MCAT Biology : Muscle Stimulation and Contraction

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #41 : Musculoskeletal System And Muscle Tissue

A sarcoplasmic reticulum is found within a muscle cell. The sarcoplasmic reticulum is a modified version of the endoplasmic reticulum.

What is the modified characteristic of a sarcoplasmic reticulum?

Possible Answers:

The sarcoplasmic reticulum contains large amounts of chlorine ions

The sarcoplasmic reticulum contains large amounts of sodium ions

The sarcoplasmic reticulum contains large amounts of potassium ions

The sarcoplasmic reticulum contains large amounts of calcium ions

Correct answer:

The sarcoplasmic reticulum contains large amounts of calcium ions

Explanation:

The sarcoplasmic reticulum contains a large amount of Ca2+ ions. This calcium is released from the sarcoplasmic reticulum when an electrical signal is sent to the cell. This release of calcium allows for contraction.

Example Question #242 : Biology

Which of the following is NOT a function of the muscular system?

Possible Answers:

Movement of substances in the body

Body stabilization

Heat generation

Storage of calcium

Correct answer:

Storage of calcium

Explanation:

The muscular system has a variety of functions. It helps regulate the temperature of the body by generating heat through contraction; this is why we shiver when we are cold. It helps push blood and lymph throughout the blood vessels via the action of smooth muscle, as well as cardiac muscle. Skeletal muscle also maintains body stability and aids in body movement. Calcium storage is not a main function of the skeletal system, although calcium is an important ion for muscular function.

Example Question #251 : Biology

Skeletal muscle fibers are not all contracted at once by the same action potential. Instead, muscle fibers are divided into clusters that can range from two to two thousand cells. All of these muscle fibers are innervated by the same neuron; the muscle fibers and the neuron that innervates them are collectively referred to as a motor unit.

When the neurotransmitter attaches to the sarcolemma of the muscle fibers, it stimulates the release of calcium. What is the primary role of calcium in skeletal muscle?

Possible Answers:

Cocking the myosin head

Spreading the action potential throughout the muscle fibers

Allowing myosin to attach to actin

Creating a gradient for ATP production, which will power the muscle contractions

Correct answer:

Allowing myosin to attach to actin

Explanation:

Acetylcholine will stimulate sodium channels on the sarcolemma, which will consequently trigger the release of calcium ions from the sarcoplasmic reticulum. The calcium will then attach to troponin, which pulls tropomyosin away from the active site on actin. With the active site available, myosin heads are able to attach to the actin filament.

Example Question #252 : Biology

Which of the following proteins binds to calcium released from the sarcoplasmic reticulum?

Possible Answers:

Calsequestrin

ATP hydrolase

Tropomyosin

Troponin

Correct answer:

Troponin

Explanation:

Troponin binds free calcium once it is released from the sarcoplasmic reticulum, causing a conformational change in tropomyosin. This change exposes the myosin binding site on actin, allowing for cross-bridge formation and contraction.

Example Question #253 : Biology

Which of the following motor units are recruited first in a muscle contraction?

Possible Answers:

All motor units are activated simultaneously

Large motor units

Intermediate motor units

Small motor units

Correct answer:

Small motor units

Explanation:

Small motor units, typically consisting of one nerve and a few muscle cells, are recruited first. As the muscle contracts for a longer period of time or is required to lift a heavier load, intermediate and large muscle motor units are recruited. As intermediate and large muscle motor units are recruited, more action potentials begin to fire, releasing more calcium from the sarcoplasmic reticulum and increasing the overall strength of the muscle contraction.

Example Question #51 : Musculoskeletal System And Muscle Tissue

A toxin prevents calcium from being actively pumped back into the sarcoplasmic reticulum. What would you expect to be a consequence of this toxin's presence in the body?

Possible Answers:

Acetylcholine would constantly be released at the neuromuscular junction

Muscles would be unable to contract

Troponin would be unable to move tropomyosin off of the active site on actin

Myosin would be unable to enter into a high energy position before the contracting motion

Myosin would always be able to attach to actin

Correct answer:

Myosin would always be able to attach to actin

Explanation:

Before a contraction, calcium is released from the sarcoplasmic reticulum and attaches to troponin. Troponin will then remove tropomyosin from the active site on actin where myosin is able to attach.

If calcium is never pumped back into the sarcoplasmic reticulum, the active site on actin will stay exposed, which allows myosin to attach at all times.

Note that calcium is also responsible for initiating acetylcholine release from the neuron at the neuromuscular junction; however, this process involves extracellular calcium ions and is not linked to the sarcoplasmic reticulum.

Example Question #52 : Musculoskeletal System And Muscle Tissue

What is the correct movement within the sarcomere during contraction?

Possible Answers:

The I-band, the H-zone, and the A-band get smaller while the Z-lines move apart.

The I-band gets smaller, the Z-lines move closer together, and the H-zone gets wider.

The I-band and the H-zone get smaller, the Z-lines move closer together, but the A-band does not change.

None of the above

The I-band and the H-zone get smaller, the Z-lines move closer together, and the A-band gets wider.

Correct answer:

The I-band and the H-zone get smaller, the Z-lines move closer together, but the A-band does not change.

Explanation:

During muscle contraction, the I-band, the H-zone, and the area between the Z-lines get decreased in length. The A-band remains in constant length. Overall, the sarcomere constricts in size, allowing the muscle to contract.

Example Question #53 : Musculoskeletal System And Muscle Tissue

The sliding filament model of muscle contraction states all of the following EXCEPT __________.

Possible Answers:

the actin slides toward the M-line, causing the muscle fiber to contract

during the power stroke, the myosin head bends

a cross-bridge forms between actin and the myosin head

when calcium is present, myosin head detaches from actin

ATP hydrolysis causes the myosin head to unbend

Correct answer:

when calcium is present, myosin head detaches from actin

Explanation:

The sliding filament model of muscle contraction states that as the cross-bridge forms between actin and myosin, the myosin head bends (the power stroke), causing actin to move (slide) in the direction of the M-line. When all the actin filaments slide toward the M line like this, the muscle fiber contracts. Calcium is needed for the binding of myosin head to actin. ATP binding leads to the detachment of myosin head from actin. ATP hydrolysis is needed for the unbending of myosin head.

Example Question #54 : Musculoskeletal System And Muscle Tissue

Contraction of the muscle cell is initiated by calcium ions bonding to which of the following cell components?

Possible Answers:

Myosin

Actin

Tropomyosin

Troponin

Correct answer:

Troponin

Explanation:

The mechanism for troponin and tropomyosin’s interaction with calcium is comparable to a bike chain with a lock and key. Tropomyosin is the “chain” that blocks the binding sites on actin from the myosin heads. The calcium ion acts like a key to “unlock” troponin and move tropomyosin off of actin’s binding sites. This allows myosin heads to bind to actin and complete their power stroke.

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