Anatomy Exam 3: Chapter 10
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 is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE). ACh diffuses away from the synaptic cleft.
Which of the following events directly occurs due to an action potential generated on the motor neuron?
ACh is released at the synapse.
The post-synaptic reaction on the sarcolemma is short in duration. Which of the following explanations describes why the reaction of the sarcolemma is short in duration?
AChE acts to breakdown the ACh in the synaptic cleft. ACh diffuses out of the synaptic cleft.
order of events that occur during excitation-contraction coupling
AP propagates along sarcolemma AP travels down T tubules to triads Voltage-sensitive proteins open Ca2+ channels Sarcoplasmic reticulum releases Ca2+ Ca2+ levels in sarcoplasm increase
The characteristic muscle stiffness associated with rigor mortis is due to the inability of myosin filaments to detach from the active site on actin filaments. What molecule is essential for this detachment?
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.
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.
Tamara's muscle weakness and fatigue becomes progressively worse over the course of the day. This is a hallmark sign of myasthenia gravis, an autoimmune disorder that affects the ability of a motor neuron to communicate with a muscle fiber. What synaptic events must happen first for excitation to occur?
Acetylcholine is released from the axon terminal and diffuses across the synapse to bind to a receptor in the surface of the motor end plate.
The injection that dramatically improved Tamara's symptoms was Tensilon, an acetylcholinesterase inhibitor. What is acetylcholinesterase, and why was this injection effective in relieving Tamara's symptoms?
Acetylcholinesterase is the enzyme that clears acetylcholine from the synapse. Inhibiting this enzyme will enable acetylcholine to be available for a longer period to bind to receptors.
What causes the release of calcium ions into the sarcoplasm from the terminal cisterns?
An action potential traveling along the t tubule
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.
order of events that occur at the neuromuscular junction after the action potential reaches the axon terminal.
Ca2+ enters theaxon terminal. Synaptic vesicles release ACh. ACh binds to ACh receptors. Ligand-gated cation channels open. Na+ enters andK+ exits. Membrane potentialis less negative.
The cross bridge cycle starts when _________.
Ca2+ from the sarcoplasmic reticulum binds to troponin
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 statements about excitation-contraction coupling is incorrect?
Calcium ions travel through the transverse tubule.
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
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.
How is delayed-onset muscle soreness (DOMS) different from muscle fatigue?
Delayed-onset muscle soreness occurs a day or more after the physical exertion.
sequence of events in excitation-contraction coupling from first to last
Depolarization of the junctional folds triggers an action potential. Action potential travels along the sarcolemma. Action potential along the T tubules opens calcium channels. Calcium ions flood the sarcoplasm. Calcium ions bind to troponin. Troponin changes shape. Tropomyosin moves off of actin binding sites. Myosin heads form cross bridges with actin.
Which of the following is a way that muscle fibers get energy needed for contraction?
During peak activity levels, glycolysis is the only pathway by which ATP can be produced to supply energy to the muscle.
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.
Myasthenia gravis is an autoimmune disorder that results in the production of antibodies that either block or cause the destruction of the ACh receptor. Why do you think that Tamara's symptoms become worse as the day progresses?
In the morning, the neuronal reserves of acetylcholine are larger, allowing for sufficient neurotransmitter release and resulting in effective muscle signaling. As the day progresses and the stores of acetylcholine become depleted, muscle function becomes progressively worse.
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.
Which of the following processes produces 36 ATP?
Krebs cycle and oxidative phosphorylation
What structure most directly stimulates a skeletal muscle fiber to contract?
Motor neuron
Which of the following statements is true concerning motor units?
Movement of the eyeball requires fewer muscle fibers with less tension being produced.
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.
Inadequate calcium in the neuromuscular junction would directly affect which of the following processes?
Release of acetylcholine from the synaptic vesicles
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.
The muscle action potentials that initiate contraction are transmitted from the sarcolemma into the interior of the muscle fiber by __________.
T tubules
order of steps that occur during a single cross bridge cycle
The activated myosin head binds to actin, forming a cross bridge. ADP is released and myosinslides the thin filament toward the center of the sarcomere. ATP binds to the myosin head and detaches it from actin. ATP is hydrolyzed to ADP and Pi and the energy released re-cocks the myosin head.
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.
Identify the incorrect statement about a single motor unit.
The more neurons involved, the more powerful the contraction.
Malignant hyperthermia (MH) is a rare genetic disease in which the sarcoplasmic reticulum leaks calcium when the patient is put under general anesthesia. Which of the following best describes how anesthesia would affect the skeletal muscles of a patient with MH?
The muscles would contract because of calcium binding to troponin.
What happens immediately after the myosin head binds to the active site on actin?
The myosin head pivots, moving the actin strand.
Which of the following statements is true? Acetylcholinesterase (AChE) is stored in vesicles within the presynaptic motor neuron. The neurotransmitter is stored in the presynaptic motor neuron. The sarcolemma is the presynaptic membrane. The acetylcholine (ACh) receptors are located on the myofibril.
The neurotransmitter is stored in the presynaptic motor neuron.
What structure is the functional unit of contraction in a skeletal muscle fiber?
The sarcomere
The soleus muscle is very red in color. Which of these statements about soleus muscle fibers is FALSE?
They are large in diameter.
Part complete Why do the disease tetanus and the normal sustained muscle contraction called tetanus share the same name?
They both eliminate the relaxation phase in the affected muscle fibers.
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.
Myasthenia gravis is a disease resulting from an autoimmune attack on the ACh receptors of the motor end plate. Binding of antibodies to the ACh receptors results in generalized muscle weakness that progresses as more ACh receptors are destroyed. Which of the following medications would help alleviate the muscle weakness?
a drug that binds to and inactivates acetylcholinesterase (neostigmine)
Which of the following is important to increasing efficiency in tension production?
a shortened relaxation phase
calcium channel
a type of voltage-gated ion channel located on the axon terminal.
sodium channel
a type of voltage-gated ion channel located on the sarcolemma of the muscle fiber.
In a neuromuscular junction, synaptic vesicles in the motor neuron contain which neurotransmitter?
acetylcholine (ACh)
Which of the following situations could contribute to prolonged muscle contraction?
acetylcholinesterase not being produced
A myosin head binds to which molecule to form a cross bridge?
actin
Myosin molecules form cross-bridges when they attach to __________.
actin
Which of the following proteins contains the active site involved in cross-bridge formation?
actin
sequence of events at the NMJ that initiate an action potential in the muscle fiber
action potential arrives at axon terminal calcium ions enter synaptic vesicles fuse to membrane acetylcholine released to synaptic cleft acetylcholine binds to receptors on junctional folds junctional folds depolarize action potential initiated on sarcolemma
The most important factor in decreasing the intracellular concentration of calcium ion after contraction is
active transport of calcium into the sarcoplasmic reticulum.
Muscles are attached to bones by tendons or __________.
aponeuroses
A(n) ________ can be described as a broad tendinous sheet.
aponeurosis
What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell?
arrival of an action potential
What causes the myosin head to disconnect from actin?
binding of ATP
tropomyosin
blocks binding sites on actin
The role of acetylcholinesterase is to
break down acetylcholine into acetate and choline components
The sarcoplasmic reticulum contains __________.
calcium
Triads in skeletal muscle fibers function in __________.
calcium ion release
what is released from the terminal cisterna
calcium ions
order of the steps of the contraction cycle
calcium ions arrive calcium binds to troponin myosin heads bind to actin active sites myosin head pivots (power stroke) and ADP and P released ATP binds to myosin head and breaks link free myosin head re cocks- ADP and P released
toponin
change(s) shape upon binding with calcium ions.
T tubule
conduct(s) action potentials throughout the interior of the muscle fiber.
Muscle tissue, one of the four basic tissue groups, consists chiefly of cells that are highly specialized for
contraction
Fatigued muscles signify __________.
decreased pH
What is the type of chemical reaction used to rebuild ADP into ATP?
dehydration synthesis
ATP binding leads to which of the following actions?
detaching and resetting cross-bridges
Conduction of an action potential along the sarcolemma depends upon ___________.
diffusion of sodium ions through voltage-gated channels
t tubules function
distribute action potentials throughout the interior of the skeletal muscle cell
Titin is a(n) __________.
elastic protein
Acetylcholinesterase
enzyme located in the synaptic cleft that breaks down acetylcholine.
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
Activities involving eccentric contractions more commonly lead to delayed-onset muscle soreness than concentric or isometric contractions. Which of the following activities is thus a candidate for delayed-onset muscle soreness?
extension of the biceps brachii
Inside a muscle, bundles of single muscle fibers form __________.
fascicles
Muscles that move the eyeball have ________ fibers.
fast
Large-diameter, densely packed myofibrils, large glycogen reserves, and few mitochondria are characteristics of
fast fibers.
Which of the following processes produces molecules of ATP and has two pyruvic acid molecules as end products?
glycolysis
Which of the following allows muscles to return to their original shape during relaxation?
gravity, opposing muscle contractions, and elastic forces
A fascicle is a
group of muscle fibers that are encased in the perimysium.
Cardiac muscles cells have a sarcolemma that is more permeable to calcium, which causes them to __________.
have contractions that last longer than skeletal muscle fibers
A weight-lifter strains to lift a heavy weight and there is no movement of the person's arms holding on to the weight. This type of contraction is called a(n) ________ contraction
isometric
Myofibrils are __________.
made of a series of sarcomeres
what are recognized functions of skeletal muscle?
maintain body temperature maintain posture guard body entrances and exits produce movement
synaptic vesicle
membranous sac located in the axon terminal that contains neurotransmitter.
Bacteria that cause tetanus release a neurotoxin. Which of the following components of the neuromuscular junction (NMJ) does this neurotoxin affect directly?
motor neurons
A single motor neuron together with all the muscle fibers it innervates is called a(n)
motor unit.
Interactions between actin and myosin filaments of the sarcomere are responsible for
muscle contraction.
During the recovery period following exercise, all of the following are true except
muscle fibers are unable to contract.
In an isotonic contraction,
muscle tension exceeds the load and the muscle lifts the load
In which of the following would the motor units have the fewest muscle fibers?
muscles that control the eyes
where in the skeletal muscle fiber is atp consumed
myofibrils and sarcoplasmic reticulum
Thick filaments are made of the protein
myosin
Which of the following is involved in the power stroke?
myosin
What area of the thick filament binds to actin once actin's binding sites are exposed?
myosin cross-bridge (head)
Acetylcholine
neurotransmitter that stimulates skeletal muscle contraction.
acetylcholine receptor
neurotransmitter that stimulates skeletal muscle contraction.
During anaerobic glycolysis,
pyruvic acid is produced. ATP is produced. oxygen is not consumed. carbohydrate is metabolized.
junctional folds
receive(s) stimulus from the motor neuron.
What can the nervous system do to increase muscle tension?
recruit larger motor units increase the number of active motor units increase stimulation frequency
The increase in muscle tension that is produced by increasing the number of active motor units is called
recruitment.
A patient takes a medication that blocks ACh receptors of skeletal muscle fibers. What is this drug's effect on skeletal muscle contraction?
reduces the muscle's ability for contraction
What causes the power stroke?
release of ADP and Pi
terminal cistern
release(s) calcium ions into the sarcoplasm.
After death, muscle fibers run out of ATP and calcium begins to leak from the sarcoplasmic reticulum into the sarcoplasm. This results in a condition known as
rigor mortis
The repeating unit of a skeletal muscle fiber is the
sarcomere
Z lines define the edges of which of the following?
sarcomeres
In response to an action potential along the transverse tubules, the __________ release(s) calcium ions into the sarcoplasm.
sarcoplasmic reticulum
Which organelle completely surrounds each myofibril inside a muscle fiber?
sarcoplasmic reticulum
Since each myofibril is attached at either end of the muscle fiber, when sarcomeres shorten, the muscle fiber
shortens.
How is acetylcholine (ACh) removed from the synaptic cleft?
simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme)
The type of muscle fiber that is most resistant to fatigue is the ________ fiber.
slow
function of sarcoplasmic reticulum
stores calcium (Ca+)
The narrow space between the synaptic terminal and the muscle fiber is the
synaptic cleft
At each end of the muscle, the collagen fibers of the epimysium, perimysium, and endomysium, come together to form a
tendon
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
Which of the following hormones directly stimulates growth of muscle tissue, leading to increased muscle mass?
testosterone
Z line
the boundary between adjacent sarcomeres
Which of the following most correctly describes excitation in the context of excitation-contraction coupling in skeletal muscle?
the generation of an action potential in the sarcolemma
Acetylcholine binds to its receptor in the sarcolemma and triggers __________.
the opening of ligand-gated cation channels
M line
the point of connection for adjacent tails of the thick filaments
H band
the region of the resting sarcomere that only contains thick filaments
I band
the region of the sarcomere that contains only thin filaments
synaptic cleft
the space between the axon terminal and junctional folds.
Increased muscle fiber endurance can be produced by __________.
the type of activity performed
Myosin molecules form what part of the sarcomere?
thick filament
Which of the following best describes the term Z line?
thin filaments are anchored here
Which of the following phrases best describes how excitation is coupled to contraction in skeletal muscle fibers?
through calcium release from the sarcoplasmic reticulum
What is name given to the regularly spaced infoldings of the sarcolemma?
transverse or T tubules
T tubules and the terminal cisternae are clustered into structures called __________.
triads
When the sarcomere is at rest, what is covering the active sites on actin?
tropomyosin
Which of the following causes the active site on actin to be exposed or uncovered?
tropomyosin shifting position
The binding of calcium to which molecule causes the myosin binding sites to be exposed?
troponin
To what regulatory protein does calcium bind during the initiation of the contraction cycle in skeletal muscle fibers?
troponin
When calcium is released inside a muscle cell, what does it bind to?
troponin
At rest, the tropomyosin molecule is held in place by
troponin molecules.
The rapid rise and fall in force produced by a muscle fiber after a single action potential is a(n)
twitch.
Each thin filament consists of
two actin protein strands coiled helically around each other.
When an action potential arrives at the axon terminal of a motor neuron, which ion channels open?
voltage-gated calcium channels
If a second stimulus arrives before the relaxation phase has ended, a second, more powerful contraction occurs. This addition of one twitch to another is called
wave summation.
Which type of muscle fiber has a large quantity of glycogen and mainly uses glycolysis to synthesize ATP?
white fast twitch fibers
