Anatomy and Physiology Chapter 9 Mastering Questions

Which of the following is CORRECTLY paired?

skeletal muscle: voluntary control

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?

troponin

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

actin

What causes the myosin head to disconnect from actin?

binding of ATP

What causes the power stroke?

release of ADP and Pi

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

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

exocytosis

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

Binding of the neurotransmitter causes chemically gated sodium channels to open in the motor end plate (junctional folds of the sarcolemma) and sodium enters the cell.

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

simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme)

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 cisterns (cisternae) of the sarcoplasmic reticulum

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.

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.

The region between which two points corresponds to the entire A band?

2 and 6

In which phase in the figure would the net movement of Ca2+ into the sarcoplasmic reticulum (SR) be greatest?

C

Excitation-contraction coupling includes all EXCEPT which of the following events?

release of acetylcholine from axon terminals at the neuromuscular junction

What is a cross bridge cycle?

It is the cycle in which an energized myosin head binds to actin and performs a power stroke, then binds to ATP in order to detach and re-energize.

Which muscle fiber type is best suited for endurance activities, such as long-distance jogging?

slow oxidative fibers

Which of the following factors influence the velocity and duration of muscle contraction?

load placed on the muscle

Cross bridge formation between myosin heads and actin molecules is caused by the elevation of calcium ion concentration in the cytosol. During rigor mortis, this elevation of calcium ion concentration in the cytosol is permanent because ________.

mitochondria stop producing ATP molecules required by the sarcoplasmic reticulum's calcium ion pumps

In a bedridden patient recovering from a badly fractured femur, disuse atrophy in the thigh muscles is caused by _________.

decreased synthesis of muscle proteins and/or increased breakdown of muscle proteins

What special feature of smooth muscle allows it to stretch without immediately resulting in a strong contraction?

stress-relaxation response

Duchenne muscular dystrophy could theoretically be cured if a technique was developed that would _________.

none of the above

T or F? In the muscles of the limbs, the origin usually lies proximal to the insertion.

T

Muscle tissue does NOT ________.

produce blood cells

Which type of muscle CANNOT contract without being stimulated by the nervous system?

skeletal

Which muscle characteristic describes the ability of muscle to respond to a stimulus?

excitability

Muscle tissue has all of the following properties except ________.

secretion

T or F? Muscle contraction will always promote movement of body parts regardless of how they are attached.

F

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.

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?

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?

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 which of the following 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.

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

Which 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?

After the myosin head detaches, energy from 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.

Skeletal muscle cells are grouped into bundles called __________.

fascicles

Synaptic vesicles at the neuromuscular junction contain __________.

acetylcholine

ACh receptors are found mainly in the __________.

sarcolemma

What most directly causes synaptic vesicles to release acetylcholine into the synaptic cleft?

calcium entering the axon terminal

Acetylcholine receptors are best characterized as what type of channel?

chemically gated Na+-K+ channels

When the chemically gated ion channels open, which ion is mainly responsible for depolarizing the sarcolemma?

sodium

What is the primary mechanism by which ACh is cleared from the synaptic cleft?

broken down by acetylcholinesterase

What would happen if acetylcholine was not removed from the synaptic cleft?

Multiple action potentials would occur in the muscle fiber.

What is the type of chemical reaction used to rebuild ADP into ATP?

dehydration synthesis

Which of the following processes produces molecules of ATP and has two pyruvic acid molecules as end products?

glycolysis

Which of the following processes produces 36 ATP?

Krebs cycle and oxidative phosphorylation

The "rest and recovery" period, where the muscle restores depleted reserves, includes all of the following processes EXCEPT __________.

Pyruvic acid is converted back to lactic acid.

Which type of muscle fiber has a large quantity of glycogen and mainly uses glycolysis to synthesize ATP?

white fast twitch fibers

myofilament with a knob-like head:

Myosin

myofilament stiffened and stabilized by tropomyosin:

Actin

the cytoplasm of a skeletal muscle fiber:

Sarcoplasm

bundle of skeletal muscle fibers enclosed by connective tissue called perimysium:

Fascicle

membrane of muscle cell:

Sarcolemma

blocks myosin-binding sites on actin:

tropomyosin

long cylindrical cells:

skeletal muscle fibers

contractile unit:

sarcomere

made up of several sarcomeres:

myofibrils

Location of higher concentration of Ca2+ needed for cross bridge formation and cycling:

cytoplasm

Actin status to begin cross bridge formation:

actin binding sites exposed

Molecule(s) bound to myosin head at start of cross bridge cycle:

ADP and Pi

Energy state of myosin for beginning cross bridge formation:

energized/cocked myosin head

The molecules released just before power stroke:

ADP and Pi

Actin behavior during the power stroke:

ratchets toward the M line

Myosin head energy status during the power stroke:

pivots and returns to low energy

The bond between the actin and myosin head weakens due to

ATP

Only __________ of the myosin heads are detached from actin at any one time while calcium is present and cycling continues.

one-half

__________ is not bound to myosin during the detachment step.

ADP

Status of ATP:

hydrolyzed to ADP and Pi

Status of myosin head:

returns to the energized position

Status of actin and myosin:

actin and myosin are detached

A muscle cell runs out of ATP. Even though these are cyclic reactions, what step of the cross bridge cycle given is most directly inhibited or terminated?

cross bridge detachment

A person dies and within hours, the skeletal muscles develop a locked contraction known as rigor mortis. Calcium ions leak from the sarcoplasmic reticulum into cytoplasm. From your knowledge of cross-bridge cycling, what best explains this rigor?

Cross bridge detachment cannot occur. Detachment requires ATP, which is only produced during life.

Which of the following is true concerning the anatomy of a skeletal muscle fiber?

Myofibrils contain thick and thin filaments.

The calcium that initiates skeletal muscle contraction is released from what structure(s)?

terminal cisternae

Which of the following are composed of myosin?

thick filaments

In muscle fibers, which regulatory protein blocks the attachment of myosin heads to actin?

tropomyosin

What event most directly triggers the release of calcium from the terminal cisternae?

action potential propagating down the T tubule

How do calcium ions initiate contraction in skeletal muscle fibers?

Calcium ions bind to troponin, changing troponin's shape.

Which of the following best describes the events of "excitation" in "excitation-contraction coupling"?

propagation of the action potential along the sarcolemma