A&P - Chapter 9: Muscles and Muscle Tissue

Which protein inhibits interaction between actin and myosin to prevent skeletal muscle contraction; and which ions remove the inhibition?

tropomyosin; calcium ions

The type of muscle found in the walls of most hollow organs is ________.

unitary smooth muscle

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

The energy comes from the hydrolysis of ATP.

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

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

cross bridge formation

The distance between Z discs ________ during muscle contraction.

decreases

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

transverse or T tubules

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

tropomyosin

Although there are no sarcomeres, smooth muscle still possesses thick and thin filaments.

true

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

voltage-gated calcium channels

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

acetylcholine (ACh)

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 statements 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

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.

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.

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

Myofibrils contain thick and thin filaments.

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.

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

Calcium ions

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.

How do calcium ions initiate contraction in skeletal muscle fibers?

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

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.

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 only produced during life.

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.

How does the shape change of these proteins lead to contraction?

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

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

Multiple action potentials would occur in the muscle fiber.

Which 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.

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.

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

The sarcomere

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.

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

propagation of the action potential along the sarcolemma

The response of a motor unit to a single action potential of its motor neuron is called ________.

a muscle twitch

Synaptic vesicles at the neuromuscular junction contain __________.

acetylcholine

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

acetylcholinesterase (AChE; an enzyme)

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

actin

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

action potential propagating down the T tubule

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

arrival of an action potential

What is the role of calcium ions in muscle contraction?

bind to regulatory sites on troponin to remove contraction inhibition

What causes the myosin head to disconnect from actin?

binding of ATP

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

binding of calcium to the protein complex

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

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head

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

excitability

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

What energizes the power stroke?

hydrolysis of ATP

The major role of the sarcoplasmic reticulum is to regulate ________.

intracellular levels of Ca2+

Immediately following the arrival of the stimulus at a skeletal muscle cell there is a short period called the ________ period during which the neurotransmitter is released by exocytosis, diffuses across the synaptic cleft, and binds to its receptors.

latent

Thick myofilaments are made of ________.

myosin

What, specifically, is a cross bridge?

myosin binding to actin

What is the primary function of wave summation?

produce smooth, continuous muscle contraction

Muscle tissue does NOT ________.

product blood cells

ACh receptors are found mainly in the __________.

sarcolemma

Which of the following are composed of myosin?

thick filaments

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

skeletal

Which of the following is CORRECTLY paired?

skeletal muscle: voluntary control

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

sodium

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

terminal cisternae

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

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

the opening of ligand-gated cation channels

The mechanism of contraction in smooth muscle is different from skeletal muscle in that ________.

the site of calcium regulation differs