Electrical activity of the heart

Question 1

What does cardiac muscle form ?

Answer 1

A functional syncytium

Question 2

What is the calcium store in muscle ?

Answer 2

Sarcoplasmic reticulum

Question 3

What forms the electrical connection in the heart ?

Answer 3

Gap junctions

Question 4

What are the intercalating disks formed by ?

Answer 4

Desmosome followed by gap junction. Desmosome ensures that cells contract at the same time

Question 5

What are the cells called with an unstable resting membrane potential ?

Answer 5

Pacemakers

Question 6

What causes the resting membrane potential in non-pacemaker APs ?

Answer 6

High resting permeability for potassium

Question 7

What causes the initial depolarisation in non-pacemaker APs ?

Answer 7

Increase in permeability for sodium

Question 8

What causes the plateau in non-pacemaker APs ?

Answer 8

Increase in permeability for calcium (L - type channels) and decrease in permeability for potassium

Question 9

What causes the repolarisation in non-pacemaker APs ?

Answer 9

Decrease in permeability for calcium and increase in permeability for potassium

Question 10

What causes the AP in pacemaker APs ?

Answer 10

Increase in permeability for calcium (L-type receptors)

Question 11

What causes the pacemaker potential (pre-potential) in pacemaker APs ?

Answer 11

Gradual decrease in permeability for potassium
Early increase for permeability for sodium (Pf channels)
Late increase in calcium in (T-type channels)

Question 12

What does the pacemaker explain ?

Answer 12

autorhythmicity

Question 13

What is the function of the SAN ?

Answer 13

Pacemaker - 0.5 metres per second

Question 14

What is the annulus fibrosus ?

Answer 14

Non- conducting insulator between the atria and the ventricles, Action potential can only pass through the atrioventricular node

Question 15

What is the atrioventricular node ?

Answer 15

A delay box - 0.05 m/s

Question 16

What is the function of bundle of his ?

Answer 16

Bundle of his ensures all the ventricle contracts at the same time

Question 17

What is the function of purkinjie fibres ?

Answer 17

Rapid conducting system, innervate ventricles - ~5m/sec

Question 18

What is the extracellular effect of an action potential in a single myocyte ?

Answer 18

Evokes a very small extracellular electrical potential

Question 19

How are large extra-cellular electrical waves created ?

Answer 19

Lots of small extracellular electrical potentials evoked by many cells depolarising and repolarising at the same time can summate to create large extracellular electrical waves

Question 20

What does the P wave correspond to ?

Answer 20

Atrial depolarisation

Question 21

What does the QRS complex correspond to ?

Answer 21

Ventricular depolarisation

Question 22

What does the T wave correspond to ?

Answer 22

Ventricular repolarisation

Question 23

What are blocks the result of ?

Answer 23

Issues in conducting

Question 24

What are flutters and fibrillations due to ?

Answer 24

Errors in rhythm

Question 25

What is characteristic of atrial fibrillation on an ECG ?

Answer 25

No distinguishable P wave

Question 26

How does a defibrillator work ?

Answer 26

Defibrillator puts all the cells into their refractory state – they are now ready to receive the next action potential

Question 27

How does hyperkalaemia affect the electrical activity of the heart ?

Answer 27

Fibrillation and heart block - reduces concentration gradient for potassium – cell starts to depolarise, spontaneous firing of action potentials

Question 28

What is the affect of hypokalaemia on electrical activity of the heart ?

Answer 28

fibrillation & heart block (anomalous) – cells start to hyperpolarise – loads of channels open and the cell depolarises