MCAT Biology : Sarcomeres

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #211 : Biology

Which of the following is true of actin and myosin filaments?

Possible Answers:

Some muscle cells do not contain myosin, but all muscle cells contain actin

Some muscle cells do not contain actin, but all muscle cells contain myosin

Troponin, calcium, and tropomyosin interact with the thick filaments

Actin molecules are classified as thin filaments, whereas myosin molecules are classified as thick filaments

Correct answer:

Actin molecules are classified as thin filaments, whereas myosin molecules are classified as thick filaments

Explanation:

Actin and myosin filaments are essential contractile elements found in muscle cells. They are essential because they conduct muscle contraction. A molecule of actin is made up of small microfilaments, which give them a very thin appearance. Myosin is made up of long polypeptide chains that join together to form a thick filament; therefore, actin molecules are classified as thin filaments, whereas myosin molecules are classified as thick filaments.

All muscle cells, regardless of type, contain both actin and myosin filaments. Muscle contraction is not possible without the presence of both contractile elements. Organization of these molecules can vary, as smooth muscle does not contain striations, but the molecules are still responsible for contractile actions. Troponin, calcium, and tropomyosin are all required to initiate the contact between myosin and actin. Calcium binds to troponin, which subsequently removes tropomyosin from actin (thin filaments). None of these interact with myosin, the thick filaments.

Example Question #212 : Biology

Duchenne Muscular Dystrophy is an X-linked recessive genetic disorder, resulting in the loss of the dystrophin protein. In healthy muscle, dystrophin localizes to the sarcolemma and helps anchor the muscle fiber to the basal lamina. The loss of this protein results in progressive muscle weakness, and eventually death.

In the muscle fibers, the effects of the disease can be exacerbated by auto-immune interference. Weakness of the sarcolemma leads to damage and tears in the membrane. The body’s immune system recognizes the damage and attempts to repair it. However, since the damage exists as a chronic condition, leukocytes begin to present the damaged protein fragments as antigens, stimulating a targeted attack on the damaged parts of the muscle fiber. The attack causes inflammation, fibrosis, and necrosis, further weakening the muscle.

Studies have shown that despite the severe pathology of the muscle fibers, the innervation of the muscle is unaffected.

A healthy muscle will have the most contractile force when __________.

Possible Answers:

it is shortened, allowing for additional actin-myosin overlap

it is at rest, allowing for the maximum number of cross-bridges

it is lengthened, allowing for more distance to contract

it is shortened, then lengthened. A muscle that has been compressed first helps activate titin

it is lengthened, then shortened. A muscle that has been stretched first will contract more strongly.

Correct answer:

it is at rest, allowing for the maximum number of cross-bridges

Explanation:

When a muscle is shortened, the force decreases as the filaments slide past one another and lose room to form new cross-bridges. When a muscle is lengthened, there is less filament overlap which leads to fewer cross-bridges. Stretching a muscle before contracting it will not affect the force produced, nor will shortening a muscle before lengthening it. Titin is a protein responsible for some of the elastic properties of muscle, but is not involved in force production.

At rest, the muscle has the potential to form the maximum number of cross-bridges, resulting in the maximum amount of force production. For further review, go over the length-tension curve for a muscle fiber.

Example Question #213 : Biology

Duchenne Muscular Dystrophy is an X-linked recessive genetic disorder, resulting in the loss of the dystrophin protein. In healthy muscle, dystrophin localizes to the sarcolemma and helps anchor the muscle fiber to the basal lamina. The loss of this protein results in progressive muscle weakness, and eventually death.

In the muscle fibers, the effects of the disease can be exacerbated by auto-immune interference. Weakness of the sarcolemma leads to damage and tears in the membrane. The body’s immune system recognizes the damage and attempts to repair it. However, since the damage exists as a chronic condition, leukocytes begin to present the damaged protein fragments as antigens, stimulating a targeted attack on the damaged parts of the muscle fiber. The attack causes inflammation, fibrosis, and necrosis, further weakening the muscle.

Studies have shown that despite the severe pathology of the muscle fibers, the innervation of the muscle is unaffected.

A muscle fiber is divided into sarcomeres. The region of the sarcomere corresponding to the myosin filament is the __________.

Possible Answers:

M-line

I-band

Z-disc

H-band

A-band

Correct answer:

A-band

Explanation:

A sarcomere is one contractile unit of a muscle fiber, and contains two half-filaments of actin and a full filament of myosin. The ends of the sarcomere are the Z-discs and the center is the center is the M-line (the middle of the myosin filament). The H-band lies between the two half-actin filaments where there is only myosin; however, it does not correspond to the full myosin filament. The I band corresponds to the region where only actin is present and the A-band correspond to the full length of the myosin filament.

Example Question #214 : Biology

During a muscular contraction, which of the following elements maintains constant length?

Possible Answers:

A band

H zone

Sarcomere

I band

N line

Correct answer:

A band

Explanation:

The A band is the only element that remains constant during a muscle contraction. It represents the segment of the sarcomere that contains the length of the thick filament. The H zone refers to the part of the sarcomere where there are only thick filaments, and no superimposing thin filaments. Conversely, the I band refers to the area where there are only thin filaments and no superimposing thick filaments. As filaments overlap, both the H zone and I band will shorten. The N line does not exist in musculoskeletal physiology.

Example Question #215 : Biology

What is a sarcomere?

Possible Answers:

A gastric muscle

A myosin fibril

The smallest unit of a smooth muscle

The contractile unit of a muscle cell

The fibers that make up skeletal muscles

Correct answer:

The contractile unit of a muscle cell

Explanation:

A sarcomere is the functional unit of the skeletal or cardiac muscle cell, and is made of interlocking myofibrils. A sarcomere is the smallest unit in the muscle cell to contract and relax.

Note that smooth muscle cells still contract using actin and myosin filaments, but do not organize these filaments into sarcomeres as skeletal and cardiac muscle do. This is why smooth muscle is not striated.

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