Chapter 9 muscles and Muscles tissues
The cross bridge cycle starts when _________.
Ca2+ from the sarcoplasmic reticulum binds to troponin
A motor neuron and all the muscle cells that it stimulates are referred to as a motor end plate.
False
During isometric contraction, the energy used appears as movement.
False
Muscle contraction will always promote movement of body parts regardless of how they are attached.
False
Once a motor neuron has fired, all the muscle fibers in a muscle contract.
False
bundle of skeletal muscle fibers enclosed by connective tissue called perimysium
Fascicle
What would happen if acetylcholine was not removed from the synaptic cleft?
Multiple action potentials would occur in the muscle fiber.
Blocks myosin-binding sites on actin:
tropomyosin
In muscle fibers, which regulatory protein blocks the attachment of myosin heads to actin?
tropomyosin
The binding of calcium to which molecule causes the myosin binding sites to be exposed?
troponin
The interaction between which protein and ion initiates muscle coupling?
troponin; calcium ions
The smallest contractile unit within skeletal muscle would correspond to the distance between which two points in the figure?
1 and 7
The region between which two points corresponds to the entire A (dark) band?
2 and 6
The molecular interaction described as a cross bridge involves the binding of which two proteins?
A and 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
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.
Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction?
ACh diffuses away from the synaptic cleft. ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE).
Which step precedes all of the other listed steps?
ACh is released by the motor neuron.
is not bound to myosin during the detachment step.
ADP
The molecules released just before power stroke: ADP and Pi
ADP and PI
Molecule(s) bound to the myosin head at the start of the cross bridge cycle:
ADP and pi
The bond between the actin and myosin head weakens as a result of
ATP
Which event causes cross bridge detachment?
ATP binding to the myosin head
After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment?
ATP binds to the myosin head
Action potential propagation in a skeletal muscle fiber ceases when acetylcholine is removed from the synaptic cleft. Which of the following mechanisms ensures a rapid and efficient removal of acetylcholine?
Acetylcholine is degraded by acetylcholinesterase.
The neuromuscular junction is a well-studied example of a chemical synapse. Which of the following statements describes a critical event that occurs at the neuromuscular junction?
Acetylcholine is released by axon terminals of the motor neuron.
: myofilament stiffened and stabilized by tropomyosin
Actin
How/when does the myosin head cock back to store energy for the next cycle?
After the myosin head detaches, energy from ATP hydrolysis is used to re-cock the myosin head.
Which of the choices below correctly describes how an action potential generated at the neuromuscular junction (NMJ) is converted to excitation in the muscle fiber?
An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh to sarcolemma receptors initiates graded potentials.
In which phase of the muscle twitch shown in the figure would the maximum amount of ATP be consumed by myosin head groups?
B
Which of the structures is surrounded by the connective tissue sheath known as the perimysium?
B
Which of the following interactions must occur first so that the others can take place?
B binds to troponin.
Which statement accurately describes the event indicated by B?
Binding of acetylcholine to a receptor triggers the opening of an ion channel.
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 (junctional folds of the sarcolemma) and sodium enters the cell.
In which phase in the figure would the net movement of Ca2+ INTO the sarcoplasmic reticulum (SR) be greatest?
C
Which protein functions as a motor protein that applies the power stroke during muscle contraction?
C
Which structure in the figure corresponds to a single skeletal muscle cell?
C
What is the role of calcium in the cross bridge cycle?
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 sarcoplasmic reticulum.
What specific event triggers the uncovering of the myosin binding site on actin?
Calcium ions bind to troponin and change its shape.
How do calcium ions initiate contraction in skeletal muscle fibers?
Calcium ions bind to troponin, changing troponin's shape.
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.
Which of the following is true about smooth muscle?
Certain smooth muscle cells can actually divide to increase their numbers.
When does cross bridge cycling end?
Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.
A person dies, and within hours, the skeletal muscles develop a locked contraction known as rigor mortis. Calcium ions leak from the sarcoplasmic reticulum into cytoplasm. From your knowledge of cross bridge cycling, what best explains this rigor?
Cross bridge detachment cannot occur. Detachment requires ATP, which is produced only during life.
Location of higher concentration of Ca2+ needed for cross bridge formation and cycling:
Cytoplasm
Energy state of myosin for beginning cross bridge formation:
Energized myosin head
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.
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
Which of the following processes produces the most ATP?
Krebs cycle and oxidative phosphorylation
Which of the following is true concerning the anatomy of a skeletal muscle fiber?
Myofibrils contain thick and thin filaments.
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.
If both motor neurons shown in this figure were to develop action potentials and stimulate muscle fibers, would all the muscle cells shown here contract?
No, because neurons in this figure do not innervate every muscle cell shown.
Troponin, a major protein in thin filaments, is a globular protein with three polypeptide subunits. Which of the following is NOT a function of the troponin?
One subunit binds to potassium ions.
Only ------- of the myosin heads are detached from actin at any one time while calcium is present and cycling continues.
One- half
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 of the following statements is true?
Skeletal muscle cells are long and cylindrical with many nuclei.
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 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.
In a neuromuscular junction, synaptic vesicles in the motor neuron contain which neurotransmitter?
acetylcholine (ACh)
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 result would be expected if an additional stimulus, equal in intensity to the first, were to be applied to the muscle at the 60 millisecond (ms) time point?
The muscle would increase in tension to a level greater than that measured at the beginning of phase C.
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
Which of the following describes the cells of unitary smooth muscle?
They exhibit spontaneous action potentials.
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.
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.
A contraction in which the muscle does not shorten but its tension increases is called isometric contraction.
True
A skeletal muscle contracts with varying force and length of time in response to the body's needs at the time.
True
Although there are no sarcomeres, smooth muscle still possesses thick and thin filaments.
True
An increase in the calcium ion level in the sarcoplasm starts the sliding of the thin filaments. When the level of calcium ions declines, sliding stops.
True
Cardiac muscle has a limited regenerative capacity.
True
Cells of unitary smooth muscle are found in the longitudinal and circular muscle layers of the intestine.
True
Excitability is the ability of a cell to receive and respond to stimulus by changing its membrane potential.
True
Muscle cells store more creatine phosphate than ATP resulting in the muscle having a reserve source of energy.
True
Muscle tone is the small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units.
True
One of the functions of skeletal muscle contraction is production of heat.
True
Smooth muscles relax when intracellular Ca2+ levels drop but may not cease contractions.
True
The alternating contraction and relaxation of the longitudinal and circular layers mixing substances in the lumen and squeezing them through an organ's internal pathway is characteristic of smooth muscle.
True
The force of muscle contraction is controlled by multiple motor unit summation or recruitment.
True
The thin filaments (actin) contain a polypeptide subunit G actin that bears active sites for myosin attachment.
True
When a muscle fiber contracts, the I bands diminish in size, the H zones disappear, and the A bands do not diminish in length.
True
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.
Most skeletal muscles contain ________.
a mixture of fiber types
Synaptic vesicles at the neuromuscular junction contain __________.
acetylcholine
Muscle tone is ________.
a state of sustained partial contraction
A myosin head binds to which molecule to form a cross bridge?
actin
Which protein is indicated by the letter A?
actin
. Status of actin and myosin:
actin and myosin detached
The sliding filament model of contraction involves ________.
actin and myosin sliding past each other and partially overlapping
During muscle contraction, myosin cross bridges attach to which active sites?
actin filaments
. 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
What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell?
arrival of an action potential
Why would inflammation of the knee joint be a "good thing" when there's been an acute quadriceps injury?
because fluid accumulation reduces joint movement
Proper application of RICE--rest, ice, compression, and elevation--can effectively control inflammation due to an ankle sprain. Why would compression provided by a correctly wrapped elastic Ace bandage be helpful in preventing further inflammation?
because it immobilizes/stabilizes an injured ankle
What causes the myosin head to disconnect from actin?
binding of ATP
What is the primary mechanism by which ACh is cleared from the synaptic cleft?
broken down by acetylcholinesterase
What most directly causes synaptic vesicles to release acetylcholine into the synaptic cleft?
calcium entering the axon terminal
In an isotonic contraction, the muscle ________.
changes in length and moves the "load"
Acetylcholine receptors are best characterized as what type of channel?
chemically gated Na+-K+ channels
The ability of muscle to shorten forcibly when adequately stimulated is known as ________, and sets muscle apart from other tissue types.
contractility
A muscle cell runs out of ATP. Even though these are cyclic reactions, what step of the cross bridge cycle given is most directly inhibited or terminated?
cross bridge detachment
Which of the following best describes the events of "contraction" in "excitation-contraction coupling"?
cross bridge formation
What is the type of chemical reaction used to rebuild ADP into ATP?
dehydration synthesis
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
Where in the cross bridge cycle does ATP hydrolysis occur?
during the cocking of the myosin head
Which of the following surrounds an individual muscle cell?
endomysium
Which of the following properties is most directly associated with changes to a muscle cell's membrane potential (the voltage across the plasma membrane)?
excitability
What cellular event is indicated by A?
exocytosis
What means of membrane transport is used to release the neurotransmitter into the synaptic cleft?
exocytosis
Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________.
extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber
Skeletal muscle cells are grouped into bundles called __________.
fascicles
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?
glycolysis
What is the functional role of the T tubules?
hold cross bridges in place in a resting muscle
Status of ATP
hydrolyzed to ADP and Pi
The strongest muscle contractions are normally achieved by ________.
increasing the stimulation up to the maximal stimulus
What is the function of the filaments found in ranges 1-2 and 6-7 but no other ranges?
protection from overstretching
During vigorous exercise, there may be insufficient oxygen available to completely break down pyruvic acid for energy. As a result, the pyruvic acid is converted to ________.
lactic acid
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 a factor that affects the velocity and duration of muscle contraction?
load on the fiber
Cross bridge formation between myosin heads and actin molecules is caused by the elevation of calcium ion concentration in the cytosol. During rigor mortis, this elevation of calcium ion concentration in the cytosol is permanent because ________.
mitochondria stop producing ATP molecules required by the sarcoplasmic reticulum's calcium ion pumps
What part of the sarcolemma contains acetylcholine receptors?
motor end plate
Which of the following is the correct sequence of events for muscle contractions?
motor neuron action potential, neurotransmitter release, muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke, sliding of myofilaments
Each neuron shown in this figure innervates a group of muscle fibers. What is the term for a group of muscle fibers and the single neuron that innervates them?
motor unit
Which of the following would be recruited later in muscle stimulation when contractile strength increases?
motor units with larger, less excitable neurons
Made up of several sarcomeres:
myofibrils
The contractile units of skeletal muscles are ________.
myofibrils
The oxygen-binding protein found in muscle cells is ________.
myoglobin
Which protein is indicated by the letter E?
myosin
myofilament with a knob-like head
myosin
What, specifically, is a cross bridge?
myosin binding to actin
Rigor mortis occurs because ________.
no ATP is available to release attached actin and myosin molecules
Myosin head energy status during the power stroke:
pivots and returns to low energy
Which of the following best describes the events of "excitation" in "excitation-contraction coupling"?
propagation of the action potential along the sarcolemma
Actin behavior during the power stroke: ratchets toward the M line
ratchets toward the M line
If both of the neurons in the figure were activated, more muscle fibers would contract than if either neuron alone were active. This mechanism for control of the force of muscle contraction is known as __________.
recruitment
What causes the power stroke?
release of ADP and Pi
the cytoplasm of a skeletal muscle fiber
sarcoplasm
. Status of the myosin head:
returns to the energized position
ACh receptors are found mainly in the __________.
sarcolemma
membrane of muscle cell
sarcolemma
Contractile unit:
sarcomere
What structure in skeletal muscle cells functions in calcium storage?
sarcoplasmic reticulum
Which of the following is NOT a normal function of muscle tissue?
secreting hormones
How is acetylcholine (ACh) removed from the synaptic cleft?
simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme)
Of the following muscle types, which has the longest muscle cells and has obvious stripes called striations?
skeletal muscle
Long, cylindrical cells:
skeletal muscle fibers
Which muscle cells have the greatest ability to regenerate?
smooth
When the chemically gated ion channels open, which ion is mainly responsible for depolarizing the sarcolemma?
sodium
Myoglobin ________.
stores oxygen in muscle cells
Creatine phosphate functions in the muscle cell by ________.
storing energy that will be transferred to ADP to resynthesize ATP
The connective tissue that covers structure A is continuous with which of the following?
tendon
The calcium that initiates skeletal muscle contraction is released from what structure(s)?
terminal cisternae
The action potential on the muscle cell leads to contraction due to the release of calcium ions. Where are calcium ions stored in the muscle cell?
terminal cisterns (cisternae) of the sarcoplasmic reticulum
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
Acetylcholine binds to its receptor in the sarcolemma and triggers __________.
the opening of ligand-gated cation channels
Which of the following regions best identifies where myosin would have maximum cross-bridge access to actin?
the region between 2 and 3
Smooth muscle is characterized by all of the following EXCEPT ________.
there are more thick filaments than thin filaments
Which of the following are composed of myosin?
thick filaments
An enzyme known as acetylcholinesterase is present in the synaptic cleft. What is its role?
to break down acetylcholine
What is name given to the regularly spaced infoldings of the sarcolemma?
transverse or T tubules
When an action potential arrives at the axon terminal of a motor neuron, which ion channels open?
voltage-gated calcium channels
Which type of muscle fiber has a large quantity of glycogen and mainly uses glycolysis to synthesize ATP?
white fast twitch fibers
