L14 Mechanisms of Contraction of the Heart Flashcards
Is cardiac striated?
Yes
True or false: Cardiomyocytes contain several nuclei and relatively large numbers of mitochondria
False.
Cardiomyocytes contain a single centrally-located nucleus (mononucleated) and large numbers of mitochondria
Describe the junctions between adjacent cardiomyocytes.
Within the intercalated discs are desmosomes and gap junctions. The gap junctions enable electrical signals to pass between cells.
True or false: Both skeletal muscle and cardiac muscle share the same contraction mechanism
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.
What is the name of the thin line that separates each sarcomere?
Z band
Which component(s) of the sarcomere attaches to the Z line?
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.
The I band is formed from what substance?
Actin - it shows up as a light band on the sarcomere because actin is a thin filament.
The A band is formed from what substance?
Myosin-II
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__.
1) actin
2) helix
3) myosin
4) tropomyosin
5) troponin
Which molecule masks to myosin binding sites by attaching itself to groove of the actin helix?
Tropomyosin
Binds to actin via troponin molecules
Troponin is a complex of three polypeptides. What are it’s subunits and what does each one do?
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.
How does Troponin C affect muscle contraction?
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.
Which end of the myosin-II filament forms the motor head domain?
The amino terminal
Which end of the myosin-II filament forms the elongated tail?
The carboxyl-ended section
True or false: In the sarcolemma, myosin exists as a dimer by forming an alpha helix with a second myosin chain
True.
The dimers can then polymerise into stable bipolar filaments with head groups at either end (head is the amino terminal)
What happens to the relationship between actin and myosin filaments during contraction?
What happens to the size of the gap between the Z-lines?
Thin filaments (actin) and thick filaments (myosin) overlap.
Gap between Z-lines shortens.
True or false: When cardiac muscles contract, ATP is converted to ADP.
True
How is muscle contraction achieved?
- Stored calcium is released from the sarcoplasmic reticulum
- Four calcium ions bind with troponin-C
- Results in conformational change in troponin complex, which moves tropomyosin away from the myosin binding sites
- Myosin heads bind to actin molecules
During the contraction cycle of a sarcomere, release of a phosphate group from the myosin head allows what to occur?
Myosin head to bind to actin, forming crossbridges.
What factors determine whether or not the contraction cycle of a sarcomere continues?
ATP and Ca2+ levels.
Contraction cycle continues if ATP is available and Ca2+ levels in sarcoplasm is high.
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
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.
At rest, the heart derives most of its ATP from what source?
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.
Though not the main source, the heart can also produce some ATP as a result of the action of creatine kinase. Why?
Creatine kinase functions by catalysing the transfer of a phosphate group from creatine phosphate to ADP.
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.
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.
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.
Re-entry.
Re-entry is the electrophysiologic mechanism responsible for the majority of clinically important arrhythmias.
Which cardiac rhythm disturbance is being described:
Spontaneous depolarisation of cells occurs in the wrong region of the heart.
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.
Which cardiac rhythm disturbance is being described:
AV node becomes electrically isolated.
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.
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
C) Implanted pacemaker
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
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.
