Electrical Properties of The Heart

Question 1

What is the calcium store in muscle?

Answer 1

Sarcoplasmic reticulum

Question 2

What forms the electrical connection in the heart?

Answer 2

Gap junctions

Question 3

What are the intercalating disks formed by?

Answer 3

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

Question 4

Why does cardia muscle have a long action potential?

Answer 4

Long refractory period ensures there is no tetanic contraction. Means Ca2+ entry from outside the cell can regulate contraction.

Question 5

How does Ca2+ entry from outside the cell regulate contraction?

Answer 5

impaired thermogenesis and energy expenditure.

Question 6

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

Answer 6

Pacemakers

Question 7

What causes the wave of electrical activity in the non-pacemaker action potential?

Resting membrane potential?

Initial depolarisation?

Plateau?

Repolarisation?

Answer 7

Resting membrane potential - High resting permeability for potassium

Initial depolarisation - Increase in permeability for sodium

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

Repolarisation - Decrease in permeability for calcium and increase in permeability for potassium

Question 8

What causes the wave of electrical activity in the pacemaker action potential?

Action potential?

Pacemaker potential (or pre-potential)?

Answer 8

Action potential - increase in permeability for calcium (L-type receptors)

Pacemaker potential (pacemaker potential):

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

Question 9

When do sodium pF channels open?

Answer 9

During repolarisation of the previous action potential

Question 10

How can drugs affect electrical activity of the heart?

Answer 10

–Ca2+-channel blockers – decrease force of contraction – target L type calcium channels – fewer cross bridges formed – smaller strength of contraction

–Cardiac glycocides – increase force of contraction – More calcium comes in - More cross bridges formed– stronger strength of contraction

Question 11

How does temperature affect the electrical activity of the heart?

Answer 11

–increases ~10 beats/min/ºC

Question 12

How does hyperkalemia affect the electrical activity of the heart?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

–Increased HR & force of contraction – more calcium comes in when the channels open – increase force of contraction.

Question 15

What is the effect of hypocalcaemia on electrical activity of the heart?

Answer 15

–Decreased HR & force of contraction

Question 16

What is the function of the sinoatrial node?

Answer 16

Pacemaker - 0.5 metres per second

Question 17

Describe the annulus fibrosis

Answer 17

Non- conduciting insulator between the atria and the ventricles. Action potential can only pass through the atroventricular node.

Question 18

Describe the stages shown

Answer 18

Question 19

What is the arterioventricular node?

Answer 19

A delay box - 0.05 m/s

Question 20

What is the function of the bundle of his?

Answer 20

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

Question 21

What is the name given to the fibres responsible for innervating the ventricular cardiac muscle?

Answer 21

•Purkinje fibres

–rapid conduction system

–~ 5 m/sec

Question 22

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

Answer 22

eEokes a very small extracellular electrical potential

Question 23

How are large extra-cellular electrical waves created?

Answer 23

•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 24

What does each wave correspond to?

Answer 24

P wave corresponds to atrial depolarisation

QRS complex corresponds to ventricular depolarisation

T wave corresponds to ventricular repolarisation

Question 25

What are blocks the result of?

Answer 25

Issues in conduction

Question 26

What are flutters and fibrilations due to?

Answer 26

Errors in rhythm

Question 27

What is characteristic of atrial fibrillation on an ECG?

Answer 27

No distinguishable P wave

Question 28

How does a defibrillator work?

Answer 28

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