Lab 7 Homework/Practice Quiz

A motor unit is defined as _______. a single neuron and a single muscle fiber a nerve and a muscle a single muscle fiber and all its axon terminals the axon terminals of a single motor neuron and all of the muscle fibers that it stimulates

a nerve and a muscle

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

actin Yes, the myosin head binds to actin, the major component of thin filaments.

What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell? arrival of an action potential ATP troponin calcium ion pump

arrival of an action potential Yes, an action potential in the T tubule causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum.

What causes the myosin head to disconnect from actin? hydrolysis of ATP binding of ATP binding of troponin binding of calcium

binding of ATP Yes, the binding of ATP causes the myosin head to disconnect from actin.

What causes muscle contractions to be graded? increasing stimulus strength increasing stimulus frequency both increasing stimulus strength and increasing stimulus frequency

both increasing stimulus strength and increasing stimulus frequency

Where in the cross bridge cycle does ATP hydrolysis occur? during the power stroke during the movement of tropomyosin to expose the active sites on actin during the removal of calcium from troponin during the cocking of the myosin head

during the cocking of the myosin head As ATP is broken down, its energy is used to cock the myosin head in preparation for the next power stroke.

What, specifically, is a cross bridge? calcium binding to troponin tropomyosin covering the active sites on actin ATP binding to the myosin head myosin binding to actin

myosin binding to actin The attachment of a myosin head from the thick filament to an active site on actin on the thin filament is a cross bridge. As soon as the cross bridge forms, the power stroke occurs, moving the thin filament toward the center of the sarcomere.

What causes the power stroke? calcium binding of ATP release of ADP and Pi hydrolysis of ATP

release of ADP and Pi

Part B NO force is generated during which of the following? the latent period the contraction phase a muscle twitch the relaxation phase

the latent period

Which of the following is proportional to the amount of tension produced by a skeletal muscle? the length of the latent period the threshold voltage the number of motor units activated the length of the relaxation phase

the number of motor units activated

Part C What is the minimum voltage needed to generate active force in the skeletal muscle? stimulus voltage contraction voltage recruitment voltage threshold voltage

threshold voltage

What is name given to the regularly spaced infoldings of the sarcolemma? motor endplates transverse or T tubules terminal cisternae sarcoplasmic reticulum

transverse or T tubules Yes! T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior.

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

troponin Yes, when calcium binds to troponin, troponin releases tropomyosin, exposing the myosin binding sites.

A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected? Voltage-gated sodium channels. Potassium leak channels. Myosin cross-bridge binding sites. A series of proteins that control calcium release.

A series of proteins that control calcium release. Yes! When action potentials propagate along T-tubules, a voltage-sensitive protein changes shape and triggers a different protein to open it's channels, resulting in the release of calcium from the terminal cisternae.

Which event causes cross bridge detachment? ATP binding to the myosin head release of calcium from troponin nervous input ends release of ADP and inorganic phosphate from the myosin head

ATP binding to the myosin head As ATP binds, the myosin head releases from the active site on actin

What is the role of calcium in the cross bridge cycle? Calcium binds to troponin, altering its shape. Calcium binds to myosin, causing the myosin head to release from the actin myofilament. Calcium binds to active sites on actin, forming the cross bridge. Calcium binds to troponin, exposing the active site on troponin.

Calcium binds to troponin, altering its shape. Calcium binding to troponin causes tropomyosin to move away from the active sites on actin.

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? Acetylcholine. Action potentials. Sodium ions. Calcium ions.

Calcium ions. Yes! Action potentials propagating down the T-tubule cause a voltage-sensitive protein to change shape. This shape change opens calcium release channels in the sarcoplasmic reticulum, allowing calcium ions to flood the sarcoplasm. This flood of calcium ions is directly responsible for the coupling of excitation to contraction in skeletal muscle fibers.

Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction? Action potentials propagate into the interior of the skeletal muscle fiber. Voltage-sensitive proteins change shape. Calcium release from the sarcoplasmic reticulum initiates the contraction. Sodium release from the sarcoplasmic reticulum initiates the contraction.

Calcium release from the sarcoplasmic reticulum initiates the contraction. Yes! Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle cells. It is especially abundant and convoluted in skeletal muscle cells. It functions in the storage, release, and reuptake of calcium ions.

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 refers to the release of calcium ions from the sarcoplasmic reticulum. Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. Excitation refers to the propagation of action potentials along the axon of a motor neuron. Excitation refers to the shape change that occurs in voltage-sensitive proteins in the sarcolemma.

Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. Yes! These action potentials set off a series of events that lead to a contraction.

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. Tropomyosin would not move, and the active sites on actin would not be exposed. Myosin heads would remain attached to actin, unable to perform the power stroke. Myosin heads would remain attached to actin, unable to detach.

Myosin heads would remain detached, unable to cock. The hydrolysis of ATP is required for the cocking of the myosin head. ATP would still bind to myosin, causing cross bridge detachment, but myosin would be stuck in this step of the cross bridge cycle.

What role does tropomyosin play in the cross bridge cycle? Tropomyosin moves the actin filament relative to the myosin filament. Tropomyosin pushes the myosin head away, causing cross bridge detachment. Tropomyosin binds to calcium, causing muscle relaxation. The displacement of tropomyosin exposes the active sites of actin, allowing cross bridges to form.

The displacement of tropomyosin exposes the active sites of actin, allowing cross bridges to form. Tropomyosin covers active sites in relaxed muscle. When tropomyosin is displaced, the active sites are exposed for cross bridge formation.

During contraction, what prevents actin myofilaments from sliding backward when a myosin head releases? The cross bridge remains in place, preventing the actin myofilament from sliding. There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching. Calcium blocks the active sites on actin. The actin myofilament can only move in one direction relative to the myosin filament.

There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching. During contraction, about half of the myosin heads are attached, preventing the actin myofilament from sliding backwards when any single myosin head detaches. The situation is analogous to a game of tug-of-war. In tug-of-war, individual hands release after they pull on the rope, but not all hands release at the same time

How does troponin facilitate cross bridge formation? Troponin gathers excess calcium that might otherwise block actin's progress. Troponin hydrolyzes ATP, which provides the energy necessary for cross bridges to form. Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin. Troponin moves away from the active sites on actin, permitting cross bridge formation.

Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin. For cross bridges to form, tropomyosin must not block the active sites. The position of tropomyosin is controlled by the regulatory protein troponin. This protein-protein interaction couples the binding of calcium (to troponin) to the exposure of active sites.

What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated? A skeletal muscle fiber is innervated by multiple motor neurons. A motor neuron typically innervates only one skeletal muscle fiber. Motor neurons always innervate thousands of skeletal muscle fibers. Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron.

Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. Yes! There are many more skeletal muscle fibers than there are motor neurons. The ratio of neurons to fibers varies from approximately one to ten to approximately one to thousands