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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 shape change that occurs in voltage-sensitive proteins in the sarcolemma.
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 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.
Sodium release from the sarcoplasmic reticulum initiates the contraction.
Action potentials propagate into the interior of the skeletal muscle fiber.
Voltage-sensitive proteins change shape.


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.
Myosin cross-bridge binding sites.
Potassium leak channels.
A series of proteins that control calcium release.


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

terminal cisternae
sarcoplasmic reticulum
motor endplates
transverse or T tubules


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

Action potentials.
Calcium ions.
Sodium ions.


What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated?

Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron.
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.


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

calcium ion pump
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 calcium
binding of troponin
binding of ATP
hydrolysis of ATP


What energizes the power stroke?

hydrolysis of ATP
binding of ATP


In a neuromuscular junction, synaptic vesicles in the motor neuron contain which neurotransmitter?

acetylcholine (ACh)


When an action potential arrives at the axon terminal of a motor neuron, which ion channels open?

voltage-gated calcium channels
voltage-gated potassium channels
voltage-gated sodium channels
chemically gated calcium channels


What means of membrane transport is used to release the neurotransmitter into the synaptic cleft?

a protein carrier
a channel


The binding of the neurotransmitter to receptors on the motor end plate causes which of the following to occur?

Binding causes voltage-gated sodium channels to open in the motor endplate.
Binding causes chemically gated potassium channels to open in the motor end plate.
Binding causes potassium voltage-gated channels to open in the motor endplate.
Binding of the neurotransmitter causes chemically gated sodium channels to open in the motor end plate.


How is acetylcholine (ACh) removed from the synaptic cleft?

diffusion away from the synaptic cleft
acetylcholinesterase (AChE; an enzyme)
a reuptake pump on the axon terminal


The action potential on the muscle cell leads to contraction due to the release of calcium ions. Where are calcium ions stored in the muscle cell?

terminal cisternae of the sarcoplasmic reticulum
T tubule


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

Calcium binds to troponin, exposing the active site on troponin.
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, 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.
Tropomyosin binds to calcium, causing muscle relaxation.
Tropomyosin moves the actin filament relative to the myosin filament.
Tropomyosin pushes the myosin head away, causing cross bridge detachment.


How does troponin facilitate cross bridge formation?

Troponin hydrolyzes ATP, which provides the energy necessary for cross bridges to form.
Troponin gathers excess calcium that might otherwise block actin's progress.
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.


What, specifically, is a cross bridge?

ATP binding to the myosin head
tropomyosin covering the active sites on actin
calcium binding to troponin
myosin binding to actin


Which event causes cross bridge detachment?

release of calcium from troponin
release of ADP and inorganic phosphate from the myosin head
nervous input ends
ATP binding to the myosin head


Where in the cross bridge cycle does ATP hydrolysis occur?

during the power stroke
during the cocking of the myosin head
during the removal of calcium from troponin
during the movement of tropomyosin to expose the active sites on actin


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

The power stroke cocks the myosin head.
The sliding of the actin myofilament during the power stroke re-cocks myosin heads that have previously delivered their power stroke.
After the myosin head detaches, energy from ATP hydrolysis is used to re-cock the myosin head.
when ADP is released from 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?

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


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

The actin myofilament can only move in one direction relative to the myosin filament.
The cross bridge remains in place, preventing the actin myofilament from sliding.
Calcium blocks the active sites on actin.
There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching.


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