Cardiac Physiology

Question 1

What are the main features of the conduction system of the heart?

Answer 1

SAN
AV node
AV bundle
Right t& left bundle fibres 
Purkinje fibres

Question 2

What does the SAN do?

Answer 2

Generate spontaneous A.Ps

Pass them to atrial muscle cells + AV node

Question 3

What is the structure of SAN?

Answer 3

Specialised cardiac cells

Question 4

What does the AV node do?

Answer 4

A.Ps conducted more slowly

= ventricles receive signal to contract after atria contract

Question 5

What does AV bundle do?

Answer 5

Pass through hole in cardiac skeleton to reach interventricular septum

Question 6

What does the right & left bundle branches do?

Answer 6

Extend beneath endocardium to apices of right + left ventricles

Question 7

What do Purkinje fibres do?

Answer 7

Conduct A.Ps to ventricular muscle cells (myocardium)

Question 8

What is the structure of Purkinje fibres?

Answer 8

Large diameter cardiac muscle cells with few myofibrils

Many gap junctions

Question 9

Describe conduction system of heart

Answer 9

Depolarisation from SAN across atria
Conduct to AVN
Conduction slow in AVN
Atrioventricular bundle of His connects to branches
Purkinje fibres = terminal bundle branches
Myocardial fibres

Question 10

Why is conduction slow in AVN?

Answer 10

To allow atria to empty into ventricles before ventricular systole

Question 11

Describe structure of cardiac muscles

Answer 11

Faintly striated
Branched, mono-nucleated cells
Connected by intercalated discs ( gap junctions + desosomes)

Question 12

Why is there slow propagation in cardiac muscles?

Answer 12

Gap junctions + small diameter

Question 13

Why is propagation faster in skeletal muscles?

Answer 13

Larger diameter fibres

Question 14

Where does A.P occur in cardiac muscles?

Answer 14

Cell to cell

Question 15

Where does A.P occur in skeletal muscles?

Answer 15

Along length of single fibre

Question 16

Describe sliding filament theory

Answer 16

Ca2+ binds to TN-C on thin filament
Exposes site on actin = can bind to myosin head
ATP hydrolysis supplies energy for actin-myosin conformational change
“Ratcheting” of actin-myosin + shortening of sacromere occurs
Ca2+ dissociates from TN-C + myosin unbinds with energy from ATP
Cycle ends when ATP binds to myosin + sacromere returns to original length

Question 17

Describe cardiomyocyte contraction

Answer 17

Ca2+ enters through L-type channel
Ca2+-induced release occurs
Stimulates Ca2+ release from SR
Intracellular [Ca2+] increases 
Ca2+ interacts with troponin-C 
Myosin binding site free on actin
Actin moves over myosin = myocyte contraction

Question 18

What happens after cardiomyocyte contraction has taken place?

Answer 18

Ca2+ reabsorbed into SR via sarco-endoplasmic reticulum Ca-ATPase pump
Removed from cell by Na+/Ca2+ exchanger + Ca2+ pump
ATP required to unbind myosin from action + resent sarcomere

Question 19

What is the charge of the inside of cell?

Answer 19

Negative

Question 20

What is charge after A.P?

Answer 20

Positive

Question 21

What are the three factors that control charge?

Answer 21

Conc of ions in + out cell
Ion conductance
Activity of electrogenic pumps

Question 22

What is value of resting potential?

Answer 22

-90mV

Question 23

What is value of depolarisation threshold value?

Answer 23

-65mV

Question 24

How many phases is there?

Answer 24

5

Question 25

What happens in phase 0?

Answer 25

Opens Na+ gates Membrane potential increases +40mV Na+ gated "inactivated" = Na+ permeability falls

Question 26

What happens in phase 1?

Answer 26

Ca2+ gates open = inward flow of Ca2+

Question 27

What happens in phase 2?

Answer 27

K+ balances Ca2+ flow = plateau Generates cardiomyocyte contraction Ca2+ channels inactivated

Question 28

What happens in phase 3? | REPOLARISATION

Answer 28

K+ channels reopen | Membrane potential falls

Question 29

What happens in phase 4? | Resting potential

Answer 29

Return to -90mV

Question 30

What is pacemaker potential?

Answer 30

After A.P, membrane potential drifts up until threshold met | Na+ channels open = AP triggered

Question 31

Where is the SAN?

Answer 31

Right atrium | close to vena cava

Question 32

Describe A.P on SAN pacemaker cells

Answer 32

``` Phase 4: spontaneous depolarisation Inward movement of Na+, outward K+ K+ movement decays with time Pacemaker potential depolarises to -55mV Ca2+ inward current accelerates threshold to -40mV Phase 0: depolarisation Ca2+ increases = L-type Ca2+ channels Phase 3: repolarisation K+ channels open + help recognise cell as Ca2+ channel inactive ```

Question 33

Describe parasympathetic regulation of cardiac cycle

Answer 33

Cardiomyocyte centre (vagus nerve) Nerve branches to SA + AV nodes Secrete acetylcholine; hyperpolarises heart Decreases HR

Question 34

Describe sympathetic regulation of cardiac cycle

Answer 34

Cardioaccelerator centre activates sympathetic neurons (cardiac nerves) Secreted norepinephrine Increases force of contractions = increases HR

Question 35

What is a P-wave?

Answer 35

Depolarisation of atrial myocardium

Question 36

What does P-wave show on electrocardiogram?

Answer 36

Signals onset of atrial contraction | On ECG = positive, not large or peaked

Question 37

What is QRS complex?

Answer 37

Ventricular depolarisation

Question 38

What does QRS complex show on electrocardiogram?

Answer 38

Signals onset of ventricular contraction | On ECG = negative deflection

Question 39

What is a T-wave?

Answer 39

Repolarisation of ventricles

Question 40

What is the PR interval?

Answer 40

Start of atrial depolarisation to start of ventricular depolarisation = contract + reflex

Question 41

What does PR interval indicate?

Answer 41

Damage to conducting pathway or AVN

Question 42

What is the QT interval?

Answer 42

Time required for ventricles to undergo single cycle of depolarisation + repolarisation

Question 43

What is the QT interval lengthened by?

Answer 43

Electrolyte disturbances, conduction problems, ischemia + damage