L14 Mechanisms of Contraction of the Heart

Question 1

Is cardiac striated?

Answer 1

Yes

Question 2

True or false: Cardiomyocytes contain several nuclei and relatively large numbers of mitochondria

Answer 2

False.

Cardiomyocytes contain a single centrally-located nucleus (mononucleated) and large numbers of mitochondria

Question 3

Describe the junctions between adjacent cardiomyocytes.

Answer 3

Within the intercalated discs are desmosomes and gap junctions. The gap junctions enable electrical signals to pass between cells.

Question 4

True or false: Both skeletal muscle and cardiac muscle share the same contraction mechanism

Answer 4

True.

Skeletal muscle and heart muscle share the same contraction mechanism, and this mechanism is described by what is known as the sliding filament model.

Question 5

What is the name of the thin line that separates each sarcomere?

Answer 5

Z band

Question 6

Which component(s) of the sarcomere attaches to the Z line?

Answer 6

Think actin filaments that form the I band attach to the Z disc on the Z line.

Titin filaments also attach to the Z line via the Z discs.

Question 7

The I band is formed from what substance?

Answer 7

Actin - it shows up as a light band on the sarcomere because actin is a thin filament.

Question 8

The A band is formed from what substance?

Answer 8

Myosin-II

Question 9

The I band consists of a chain of globular __1__ molecules joined together to form a __2__ shape. Each molecule has a binding site for a __3__ head. At every 7th molecule, it is also coupled to two other proteins: __4__ and __5__.

Answer 9

1) actin
2) helix
3) myosin
4) tropomyosin
5) troponin

Question 10

Which molecule masks to myosin binding sites by attaching itself to groove of the actin helix?

Answer 10

Tropomyosin

Binds to actin via troponin molecules

Question 11

Troponin is a complex of three polypeptides. What are it’s subunits and what does each one do?

Answer 11

Troponin T: Binds to tropomyosin

Troponin I: Binds to actin

Troponin C: Calcium-sensitive, binding 4 Ca2+ changes shape of troponin and opens myosin binding site on actin.

Question 12

How does Troponin C affect muscle contraction?

Answer 12

Contraction is blocked when troponin I and T are added to tropomyosin and actin (blocks myosin binding site), but on addition of troponin C the troponin complex becomes calcium sensitive.

On binding with 4 Ca2+ ions, troponin goes through a conformational change moving the tropomyosin from the myosin binding site.

Question 13

Which end of the myosin-II filament forms the motor head domain?

Answer 13

The amino terminal

Question 14

Which end of the myosin-II filament forms the elongated tail?

Answer 14

The carboxyl-ended section

Question 15

True or false: In the sarcolemma, myosin exists as a dimer by forming an alpha helix with a second myosin chain

Answer 15

True.

The dimers can then polymerise into stable bipolar filaments with head groups at either end (head is the amino terminal)

Question 16

What happens to the relationship between actin and myosin filaments during contraction?

What happens to the size of the gap between the Z-lines?

Answer 16

Thin filaments (actin) and thick filaments (myosin) overlap.

Gap between Z-lines shortens.

Question 17

True or false: When cardiac muscles contract, ATP is converted to ADP.

Answer 17

True

Question 18

How is muscle contraction achieved?

Answer 18

1. Stored calcium is released from the sarcoplasmic reticulum
2. Four calcium ions bind with troponin-C
3. Results in conformational change in troponin complex, which moves tropomyosin away from the myosin binding sites
4. Myosin heads bind to actin molecules

Question 19

During the contraction cycle of a sarcomere, release of a phosphate group from the myosin head allows what to occur?

Answer 19

Myosin head to bind to actin, forming crossbridges.

Question 20

What factors determine whether or not the contraction cycle of a sarcomere continues?

Answer 20

ATP and Ca2+ levels.

Contraction cycle continues if ATP is available and Ca2+ levels in sarcoplasm is high.

Question 21

Cardiac muscle produces ATP by aerobic oxidation of:

A) fatty acids
B) glucose
C) lactic acid
D) amino acids
E) ketones
F) B, D, and E only
G) All of the above

Answer 21

G) All of the above

Cardiac muscle produces most of its ATP by aerobic oxidation of fatty acids, glucose, lactic acid, amino acids and ketones.

Question 22

At rest, the heart derives most of its ATP from what source?

Answer 22

Metabolism of fatty acids (60%)

Also derives about 35% from glucose metabolism. During exercise the heart can use a lot of lactic acid to form ATP.

Question 23

Though not the main source, the heart can also produce some ATP as a result of the action of creatine kinase. Why?

Answer 23

Creatine kinase functions by catalysing the transfer of a phosphate group from creatine phosphate to ADP.

Question 24

Which cardiac rhythm disturbance is being described:

Repetitive myocyte activity that is not driven by potentials arising from other cells. Arises after a normal action potential, and occurs because intracellular calcium concentrations increase beyond the normal range.

Answer 24

Delayed Afterdepolarisation.

DADs begin during phase 4, after repolarization is completed but before another action potential would normally occur via the normal conduction systems of the heart.

Question 25

Which cardiac rhythm disturbance is being described: Propagating impulse fails to die out after normal activation of contraction and continues to re-excite the heart after the refractory period has ended. Caused by unusual anatomical variations that form a conductive ring, or damage that results in slow conducting pathways.

Answer 25

Re-entry. Re-entry is the electrophysiologic mechanism responsible for the majority of clinically important arrhythmias.

Question 26

Which cardiac rhythm disturbance is being described: Spontaneous depolarisation of cells occurs in the wrong region of the heart.

Answer 26

Abnormal Pacemaker Activity. Ectopic pacemakers can develop in the heart away from the nodes. Ectopic pacemakers can be induced by excessive sympathetic stimulation, caffeine, nicotine, electrolyte imbalances and hypoxia. Ischemia can also cause depolarisation: decreased sodium pump activity results in the cells being maintained at a low resting membrane potential.

Question 27

Which cardiac rhythm disturbance is being described: AV node becomes electrically isolated.

Answer 27

Heart Block. Can be partial (ventricles contract after 2 or three atrial contractions), or total (atria and ventricles contract independently). Sporadic total AV node block can also occur, and this results in sudden periods of unconsciousness.

Question 28

How is bradycardia treated? ``` A) Ablation B) Calcium channel blockers C) Implanted pacemaker D) Beta-blockers E) A, B and D only F) All of the above ```

Answer 28

C) Implanted pacemaker

Question 29

How is tachycardia treated? ``` A) Ablation B) Calcium channel blockers C) Implanted pacemaker D) Beta-blockers E) A, B and D only F) All of the above ```

Answer 29

E) A, B and D only Other drugs used to treat tachycardia include sodium channel blockers, b-adrenoceptor antagonists, and drugs that extend the refractory period.