Human Anatomy and Physiology : Help with Sarcomere Physiology

Study concepts, example questions & explanations for Human Anatomy and Physiology

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Example Questions

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Example Question #1 : Help With Sarcomere Physiology

An investigational drug prevents skeletal muscle contraction by preventing ATP hydrolysis at the active site in muscle tissue. Where is this drug most likely to act?

Possible Answers:

Myosin head

Actin head

Intermediate filament tail

Actin tail

Myosin tail

Correct answer:

Myosin head

Explanation:

ATP binds to myosin head regions and is hydrolyzed to ADP and inorganic phosphate when muscle relaxes. The release of these products allows the contraction to occur as the myosin head changes position. The binding of new ATP releases the myosin head from actin, and allows the muscle to relax prior to another round of hydrolysis. This explains why, in the absence of adequate ATP, muscle can remain in a contracted state. This phenomenon, when seen in the deceased, is called rigor mortis.

Example Question #1 : Help With Sarcomere Physiology

Ions are key in mediating muscle contraction. Which of the following structures interacts directly with ions to expose actin binding sites in contracting muscle?

Possible Answers:

Myosin

Cross-bridges

Tropomyosin

Troponin

Myofibrils

Correct answer:

Troponin

Explanation:

Troponin (also called troponin C) is the most direct structure that interacts with ions involved in initiating muscle contractions. Once bound to calcium ions, troponin facilitates the movement of tropomyosin away from actin binding sites, allowing myosin to bind and, ultimately, contract. Without the binding of calcium ions to troponin, the myosin-binding site on actin remains obscured by tropomyosin and contraction cannot occur.

Example Question #2 : Help With Sarcomere Physiology

Which of the following sarcomere portions does not decrease in length during muscular contraction?

Possible Answers:

A band

I band

None of these portions decrease during contraction

All of these portions decrease during contraction

H zone

Correct answer:

A band

Explanation:

During muscular contraction, the myosin heads pull the actin filaments toward one another resulting in a shortened sarcomere. While the I band and H zone will disappear or shorten, the A band length will remain unchanged. This is because the A band corresponds to the full length of the myosin filament, or thick filament. Since the myosin filament does not actually change length, the A band remains constant.

The I band corresponds to the region of action that does not overlap with myosin. The length of the actin filament does not change during contraction, but the region of overlap increases. This results in a decrease of the non-overlapped I band.

The H zone refers to the region of myosin that is not overlapped by action. As the region of overlap grows, the H zone shrinks.

Example Question #4 : Help With Sarcomere Physiology

In excitation-contraction coupling in skeletal muscle, the calcium released from the smooth endoplasmic reticulum binds to which of the following?

Possible Answers:

Calmodulin

Tropomyosin

Myosin

Troponin

Actin

Correct answer:

Troponin

Explanation:

When calcium is released in muscle cells, it binds to troponin. This binding allows tropomyosin to change its orientation, exposing the myosin-binding sites on actin. Myosin heads can then bind to actin, and contraction can occur.

Calmodulin is a molecule that can bind calcium; however, it plays important roles in cell signal cascades and is not involved in skeletal muscle contraction.

Example Question #3 : Help With Sarcomere Physiology

In a muscle cell, the H-zone has __________.

Possible Answers:

Z-discs

thick filaments only

thin filaments only

thick and thin filaments overlapping

Correct answer:

thick filaments only

Explanation:

The H-zone is an area made up of only thick filaments (myosin). The I-band is thin filaments (actin) only, and the A-band is where there are thick and thin filaments. The Z-disc is dividing feature between sarcomeres and appears as dark lines in electron micrographs. 

Example Question #6 : Help With Sarcomere Physiology

During muscle contraction, which band(s) of the sarcomere shorten(s)?

Possible Answers:

The I-band only

The H-band and I-band

The H-band only 

The A-band only

The A-band and H-band

Correct answer:

The H-band and I-band

Explanation:

Muscle contraction results in both the H-band and I-bands shortening, but the A-band remains the same length (A band is Always the same). The Z-band is a static structure and doesn't change size.  

Example Question #4 : Help With Sarcomere Physiology

Where within the sarcomere is myosin contained?

Possible Answers:

Thin filaments

Transverse tubules

Sarcoplasmic reticulum

Thick filaments

Correct answer:

Thick filaments

Explanation:

Thick filaments contain myosin. Thick filaments are present in the A band in the center of the sarcomere. Myosin has six polypeptide chains, including one pair of heavy chains and two pair if light chains. Each myosin molecule has two “heads” attached to a single “tail.” The myosin heads are capable of ATP hydrolysis and bind ATP and actin, and are involved in cross bridge formation.

Example Question #5 : Help With Sarcomere Physiology

Troponin binds with which ion?

Possible Answers:

Correct answer:

Explanation:

Troponin is the regulatory protein that permits cross bridge formation when it binds . Troponin is a complex of three globular proteins. Troponin C () is the  binding protein, that when bound to , permits the interaction of actin and myosin via conformational change that reveals the myosin binding site on actin, and thus muscular contraction.

Example Question #6 : Help With Sarcomere Physiology

Where can you find a ryanodine receptor?

Possible Answers:

Transverse tubules

A band

Thick chain

Sarcoplasmic reticulum

Correct answer:

Sarcoplasmic reticulum

Explanation:

The ryanodine receptor is a calcium channel that is responsible for the release of calcium ions. It is located in the sarcoplasmic reticulum. The release of calcium is a required step in muscle contraction. The ryanodine receptor pumps calcium ions from the intracellular fluid into the interior of the sarcoplasmic reticulum, this process keeps the intracellular calcium ions low and creates a concentration gradient. 

Example Question #7 : Help With Sarcomere Physiology

Muscle myofibrils are arranged longitudinally into sarcomeres. Which of the following correctly lists the boundaries of a single sarcomere?

Possible Answers:

M line to H band

Z line to M line 

Z line to Z line

Z line to H band

Correct answer:

Z line to Z line

Explanation:

Each myofibril contains interdigitating thick and thin filaments arranged longitudinally in sarcomeres. Repeating units of sarcomeres account for the unique banding pattern in striated muscle. A sarcomere runs from Z line to Z line.

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