Chapter 9 Homework

The smallest contractile unit within skeletal muscle would correspond to the distance between which two points in the figure?

1 and 7

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

2 and 6

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

Which of the following choices best summarizes excitation-contraction coupling?

A series of events in which an electrical stimulus is conveyed to a muscle fiber to enact 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.

Which of the following best describes how ACh changes the ion permeability of the sarcolemma?

ACh binds to receptors in the junctional folds.

Which event causes cross bridge detachment?

ATP binding to the myosin head

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.

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.

Which of the following best describes the role of acetylcholinesterase molecules at the neuromuscular junction?

Acetylcholinesterase breaks down ACh, which allows chemically gated ion channels to close.

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.

Which of the choices below correctly describes how an action potential generated at the neuromuscular junction (NMJ) is converted to excitation in the muscle fiber?

An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh to sarcolemma receptors initiates graded potentials.

What causes the release of calcium ions into the sarcoplasm from the terminal cisterns?

An action potential traveling along the t tubule Submit

In which phase of the muscle twitch shown in the figure would the maximum amount of ATP be consumed by myosin head groups?

B

Which of the following corresponds to a single fascicle?

B

Which of the structures is surrounded by the connective tissue sheath known as the perimysium?

B

Which of the following interactions must occur first so that the others can take place?

B binds to troponin.

Which statement accurately describes the event indicated by B?

Binding of acetylcholine to a receptor triggers the opening of an ion channel.

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.

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

C

Which protein functions as a motor protein that applies the power stroke during muscle contraction?

C

Which structure in the figure corresponds to a single skeletal muscle cell?

C

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin

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

Calcium binds to troponin, altering its shape.

Which selection correctly describes the role of calcium in coupling?

Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin.

Which of the following is responsible for muscle relaxation?

Calcium ions are removed from the sarcoplasm by active transport.

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.

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

Calcium ions bind to troponin and change its shape.

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

Calcium ions.

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.

Which statement best describes the importance of calcium in skeletal muscle contraction?

Calcium released from the sarcoplasmic reticulum binds to troponin to initiate muscle contraction.

Which statement best describes what might happen if a child with malignant hyperthermia is given the wrong anesthetic and the sarcoplasmic reticulum releases all its calcium?

Continuous cross bridge cycling causes the muscle to contract too much.

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.

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 produced only during life. Submit

Curare is a poisonous plant extract. Curare molecules have a chemical structure like the neurotransmitter ACh. Curare can bind to the ACh receptor site on the chemically gated ion channels in the motor end plate. Even though curare will bind to the receptor site it will not open the ion channel and no ions will pass through. What do you think the symptoms of curare poisoning would look like?

Curare will only affect muscles with ACh receptors, paralyzing them.

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.

Which of the following is NOT a role of ATP in muscle contraction?

Exposing myosin binding sites on actin

The action potential propagates along the sarcolemma. As the action potential spreads down the T tubules of the triads, voltage-sensitive tubule proteins change shape. How does the shape change of these proteins lead to contraction?

It allows calcium to exit the sarcoplasmic reticulum and enter the cytosol.

EGTA is a substance that binds calcium ions. Imagine an experimental setup with a motor neuron and a muscle fiber. Stimulation of the motor neuron causes contraction of the muscle fiber through activity at the neuromuscular junction and excitation-contraction coupling. Now, inject the muscle fiber with EGTA. Which of the following effects would EGTA have on excitation-contraction coupling after the neuron releases acetylcholine at the neuromuscular junction?

It would prevent myosin from forming cross bridges with actin.

Which of the following processes produces 36 ATP?

Krebs cycle and oxidative phosphorylation

What structure most directly stimulates a skeletal muscle fiber to contract?

Motor neuron

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

Multiple action potentials would occur in the muscle fiber.

Which selection best describes the initial event in contraction?

Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges.

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.

If a muscle fiber were to suddenly and permanently stop producing ATP the fiber would no longer be able to actively transport calcium out of the cytoplasm (sarcoplasm) and the intracellular calcium concentration would rise. Which of the following would you expect to happen?

Myosin would be able to bind to the exposed binding sites on thin filaments but it would not be able to detach.

If both motor neurons shown in this figure were to develop action potentials and stimulate muscle fibers, would all the muscle cells shown here contract?

No, because neurons in this figure do not innervate every muscle cell shown.

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.

Inadequate calcium in the neuromuscular junction would directly affect which of the following processes?

Release of acetylcholine from the synaptic vesicles

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.

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 the myosin head obtain the energy required for activation?

The energy comes from the hydrolysis of ATP.

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.

Based on what you know of the relationship between the thick and the thin filaments, what would happen if a disorder existed that caused a person to produce no tropomyosin?

The muscle tissues would never be able to relax.

What result would be expected if an additional stimulus, equal in intensity to the first, were to be applied to the muscle at the 60 millisecond (ms) time point?

The muscle would increase in tension to a level greater than that measured at the beginning of phase C.

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

The sarcomere

If troponin is a component of both cardiac and skeletal muscle, why is an elevated plasma troponin level useful in diagnosing myocardial damage?

The subunits of the troponin in cardiac muscle are unique to heart muscle.

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.

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 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.

Myasthenia gravis is a disease resulting from an autoimmune attack on the ACh receptors of the motor end plate. Binding of antibodies to the ACh receptors results in generalized muscle weakness that progresses as more ACh receptors are destroyed. Which of the following medications would help alleviate the muscle weakness?

a drug that binds to and inactivates acetylcholinesterase (neostigmine)

Synaptic vesicles at the neuromuscular junction contain __________.

acetylcholine

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

acetylcholine (ACh)

Which protein is indicated by the letter A?

actin

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

broken down by acetylcholinesterase

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

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

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

dehydration synthesis

What event directly triggers the release of neurotransmitter shown in A?

diffusion of Ca2+ into the axon terminal

Conduction of an action potential along the sarcolemma depends upon ___________.

diffusion of sodium ions through voltage-gated channels

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head

What cellular event is indicated by A?

exocytosis

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

exocytosis

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

Skeletal muscle cells are grouped into bundles called __________.

fascicles

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

glycolysis

Each neuron shown in this figure innervates a group of muscle fibers. What is the term for a group of muscle fibers and the single neuron that innervates them?

motor unit

Which protein is indicated by the letter E?

myosin

What, specifically, is a cross bridge?

myosin binding to actin

What is the function of the filaments found in ranges 1-2 and 6-7 but no other ranges?

protection from overstretching

If both of the neurons in the figure were activated, more muscle fibers would contract than if either neuron alone were active. This mechanism for control of the force of muscle contraction is known as __________.

recruitment

ACh receptors are found mainly in the __________.

sarcolemma

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

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

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

sodium

The connective tissue that covers structure A is continuous with which of the following?

tendon

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

A toxin released by certain bacteria can block the release of neurotransmitters into a neuromuscular synapse. What would result from such a block?

the loss of ability to contract the muscle

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

the opening of ligand-gated cation channels

Which of the following regions best identifies where myosin would have maximum cross-bridge access to actin?

the region between 2 and 3

An enzyme known as acetylcholinesterase is present in the synaptic cleft. What is its role?

to break down acetylcholine

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

transverse or T tubules

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

voltage-gated calcium channels

Myasthenia gravis is a disease that is believed to be caused by autoimmune disorder, resulting in the loss of ACh receptors at the motor end plate of muscle fibers. Which of the following is likely to be a symptom of myasthenia gravis?

weakness of muscle

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

white fast twitch fibers

Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction?

• ACh diffuses away from the synaptic cleft. • ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE).