MCAT Biology : Muscle Stimulation and Contraction

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #229 : Biology

What is the role of the sarcoplasmic reticulum prior to a muscle contraction?

Possible Answers:

It releases calcium once an action potential reaches the sarcolemma

It creates the proteins needed to cover the actin filaments

It releases calcium ions by active transport

It actively pumps calcium ions into its lumen

Correct answer:

It releases calcium once an action potential reaches the sarcolemma

Explanation:

The sarcoplasmic reticulum is responsible for the storage of calcium ions that are used in muscle contraction. Prior to a contraction, an action potential will reach the sarcoplasmic reticulum, making it permeable to calcium ions. At the end of the contraction, the sarcoplasmic reticulum will actively pump calcium ions back into its lumen.

The sarcoplasmic reticulum is not involved in protein synthesis.

Example Question #230 : Biology

The sarcoplasmic reticulum has the ability to aid in muscle contraction by storing large amounts of which ion?

Possible Answers:

Correct answer:

Explanation:

Calcium plays a huge role in the regulation of muscle contraction. Without the presence of calcium, the myosin binding sites on the actin filaments are blocked by tropomyosin and muscle contraction cannot occur. Stimulation from the nervous system causes a chain reaction that releases large stores of calcium from the sarcoplasmic reticulum to regulate muscle contraction.

Sodium ions play an essential role in initiating the chain reaction that eventually leads to calcium release, but is not stored in the sarcoplasmic reticulum. Potassium plays a role in regulating membrane potential, but also is not stored in the sarcoplasmic reticulum. Protons are essential to mitochondrial function and play a crucial role in myocyte metabolism, but are not linked to the sarcoplasmic reticulum or contractile function.

Example Question #251 : Mcat Biological Sciences

The sarcoplasmic reticulum is a special type of endoplasmic reticulum. Based on this information, which of the following is associated with a sarcoplasmic reticulum?

I. Network of tubules

II. Digestive enzymes

III. Vesicles

Possible Answers:

I only

II only

I and III

I, II, and III

Correct answer:

I and III

Explanation:

The question states that the sarcoplasmic reticulum is a specialized endoplasmic reticulum. This means that the structures that make up the sarcoplasmic reticulum must be similar to the endoplasmic reticulum. Recall that both endoplasmic reticulum (rough and smooth) are made up of a network of tubules. Similarly, both structures contain vesicles that transport processed molecules to a target location (proteins in rough endoplasmic reticulum and lipids in smooth endoplasmic reticulum); therefore, the sarcoplasmic reticulum must contain vesicles and a network of tubules.

The sarcoplasmic reticulum does not contain digestive enzymes, nor does the endoplasmic reticulum. Digestive enzymes are usually found in degradative organelles, such as lysosomes.

Example Question #21 : Musculoskeletal System And Muscle Tissue

A person has a mutation that produces abnormal sarcoplasmic reticulum. Which of the following will not be a consequence of this mutation?

Possible Answers:

The myosin heads will not bind the actin filaments

Tropomyosin will remain bound to actin

Calcium ions will not be released into a synapse, and propagation of action potentials will stop

Troponin will not be activated

Correct answer:

Calcium ions will not be released into a synapse, and propagation of action potentials will stop

Explanation:

The main function of the sarcoplasmic reticulum is to store and release calcium ions. Calcium ions released into the cytoplasm of a muscle cell activate troponin. Activated troponin removes tropomyosin from actin, which opens up the myosin active site on actin. Myosin head binds to actin, which causes muscle contraction. 

The sarcoplasmic reticulum does not play a role in releasing calcium ions into synapses during action potential propagation. The calcium ions in synapses are released via vesicles in the presynaptic neuron; therefore, an abnormal sarcoplasmic reticulum will not stop the release of calcium ions in neuronal synapses.

Example Question #11 : Muscle Stimulation And Contraction

Which of the following is false about the sarcoplasmic reticulum?

Possible Answers:

A change in membrane potential causes the sarcoplasmic reticulum to become more permeable to calcium ions

The sarcoplasmic reticulum releases calcium ions into the cytoplasm of the muscle cell

The sarcoplasmic reticulum is found only in voluntary muscle cells

The sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum

Correct answer:

The sarcoplasmic reticulum is found only in voluntary muscle cells

Explanation:

The sarcoplasmic reticulum is an organelle that closely resembles the smooth endoplasmic reticulum. They are structurally similar, but have very different functions. The main function of the sarcoplasmic reticulum is to store and release calcium, whereas the smooth endoplasmic reticulum functions in lipid synthesis.

Muscle contraction is usually initiated by an action potential. The first step in muscle contraction is the release of calcium ions from the sarcoplasmic reticulum; therefore, a change in membrane potential (action potential) will stimulate the sarcoplasmic reticulum to become more permeable to calcium ions. Once stimulated, the sarcoplasmic reticulum releases calcium ions into the cytoplasm, where the calcium ions interact with troponin and remove tropomyosin from actin. This sequence allows for myosin to bind actin and shorten the sarcomere, resulting in contraction.

The sarcoplasmic reticulum is actually found in all three types of muscle cells: smooth muscle, cardiac muscle, and skeletal muscle. Recall that skeletal muscles are voluntary, whereas smooth and cardiac muscles are involuntary; therefore, sarcoplasmic reticulum is found in both voluntary and involuntary muscles.

Example Question #254 : Mcat Biological Sciences

Binding of myosin head to actin generates muscle contraction. After muscle contraction the myosin head has to be removed from the actin. Which of the following accomplishes this task?

Possible Answers:

Binding of ATP to tropomyosin

Binding of calcium to troponin

Binding of ATP to myosin head

Binding of troponin to actin

Correct answer:

Binding of ATP to myosin head

Explanation:

Remember that troponin, tropomyosin, and calcium are not involved in the detachment of myosin head from actin; they are involved in the attachment of myosin head to actin. The molecule required for muscle relaxation (detachment of myosin head from actin) is ATP. After muscle contraction, a molecule of ATP binds to the myosin head and signals it to detach from the active site on actin.  After detachment, myosin head converts the ATP to ADP and inorganic phosphate and the cycle of muscle contraction and muscle relaxation continues.

Example Question #255 : Mcat Biological Sciences

The contraction of muscle by actin and myosin is described by which biological theory?

Possible Answers:

Sliding filament theory

Cross-bridge theory

Central Dogma theory

Endosymbiotic theory

Correct answer:

Sliding filament theory

Explanation:

The theory that describes the movement of actin and myosin is the sliding filament theory. This theory proposes that the myosin filaments slide relative to the actin filaments and shorten the length of sarcomere. A sarcomere is the basic unit of muscle; therefore, when myosin filaments shorten the length of the sarcomere, the muscle contracts. 

Cross-bridge theory is not an actual theory. A cross-bridge is a term used to describe the linkage between actin and myosin. Endosymbiotic theory states that mitochondria and chloroplasts were originally prokaryotes that evolved by forming a symbiotic relationship with eukaryotic cells. This explains why mitochondria and chloroplasts have their own unique DNA. Finally, the Central Dogma theory describes the flow of genetic information in a living organism. It states that genetic information is transferred through the processes of replication, transcription, and translation.

Example Question #256 : Mcat Biological Sciences

A muscle will not have all of its fibers contract at once. Instead, the muscle is divided into multiple bundles of muscle fibers, with a neuron innervating all of the fibers in a given bundle. Each collection of fibers controlled by a single neuron is referred to as a motor unit.

Which of the following statements is false when discussing motor units?

Possible Answers:

Muscles requiring finer, more precise motions are composed of smaller motor units.

The neuron will create an action potential in the muscle by releasing acetylcholine into the synaptic cleft.

When lifting an object, motor units are recruited as needed in order to provide a smooth, controlled motion.

Larger motor units are typically activated first in order to provide immediate stability.

Correct answer:

Larger motor units are typically activated first in order to provide immediate stability.

Explanation:

Smaller motor units are activated first during muscular contraction. If more force is needed, larger motor units will be recruited in order to provide the necessary force.

Example Question #257 : Mcat Biological Sciences

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.

Which of the following statements is true concerning motor units?

Possible Answers:

Finger muscles are typically composed of smaller motor units

Large motor units are innervated first in a muscle, followed by smaller units in order to have a smooth contraction

The neuron releases epinephrine across the synaptic cleft in order to stimulate an action potential in the sarcolemma

Motor unit neurons only transmit one action potential during a contraction

Correct answer:

Finger muscles are typically composed of smaller motor units

Explanation:

During a contraction, smaller motor units are typically fired first, followed by larger units in order to have a smooth, controlled movement. Movements that require fine, controlled motion, such as the muscles of the fingers, will be composed of smaller motor units.

The neurotransmitter associated with skeletal muscle is acetylcholine, not epinephrine. A single action potential may initiate contraction of a motor unit, but the neuron must continue to fire in order to sustain the contraction.

Example Question #261 : Mcat Biological Sciences

Rigor mortis, a recognizable sign of death, is the stiffness observed in the muscle of an individual who has just passed away. On a molecular level, what causes rigor mortis?

Possible Answers:

The body no longer produces ATP, which is necessary to cause the myosin heads to detach from actin

The sarcoplasmic reticulum no longer releases calcium, which causes continued contraction

The body no longer produces ADP, which is necessary to cause the myosin heads to detach from actin

The sarcoplasmic reticulum no longer sequesters calcium, which causes continued contraction

Actin is no longer responsive to myosin

Correct answer:

The body no longer produces ATP, which is necessary to cause the myosin heads to detach from actin

Explanation:

After the myosin head has attached to the actin filament, a power stroke occurs, which causes the "sliding filament theory" (contraction).This process occurs in a cycle as long as two conditions are present: calcium must be available to bind to troponin, revealing the binding sites on actin, and ATP must be available for the movement of the myosin head. When an individual is no longer alive, calcium is no longer sequestered and remains available to bind to troponin, revealing the binding sites. This would allow continued normal contraction, but is not the cause of sustained contraction seen in rigor mortis. After death, cellular metabolism no longer produces ATP, and stores of ATP are quickly depleted. This results in a break in the contraction cycle. ATP is necessary to detach the myosin head from the actin filament. Without ATP present, the myosin head remains bound and the contraction is sustained. The depletion of ATP is thus the cause of rigor mortis, causing stiffness due to myosin's inability to detach from actin.

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