UNIT 3: Muscular System (Mylab and Mastering)

ion entering muscle fiber through open chemically gated ion channels

Na+

It diffuses across the cell membrane resulting in depolarization.

Sodium ions

A protein that changes shape as a direct result of an action potential

Voltage-sensitive tubule protein

bundle of skeletal muscle fibers enclosed by connective tissue called perimysium

fascicle

Status of ATP:

hydrolyzed to ADP and Pi

Rigor mortis occurs because ________.

no ATP is available to release attached actin and myosin molecules

protein embedded in the sarcolemma

ACh receptor

is not bound to myosin during the detachment step.

ADP

Molecule(s) bound to the myosin head at the start of the cross bridge cycle

ADP and Pi

Which event causes cross bridge detachment?

ATP binding to the myosin head (As ATP binds, the myosin head releases from the active site on actin)

A neurotransmitter released at motor end plates by the axon terminals

Acetylcholine

In a neuromuscular junction, synaptic vesicles in the motor neuron contain which neurotransmitter? A. acetylcholine (ACh) B. dopamine C. norepinephrine D. serotonin

Acetylcholine (ACh)

Which of the following best describes the role of acetylcholinesterase molecules at the neuromuscular junction? A. Acetylcholinesterase molecules close once ACh is released, which decreases the sarcolemma's permeability to Na+ and K+. B. Acetylcholinesterase binds ACh, preventing it from binding to chemically gated ion channels. C. Acetylcholinesterase prevents Na+ and K+ from passing through chemically gated ion channels. D. Acetylcholinesterase breaks down ACh, which allows chemically gated ion channels to close.

Acetylcholinesterase breaks down ACh, which allows chemically gated ion channels to close. (The chemical signal at the neuromuscular junction is quite brief. As soon as ACh is released from the axon terminal, acetylcholinesterase molecules in the synaptic cleft begin to break it down. This ensures that ACh receptors are open only for the brief amount of time required to initiate an action potential in the muscle fiber.)

Which of the following is most directly required to initiate the coupling of myosin to actin? A. ATP B. Glucose C. Ca++ D. ACh

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

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin (The release of Ca2+ is triggered by the propagation of an action potential along a skeletal muscle fiber. Ca2+ is released from the sarcoplasmic reticulum and into the sarcoplasm of the muscle fiber. When Ca2+ concentration is high in the sarcoplasm, Ca2+ binds to troponin causing change in its shape. This shape change alters the position of tropomyosin and moves it away from myosin binding sites on actin, thus allowing the myosin head to bind actin and form a cross bridge.)

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

Calcium binds to troponin, altering its shape. (Calcium binding to troponin causes tropomyosin to move away from the active sites on actin.)

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? A. Action potentials B. Acetylcholine C. Sodium ions D. Calcium ions

Calcium ions. (Action potentials propagating down the T-tubule cause a voltage-sensitive protein to change shape. This shape change opens calcium release channels in the sarcoplasmic reticulum, allowing calcium ions to flood the sarcoplasm. This flood of calcium ions is directly responsible for the coupling of excitation to contraction in skeletal muscle fibers.)

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. (Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle cells. It is especially abundant and convoluted in skeletal muscle cells. It functions in the storage, release, and reuptake of calcium ions.)

Which of the following is true about smooth muscle? A. Smooth muscle has well-developed T tubules at the site of invagination. B. Smooth muscle cannot stretch as much as skeletal muscle. C. Smooth muscle, in contrast to skeletal muscle, cannot synthesize or secrete any connective tissue elements. D. Certain smooth muscle cells can actually divide to increase their numbers.

Certain smooth muscle cells can actually divide to increase their numbers.

______ is a missing protein in Duschenne 19s muscular dystrophy.

Dystrophin

Contain abundant amounts of glycogen.

Fast (oxidative or glycolytic), fatigable fibers

Which of the following statements is NOT true regarding ATP production in muscles during periods of prolonged energy use, such as exercise? A.Prolonged activity requires the use of aerobic pathways for ATP regeneration. B.In the absence of oxygen, creatine phosphate can drive aerobic respiration pathways for a few minutes. C.As your blood sugar drops at any time during prolonged activity, glycogen stores in your muscles can be broken down to supply a glucose source. D.In the absence of oxygen, anaerobic pathways provide minimal ATP regeneration for less than a minute.

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 doesnt need an oxygen deficit for creatine phosphate to be activated, it just needs to be short on ATP.)

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. (As the action potential propagates, it changes the shape of T tubule proteins. These proteins are linked to calcium channels in the terminal cisterns of the sarcoplasmic reticulum. When these proteins' calcium channels open, a massive amount of calcium flows into the cytosol.)

Which of the following processes produces the most ATP? A. glycolysis B. Krebs cycle and oxidative phosphorylation C. hydrolysis of creatine phosphate

Krebs cycle and oxidative phosphorylation (around 30 or more ATPs are produced for each glucose molecule. This process, which takes place in the mitochondria, is considered aerobic respiration because oxygen is required.)

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

Motor neuron (The motor neuron transmits action potentials from the brain or spinal cord to muscle fibers by releasing acetylcholine at the neuromuscular junction.)

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

Multiple action potentials would occur in the muscle fiber. (Action potentials will not cease until acetylcholine is removed from the synaptic cleft. Therefore, the constant presence of acetylcholine would cause multiple muscle action potentials and near-constant muscle contraction.)

Determined by alterating motor units of a muscle organ even when the muscle is at rest

Muscle tone

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. (The hydrolysis of ATP is required for the cocking of the myosin head. ATP would still bind to myosin, causing cross bridge detachment, but myosin would be stuck in this step of the cross bridge cycle.)

Only_____ of the myosin heads are detached from actin at any one time while calcium is present present and cycling continues

One-half

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

Release of acetylcholine from the synaptic vesicles (Calcium ions enter the axon terminal when voltage-gated calcium channels open in response to the arrival of an action potential. The presence of calcium causes synaptic vesicles to release acetylcholine into the synaptic cleft.)

A relatively high percentage are found in successful marathon runners.

Slow (oxidative), fatigue-resistant fibers

Abundant in muscles used to maintain posture.

Slow (oxidative), fatigue-resistant fibers

A structure that forms a path from the membrane to the cell interior

T-tubule

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. (The resting membrane potential of all cells is negative (inside compared to outside). Therefore, given the direction of the chemical and electrical gradients, more sodium ions diffuse inward than potassium ions diffuse outward.)

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.)

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. (During contraction, about haif of the myosin heads are altached, preventing the adin myoflament from sliding backwards when any single myosin head detaches. The situation is amalogous to a game of lug-of-mar in tug-of-war, individual hands release after they pull on the rope, but not all hands release at the same time)

Which of the following describes the cells of unitary smooth muscle? A. They exhibit spontaneous action potentials. B. They depend upon recruitment using the autonomic nervous system. C. They are used for vision and hair raising. D. They consist of muscle fibers that are structurally independent of each other.

They exhibit spontaneous action potentials.

______ has an affinity for myosin binding sites in the absence of calcium.

Tropomyosin

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. (For cross bridges to form, tropomyosin must not block the active sites. The position of tropomyosin is controlled by the regulatory protein troponin. This protein-protein interaction couples the binding of calcium (to troponin) to the exposure of active sites.)

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.

Synaptic vesicles at the neuromuscular junction contain __________.

acetylcholine (Acetylcholine (ACh) is the neurotransmitter contained in synaptic vesicles at the neuromuscular junction. ACh allows a motor neuron to stimulate the sarcolemma of a skeletal muscle fiber.)

myofilament stiffened and stabilized by tropomyosin

actin

The sliding filament model of contraction involves ________.

actin and myosin sliding past each other and partially overlapping

Actin status to begin cross bridge formation

actin-binding sites exposed

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

action potential propagating down the T tubule (The action potential propagating down the T tubule causes voltage sensitive tubule proteins to change shape, which opens calcium release channels in the terminal cisternae.)

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

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

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

broken down by acetylcholinesterase (Acetylcholinesterase breaks down acetylcholine in the synaptic cleft. Inhibition of acetylcholinesterase, as some poisons can do, causes repeated muscle action potentials and near-constant muscle contraction.)

The influx of which ion is directly responsible for triggering the release of acetylcholine from a motor neuron?

calcium (When an action potential from a neuron reaches an axon terminal, voltage-gated calcium channels open. The influx of calcium triggers exocytosis of acetylcholine.)

a type of voltage-gated ion channel located on the axon terminal.

calcium channel

What most directly causes synaptic vesicles to release acetylcholine into the synaptic cleft?

calcium entering the axon terminal (Calcium entry causes synaptic vesicles to release acetylcholine into the synaptic cleft. Each synaptic vesicle contains thousands of acetylcholine molecules.)

The sliding of the myofilaments is directly initiated by the availability of which chemical at the myofibril?

calcium ions (Calcium ions bind to the troponin-tropomyosin complex on actin. When calcium binds to troponin, a conformational shape change occurs in the protein complex, which results in the availability of the active sites on actin.)

What is the functional role of the T tubules?

carry action potentials from the sarcolemma to triads

Acetylcholine receptors are best characterized as what type of channel?

chemically gated Na+-K+ channels (Chemically gated ion channels open due to the binding of a neurotransmitter, such as acetylcholine. Chemically gated Na+-K+ channels allow the diffusion of Na+ into the muscle fiber and K+ out of the muscle fiber)

Location of higher concentration of Ca2+ needed for cross bridge formation and cycling

cytoplasm

The strongest muscle contractions are normally achieved by ________.

increasing the stimulation up to the maximal stimulus

The prefix ______ means equal or same.

iso-

When muscle tension develops but the load is not moved.

isometric contraction

When the muscle tension developed overcomes the load and muscle shortening occurs.

isotonic contraction

portion of sarcolemma containing ACh receptors

junctional folds

receive(s) stimulus from the motor neuron.

junctional folds

What, specifically, is a cross bridge?

myosin binding to actin (The attachment of a myosin head from the thick filament to an active site on actin on the thin filament is a cross bridge. As soon as the cross bridge forms, the power stroke occurs, moving the thin filament toward the center of the sarcomere.)

Hypothetically, if a muscle were stretched to the point where thick and thin filaments no longer overlapped, ________.

no muscle tension could be generated

Duchenne muscular dystrophy could theoretically be cured if a technique was developed that would _________. A. strengthen the dystrophin proteins that are present in the patient's muscle fibers B. double the existing number of dystrophin molecules in the patient's muscle fibers C. strengthen the thick and thin filaments in the patient's muscle fibers D. none of the above

none of the above (Duchenne muscular dystrophy is caused by a mutation in the dystrophin gene. Without a normal copy of DNA for transcription, there can never be a normal protein translated. This genetic mutation causes the protein to be either abnormal in structure or completely absent.)

Myosin head energy status during the power stroke

pivots and returns to low energy

What chemical does NOT need to be present in the extracellular fluid for a signal to travel from neuron to muscle cell? A. Acetylcholine B. Potassium fun C. Sodium D. Calcium

potassium (The potassium ion concentration is higher in the cytosol than in the extracellular fluid. Potassium ions are released during stimulation.)

What is the primary function of wave summation?

produce smooth, continuous muscle contraction

Actin behavior during the power stroke:

ratchets toward the M line

Status of the myosin head:

returns to the energized position

Slow oxidative muscle fibers are best suited for________

running a marathon (Slow oxidative muscle fibers are best suited for endurance activities, such as long distance running, cycling, or rowing.)

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

simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme) (Acetylcholinesterase breaks down acetylcholine in the synaptic cleft.)

a type of voltage-gated ion channel located on the sarcolemma of the muscle fiber.

sodium channel

Creatine phosphate functions in the muscle cell by______

storing energy that will be transferred to ADP to resynthesize ATP

the space between the axon terminal and junctional folds.

synaptic cleft

container of many neurotransmitter molecules

synaptic vesicle

The force of a muscle contraction is NOT affected by________ A. the size of the muscle fibers stimulated B. the number of muscle fibers stimulated C. the degree of muscle stretch D. the amount of ATP stored in the muscle cells E. the frequency of the stimulation

the amount of ATP stored in the muscle cells (The force of a muscle contraction is NOT affected by the amount of ATP stored in the muscle cells. Instead of relying on storage of ATP, muscle cells use ATP regenerating pathways, such as glycolysis, to meet the ATP demands of muscle contraction.)

Acetylcholine binds to its receptor in the sarcolemma and triggers_____

the opening of ligand-gated cation channels (These channels permit sodium ions to diffuse inward and potassium ions to diffuse outward.)

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.)

The prefix ______ means turning, change, or affinity for.

tropo-

block(s) binding sites on actin.

tropomyosin

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.)

Rigor mortis refers to the changes in muscle tissue that occur after death. The muscles become very stiff or rigid and are frozen in a contracted state. Rigor mortis usually sets in within four hours, first in the face and generally smaller muscles. The body reaches maximum stiffness in twelve to forty-eight hours. However, this time may vary due to environmental conditions, with cooler conditions inhibiting rigor mortis. Which of the following images would depict the state of the cross bridge cycle during rigor mortis?

(After death, the integrity of the sarcoplasmic reticulum breaks down, releasing calcium into the sarcoplasm. This allows for the uncovering of myosin binding sites on actin. Cross bridge formation will result. However, without adequate ATP being produced through aerobic cellular respiration, cross bridge detachment cannot occur, and the muscles become stiff and locked in a contracted state.)

Conclusion/Synthesis Click and drag the description of the part of the excitation-contraction coupling to the correct order of occurrence from left to right. Do not overlap any steps.

(Coupling begins with an action potential after the arrival of ACh and graded potentials, and it ends before cross bridge cycling.)

Signals flowing through the neuromuscular junction pass through several structures in a single direction. Arrange the structures below in the order in which signals travel through them.

(Overall, an electrical signal in the motor neuron's axon leads to an electrical signal in the muscle fiber's sarcolemma. However, the electrical signal is briefly converted to a chemical signal as the information crosses the synaptic cleft.)

Sequence of Events During Excitation-Contraction Coupling The conversion of an action potential (AP) generated by a motor neuron to contraction of skeletal muscle fiber is called excitation-contraction coupling. This activity will test your understanding of the sequence of events that occur during excitation-contraction coupling.

(Recall from the video that a single motor neuron can conduct action potentials to several skeletal muscle fibers within a muscle. Excitation-contraction coupling converts these action potentials in a muscle fiber to a contraction. As muscle fibers contract, the entire muscle shortens and pulls the tendons on either end of the muscle, producing muscle tension.)

The site where a motor neuron excites a skeletal muse fiber is called the neuromuscular junction. This activity will test your understanding of the sequence of events that occur at the neuromuscular junction. Using the images for help, place in the correct order of occurrence from left to right the events that occur at the neuromuscular junction after the action potential reaches the axon terminal.

(The neuromuscular junction is a chemical synapse. An electrical signal (known as an action potential) from the motor neuron is converted into a chemical signal (ACh release) The skeletal muscle fiber responds to the chemical signal (ACh) by depolarizing and initiating an action potential. This action potential is propagated along the sarcolemma of the muscle fiber, which triggers sliding of the myofilaments for muscle contraction.)

The sarcomere shortens when the myosin heads of the thick filaments, in a cocked position, form cross bridges with the actin molecules in thin filaments. This activity will test your understanding of the steps that occur in one complete cross bridge cycle. Place the steps that occur during a single cross bridge cycle in the correct order from left to right.

(The thin filaments are pulled toward the center of the sarcomere by the myosin heads of the thick filament. This shortens the sarcomeres in the muscle fiber and causes the whole skeletal muscle to contract. The cross bridge cycle ends when Ca2+ are actively transported back into the sarcoplasmic reticulum. The cycle repeats as long as the binding sites on actin remain exposed, and both Ca2+ and ATP are available.)

The smallest contractile unit within skeletal muscle would correspond to the distance between which two points in the figure? A. 1 and 7 B. 3 and 5 C. 1 and 3 D. 2 and 6

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

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).)

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 (As soon as the activated myosin head forms a cross bridge with actin, the power stroke begins.)

Which of the following choices best summarizes excitation-contraction coupling? A series of events in which an action potential triggers the release of chemical messengers B. A series of events in which actin binds myosin to enact contraction C. A series of events in which calcium gradients are used to convey a signal to the muscle fiber to enact contraction D. A series of events in which an electrical stimulus is conveyed to a muscle fiber to enact contraction

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. (When action potentials propagate along T-tubules, a voltage-sensitive protein changes shape and triggers a different protein to open it's channels, resulting in the release of calcium from the terminal cisternae.)

Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction? A. ACh diffuses away from the synaptic cleft. B. ACh binds to ACh receptors. C. ACh is taken up by the axon terminal via endocytosis. D. ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE).

A. ACh diffuses away from the synaptic cleft. D. ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE). (The chemical signal (ACh) at the neuromuscular junction is very short Soon after ACh is released from the axon terminal, it is rapidly degraded by AChE or diffuses out of the synaptic deft. These two mechanisms ensure that just enough ACh binds to its receptor to intlate an action potential and that the muscle fiber's response is quickly terminated)

_____ binds to ACh receptors, causing them to open chemically gated ion channels

ACh

Which of the following best describes how ACh changes the ion permeability of the sarcolemma? A. ACh binds to receptors in the junctional folds. B. ACh entry into the muscle fiber causes Ca2+ to be released by the sarcoplasmic reticulum. C. ACh entry into the axon terminal causes neurotransmitter release. D. ACh activates acetylcholinesterase.

ACh binds to receptors in the junctional folds. (The junctional folds of the muscle fiber's sarcolemma contain ACh receptors. These receptors are chemically gated ion channels. When ACh binds to these receptors, they open, allowing ions to diffuse across the sarcolemma. This represents an increase in the sarcolemma's ion permeability.)

The molecules released just before power stroke

ADP and Pi

The bond between the actin and myosin head weakens as a result of______

ATP

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 (The binding of ATP to the myosin head weakens the bond between myosin and actin, forcing the myosin head to detach. ATP also provides the energy for the next power stroke)

Which of the following occurs during cross bridge cycling? A. Actin binds to myosin, pulling myosin heads closer to the M line of the sarcomere. B. ATP is hydrolyzed in order to put the myosin head in a high-energy position. C. ATP is hydrolyzed to activate the binding of myosin to active sites on actin. D. The troponin-tropomyosin complex blocks the active sites on actin.

ATP is hydrolyzed in order to put the myosin head in a high-energy position. (ATP must be available to ready the myosin molecule in its "cocked" position.)

neurotransmitter at neuromuscular junction

Acetylcholine (ACh)

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

Acetylcholine is degraded by acetylcholinesterase. (Acetylcholinesterase is an enzyme that degrades acetylcholine. 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? A. When the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal. B. Acetylcholine is released by axon terminals of the motor neuron. C. Acetylcholine is released and moves across the synaptic cleft bound to a transport protein D. 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 into the synaptic cleft via exocytosis.)

Breaks down ACh into its building blocks, rendering it inactive.

Acetylcholinesterase

What is the fate of acetylcholine (Ach) after it binds to Ach receptors?

Ach is broken down by acetylcholinesterase. (The effects of acetylcholine must be terminated to prevent sustained muscle cell contraction.)

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. (In addition to breaking the cross bridge, ATP provides energy to cock the heads back after the previous power stroke.)

Consider how the action potential that initiates contraction is delivered to the muscle cell. 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? A. An action potential in the motor neuron directly triggers an action potential at the sarcolemma. B. ACh binds at a receptor in the motor neuron, which initiates graded potentials. C. An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh at the membrane receptor initiates a direct action potential. D. 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.

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. (An action potential traveling down a motor neuron arrives at the axon terminal and causes exocytosis of the neurotransmitter ACh into the synaptic cleft. ACh diffuses into the synaptic cleft, binds to the receptor proteins on the junctional folds of the muscle sarcolemma (motor end plate), and initiates graded potentials. These graded potentials sum to an action potential, thus initiating excitement of the muscle fiber)

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

An action potential traveling along the t tubule (in response to an action potential traveling along the t tubule, voltage-sensitive proteins change shape, opening calcium channels in the adjacent terminal cisterns of the sarcoplasmic reticulum.)

What action is directly responsible for the generation of an action potential at the sarcolemma?

An end plate potential sufficient to reach threshold must be produced. (If an end plate potential occurs, voltage-gated sodium channels open, propagating an action potential that will be conducted along the sarcolemma.)

Which of the following interactions must occur first so that the others can take place? A. D is shifted from actin's binding sites. B. C binds to actin. C. B binds to troponin. D. C releases ADP and P.

B binds to troponin. (Binding of calcium (B) to troponin removes the blocking action of tropomyosin along the thin myofilament. This allows myosin to bind to actin and form the cross bridge illustrated in this figure.)

Which statement accurately describes the event indicated by B? A. Diffusion of Ca?* into the muscle fiber triggers the diffusion of acetylcholine out of the muscle fiber. B. Binding of acetylcholine to a receptor triggers the opening of an ion channel. C. The binding of acetylcholine directly causes the formation of a wave of depolarization. D. Diffusion of acetylcholine into the muscle fiber triggers the opening of an ion channel.

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

The binding of the neurotransmitter to receptors on the motor end plate causes which of the following to occur? A. Binding causes potassium voltage-gated channels to open in the motor end plate (junctional folds of the sarcolemma) and potassium enters the cell. B. 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. C. Binding causes voltage-gated sodium channels to open in the motor end plate (junctional folds of the sarcolemma) and sodium enters the cell. D. Binding causes chemically gated potassium channels to open in the motor end plate junctional folds of the sarcolemma) and potassium enters the cell.

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. (sodium enters the cell and causes depolarization. A small amount of potassium also leaves the motor end plate (unctional folds of the sarcolemma).)

ion triggering exocytosis

Ca2+

Which selection correctly describes the role of calcium in coupling? A. Calcium binds to actin, which triggers it to bind to myosin. B. Calcium binds to actin, which triggers troponin to fall off and expose myosin-binding sites. C. Calcium binds to tropomyosin, which moves tropomyosin and exposes the myosin-binding sites on actin. D. Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin.

Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin. (The thin filament in a sarcomere is composed of actin, troponin, and tropomyosin. Troponin and tropomyosin are attached to one another, both overlaying actin. When a muscle is relaxed, tropomyosin blocks actin's myosin-binding sites. Calcium binds to troponin, initiating a shape change that removes the blocking action of tropomyosin. This exposes the myosin-binding sites on actin to the myosin heads for cross bridging.)

Activates synaptic vesicles in axon terminals to fuse with plasma membrane of axon terminal.

Calcium ions

The final chemical messenger and "trigger" for muscle contraction. It binds to troponin.

Calcium ions

Which of the following is responsible for preventing a muscle from contracting? A. Calcium ions are released from the terminal cisterns during depolarization of the T tubule. B. ATP binds to the myosin head to cause cross bridge detachment. C. Calcium ions are removed from the sarcoplasm by active transport. D. Sodium ions enter into the sarcoplasm through voltage-gated channels

Calcium ions are removed from the sarcoplasm by active transport. (Removal of calcium ions lowers the calcium ion concentration below the threshold for muscle contraction.)

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 (Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle fibers. The sarcoplasmic reticulum is very elaborate in skeletal muscle fibers, allowing for significant storage of calcium ions.)

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

Calcium ions bind to troponin and change its shape. (The shape change caused by the binding of calcium to troponin shifts tropomyosin away from the myosin binding sites on actin)

How do calcium ions initiate contraction in skeletal muscle fibers?

Calcium ions bind to troponin, changing troponin's shape. (The shape change in troponin, caused by calcium binding, causes a shift in the position of tropomyosin along the thin filament. This exposes active sites to myosin and allows cross bridges to form. The cycling of cross bridges is what creates tension during contraction.)

The swollen chamber of an organelle that lies on either side of a T tubule

Cistern

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. (The sarcoplasmic reticulum contains Ca2+-ATPases that actively transport Ca2+ into the SR. Without Ca2+, troponin returns to its resting shape, and tropomyosin glides over and covers the myosin binding sites on actin.)

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 (When ATP is added to the myosin, myosin detaches from actin.)

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 rigors? A. Cross bridge formation cannot occur. ATP, which is produced only during life, must to be bound to the myosin head to permit cross bridging B. The cross bridge is locked up because ATP directly attaches myosin to actin. C. The cross bridge is locked up because the power stroke requires ATP, which is produced only during life. D. Cross bridge detachment cannot occur. Detachment requires ATP, which is produced only during life.

Cross bridge detachment cannot occur. Detachment requires ATP, which is produced only during life. (After ATP attaches to the myosin head, the bond between actin and myosin is weakened and the cross bridge breaks.)

The prefix ______ means abnormal, diseased.

Dys-

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

Eccentric (During isotonic, eccentric contractions, musce 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).

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. (These action potentials set off a series of events that lead to a contraction.)

Which of the following is NOT a role of ATP in muscle contraction? A. Transporting calcium ions into the sarcoplasmic reticulum B. Exposing myosin binding sites on actin C. Detaching the cross bridge from actin

Exposing myosin binding sites on actin (Exposure of myosin binding sites does not utilize ATP. During excitation-contraction coupling, calcium ions released from the terminal cisterns of the sarcoplasmic reticulum flood into the sarcoplasm. Calcium ions bind to troponin, which changes its conformation to slide tropomyosin away from the myosin binding sites. Therefore, calcium ions, not ATP, are responsible for exposure of the myosin binding sites on actin.)

Have very fast-acting myosin ATPases and depend upon anaerobic metabolism during contraction.

Fast (oxidative or glycolytic), fatigable fibers

During an exercise where muscles do not shorten,______ contractions have occurred.

Isometric

If the cell could no longer produce ATP, what would be the effect on the sarcoplasmic reticulum?

It would be unable to concentrate Ca++ in the sarcoplasmic reticulum. (To concentrate Ca++ in the sarcoplasmic reticulum, active transport must occur. With no ATP, there would be no active transport.)

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. (Calcium ions are the chemical link between the excitation signal or action potential and contraction. Once released from the terminal cisterns, calcium ions bind to troponin allowing the movement of tropomyosin, and cross bridge formation. If GTA binds to the caldium ions once they are released, then calcium ions are not avalable for binding to troponin. Therefore, myosin binding sites will not be exposed, which prevents muscle contraction.)

_____ diffuses out of the muscle fiber through open chemically gated ion channels.

K+

Which of the following is true concerning the anatomy of a skeletal muscle fiber? A. The sarcolemma is the muscle fiber's cytoplasm. B. Myofibrils contain thick and thin filaments. C. T tubules are extensions of the sarcoplasmic reticulum. D. A triad consists of a T tubule and the nearby sarcolemma.

Myofibrils contain thick and thin filaments. (Each skeletal muscle fiber contains many myofibrils. And each of these myofibrils contains many thick and thin filaments (myofilaments). These myofilaments are arranged as numerous sarcomeres within the myofibril.)

The two proteins directly involved in muscle contraction are broadly called ______

Myofilament

Which selection best describes the initial event in contraction? A. Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges. B. Calcium binds actin to myosin to begin the cross bridge cycle. C. The myofilaments slide closer together. D. Tropomyosin binds to myosin heads and actin bridges with tropomyosin.

Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges. (Contraction begins when myosin binds to actin and forms cross bridges.)

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? A. The fiber would twitch uncontrollably due to excessive calcium bound to troponin B. Calcium would be transported to the sarcoplasmic reticulum therefore contractions would cease C. Myosin would be able to bind to the exposed binding sites on thin filaments but it would not be able to detach. D. No change would occur in a muscle that was relaxed to begin with

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

The end plate potential is primarily, and most directly, caused by the movement of

Na+

The end plate potential is primarily, and most directly, caused by the movement of _____

Na+

_____ diffuses into the muscle fiber through open chemically gated ion channels.

Na+

Where the axon of a motor neuron connects with the muscle fibers.

Neuromuscular junction

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? A. One subunit attaches troponin to actin. B. One subunit binds tropomyosin and helps position it on actin. C. One subunit binds to calcium ions. D. One subunit binds to potassium ions.

One subunit binds to potassium ions.

The "rest and recovery" period, where the muscle restores depleted reserves, includes all of the following processes EXCEPT __________. A. Glycogen is synthesized from glucose molecules B. Pyruvic acid is converted back to lactic acid. C. ATP is used to rephosphorylate creatine into creatine phosphate D. Oxygen rebinds to myoglobin

Pyruvic acid is converted back to lactic acid. (this is NOT a part of the "rest and recovery" period. When oxygen is available, lactic acid is converted back to pyruvic acid (not vice versa) that then enters the Krebs cycle. Lactic acid is the end product of the anaerobic pathway.)

Depends on oxygen delivery and aerobic mechanisms.

Slow (oxidative), fatigue-resistant fibers

Red fibers, the smallest of the fiber types.

Slow (oxidative), fatigue-resistant fibers

Which muscle fiber type is best suited for endurance activities, such as long-distance jogging?

Slow oxidative fibers (Slow oxidative fibers are best suited for endurance activities because they produce ATP aerobically and are fatigue-resistant.)

Calcium entry into the axon terminal triggers which of the following events? A. Acetylcholine binds to its receptor. B. Acetylcholine is released into the cleft by active transporters in the plasma membrane of the axon terminal. C. Cation channels open and sodium ions enter the axon terminal while potassium ions exit the axon terminal. D. Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine.

Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine. (When synaptic vesicles fuse to the plasma membrane, acetylcholine is released via exocytosis)

conduct(s) action potentials throughout the interior of the muscle fiber.

T tubule

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 covers active sites in relaxed muscle. When tropomy sin is displaced, the active sites are exposed for cross bridge formation.)

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

The energy comes from the hydrolysis of ATP. (Myosin is a large, complex protein with a binding site for actin. It also contains an ATPase. The energy released during the hydrolysis of ATP activates the myosin head)

One brief stimulus of a skeletal muscle may result in a single isolated contractile event, which is called a twitch. In a living organism, one twitch can generate max tension in a whole muscle. Are these statements true or false?

The first statement is true; the second statement is false. (A single stimulation may result in a single muscle twitch. However, a single twitch would not generate any meaningful tension in a skeletal muscle.)

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? A. Since the exercise was mostly aerobic exercise, the runners' bodies have not yet realized the run is over. B. The runners' fast oxidative muscles are so slow to utilize oxygen, they only begin aerobic respiration by the time the run has finished. C. The runners' use of stored oxygen, glucose, and creatine phosphate is being replenished and this requires a prolonged increase of oxygen intake. D. Slow oxidative fibers are recruited last and have only started to work at completion of the run.

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

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

The sarcomere (A sarcomere is a regular arrangement of thin and thick myofilaments that extends from one Z disc to the next. A myofibril consists of a series of sarcomeres)

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. (Although both types of striated muscle contain troponin, the subunits found in cardiac muscle are different from those in skeletal muscle. The laboratory test used to diagnose cardiac muscle damage is specific for those subunits.)

Which of the following best describes the role of muscle fiber triads? A. Acetylcholine, T tubules, and actin create a functional triad that allows acetylcholine to penetrate the sarcolemma, bind to actin, and activate a muscle fiber contraction. B. Two axon terminals and the one muscle fiber they activate compose a functional neuromuscular triad, or the functional unit of muscle. C. The triad allows an action potential to activate voltage-sensitive proteins in a T tubule, which in turn opens calcium channels in the paired terminal cisternae, allowing calcium to flood the sarcomere. D. A triad is made up of troponin, tropomyosin, and actin. They function together to control when myosin pulls on actin.

The triad allows an action potential to activate voltage-sensitive proteins in a T tubule, which in turn opens calcium channels in the paired terminal cisternae, allowing calcium to flood the sarcomere. (A triad describes the grouping of two terminal cisternae with a T tubule that ensures an action potential causes the release of calcium within the myofibril.)

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

Transverse or T tubules (T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior.)

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

Tropomyosin (Tropomyosin covers the active site on actin, preventing cross bridge formation.)

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.

Which description is most appropriate for a myofibril? A. a single group of long muscle cells surrounded by a membrane B. a single long protein structure found in the sarcomeres of a muscle cell C. a single long, thin organelle containing many proteins D. a single long cell found within a muscle

a single long, thin organelle containing many proteins (A fibril is a very small, fiber-like structure. Since muscle cells are called muscle fibers, the myofibrils are the smaller, fiber-like organelles within a muscle fiber.)

Muscle tone is ________.

a state of sustained partial contraction

neurotransmitter that stimulates skeletal muscle contraction.

acetylcholine

a type of chemically-gated ion channel located on the junctional folds of the muscle fiber.

acetylcholine receptor

Although all the anatomical parts of muscle work together to give it it's characteristics, which of the following proteins listed below would be most associated with the characteristics of excitability? A. the Na+-K+ pump B. acetylcholine receptors in the motor end plate C. elastic (titin) filaments D. actin of thin filaments

acetylcholine receptors in the motor end plate

enzyme located in the synaptic cleft that breaks down acetylcholine.

acetylcholinesterase

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

acetylcholinesterase breaks apart the ACh

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

actin (the myosin head binds to actin, the major component of thin filaments.)

Status of actin and myosin:

actin and myosin detached

Match the sport to the energy system primarily used to perform the activity: Marathons

aerobic pathway

Match the sport to the energy system primarily used to perform the activity: 25 meter swim

anaerobic pathway

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

arrival of an action potential (an action potential in the T tubule causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum.)

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- Ca++ binds to troponin, d- Troponin removes tropomyosin from G actin, f- Myosin binds to actin, a- Myosin generates a power stroke, e- The sarcomere shortens, c- ATP recharges the myosin head (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.)

What causes the myosin head to disconnect from actin?

binding of ATP (the binding of ATP causes the myosin head to disconnect from actin.)

A muscle fiber represents which organizational level of anatomy?

cellular level (Muscle fibers are simply muscle cells. But muscle cells are unique in many ways, and they are often very long, which is why they are described as fibers.)

In an isotonic contraction, the muscle ________.

changes in length and moves the "load"

Myofilaments represent which organizational level of anatomy?

chemical level (The chemical level of anatomy refers to the atoms and molecules that make up the parts of the body. Myofilaments are protein structures. Proteins are large molecules.)

Which of the following best describes the events of contraction" in "excitation-contraction coupling"? A. release of calcium from the terminal cisternae B. cross bridge formation C. sliding of myofilaments D. propagation of the action potential along the sarcolemma of the muscle fiber

cross bridge formation ("Excitation-contraction coupling" connects muscle fiber excitation to the muscle fiber contraction (cross bridge formation). During contraction, myosin heads form cross bridges many times—with each cross bridge generating a small amount of tension in the muscle fiber.)

In a bedridden patient recovering from a badly fractured femur, disuse atrophy in the thigh muscles is caused by _________.

decreased synthesis of muscle proteins and/or increased breakdown of muscle proteins (Muscle tissue grows and heals in response to stress. Without the stress of exercise and normal daily activities, muscle tissue degenerates.)

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

dehydration synthesis (a water molecule is removed, thus it is called dehydration synthesis. Building ATP from ADP requires a synthetic enzyme plus a source of energy to rebuild the high energy bond.)

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

diffusion of Ca2+ into the axon terminal (A nerve impuise 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.)

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

diffusion of sodium ions through voltage-gated channels (Action potentials are propagated along the sarcolemma by the sequential opening of voltage-gated sodium channels embedded in the membrane. As the initial set of sodium channels open, sodium ions diffuse inward, depolarizing the sarcolemma of the adjacent segment to threshold. This triggers the generation of the action potential at the new segment, and the process continues along the sarcolemma, conducting the action potential toward the t tubules.)

Match the sport to the energy system primarily used to perform the activity: Weight lifting

direct phosphorylation

The energy reserves for which of the ATP regenerating pathways will be depleted first during an extended period of light to moderate exercise?

direct phosphorylation of ADP by creatine phosphate (This rapid ATP-synthesizing pathway only lasts for about 15 seconds.)

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 myofilament)

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head (As ATP is broken down, its energy is used to cock the myosin head in preparation for the next power stroke.)

Although all the anatomical parts of muscle work together to give it its characteristics, which of the following proteins listed below would be most associated with the characteristics of extensibility? A. elastic (titin) filaments B. potassium (K+) leak channels C. thick (myosin) filaments D. acetylcholine receptors in the motor end plate

elastic (titin) filaments

Energy state of myosin for beginning cross bridge formation

energized/cocked myosin head

Which membrane surrounds the others?

epimysium (Epi means "over," "above," or "outer." Epimysium, therefore, is the outermost membrane.)

What cellular event is indicated by A?

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

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

exocytosis (the synaptic vesicles (where the neurotransmitter is stored) merge with the membrane and release the neurotransmitter by 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 (The cell bodies of motor neurons to muscles in the head and neck are located in the brain. The cell bodies of motor neurons to the rest of our muscles are located in the spinal cord.)

Skeletal muscle cells are grouped into bundles called __________.

fascicles (Skeletal muscle cells (or muscle fibers) are grouped into fascicles that resemble bundles of sticks.)

Exhaustion of glycogen storage within a muscle fiber would have the biggest effect on______

fast glycolytic fibers

Addition of more myoglobin to a muscle fiber would have the largest effect on______

fast oxidative and slow oxidative fibers

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

glycolysis

Which of the following processes produces molecules of ATP and has two pyruvic acid molecules as end products? A. glycolysis B. Krebs cycle and oxidative phosphorylation C. hydrolysis of creatine phosphate

glycolysis (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.)

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

Which of the following is the correct order for the phases of a muscle twitch? A. latent, contraction, relaxation B. relaxation, contraction, latent C. contraction, relaxation, latent D. latent, relaxation, contraction

latent, contraction, relaxation

Which of the following is a factor that affects the velocity and duration of muscle contraction? A. muscle length B. load on the fiber C. number of muscle fibers stimulated D. size of the muscle fibers stimulated

load on the fiber

Which of the following factors influence the velocity and duration of muscle contraction? A. length of muscle fibers activated B. frequency of stimulation C. load placed on the muscle D. muscle fiber size

load placed on the muscle (Speed or 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)

Which of the following is the correct sequence of events for muscle contractions? A. neurotransmitter release, motor neuron action potential, muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke B.muscle cell action potential, neurotransmitter release, ATP-driven power stroke, calcium ion release from SR, sliding of myofilaments C. neurotransmitter release, muscle cell action potential, motor neuron action potential, release of calcium ions from SR, sliding of myofilaments, ATP-driven power stroke D. motor neuron action potential, neurotransmitter release, muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke, sliding of myofilaments

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

Which of the following would be recruited later in muscle stimulation when contractile strength increases? A. many small motor units with the ability to stimulate other motor units B. large motor units with small, highly excitable neurons C. motor units with the longest muscle fibers D. motor units with larger, less excitable neurons

motor units with larger, less excitable neurons

How a smooth increase in muscle force is produced.

multiple motor unit summation

Which term best identifies a muscle cell? A. muscle fascicle B. muscle fiber C. sarcomere D. myofibril

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

The prefix ______ means muscle.

myo-

Made up of several sarcomeres

myofibrils

The contractile units of skeletal muscles are ________.

myofibrils

A protein with heads that attach in cross bridging

myosin

myofilament with a knob-like head

myosin

Which of the following best describes the events of "excitation" in "excitation-contraction coupling?" A. cross bridge formation B. release of calcium from the terminal cisternae C. movement of tropomyosin away from the active sites on actin D. propagation of the action potential along the sarcolemma

propagation of the action potential along the sarcolemma ("Excitation-contraction coupling" connects muscle fiber excitation to muscle fiber contraction. The action potential propagating along the sarcolemma represents the "excitation" of the muscle fiber. The ensuing cross bridge formation represents the "contraction" of the muscle fiber.)

When a muscle is unable to respond to stimuli temporarily, it is in which of the following periods? A. latent period B. fatigue period C. relaxation period D.refractory period

refractory period

What causes the power stroke?

release of ADP and Pi (the hydrolysis of ATP provides the energy for the power stroke. Energy is transferred from ATP to the myosin head.)

Which of the following events triggers the subsequent steps of excitation-contraction coupling? A. release of acetylcholine from axon terminals at the neuromuscular junction B. release of calcium ions from the terminal cisterns of the sarcoplasmic reticulum D. propagation of an action potential along the sarcolemma and T tubules D. binding of calcium ions to troponin, which removes the blocking action of tropomyosin

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 prefix ______ means flesh.

sarco-

The cell membrane of a muscle fiber is the ______

sarcolemma

membrane of muscle cell

sarcolemma

membrane surrounding muscle fiber

sarcolemma

ACh receptors are found mainly in the __________.

sarcolemma (The sarcolemma contains ACh receptors. The opening of these ACh receptors depolarizes the muscle fiber, which leads to the generation of a muscle action potential.)

Contractile unit

sarcomere

The __________ shorten(s) during muscle contraction

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

the cytoplasm of a skeletal muscle fiber

sarcoplasm

long, cylindrical cells

skeletal muscle fibers

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

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

sodium (The inward diffusion of sodium ions, through acetylcholine receptors, depolarizes the sarcolemma. Potassium also diffuses through acetylcholine receptors, but less potassium diffuses than sodium, and the outward movement of potassium is not depolarizing.)

Myoglobin ________.

stores oxygen in muscle cells

Unlike skeletal muscle, smooth muscle may spontaneously contract when it is stretched. What feature of smooth muscle allows it to stretch without immediately resulting in a strong contraction?

stress-relaxation response (Stretching of smooth muscle provokes contraction; however, the increased tension persists only briefly, and soon the muscle adapts to its new length and relaxes, while still retaining the ability to contract on demand. The stress-relaxation response of smooth muscle allows a hollow organ to fill or expand slowly to accommodate a greater volume without promoting strong contractions that would expel its contents.)

location of acetylcholinesterase

synaptic cleft

membranous sac located in the axon terminal that contains neurotransmitter.

synaptic vesicle

release(s) calcium ions into the sarcoplasm.

terminal cistern

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

terminal cisternae (The terminal cisternae release calcium, which is the "go" signal for a muscle contraction. The terminal cisternae are the enlarged ends of the sarcoplasmic reticulum.)

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

Continued sustained smooth contraction due to rapid stimulation.

tetanus

When a sarcomere contracts and thin filaments move over thick filaments you would expect to see______

the I bands to appear smaller

What does excess postexercise oxygen consumption represent?

the difference between the amount of oxygen needed for totally aerobic muscle activity and the amount actually used

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)

An action potential is propagated down the sarcolemma as a result of which of the following? A. the closing of potassium channels B. the opening of voltage-gated potassium channels C. the opening of voltage-gated sodium channels D. the closing of voltage-gated sodium channels

the opening of voltage-gated sodium channels (As voltage-gated sodium channels are opened, the influx of positively charged sodium ions locally changes the charge of the sarcolemma. This opens more voltage-gated sodium channels farther down the sarcolemma.)

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

the site of calcium binding site differs

Although all the anatomical parts of muscle work together to give it it's characteristics, which of the following proteins listed below would be most associated with the characteristics of contractility? A. potassium (K+) leak channels B. thick (myosin) filaments C. voltage gated sodium channels D. elastic (titin) filaments

thick (myosin) filaments

Which of the following are composed of myosin? A. thin filaments B. thick filaments C. intermediate filaments D. tropomyosin

thick filaments (Each thick filament is made of hundreds of myosin molecules.)

A potential benefit of recruiting slow oxidative fibers for contraction before recruiting fast oxidative and fast glycolytic fibers might be_______

to allow for fine control with delicate contractile force with a small stimulus

blocks myosin binding sites on actin

tropomyosin

The protein that binds calcium

troponin

change(s) shape upon binding with calcium ions.

troponin

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

troponin (when calcium binds to troponin, troponin releases tropomyosin, exposing the myosin binding sites.)

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

voltage-gated calcium channels (the action potential opens voltage-gated calcium channels and calcium rushes into the axon terminal, leading to the release of the neurotransmitter.)

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).