Mechanisms of contraction of the heart

Question 1

What do gap junctions between intercalated discs do?

Answer 1

Provide physical connections that enable electrical signals to be passed from cell to cell

Question 2

What is the contraction mechanism in the heart?

Answer 2

Sliding filament model

Question 3

What are actin and myosin arranged into?

Answer 3

Myofibrils

Question 4

Describe the actin I band molecule?

Answer 4

Globular actin molecules joined to form a helix - has a binding site for myosin head and coupled to troponin and tropomyosin at every 7th molecule

Question 5

What is Troponin?

Answer 5

3 polypeptides T - tropomyosin, I - actin , C- calcium

Question 6

How many Calcium ions need to bind to troponin for a conformational change to move tropomyosin?

Answer 6

4

Question 7

Describe Myosin? A band

Answer 7

myosin II
2 identical heavy and light chains
Amino terminal - motor head
Carboxyl end - elongated tail forms alpha helix with 2nd heavy chain to make a dimer

Question 8

What do dimers undergo?

Answer 8

Polymerisation

Question 9

How is contraction achieved?

Answer 9

Through excitation -released calcium from SR - bind to Troponin C so myosin heads bind to actin binding site

Question 10

How is muscle contraction driven?

Answer 10

ATP to ADP

Question 11

How does cardiac muscle produce ATP?

Answer 11

aerobic oxidation of fatty acids, glucose lactic acid, AA and Ketones

Question 12

How does cardiac muscle gain oxygen for ATP?

Answer 12

oxygen diffuses from blood pumped through coronary blood circulatory system at rest majority of atp from metabolism of fatty acids and glucose

Question 13

What happens to heart during exercise?

Answer 13

use lactic acid to increase metabolism

Question 14

How else does heart muscle produce ATP in regards to enzyme reaction?

Answer 14

Intracellular enxyme of Creatin Kinase CK - catalyses transfer of phosphate from creatin phosphate to ADP

Question 15

How many mechanisms results in disturbances in cardiac rhythm? (dysrhythmia)

Answer 15

4 - delayed after depolarisation, re entry, abnormal pacemaker activity and heart block

Question 16

What happens in delayed after depolarisation?

Answer 16

repetitive myocyte activity not driven by potentials arising from other cells - after normal AP and occurs due to intracellular calcium conc increasing normal range

Question 17

What happens upon re-entry?

Answer 17

cardiac AP dies out in ventricles -unusual anatomical variations form a network of cells that form a conductive ring - also to slow conducting pathways of myocardial damage

Question 18

What happens with abnormal pacemaker activity?

Answer 18

pacemaker elsewhere in the heart - ectopic pacemakers by excess sympthetic stimulation (caffeins hypoxia)
depolarisation results from ischemia from decreases sodium pump activity low RMP

Question 19

What happens in heart block?

Answer 19

AV node electrically isolated
partial in every 2 to 3 atrial contractions the ventricle will contract independently
Sporadic total AV node block causes random unconsiousness

Question 20

How is bradycardia treated?

Answer 20

Pacemaker systems

Question 21

How is Tachycardia treated?

Answer 21

oblation procedures with range of drugs - block voltage sensitive sodium channels
B-adrenoreceptor antagonists
elongate refractory period
calcium channel blockers