3. Origin + spread of heartbeat

Question 1

Ohms Law

Answer 1

V=IR

Question 2

Pressure

Answer 2

F x R (flow x resistance)

Question 3

Systemic Pressure

Answer 3

CO x TPR

Question 4

Cardiac Output

Answer 4

HR x SV

Question 5

Sinoatrial (SA) node

Answer 5

100/min

Question 6

Atrioventricular (AV) node

Answer 6

50/min

Question 7

Bundle of His/Purkinje fibres

Answer 7

30/min

Question 8

SA node AP

CaKNa

Answer 8

Phase 0 = Ca2+ influx

Phase3 = Ca2+ channels inactivate, Delayed K+ efflux

Phase 4 = Na+ influx, Ca2+ channels recover from inactivation, pumps restore ion gradients

Question 9

Phase 4 SA Node

Answer 9

HCN-gated channels

mediates a ‘funny current’

simultaneous K+ efflux and Na+ influx

Na influx dominates and membrane slowly depolarises to threshold

Question 10

Sympathetic change of HR

Answer 10

noradrenaline

acting on b1 adrenoceptors

·Increases opening of HCN channels

•Increase Na+ influx; If

·Opens Ca2+ channels

•Increased Ca2+ influx

·Increase in slope of prepotential hence HR

•Tachycardia

Question 11

Parasympathetic change of HR

Answer 11

ach

muscarinic cholinergic receptors

·Decreases opening of HCN channels

•Decreased Na influx

·Slows opening of Ca2+ channels

• Decreased Ca2+ influx
• Increased K+ efflux

·reduces slope of prepotential hence, fall in HR

•Bradycardia

Question 12

Ventricular Myocyte AP

NaKCa

Answer 12

Phase 0 = Na+ influx

Phase 1 = Na+ channels inactivate, fast K+ efflux

Phase 2 = Ca2+ influx

Phase 3 = Ca2+ channels inactivate, delayed K+ efflux

Phase 4 = Na+ and Ca2+ channels recover from inactivation, pumps restore ion gradients

Question 13

Effect of ·High [K+] on heart

Answer 13

↓ K+ gradient across cell membrane

↓ K+ efflux

Cell initially more excitable

Closure of h gate

Conduction problems as Na channel cannot reset

Cannot generate another action potential upstroke

Arrest in diastole and death

Question 14

Effect of low [Na+] on heart

Answer 14

Little effect on resting membrane potential

↓ Na+ concentration gradient

↓ Na+ influx

↓ Height of action potential, may not reach threshold

Conduction problems

Eventual arrest in diastole and death

Question 15

Effect of ·High [Ca2+] on heart

Answer 15

↑ Ca2+ concentration gradient

↑ Ca2+ influx

Big force contraction

[Ca2+] too high to pump out enough to relax cell

Arrest in systole