MCAT Biology : Muscle Stimulation and Contraction

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #262 : Mcat Biological Sciences

What role does calcium play during muscle contraction?

Possible Answers:

It binds to tropomyosin

It binds to ATP

It binds to myosin

It binds to actin

It binds to troponin

Correct answer:

It binds to troponin

Explanation:

Calcium is released from the sarcoplasmic reticulum and binds to troponin. At rest, troponin interacts with tropomyosin to block the active sites on actin, preventing myosin from binding. When calcium binds troponin, it causes a conformational change in tropomyosin. This allows the myosin heads to bind to the actin active sites, initiating the contraction process. ATP is used to cause the dissociation of the myosin head from the actin filament, and is not involved in initiating actin-myosin interaction.

Example Question #263 : Mcat Biological Sciences

Which neurotransmitter is released at the neuromuscular junctions of skeletal muscle?

Possible Answers:

Serotonin

Dopamine

Norepinephrine

Acetylcholine 

Correct answer:

Acetylcholine 

Explanation:

Acetylcholine (ACh) is released from neurons at the neuromuscular junction. Once ACh binds to its receptor in the muscle T-tubule, the sarcolemma is depolarized and calcium can be released from the sarcoplasmic reticulum, triggering muscle contraction.

Example Question #21 : Muscle Stimulation And Contraction

The force of a muscle contraction is not proportional to which of the following?

Possible Answers:

Number of motor units recruited 

Size of the action potential

Size of motor units recruited

Number of action potentials

Correct answer:

Size of the action potential

Explanation:

The force of a muscle contraction can be manipulated in several ways. When a muscle is stimulated by an action potential, all fibers within a given motor unit are activated together. Increasing the number of motor units will increase the percentage of the muscle mass that is contracting, increasing the force. Increasing the size of the motor units will have the same effect. Increasing the number of action potentials will allow for sustained contraction.

Action potentials themselves are all-or-nothing; they either happen or they do not. There is no such thing as a large or small action potential. Several action potentials can arrive simultaneously, causing a summation effect, but the size of an action potential on its own cannot affect the force of a muscle contraction.

Example Question #21 : Muscle Stimulation And Contraction

In the muscle unit, the action potential travels down what specialized structure to eventually allow for muscle contraction?

Possible Answers:

T-tubule

Sarcoplasmic reticulum

Endoplasmic reticulum

Extracellular matrix

Correct answer:

T-tubule

Explanation:

The T-tubules are responsible for propagating action potentials deep down into muscle fibers, allowing for a uniform and coordinated contraction. The T-tubules are invaginations of the sarcolemma, the specialized cell membrane of muscle cells. T-tubules run adjacent to the sarcoplasmic reticulum, triggering calcium release as the tubule is depolarized. The sarcoplasmic reticulum stores calcium, while the extracellular matrix provides structural support for cells.

Example Question #266 : Mcat Biological Sciences

An action potential stimulates the muscles of the hand to grip a metal bar. This action potential would travel through which neuronal pathways?

Possible Answers:

Efferent motor pathways

Afferent sensory pathways

Afferent motor pathways

Efferent sensory pathways

Correct answer:

Efferent motor pathways

Explanation:

Motor neurons are responsible for the stimulation of muscle. Pathways traveling away from the central nervous system to the body are considered to be efferent, where pathways going from the body into the central nervous system are afferent.

All sensory signals are afferent and all motor signals are efferent. Sensory input causes stimulation of various sensors and receptors, which carry the signal to the central nervous system for processing. The central nervous system interprets the signal and generates a response, which is then sent to the appropriate muscle groups.

An action potential sent to the muscles of the hand would follow an efferent pathway (away from the central nervous system) through motor neurons.

Example Question #267 : Mcat Biological Sciences

A weightlifter is trying to increase the force of his muscle contraction in order to lift a heavy weight. Which of the following most adequately explains what is happening in his muscles?

Possible Answers:

Neurons that synapse with skeletal muscle cells are depolarizing with a larger charge

ATP is binding to troponin to move tropomyosin off of the myosin binding site

Calcium ions are binding to tropomyosin to move troponin off of the myosin binding sites

Neurons that synapse with skeletal muscle cells are depolarizing more frequently

Correct answer:

Neurons that synapse with skeletal muscle cells are depolarizing more frequently

Explanation:

It is important to remember that neurons and muscle cells both depolarize in an "all or nothing" response, meaning that the action potential is not a graded process. As such, neurons cannot produce a larger or smaller depolarizing charge in the muscle.

Instead, the strength of a neurological action potential or muscle contraction is determined by the frequency of signal firing. The strength of each individual action potential or signal cannot be changed, and remains constant; however, frequent stimulation to the same area can activate more motor units and cause a larger total contraction.

When muscle contraction occurs, a calcium ion binds to troponin to move tropomyosin off of the myosin binding site on actin. ATP then is hydrolyzed to release the myosin head from actin.

Example Question #268 : Mcat Biological Sciences

The area where the motor neuron intersects the muscle is known as the __________.

Possible Answers:

cross bridge

neuromuscular connection

neuromuscular junction

linkage

sarcomere

Correct answer:

neuromuscular junction

Explanation:

The neuromuscular junction is where the nerve fibers directly connect to the muscle to deliver signals from the brain to the muscle tissue.  "Cross bridge" refers to the linkage of actin and myosin filaments. The other answers sound similar, but are incorrect.

Example Question #269 : Mcat Biological Sciences

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.

When a healthy muscle fiber is activated, Ca2+ ions will __________.

Possible Answers:

bind dystrophin

bind myosin

bind actin

bind tropomyosin

bind troponin

Correct answer:

bind troponin

Explanation:

The troponin-tropomyosin complex wraps around actin when the muscle fiber is inactive, blocking all myosin-binding sites. When Ca2+ is released it binds to troponin, inducing a change in tropomyosin, which shifts its position to expose the myosin-binding sites on the actin filament.

Calcium does not bind any of the other listed answer choices.

Example Question #261 : Mcat Biological Sciences

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.

ATP is required for muscle contraction. Identify which of the following are true

     I. ATP binding causes myosin to release actin

     II. Actin carries an inactive ADP when myosin binds

     III. The myosin head movement to contract the muscle converts ATP to ADP

Possible Answers:

I and III only

II only

III only

I only

I, II, and III

Correct answer:

I only

Explanation:

This question requires us to know the ATP binding cycle associated with muscle contraction. I is true; binding of ATP causes myosin to release actin. When there is no ATP present, the myosin remains bound and the muscle becomes stiff (rigor mortis). II is false; actin does not bind ATP. III is also false; ATP is converted to ADP when the myosin head goes from the contracted position to the relaxed position, not the other way around.

Example Question #21 : Muscle Stimulation And Contraction

Muscle contraction is mainly powered by which chemical?

Possible Answers:

ATP

Acetylcholine

Lactic acid

Calcium ions

ADP

Correct answer:

ATP

Explanation:

ATP (adenosine triphosphate) is the primary chemical that provides the power for muscle contraction. ADP (adenosine diphosphate) is the resulting chemical when ATP is expended. ATP is required for the cross-bridge cycle. Acetylcholine is a neurotransmitter used in muscle contraction, but does not provide a power source. Lactic acid results from anaerobic production of ATP.

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