolaristion)

Answer 2

• 1st stage caused depolarisation from -60mV to -40mV
• Voltage gated Ca2+ channels open and Ca2+ enters, causing further depolarisation of the cell to +20mV

Question 3

AP in the SAN - 3rd stage (repolarisation)

Answer 3

• 2nd stage caused depolarisation to +20mV
• This results in the opening of K+ channels
• K+ moves out of the cell, loss of +ve charge causes repolarisation
• Hyperpolarisation threshold gets reached and HCN channels open again (cycle)

Question 4

Effect of catecholamine on HR

Answer 4

• Has a positive chronotropic effect
• Leads to faster repolarisation, so reaches threshold faster, so increased HR

(has its effect on the SAN cycle)

Question 5

Effect of acetylcholine on SAN cycle

Answer 5

Slows down the opening of HCN

Slows HR

Question 6

Describe the 1st stage of a myocardial action potential

Answer 6

• Resting potential is -90mV
• Depolarisation of cell, reaches threshold of -70mV
• Sodium gated fast channels open, Na+ enters the cells and depolarises to +20mV

Question 7

Describe 0st and 1st stage of myocardial action potential

Answer 7

• 1st stage causes depolarisation to +20mV
• K+ channels open, K+ leaves the cell and drops to -15mV (partial repolarisation). Na+ stops.

Question 8

Describe 2nd stage of myocardial AP (after 2nd stage repolarised to -15mV)

Answer 8

• Voltage gated Ca2+ channels open, Ca2+ moves in and counters the K+ channels, resulting in a plateau for approx 200msec

Question 9

What is the plateau stage in a myocardial AP?

Answer 9

Ca2+ and K+ channels balance out (Ca2+ moving in, K+ moving out)

Question 10

Describe the 3rd stage of a myocardial AP (after plateuau)

Answer 10

• Entry of Ca2+ into the cell results in contraction of myocyte
• Ca2+ inflow stops, K+ outflow, repolarisation until -90mV

Question 11

4th stage in myocardial AP

Answer 11

Resting potential maintained (K+ outflow)

Question 12

Describe sympathetic control of HR

Answer 12

• Increase HR (positive chronotropic)
• Increase force of contraction (positive inotropic)
• Increase CO by 200%
• Controlled by adrenaline and noradrenaline - type 1 beta adrenoreceptors, increased adenyl cyclase and increased cAMP

Question 13

Describe parasympathetic control of HR

Answer 13

Decrease HR by 30-40 bpm (negative chronotropic)
Decrease force of contraction (negative inotropic)
Decrease CO by 50%
Controlled by ACh - M2 receptors which inhibit adenyl cyclase so less cAMP