Cardiac Muscle Contractions

CARDIAC MUSCLE CONTRACTIONS

Q. 1

All of the following are true statements about electromechanical coupling, EXCEPT:

 A

In smooth muscle, Ca2+ ions bind to calmodulin

 B

Smooth muscle contraction is initiated by myosin light chain phosphorylation

 C

In cardiac muscle, Ca2+ ions bind to troponin

 D

The major source of Ca2+ ions for contraction of skeletal muscle is influx of Ca2+ ions from the extracellular space following voltage-dependent opening of L-type Ca2+ channels.

Q. 1

All of the following are true statements about electromechanical coupling, EXCEPT:

 A

In smooth muscle, Ca2+ ions bind to calmodulin

 B

Smooth muscle contraction is initiated by myosin light chain phosphorylation

 C

In cardiac muscle, Ca2+ ions bind to troponin

 D

The major source of Ca2+ ions for contraction of skeletal muscle is influx of Ca2+ ions from the extracellular space following voltage-dependent opening of L-type Ca2+ channels.

Ans. D

Explanation:

The three types of muscle cells (smooth muscle, cardiac muscle, skeletal muscle) utilize different mechanisms by which membrane depolarization results in muscle contraction, i.e., electromechanical coupling. In smooth muscle, Ca2+ ions entering through opening of voltage-gated Ca2+ channels or following release from the SR, bind to calmodulin.

The Ca2+-calmodulin complex activates myosin light chain kinase, and phosphorylation of myosin light chain initiates smooth muscle contraction. This is in contrast to cardiac and skeletal muscle, where contraction is triggered by a conformational change induced by Ca2+ handling. In cardiac muscle, opening of voltage-gated Ca2+ channels results in Ca2+. In skeletal muscle, depolarization of the T tubule triggers release of Ca2+ from the SR. 

In contrast to cardiac muscle, skeletal muscle contractions can summate during repeated stimulation, resulting in an increase in force with incomplete relaxation between the stimuli (tetanic contraction). This difference is due to the much shorter action potential duration in skeletal muscle compared to cardiac muscle.


Q. 2

Which of the following statement is CORRECT during a contraction of a cardiac muscle cell?

 A

Ca++ binds to the heads of myosin molecules, resulting in a conformational change in the myosin molecules

 B

Ca++ from the extracellular medium and the sarcoplasmic reticulum binds calmodulin, leading to activation of myosin light-chain kinase

 C

Ca++ from the extracellular medium and the sarcoplasmic reticulum binds calmodulin, leading to activation of myosin light-chain kinase

 D

Ca++ from the sarcoplasmic reticulum binds to troponin on the thin filaments

Q. 2

Which of the following statement is CORRECT during a contraction of a cardiac muscle cell?

 A

Ca++ binds to the heads of myosin molecules, resulting in a conformational change in the myosin molecules

 B

Ca++ from the extracellular medium and the sarcoplasmic reticulum binds calmodulin, leading to activation of myosin light-chain kinase

 C

Ca++ from the extracellular medium and the sarcoplasmic reticulum binds calmodulin, leading to activation of myosin light-chain kinase

 D

Ca++ from the sarcoplasmic reticulum binds to troponin on the thin filaments

Ans. D

Explanation:

In all striated muscle (including cardiac muscle), membrane depolarization results in release of calcium from the sarcoplasmic reticulum into the cytosol. This calcium can then bind to troponin associated with the actin-containing thin filaments, resulting in sliding of thick and thin filaments and sarcomere contraction. 

Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong’s Review of Medical Physiology, 24e.


Q. 3

Release of synaptic transmitter by exocytosis would be blocked most effectively by preventing the:

 A

Propagation of the action potential into the nerve terminal membrane

 B

Depolarization of the nerve terminal membrane

 C

Flow of Na+ into the nerve terminal membrane

 D

Flow of Ca2+ into the nerve terminal membrane

Q. 3

Release of synaptic transmitter by exocytosis would be blocked most effectively by preventing the:

 A

Propagation of the action potential into the nerve terminal membrane

 B

Depolarization of the nerve terminal membrane

 C

Flow of Na+ into the nerve terminal membrane

 D

Flow of Ca2+ into the nerve terminal membrane

Ans. D

Explanation:

Preventing the flow of Ca2+ into the cell would prevent the release of transmitter, because Ca2+ initiates the intracellular events leading to the docking of the vesicle to its binding site on the active zone. Although Ca2+ normally enters the cell through voltage-operated channels that are opened by the depolarization of the nerve terminal that occurs as the action potential propagates along the nerve axon, release of transmitter will not occur if Ca2+ does not enter the nerve terminal.

The flow of Na+ into the nerve terminal would depolarize the membrane and open Ca2+ channels, leading to Ca2+ entry and exocytosis. However, Na+ entry does not directly stimulate exocytosis. K+ does not affect the nerve terminal membrane.


Q. 4

Function of Phospholamban is :

 A

Regulates Na K Pump

 B

Transports calcium out of the mitrochondria

 C

Binds actin with myosin

 D

Collects calcium into the sarcoplasmic

Q. 4

Function of Phospholamban is :

 A

Regulates Na K Pump

 B

Transports calcium out of the mitrochondria

 C

Binds actin with myosin

 D

Collects calcium into the sarcoplasmic

Ans. D

Explanation:

D i.e. Collects calcium into the sarcoplasmic reticulum

Phospholamban is a protein contained within the sarcoplasmic reticulum that inhibits the activity of sarcoplasmic calcium pumpQ. Inactivation of phospholamban results in an increase in calcium sequestration by sarcoplasmic reticulum.Q

–   In cardiac muscle, the rapid sequestration of Ca2+ shortens the duration of contraction.

–   In smooth muscle, sequestration causes the muscle to relax.

–   Phospholamban has little effect on skeletal muscle contraction.


Q. 5

Conduction along membrane is dependent on all of the following EXCEPT:    

March 2013

 A

Ca2+

 B

K+

 C

Fe2+

 D

Na+

Q. 5

Conduction along membrane is dependent on all of the following EXCEPT:    

March 2013

 A

Ca2+

 B

K+

 C

Fe2+

 D

Na+

Ans. C

Explanation:

Ans. C i.e. Fe2+

A decrease in extracellular Ca2+ concentration increases the excitability of nerve and muscle cells by decreasing the amount of depolarization necessary to initiate the changes in the Na+ and K+ conductance that produce the action potential. Conversely, an increase in extracellular Ca2+ concentration can stabilize the membrane by decreasing excitability.



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