MCAT Biology : Muscles and Myocytes

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #211 : Biology

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

Possible Answers:

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

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

Troponin, calcium, and tropomyosin interact with the thick filaments

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

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 #211 : 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 at rest, allowing for the maximum number of cross-bridges

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

it is lengthened, allowing for more distance to contract

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

it is shortened, allowing for additional actin-myosin overlap

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 #12 : Musculoskeletal System And Muscle Tissue

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:

A-band

H-band

M-line

Z-disc

I-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 #211 : Biology

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

Possible Answers:

A band

N line

H zone

Sarcomere

I band

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 #11 : Sarcomeres

What is a sarcomere?

Possible Answers:

The smallest unit of a smooth muscle

A myosin fibril

The fibers that make up skeletal muscles

The contractile unit of a muscle cell

A gastric muscle

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.

Example Question #212 : Biology

An individual has muscle cells that have an abnormally high level of intracellular calcium. The physician suspects that he may have a genetic defect affecting one of his organelles. Which organelle is likely to be the cause of this condition?

Possible Answers:

Golgi body

Nucleus

Ribosomes

Mitochondria

Sarcoplasmic reticulum

Correct answer:

Sarcoplasmic reticulum

Explanation:

The sarcoplasmic reticulum, an organelle unique to muscle cells, sequesters calcium when the muscle is at rest. This calcium is released into the cytosol during stimulation, and is an integral part of contraction. The affected individual probably has a leaky sarcoplasmic reticulum, allowing the release of calcium into the cytosol and resulting in abnormally high levels of intracellular ion.

Ribosomes are used during protein synthesis and not related to muscle contraction. The nucleus also is not involved in muscle contraction. The Golgi body is involved in modification and packaging of proteins, and also not involved in muscle contraction. Mitochondria are responsible for producing ATP. While ATP is an important part of the contraction process, and mitochondria are abundant in muscle cells, a defect in the mitochondria would not directly cause an increase in intracellular calcium.

Example Question #211 : Biology

Which of the following proteins directly interacts with the myosin-binding site on actin?

Possible Answers:

ATP hydrolase

Troponin

Calsequestrin

Tropomyosin

Correct answer:

Tropomyosin

Explanation:

Tropomyosin is interwoven with actin and serves to cover the myosin-binding site in the absence of calcium. Once calcium enters the cell it interacts with troponin, which in turn causes a conformational change in tropomyosin leading to the interaction of myosin with actin and a resulting muscle contraction. Without tropomyosin in place spontaneous cross-bridges could form, leading to inappropriate muscle contraction in the absence of action potentials.

Example Question #241 : Mcat Biological Sciences

What is the specialized organelle in muscle cells that serves to sequester calcium?

Possible Answers:

Sarcomere

Sarcoplasmic reticulum

T-tubule

Sarcolemma

Correct answer:

Sarcoplasmic reticulum

Explanation:

The sarcoplasmic reticulum is the specialized organelle of the muscle cells that allows for calcium to be sequestered. Once calcium is released into the cytoplasm it interacts with troponin and tropomyosin, allowing myosin and actin to bind and cause contraction. Calcium must be sequestrated to allow for the myosin-actin bridges to be broken and reset for future contraction.

The sarcolemma is the muscle cell membrane. The T-tubules permeate the muscle cell to allow for propagation of the stimulating action potential to all parts to the cell. The sarcomeres are the contractile units of the muscle cell.

Example Question #241 : Mcat Biological Sciences

Which of the following changes will not take place in a muscle experiencing prolonged, powerful contractions?

Possible Answers:

Division of muscle cells

Increased number of sarcomeres

Enlargement of muscle cells

Increased number of mitochondria

Correct answer:

Division of muscle cells

Explanation:

Keep in mind that muscle cells do not undergo mitosis. Instead, the increase in muscle size is due to increased muscle fiber diameter. The number of sarcomeres and mitochondria in each muscle fiber will also increase over time. This increase in size, but not cell number, is called hypertrophy.

Example Question #1 : Muscle Stimulation And Contraction

What is the purpose of calcium in the muscles?

Possible Answers:

It releases the myosin head from the active site on actin

It helps move the myosin head into a high-energy position

It allows tropomyosin to be pulled away from the actin filament

It pulls tropomyosin off of the myosin heads

Correct answer:

It allows tropomyosin to be pulled away from the actin filament

Explanation:

The proteins troponin and tropomyosin are attached to the actin filaments in sarcomeres. These proteins function to block the myosin-binding site on the actin protein, preventing unnecessary contraction. When calcium is released from the sarcoplasmic reticulum, it will attach to troponin. The troponin will then pull tropomyosin away from the actin filament, which allows myosin heads to attach and cause a contraction.

ATP binds myosin to release it from the actin binding site and is converted to ADP in order to adjust the myosin head to a high-energy position.

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