BSC 3096 Chapter 12

Put the events of contraction in order

An action potential that travels down the T-tubule changes the structural confirmation of the DHP L-type Ca2+ channel Ryanodine receptor channels open Ca2+ leaves the sarcoplasmic reticulum Acting and myosin bind to one another Myosin heads utilize energy from ATP hydrolysis to produce the power stroke Actin filaments slide toward the M line, shortening the sarcomere

The relaxation of skeletal muscle relies on the activity of the ________, which decreases cytoplasmic calcium concentration. Ca2+ ATPase dihydropyridine (DHP) receptor (L-type calcium channel) acetylcholine receptor ryanodine receptor

Ca2+ ATPase

Explain the roles of troponin, tropomyosin, and Ca2+ in skeletal muscle contraction.

Ca2+ binds to troponin, which repositions tropomyosin, uncovering actin's myosin-binding sites.

Which of the following best describes the role of calcium (Ca2+ ) in the excitation-contraction coupling reaction? Ca2+ will bind to troponin, which leads to a conformational shift in tropomyosin, allowing for actin and myosin to attach. Ca2+ binds to specific receptors which control the release of ATP into the cytoplasm of the muscle fiber. Ca2+ gives the myosin heads the energy necessary for the power stroke. Ca2+ will bind directly to tropomyosin, resulting in a conformational shift which reveals the binding sites for myosin on actin.

Ca2+ will bind to troponin, which leads to a conformational shift in tropomyosin, allowing for actin and myosin to attach.

A structure that when bound to a ligand opens a divalent channel for the movement of both Na+ and K+

ACH receptor channel

Which neurotransmitter is released by somatic motor neurons?

Acetylcholine

When comparing complete tetanus with unfused (incomplete) tetanus, which is true? Complete tetanus would occur when there is more time between twitch contractions. Complete tetanus occurs at a lower frequency of stimulation than unfused tetanus. Complete tetanus involves some relaxation between twitches. Complete tetanus involves development of maximum tension.

Complete tetanus involves development of maximum tension.

Explain the sliding filament theory of contraction.

Contraction occurs when thin and thick filaments slide past each other as myosin binds to actin, swivels, and pulls actin toward the center of the sarcomere.

When does cross bridge cycling end? Cross bridge cycling ends when calcium release channels in the sarcoplasmic reticulum open. Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. Cross bridge cycling ends when ATP binds to the myosin head. Cross bridge cycling ends when calcium ions are passively transported back into the sarcoplasmic reticulum.

Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.

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 propagation of action potentials along the axon of a motor neuron. 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, in this case, refers to the propagation of action potentials along the sarcolemma.

Muscle fibers depend on a continuous supply of ATP. How do the fibers in the different types of muscle generate ATP?

Generate ATP by energy transfer from phosphocreatine. Oxidative fibers use oxygen to make ATP from glucose and fatty acids; glycolytic fibers get ATP primarily from anaerobic glycolysis.

Part H Part complete 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? View Available Hint(s) Myosin heads would remain attached to actin, unable to perform the power stroke. Tropomyosin would not move, and the active sites on actin would not be exposed. Myosin heads would remain attached to actin, unable to detach. Myosin heads would remain detached, unable to cock.

Myosin heads would remain detached, unable to cock.

What is it called when myosin crossbridges attach to actin filaments and pull them toward the middle of the sarcomere? Power stroke Filamental sliding Pull flash Rotational pulling

Power stroke

Put the Events at the Neuromuscular Junction in Order

Somatic motor neuron delivers action potential Acetylcholine is released from the axon terminal onto the motor endplate Net Na+ influx through nicotinic receptors Muscle fiber depolarizes Action potential travels down the T-tubule of the muscle fiber

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.

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 GTP. The energy comes from oxidative phophorylation. The energy comes from the hydrolysis of ATP. The energy comes from the direct phosphorylation of ADP by creatine phosphate.

The energy comes from the hydrolysis of ATP.

Briefly explain the functions of titin and nebulin.

They keep actin and myosin in alignment. Titin helps stretched muscles return to resting length.

T-tubules allow to move to the interior of the muscle fiber.

action potentials

In order for the bicep muscle to shorten, the force applied by the muscle must be ________ the load applied at the hand. greater than equal to less than

greater than

fast-twitch oxidative-glycolytic fibers

has some myoglobin is also called red muscle uses a combination of oxidative and glycolytic metabolism

fast-twitch glycolytic fibers

has the largest diameter uses anaerobic metabolism, thus fatigues quickly is used for quick, fine movements

slow-twitch oxidative fibers

has the most blood vessels has some myoglobin is also called red muscle has the most mitochondria

The force generated by a single muscle fiber can be increased by increasing frequency of action potentials only. increasing muscle length beyond optimal only. increasing frequency of action potentials and increasing muscle length beyond optimal only. increasing the asynchrony of recruitment only

increasing frequency of action potentials only.

The basic unit of contraction in an intact skeletal muscle is the . The force of contraction within a skeletal muscle is increased by additional motor units.

motor unit, recruitment

A motor unit consists of one neuron and all the skeletal muscle fibers it controls. one skeletal muscle fiber and all the neurons that control that fiber. one skeletal muscle fiber and the neuron that controls that fiber. one axon terminal branch and the myofibrils it controls.

one neuron and all the skeletal muscle fibers it controls.

Which of the following statements best describes the neuromuscular junction? Choose the best answer. the point of synapse between a motor neuron and the muscle fiber that it innervates the act of myosin and actin sliding past each other in order to produce a contraction the release of calcium from the sarcoplasmic reticulum as a result of the action potential traveling down the t-tubule the release of acetylcholine from the axon terminal of the motor neurons

the point of synapse between a motor neuron and the muscle fiber that it innervates

A single contraction-relaxation cycle in a skeletal muscle fiber is known as a(n)

twitch

An area of muscle fiber membrane that is in close association with the axon terminal of the motor neuron, contain receptors for acetylcholine

Motor end plate

What is the role of the sarcoplasmic reticulum in muscular contraction? How can smooth muscle contract when it has so little sarcoplasmic reticulum?

Stores and releases Ca2+ on command. Smooth muscle uses Ca2+ from the ECF.

What happens when calcium binds troponin? Troponin changes shape and covers the SR calcium channel, a form of negative feedback. Troponin binds to actin and causes sarcomere shortening. Troponin releases myosin and activates actin. Tropomyosin is pulled away from the actin active site.

Tropomyosin is pulled away from the actin active site.

List the letters used to label the elements of a sarcomere. Which band has a Z disk in the middle? Which is the darkest band? Why? Which element forms the boundaries of a sarcomere? Name the line that divides the A band in half. What is the function of this line?

Z disk—ends of a sarcomere. I band—Z disk in the middle. A band (thick filaments)—darkest; H zone—lighter region of A band. M line divides A band in half; thick filaments link to each other.

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 the acetylcholine receptor and prevents it from opening (curare) a drug that binds to and inactivates acetylcholinesterase (neostigmine) a drug that blocks the release of acetylcholine (botulinum toxin) a drug that prevents acetylcholine from being loaded into synaptic vesicles (vesamicol)

a drug that binds to and inactivates acetylcholinesterase (neostigmine)

The nervous system avoids muscle fatigue during submaximal contraction by asynchronous recruitment. always recruiting fast-twitch motor units. synchronous recruitment. always recruiting slow-twitch motor units.

asynchronous recruitment.

Which two muscle types are striated?

cardiac and skeletal muscle

The binding of acetylcholine to its receptor at the neuromuscular junction causes the opening of a Na+ channel. channel for both Na+ and K+. ryanodine receptor. calcium channel.

channel for both Na+ and K+.

Arrange the following skeletal muscle components in order, from outermost to innermost: sarcolemma, connective tissue sheath, thick and thin filaments, myofibrils.

connective tissue, sarcolemma, myofibrils, thick and thin filaments

As ATP binds to the myosin head at the beginning of a muscle contraction cycle, the myosin head immediately tightens its bond to actin. swivels. initiates binding with actin. detaches from actin.

detaches from actin.

Where in the cross bridge cycle does ATP hydrolysis occur? during the power stroke during the removal of calcium from troponin during the cocking of the myosin head during the movement of tropomyosin to expose the active sites on actin

during the cocking of the myosin head

Which fibers generate more force? fast-twitch fibers slow-twitch fibers

fast-twitch fibers

Motor units that control skeletal muscles involved with fine motor movements (eye muscles or the hands) have ________ muscle fibers than motor units that control more gross movements (gastrocnemius muscle of the lower leg). fewer more the same number of

fewer

Within a single fiber, the tension developed during a twitch depends upon the length of the thick filament. amplitude of the action potential. duration of the stimulus. length of the sarcomeres prior to contraction.

length of the sarcomeres prior to contraction.

Which of the following is a characteristic of slow-twitch oxidative skeletal muscle fibers? long contraction duration only long contraction duration and high capillary density high capillary density only few mitochondria only long contraction duration, few mitochondria, and high capillary density

long contraction duration and high capillary density

What, specifically, is a cross bridge? ATP binding to the myosin head calcium binding to troponin tropomyosin covering the active sites on actin myosin binding to actin

myosin binding to actin

The function of transverse tubules is to rapidly conduct action potentials to the interior of the muscle fiber. ensure a supply of glycogen throughout the muscle sarcoplasm. store Ca2+ ions inside the muscle fiber. conduct ATP molecules out of the mitochondria throughout the sarcoplasm.

rapidly conduct action potentials to the interior of the muscle fiber.

The molecular event that occurs immediately after the power stroke is the release of actin from the myosin head. binding of tropomyosin to the myosin. binding of actin to the myosin. release of ADP from the myosin. hydrolysis of ATP by the myosin head.

release of ADP from the myosin.

The factor(s) that determine the duration of a twitch in various types of fibers is the speed of the hydrolysis of ATP only. removal of Ca2+ ions from the sarcoplasm only. movement of Na+ across their membranes only. removal of Ca2+ ions from the sarcoplasm and hydrolysis of ATP. removal of Ca2+ ions from the sarcoplasm and hydrolysis of ATP and movement of Na+ across their membranes

removal of Ca2+ ions from the sarcoplasm only.

What immediately follows hydrolysis of ATP during muscle contraction? the power stroke myosin binding to actin rotation of the myosin head to the cocked position myosin release of actin

rotation of the myosin head to the cocked position

The modified endoplasmic reticulum of skeletal muscle is called the . Its role is to sequester ions.

sarcoplasmic reticulum; Ca2+ ions

Which type of muscle tissue is controlled only by somatic motor neurons?

skeletal muscle

The three types of muscle tissue found in the human body are , , and . Which type is attached to the bones, enabling it to control body movement?

smooth, cardiac, skeletal. Skeletal are attached to bones.

When a skeletal muscle cell contracts and the muscle shortens, the actin molecule swivels during the power stroke. the actin ATPase allows the actin molecule to swivel. some myosin heads are forming crossbridges as others are releasing them. the position of an actin molecule relative to a myosin molecule does not change. myosin heads generate a single power stroke.

some myosin heads are forming crossbridges as others are releasing them.

Acetylcholine binds to its receptor in the sarcolemma and triggers __________. the opening of ligand-gated anion channels the opening of voltage-gated calcium channels the opening of ligand-gated cation channels the opening of calcium-release channels

the opening of ligand-gated cation channels

Compared to fast-twitch glycolytic fibers (type IIb), slow-oxidative muscle fibers (type I) are characterized by a smaller diameter, less stored glycogen, and ________. development of tension 2-3 times quicker a greater concentration of glycolytic enzymes faster reuptake of Ca2+ into the SR the presence of myoglobin

the presence of myoglobin

A flexor is a skeletal muscle whose shortening moves attached bones medially away from one another. laterally. towards one another.

towards one another.

Which of the following statement(s) is (are) true about skeletal muscles? They constitute about 60% of a person's total body weight. They position and move the skeleton. The insertion of the muscle is more distal or mobile than the origin. They are often paired into antagonistic muscle groups called flexors and extensors.

(a) false, (b) true, (c) true, (d) true

Put these events in the correct chronological sequence: 1. End-plate potentials trigger action potentials. 2. Transverse tubules convey potentials into the interior of the cell. 3. Acetylcholine binds to receptors on the motor end plate. 4. Ca2+ is released from the sarcoplasmic reticulum. 1, 2, 3, 4 4, 3, 2, 1 3, 1, 2, 4 3, 1, 4, 2 2, 1, 4, 3

3, 1, 2, 4

During contraction, the band remains a constant length. This band is composed primarily of molecules. Which components of the sarcomere approach each other during contraction?

A band; myosin. Z disks approach each other.

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? View Available Hint(s) ATP bound to a myosin head Troponin bound to tropomyosin Calcium bound to troponin A myosin head bound to actin

A myosin head bound to actin

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

A series of proteins that control calcium release.

hich event causes cross bridge detachment? release of ADP and inorganic phosphate from the myosin head nervous input ends release of calcium from troponin ATP binding to the myosin head

ATP binding to the myosin head

List the steps of skeletal muscle contraction that require ATP.

ATP binding—myosin dissociates from actin. ATP hydrolysis—myosin head swings and binds to a new actin. Release of Pi initiates the power stroke.

After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment? ADP and inorganic phosphate are bound to the myosin head. Calcium ions bind to troponin. ATP binds to the myosin head. Acetylcholine binds to receptors in the junctional folds of the sarcolemma.

ATP binds to the myosin head.

In order to contract and relax, muscle cells require energy in the form of ________. The immediate backup energy source is ________ that requires the enzyme ________. GTP, glucose, ATPase ATP, glucose, ATPase glucose, phosphocreatine, creatine kinase ATP, phosphocreatine, creatine kinase

ATP, phosphocreatine, creatine kinase

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 transported into the postsynaptic neuron by receptor-mediated endocytosis. Acetylcholine diffuses away from the cleft. Acetylcholine is transported back into the axon terminal by a reuptake mechanism. Acetylcholine is degraded by acetylcholinesterase.

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 and moves across the synaptic cleft bound to a transport protein. When the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal. Acetylcholine binds to its receptor in the junctional folds of the sarcolemma. Its receptor is linked to a G protein. Acetylcholine is released by axon terminals of the motor neuron.

Acetylcholine is released by axon terminals of the motor neuron.

Correct sequential order

Acetylcholine is released from motor neuron Action potential travels down the T-tubule Ca2+ is released from the sarcoplasmic reticulum Myosin generates the power stroke Ca2+-ATPase channels actively pump Ca2+ back into the sarcoplasmic reticulum Ca2+ unbinds from troponin Actin filament slip back to rest position

Which of the following best describes the contraction phase of the excitation-contraction coupling reaction? Actin pushes on myosin to shorten its length, thereby shortening the muscle. Actin and myosin filaments slide past each other to shorten the sarcomere, bringing Z disks closer together. An action potential travels down the t-tubule in order to release Ca2+ from the sarcoplasmic reticulum. An action potential travels down the axon of a motor neuron in order to release acetylcholine onto the motor end plate.

Actin and myosin filaments slide past each other to shorten the sarcomere, bringing Z disks closer together.

List six proteins that make up the myofibrils. Which protein creates the power stroke for contraction?

Actin, myosin, troponin, tropomyosin, titin, and nebulin. Myosin produces the power stroke.

A change in membrane voltage that travels down the T-tubule to cause opening of Ca2+ channels

Action Potential

How does an action potential in a muscle fiber trigger a Ca2+

Action potential activates DHP receptors that open SR Ca2+ channels.

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. The power stroke cocks the myosin head. when ADP is released from 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.

Skeletal muscle moves a bone at a joint, such as flexing the forearm at the elbow. How is the opposite action produced (i.e., extension at the elbow)? The muscle that produces flexion relaxes to produce extension. An antagonistic muscle produces the opposite movement. The muscle that produces flexion extends to produce extension. An antagonistic neuron secretes a different neurotransmitter onto the same muscle to produce the opposite skeletal movement.

An antagonistic muscle produces the opposite movement.

An area that contains many synaptic vesicles filled with acetylcholine

Axon terminal of motor neuron

Match from the following list. A. A band B. I band C. Z disk D. H zone E. M line The structures that serve as the attachment site for the thin filaments and mark the boundaries for one sarcomere.

C. Z disk

Which of the following is the most direct cause of muscle relaxation? Choose the best answer. The delivery of action potentials from the motor neuron stops. The cytoplasm of the muscle fiber exhausts all of its available ATP. The voltage created along the t-tubule fades and the muscle fiber membrane returns to a resting potential value. Ca2+ unbinds from troponin, which results in tropomyosin re-covering myosin binding sites.

Ca2+ unbinds from troponin, which results in tropomyosin re-covering myosin binding sites.

What is the role of calcium in the cross bridge cycle? Calcium binds to troponin, altering its shape. Calcium binds to active sites on actin, forming the cross bridge. Calcium binds to myosin, causing the myosin head to release from the actin myofilament. Calcium binds to troponin, exposing the active site on troponin.

Calcium binds to troponin, altering its shape.

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 mitochondria. Calcium ions are stored in the transverse tubules. Calcium ions are stored in the sarcoplasmic reticulum. Calcium ions are stored in the nuclei.

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. Calcium release channels open in the sarcoplasmic reticulum, and calcium levels rise in the sarcoplasm. Sodium ions bind to troponin and change its shape. Calcium ions bind to tropomyosin and change its shape.

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? Sodium ions. Action potentials. Acetylcholine. Calcium ions.

Calcium ions.

Which of the following best summarizes the events of excitation-contraction coupling? Choose the best answer. Muscle action potentials initiate calcium signals that activate a contraction-relaxation cycle. Cross-bridges release and the muscle relaxes. An acetylcholine signal from the motor neuron is converted into an electrical signal in the muscle fiber. The actin filament slides towards the sarcomere and the muscle contracts.

Muscle action potentials initiate calcium signals that activate a contraction-relaxation cycle.

Which loss of function would occur if you introduced a chemical that functioned as an inhibitor of the ryanodine receptor channel? Choose the best answer. Ca2+ ions would not be actively pumped back into the sarcoplasmic reticulum Myosin would not be able to bind to actin in order to cause shortening of the sarcomere The axon terminal of the motor neuron would not release acetylcholine Na+ ions would not be able to flow into the muscle cell in order to depolarize it

Myosin would not be able to bind to actin in order to cause shortening of the sarcomere

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. Cation channels open and sodium ions enter the axon terminal while potassium ions exit the axon terminal. Acetylcholine is released into the cleft by active transporters in the plasma membrane of the axon terminal. Acetylcholine binds to its receptor.

Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine.

Explain how you vary the strength and effort made by your muscles in picking up a pencil versus picking up a full gallon container of milk.

The body uses different types of motor units and recruits different numbers of motor units. Small movements use motor units with fewer muscle fibers; gross movements use motor units with more fibers.

What is the motor end plate, and what kinds of receptors are found there? Explain how neurotransmitter binding to these receptors creates an action potential.

The region of a muscle fiber where the synapse occurs. Contains ACh receptors. Influx of Na+ through ACh receptor-channels depolarizes muscle.

What structure is the functional unit of contraction in a skeletal muscle fiber? The sarcomere The triad The cross bridge The junctional folds of the sarcolemma

The sarcomere

During contraction, what prevents actin myofilaments from sliding backward when a myosin head releases? Calcium blocks the active sites on actin. The cross bridge remains in place, preventing the actin myofilament from sliding. There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching. The actin myofilament can only move in one direction relative to the myosin filament.

There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching.

Which of the following occurs when a myofibril contracts? Thin filaments contract, but thick filaments slide. Thin and thick filaments contract as they slide past each other. Thick filaments contract, but thin filaments slide. Thin and thick filaments slide past each other, but do not change in length.

Thin and thick filaments slide past each other, but do not change in length.

How does troponin facilitate cross bridge formation? Troponin gathers excess calcium that might otherwise block actin's progress. Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin. Troponin moves away from the active sites on actin, permitting cross bridge formation. Troponin hydrolyzes ATP, which provides the energy necessary for cross bridges to form.

Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin.