A&P CH. 9

The molecular interaction described as a cross bridge involves the binding of which two proteins?

A and C Cross bridges between thin and thick myofilaments are formed by the specific interaction between actin (A) and myosin head groups (C).

Which step precedes all of the other listed steps?

ACh is released by the motor neuron. The first step toward generating a skeletal muscle contraction is nervous stimulation of the muscle fiber in order to generate an action potential. The site of muscle stimulation, where the nerve ending communicates with the muscle fiber, is called the neuromuscular junction.

Which of the following corresponds to a single fascicle?

B. A fascicle is an organized group of muscle fibers (cells).

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

B. Myosin heads catalyze the hydrolysis of ATP to power the sliding of myofilaments. This event would be at a maximum during the period of contraction (B).

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

B. The structure at B is covered by the perimysium. Remember that the word root peri means "around," as in "perimeter."

Which statement accurately describes the event indicated by B?

Binding of acetylcholine to a receptor triggers the opening of an ion channel. Binding of acetylcholine to its receptor opens chemically (ligand) gated ion channels that allow Na+ and K+ to diffuse across the sarcolemma.

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

C. During the period of relaxation (C), Ca2+ is transported by active transport into the SR. Decreased Ca2+ concentrations in the sarcoplasm leads to detachment of cross bridges and, consequently, decreased contractile force.

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

C. Note that a skeletal muscle cell is commonly termed a muscle fiber.

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

C. The myosin head groups (C) hydrolyze ATP to power molecular movement along the actin subunits of the thin myofilaments.

Which of the following is most directly required to initiate the coupling of myosin to actin?

Ca++ Ca++ binds to troponin, which removes tropomyosin from the myosin binding site on actin.

A motor neuron and all the muscle cells that it stimulates are referred to as a motor end plate.

False

During isometric contraction, the energy used appears as movement.

False

Once a motor neuron has fired, all the muscle fibers in a muscle contract.

False

ATP is required to cause the power stroke in the myosin head.

False ATP is required to re-cock the myosin head so that it can generate a second power stroke.

Which of the following statements is NOT true regarding ATP production in muscles during periods of prolonged energy use, such as exercise?

In the absence of oxygen, creatine phosphate can drive aerobic respiration pathways for a few minutes. This is not true for a few reasons. First, creatine phosphate directly phosphorylates ATP instead of providing any support for aerobic pathways. Second, creatine phosphate stores are used up in about 15 seconds. Third, a cell doesn't need an oxygen deficit for creatine phosphate to be activated; it just needs to be short on ATP.

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. Only five muscle fibers shown are innervated. The others would not contract unless they were stimulated by other neurons that are not shown.

Troponin, a major protein in thin filaments, is a globular protein with three polypeptide subunits. Which of the following is NOT a function of the troponin?

One subunit binds to potassium ions.

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. The tropomyosin covers the myosin binding site on actin. Without tropomyosin, the myosin would constantly have access to those binding sites.

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. The second, more forceful contraction that would occur by adding another stimulus before the muscle has completely relaxed is an example of wave summation.

The 100-meter dash is a quick and short run requiring explosive speed. On completion of the dash, the runners will continue to breathe hard for several seconds to minutes even though they are no longer running. Which of the following is the best explanation for why this is so?

The runners' use of stored oxygen, glucose, and creatine phosphate is being replenished and this requires a prolonged increase of oxygen intake.

What is the role of tropomyosin in skeletal muscles?

Tropomyosin serves as a contraction inhibitor by blocking the myosin binding sites on the actin molecules.

A contraction in which the muscle does not shorten but its tension increases is called isometric contraction.

True

A skeletal muscle contracts with varying force and length of time in response to the body's needs at the time.

True

An increase in the calcium ion level in the sarcoplasm starts the sliding of the thin filaments. When the level of calcium ions declines, sliding stops.

True

If given the exact same amount of ATP, which of the three fiber types would be able to contract for the longest amount of time?

True

One of the functions of skeletal muscle contraction is production of heat.

True

The force of muscle contraction is controlled by multiple motor unit summation or recruitment.

True

When a muscle fiber contracts, the I bands diminish in size, the H zones disappear, and the A bands do not diminish in length.

True

In the muscles of the limbs, the origin is usually the immobile muscle attachment.

True In the muscles of the limbs, the insertion is pulled toward the immobile origin.

Most skeletal muscles contain ________.

a mixture of fiber types

Which of the following statements is true?

a. Skeletal muscle cells are long and cylindrical with many nuclei. b. Cardiac muscle cells are found in the heart and large blood vessels. c. Cardiac muscle cells have many nuclei. d. Smooth muscle cells have T tubules. a. Skeletal muscle cells are long and cylindrical with many nuclei.

Which of the following are correctly paired?

a. skeletal muscle; voluntary control b. cardiac muscle; nonstriated c. smooth muscle; striated d. cardiac muscle; voluntary control a. skeletal muscle; voluntary control Skeletal muscle is under voluntary control.

After nervous stimulation stops, what prevents ACh in the synaptic cleft from continuing to stimulate contraction?

acetylcholinesterase breaks apart the ACh

Which protein is indicated by the letter A?

actin Actin is the chief component of the thin myofilaments. It is made of a set of globular proteins attached in a chain.

The sliding filament model of contraction involves ________.

actin and myosin sliding past each other and partially overlapping

During muscle contraction, myosin cross bridges attach to which active sites?

actin filaments

Identify the correct sequence of the following events. a. Myosin generates a power stroke. b. Ca++ binds to troponin. c. ATP recharges the myosin head. d. Troponin removes tropomyosin from G actin. e. The sarcomere shortens.f. Myosin binds to actin.

b, d, f, a, e, c The Ca++ binds to the troponin, which removes the tropomyosin from actin. This allows the myosin to bind to actin and generate a power stroke, shortening the sarcomere. ATP is used to recharge the myosin.

In an isotonic contraction, the muscle ________.

changes in length and moves the "load"

The ability of muscle to shorten forcibly when adequately stimulated is known as ________, and sets muscle apart from other tissue types.

contractility

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

diffusion of Ca2+ into the axon terminal A nerve impulse arrives at the axon terminal triggering the opening of Ca2+ channels, which allows for the diffusion of Ca2+ into the terminal. This in turn leads directly to the release of neurotransmitters by exocytosis.

The sliding filament model of contraction states that __________.

during contraction, the thin myofilaments slide past the thick myofilaments so that the actin and myosin myofilaments overlap to a greater degree The sliding filament model of contraction states that during contraction, the thin myofilaments slide past the thick myofilaments so that actin and myosin myofilaments overlap to a greater degree. In a relaxed muscle fiber, the thick and thin myofilaments overlap only at the ends of the A band.

A muscle that is lengthening while it produces tension is performing a(n) __________ contraction.

eccentric During isotonic, eccentric contractions, muscle lengthens as it generates tension, but not enough force, to overcome the load. We use eccentric contractions to lower objects (such as lowering a book from the shelf or lowering the barbell during a bench press).

Which of the following surrounds an individual muscle cell?

endomysium

What cell organelle plays a role in the process of regulating intracellular calcium ions for muscle contraction?

endoplasmic reticulum

What is the functional role of the T tubules?

enhance cellular communication during muscle contraction

Which of the following properties is most directly associated with changes to a muscle cell's membrane potential (the voltage across the plasma membrane)?

excitability Excitability, also termed responsiveness, is the ability of a cell to receive and respond to a stimulus by changing its membrane potential. For example, a neurotransmitter released by a nerve cell may cause the depolarization of the muscle cell's sarcolemma.

What cellular event is indicated by A?

exocytosis Neurotransmitters stored in synaptic vesicles are released into the synaptic cleft by exocytosis.

Exhaustion of glycogen storage within a muscle fiber would have the biggest effect on ________.

fast glycolytic fibers

An anaerobic metabolic pathway that results in the production of two net ATPs per glucose plus two pyruvic acid molecules is ________.

glycolysis

During vigorous exercise, there may be insufficient oxygen available to completely break down pyruvic acid for energy. As a result, the pyruvic acid is converted to ________.

lactic acid

Which of the following is the correct order for the phases of a muscle twitch?

latent, contraction, relaxation

Which of the following is a factor that affects the velocity and duration of muscle contraction?

load on the fiber

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

load placed on the muscle Speed (velocity) of shortening is a function of load and muscle-fiber type. Contraction is fastest when the load on the muscle is zero; a greater load results in a slower contraction and a shorter duration of contraction.

What part of the sarcolemma contains acetylcholine receptors?

motor end plate

Which of the following is the correct sequence of events for muscle contractions?

motor neuron action potential, neurotransmitter release, muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke, sliding of myofilaments

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 A group of muscle fibers innervated by a single neuron is called a motor unit.

During development embryonic cells will fuse to form muscle fibers. This will result in ________.

multinucleated muscle fibers that can extend as long as 30 centimeters

Which term best identifies a muscle cell?

muscle fiber Skeletal muscle cells fuse during development to form the mature, multinucleated muscle fibers.

The contractile units of skeletal muscles are ________.

myofibrils

Which protein is indicated by the letter E?

myosin Myosin tail groups form the extended regions of the thick myofilament.

Rigor mortis occurs because ________.

no ATP is available to release attached actin and myosin molecules

Approximately 80% of a muscle fiber's volume are the myofibrils. This characteristic reflects muscles ability to ________.

produce movement through contractile force

When a muscle is unable to respond to stimuli temporarily, it is in which of the following periods?

refractory period

Which of the following events triggers the subsequent steps of excitation-contraction coupling?

release of acetylcholine from axon terminals at the neuromuscular junction Release of acetylcholine from axon terminals at the neuromuscular junction is an event that precedes excitation-contraction coupling. Binding of acetylcholine to receptors in the motor end plate triggers an action potential that propagates along the sarcolemma and into the cell interior via T tubules.

The __________ shorten(s) during muscle contraction

sarcomere As actin slides over myosin, the z lines are pulled closer together, shortening the sarcomere.

What structure in skeletal muscle cells functions in calcium storage?

sarcoplasmic reticulum

Which of the following is NOT a normal function of muscle tissue?

secreting hormones Secreting hormones is a function of the endocrine system.

Which type of muscle requires somatic (voluntary) nervous stimulation for activation?

skeletal In order to contract, skeletal muscle fibers must be voluntarily stimulated by motor neurons of the somatic nervous system.

Of the following muscle types, which has the longest muscle cells and has obvious stripes called striations?

skeletal muscle

Addition of more mitochondria to a muscle fiber will have the greatest effect on ________.

slow oxidative fibers

If given the exact same amount of ATP, which of the three fiber types would be able to contract for the longest amount of time?

slow oxidative fibers

Reduction in blood flow to a muscle fiber would have the greatest effect on ________.

slow oxidative fibers

Creatine phosphate functions in the muscle cell by ________.

storing energy that will be transferred to ADP to resynthesize ATP

When a sarcomere contracts and thin filaments move over thick filaments you would expect to see ________.

the I bands to appear smaller

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 Without acetylcholine, the muscle would not be able to contract or even maintain tone.

The contractile, or functional, unit of a muscle fiber is __________.

the sarcomere The sarcomere is the contractile unit of a muscle fiber and the smallest functional unit of muscle. A sarcomere is the region of a myofibril between two successive Z discs; it primarily consists of thin and thick myofilaments.

Which muscle cell structure stores calcium ions that are used to trigger the contraction?

the sarcoplasmic reticulum The sarcoplasmic reticulum is an intracellular tubule that stores and releases calcium ions.

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

to break down acetylcholine Acetylcholinesterase breaks acetylcholine apart, terminating the synaptic transmission. Choline can be taken back into the axon terminal and reused.

The interaction between which protein and ion initiates muscle coupling?

troponin; calcium ions When calcium ions bind to troponin, troponin changes shape and allows tropomyosin to shift away from its inhibitory position. As a result, myosin heads can bind to the actin molecules and begin the cross bridge cycle.

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

1 and 7 The distance between points 1 and 7 spans a sarcomere, the smallest contractile unit of a muscle fiber.

Which of the following statements best illustrates the fact that skeletal muscle is voluntary muscle?

Skeletal muscle fibers are innervated by somatic motor neurons.