MasteringA&P CH 12 Muscles

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

voltage-gated calcium channels

The molecular event that occurs immediately after the power stroke is the

release of ADP from the myosin.

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 ion signals the beginning of skeletal muscle contraction?

Ca2+

________ is the backup energy molecule that can be rapidly converted to ATP in active skeletal muscle.

Phosphocreatine

Put these myofibers in the order they would be recruited as the body moves from a weak to a strong stimulus: 1. glycolytic fast-twitch fibers 2. fatigue-resistant oxidative fast-twitch fibers 3. fatigue-resistant slow-twitch fibers

3,2,1

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.

Which of the following best describes the contraction phase of the excitation-contraction coupling reaction?

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

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 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.

Brian decided to enter a 5-mile race designed to raise money for a class trip. He did not do any preconditioning, nor has he ever jogged before. The morning after the race, Brian had so much pain in both his legs that he was unable to go to school. What caused this pain, and why?

Brian has shin splints. This is a condition in which the anterior tibialis muscle is irritated, inflamed, and swollen. The circulation is impaired by the tight fascial wrappings, causing pain.

The contraction cycle is triggered by the rise in ________ released from the sarcoplasmic reticulum (SR).

Ca2+

The relaxation of skeletal muscle relies on the activity of the ________, which decreases cytoplasmic calcium concentration.

Ca2+ ATPase

How does the process of smooth muscle contraction differ from contraction in skeletal muscle?

Ca2+ comes from the extracellular fluid as well as the SR.

Which of the following is the most direct cause of muscle relaxation?

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

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

Calcium binds to troponin, altering its shape.

When comparing complete tetanus with unfused (incomplete) tetanus, which is true?

Complete tetanus involves development of maximum tension.

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.

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

load on the fiber

What is it called when myosin crossbridges attach to actin filaments and pull them toward the middle of the sarcomere?

power stroke

During heavy exercise, the ATP requirements of active muscle are likely to be met by metabolism of

carbohydrates

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

glycosis (Glucose is broken down in the process called glycolysis. This process takes place in the cytoplasm and does not require oxygen - hence it is called anaerobic respiration. If oxygen is available, the pyruvic acid moves into the mitochondria and glycolysis contributes to aerobic respiration)

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 happens when calcium binds troponin?

Tropomyosin is pulled away from the actin active site.

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.

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

acetylcholine (ACh)

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

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

action potential

The binding of acetylcholine to its receptor at the neuromuscular junction causes the opening of a

channel for both Na+ and K+.

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head

Slow oxidative muscle fibers are best suited for:

endurance activities.

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

Which fibers fatigue sooner?

fast-twitch fibers

Which fibers generate more force?

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

What causes the aortic semilunar valve to close?

greater pressure in the aorta than in the left ventricle

What energizes the power stroke?

hydrolysis of ATP

The force generated by a single muscle fiber can be increased by

increasing frequency of action potentials only.

Put the phases of the cardiac cycle in the correct order, starting after ventricular filling.

isovolumetric contraction, ventricular ejection, isovolumetric relaxation

Immediately following the arrival of the stimulus at a skeletal muscle cell there is a short period called the ________ period during which the events of excitation-contraction coupling occur.

latent

Within a single fiber, the tension developed during a twitch depends upon the

length of the sarcomeres prior to contraction.

Which of the following is a characteristic of slow-twitch oxidative skeletal muscle fibers?

long contraction duration and high capillary density

The speed with which force is developed by skeletal muscles is determined by the

myosin ATPase isoform.

In fast-twitch oxidative-glycolytic fibers, their rapid increases in force rely on the ________ activity where rapid relaxation requires ________ activity.

myosin ATPase, Ca2+-ATPase

What, specifically, is a cross bridge?

myosin binding to actin

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

stress-relaxation response

The inward extensions of the sarcolemma are called _____, and they conduct _____ into the interior of the muscle cell.

t-tubules; the action potential

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

Isovolumetric relaxation and ventricular filling (two phases of the cardiac cycle) take place during __________.

ventricular diastole

Which of the following is correct about the filling of the ventricles?

Most blood flows passively into the ventricles through open AV valves.

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

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

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.

Describe the pressures in the atria and ventricles that would cause the opening of the AV valves.

Pressure in the atria would be greater than the pressure in the ventricles.

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 event causes cross bridge detachment?

ATP binding to the myosin head

z disc, H zone, I band, M line etc

...

SA node, AV node, AV bundle, bundle branches, purkinje fibres on heart and graph: QRS complex, T wave, P wave, Atrial depolarisation, ventricular depolarisation and repolarisation.

...

Put excitation-contraction coupling and muscle relaxation so that they occur in the correct sequential order.

1. Acetylcholine is release from motor neuron. 2. Action potential travels down the T-tubule. 3. Ca2+ is released from the sarcoplasmic reticulum. 4. Myosin generates the power stroke. 5. Ca2+ -ATPase channels actively pump Ca2+ back into the sarcoplasmic reticulum. 6. Ca2+ unbinds from troponin. 7. Actin filament slip back to rest position.

Putting the events of contraction in order

1. An action potential that travels down the T-tubule changes the structural confirmation of the DHP L-type Ca2+ channel. 2. Ryanodine receptor channels open. 3. Ca2+ leaves the sarcoplasmic reticulum. 4. Acitn and myosin bind to one another. 5. Myosin heads utilize energy from ATP hydrolysis to produce the power stroke. 6. Actin filaments slide toward the M line, shortening the sarcomere.

Putting the events at the neuromuscular junction in order or that they occur in the correct sequential order.

1. Somatic motor neuron delivers action potential. 2. Acetylcholine is released from the axon terminal onto the motor endplate. 3. Net Na+ influx through nicotinic receptors. 4. Muscle fiber depolarizes. 5. Action potential travels down the T-tubule of the muscle fiber.

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.

3,1,2,4

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

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.

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

ACh receptor-channel

The heavy chain of myosin contains two important regions: the first acts as ________ to convert energy into movement whereas the second binds to ________.

ATPase, actin

Increased pressure in the ventricles would close what valve(s)?

AV valves only

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.This degradation results in a rapid cessation of the acetylcholine signal and a swift removal from the cleft.

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.

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 would cause muscle relaxation to occur?

Calcium ions pumped into the SR

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.

Cardiac muscle makes most of its ATP via anaerobic mechanisms.

False

Which of the following processes produces 36 ATP?

Krebs cycle and oxidative phosphorylation (36 ATPs are produced for each glucose molecule. This process, which takes place in the mitochondria, is considered aerobic respiration because oxygen is required.)

Which of the following is NOT a correct comparison of cardiac myocytes to other muscle cell types?

Like skeletal muscle, contraction of cardiac muscle is under autonomic nervous control.

Which loss of function would occur if you introduced a chemical that functioned as an inhibitor of the ryanodine receptor channel?

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

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.

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.

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

arrival of an action potential

An area that contains many synaptic vesicles filled with acetylcholine

axon terminal of motor neuron

In order for high force crossbridges to form in contracting skeletal muscle, calcium must

bind to troponin which moves the tropomyosin.

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 type of chemical reaction used to rebuild ADP into ATP?

dehydration synthesis

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

motor end plate

Which of the following structures is/are necessary to initiate the muscle action potential?

muscle fiber, acetylcholine, ACh receptor-channels, motor neuron, motor end plate

Most skeletal muscles move bones. They do this by _____ on the bone, to _____ the joint angle.

pulling; increase or decrease

The function of transverse tubules is to

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

The hydrolysis of ATP causes myosin to immediately

rotate into a position (cocked) to bind to actin.

The heart's pacemaker is the:

sinoatrial node.

The calcium calmodulin system of contraction regulation is found in:

smooth muscle only.

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

the opening of ligand-gated cation channels

Which of the following statements best describes the neuromuscular junction?

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

After death, when metabolism stops, in which step of the contractile cycle must skeletal muscles remain?

the rigor state

When a skeletal muscle generates enough force during contraction to shorten,

the sarcomere and the I band will shorten.

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

the site of calcium regulation differs

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

transverse or T tubules

The calcium-binding protein in muscle contractile units is _____.

troponin

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

white fast twitch fibers (white fast twitch fibers have high glycogen content for a readily available source of glucose for glycolysis. They appear white because of the reduced amount of myoglobin and fewer capillaries surrounding them. Because these fibers have reduced myoglobin and very few capillaries surrounding them, they have very little oxygen available for the Krebs cycle and oxidative phosphorylation. They also have fewer mitochondria (where the Krebs cycle and oxidative phosphorylation would take place).

In comparison to skeletal muscle, smooth muscle cells function over a ________ range of muscle lengths and exhibit a speed of contraction that is ________ than skeletal muscle.

wider, slower