Muscle Contraction

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Action potential propagation in a skeletal muscle fiber ceases when acetylcholine is removed from the synaptic cleft. Which of the following mechanisms ensures a rapid and efficient removal of acetylcholine?
a. Acetylcholine is transported back into the axon terminal by a reuptake mechanism.
b. Acetylcholine is transported into the postsynaptic neuron by receptor-mediated endocytosis.
c. Acetylcholine diffuses away from the cleft.
d. Acetylcholine is degraded by acetylcholinesterase.

d. Acetylcholine is degraded by acetylcholinesterase.

The neuromuscular junction is a well-studied example of a chemical synapse. Which of the following statements describes a critical event that occurs at the neuromuscular junction?
a. Acetylcholine is released and moves across the synaptic cleft bound to a transport protein.
b. When the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal.
c. Acetylcholine binds to its receptor in the junctional folds of the sarcolemma. Its receptor is linked to a G protein.
d. Acetylcholine is released by axon terminals of the motor neuron.

d. Acetylcholine is released by axon terminals of the motor neuron.

Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________.

extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber

Calcium entry into the axon terminal triggers what events?

Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine.

Acetylcholine binds to its receptor in the sarcolemma and triggers __________.

the opening of ligand-gated cation channels

Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why?

The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients.

Excitation-contraction coupling is a series of events that occur after the events of the neuromuscular junction have transpired. The term excitation refers to which step in the process?

Excitation, in this case, refers to the propagation of action potentials along the sarcolemma.

Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction?

Calcium release from the sarcoplasmic reticulum initiates the contraction.

A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected?

A series of proteins that control calcium release.

What is name given to the regularly spaced infoldings of the sarcolemma?

transverse or T tubules

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers?

Calcium ions

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge?

A myosin head bound to actin

What structure is the functional unit of contraction in a skeletal muscle fiber?

The sarcomere

Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber?

Calcium ions are stored in the sarcoplasmic reticulum.

After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment?

ATP binds to the myosin head

How does the myosin head obtain the energy required for activation?

The energy comes from the hydrolysis of ATP.

What specific event triggers the uncovering of the myosin binding site on actin?

Calcium ions bind to troponin and change its shape.

When does cross bridge cycling end?

Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.

What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell?

arrival of an action potential

The binding of calcium to which molecule causes the myosin binding sites to be exposed?


A myosin head binds to which molecule to form a cross bridge?


What causes the myosin head to disconnect from actin?

binding of ATP

What energizes the power stroke?

hydrolysis of ATP

What is the role of calcium in the cross bridge cycle?

Calcium binds to troponin, altering its shape.

What role does tropomyosin play in the cross bridge cycle?

The displacement of tropomyosin exposes the active sites of actin, allowing cross bridges to form.

How does troponin facilitate cross bridge formation?

Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin.

What, specifically, is a cross bridge?

myosin binding to actin

What event causes cross bridge detachment?

ATP binding to the myosin head

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head

How/when does the myosin head cock back to store energy for the next cycle?

ATP hydrolysis is used to re-cock the myosin head.

BMD (2,3-butanedione 2-monoximime) inhibits myosin, such that ATP can bind to myosin but myosin is unable to hydrolyze the bound ATP. What effect would BMD have on the cross bridge cycle?

Myosin heads would remain detached, unable to cock.

During contraction, what prevents actin myofilaments from sliding backward when a myosin head releases?

There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching.

Which muscle fiber type is best suited for endurance activities?

slow oxidative fibers

Which of the following is not a usual result of resistance exercise?
a. increase in the number of myofibrils within the muscle cells
b. increase in the efficiency of the circulatory system
c. increase in the efficiency of the respiratory system
d. increase in the number of muscle cells

d. increase in the number of muscle cells

Myoglobin ________.

stores oxygen in muscle cells

Immediately following the arrival of the stimulus at a skeletal muscle cell there is a short period called the ________ period during which the events of excitation-contraction coupling occur.


Creatine phosphate functions in the muscle cell by ________.

storing energy that will be transferred to ADP to resynthesize ATP

Rigor mortis occurs because _____.

no ATP is available to release attached actin and myosin molecules

During vigorous exercise, there may be insufficient oxygen available to completely break down pyruvic acid for energy. As a result, the pyruvic acid is converted to ________.

lactic acid

An anaerobic metabolic pathway that results in the production of two net ATPs per glucose plus two pyruvic acid molecules is ________.


After nervous stimulation stops, what prevents ACh in the synaptic cleft from continuing to stimulate contraction?

acetylcholinesterase destroying the ACh

Three discrete types of muscle fibers are identified on the basis of their size, speed, and endurance. Which of the following athletic endeavors best represents the use of red fibers?

a long, relaxing swim

Hypothetically, if a muscle were stretched to the point where thick and thin filaments no longer overlapped, ________.

no muscle tension would be generated

What part of the sarcolemma contains acetylcholine receptors?

motor end plate

True/False: Once a motor neuron has fired, all the muscle fibers in a muscle contract.


True/False: The force of muscle contraction is controlled by multiple motor unit summation or recruitment.


The H zone contains

thick filaments

The tension generated in a muscle fiber is directly proportional to the

number of cross bridges formed

Each myosin head has

a binding site for an ATP molecule and for an actin molecule, and the ability to swivel when powered by ATP.

The contraction cycle is triggered by the rise in __________ released from the SR.


Within a single fiber, the tension developed during a twitch depends on

the length of the sarcomeres prior to contraction

When comparing complete tetanus with unfused tetanus, which is true?

In complete tetanus, no relaxation occurs between stimuli, maximum tension is developed, and the muscle fiber is stimulated at a higher frequency.

A motor unit consists of

one neuron and the muscle fibers it controls

In muscles used for fine actions, such as controlling eye movement or use of the hand, a motor unit will have __________ muscle fibers when compared to a motor unit in muscles used for power and strength.

very few

Asynchronous recruitment

helps avoid fatigue during sustained muscle contractions, refers to different motor units taking turns maintaining muscle tension, and makes an entire muscle seem to contract smoothly.

The force generated by a single muscle fiber

can be increased by increasing the frequency of action potentials and due to summation.

A muscle fiber develops its maximum tension when it is in what position at the moment it is stimulated?

at its typical resting length

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