Muscle Physiology Flashcards
When myosin binds to actin, its ATPase activity increases.
True
False
True
Thapsigargin is a drug that inhibits sarcoplasmic reticulum calcium-ATPase. Treating skeletal muscle cells with this drug will:
reduce cytosolic calcium concentration.
prevent fused tetanic contractions.
slow twitch relaxation.
decrease the number of thick filaments.
Slow twitch relaxation
The maximal shortening velocity of a single skeletal muscle fiber is determined by:
the action potential amplitude.
the load it has to move.
its myosin isoform.
its calcium ATPase.
Its myosin isoform
The term “striated muscle” is used to describe:
the microscopic appearance of some muscles.
the location of some muscles in the body.
the anatomical arrangement of some muscles.
the presence of actin and myosin in muscle.
The microscopic appearance of some muscles
Each myofibril is surrounded by:
sarcoplasmic reticulum.
mitochondria and other organelles.
sarcolemma.
transverse tubules.
Sarcoplasmic reticulum
Which of the following is not a physiological mechanism to influence the force of a skeletal muscle contraction?
Sustained increase in cytosolic calcium concentration.
Increase the stimulation frequency.
Increase the number of recruited motor units.
Increase the action potential amplitude.
Increase the action potential amplitude
Imagine that a skeletal muscle fiber is stretched until the thick and thin filaments no longer overlap. Under those conditions, if you stimulate the muscle fiber, it will:
not use ATP.
not produce active force.
not release calcium.
fatigue faster.
not produce active force
In contrast to skeletal muscle, smooth muscle contraction:
is inhibited by action potentials.
is regulated by cytosolic calcium.
does not require myosin.
requires myosin phosphorylation.
requires myosin phosphorylation
Examine the force/velocity relationship shown above. During a maximal isometric contraction, which point would be closer to its shortening velocity?
At 0
How do action potentials in the sarcolemma induce calcium release?
Dihydropyridine receptors allow the entry of calcium to the cytosol.
Ryanodine receptors start to pump calcium to the cytosol.
Dihydropyridine receptors change the conformation of ryanodine receptors.
Calsequestrin allows the removal of calcium from the cytosol.
Dihydropyridine receptors change the conformation of ryanodine receptors
Examine the figure above; different structures are marked by lines labeled with capital letters. On this figure, which line marks the area containing only thin filaments?
I band
Skeletal muscle contraction is a calcium-regulated process. In order for a contraction to start, the following must occur:
cytosolic calcium binds to troponin.
a calcium-dependent phosphatase is activated.
all the options are correct.
calcium displaces troponin away from actin.
cytosolic calcium binds to troponin
Examine the figure above; different structures are marked by lines labeled with capital letters. On this figure, which line spans a sarcomere?
from Z disk to Z disk
Single-unit smooth muscle cells are electrically coupled by:
desmosomes.
dense bodies.
gap junctions.
neurotransmitter receptors.
gap junctions
Examine the micrograph above. According to the sliding filament theory of contraction, when a muscle contracts:
the H band expands.
the A band expands.
the A band narrows.
the I band narrows
The I band narrows
In smooth muscle, ____________ serve a similar function as ____________ in skeletal muscle sarcomeres.
caldesmon and calponin — actin and myosin
dense bodies — Z lines
gap junctions — sarcoplasmic reticulum
dense bodies — Z lines
Curare is a drug that blocks the nicotinic acetylcholine receptors on the postsynaptic membrane of neuromuscular junction. What is an expected finding in a subject receiving a large dose of curare?
Acetylcholine release will be blocked.
Endplate potentials will increase in amplitude.
Motor neurons will not have action potentials.
Skeletal muscles will not have action potentials.
Skeletal muscles will not have action potentials
What is the order of events triggered inside a skeletal muscle fiber as a result of an action potential?
(DHPR = dihydropyridine receptor; RYR = ryanodine receptor)
T-tubular RYRs open; Ca2+ flows through DHPR; Ca2+ binds to tropomyosin.
T-tubular DHPRs open; Ca2+ binds to RYR; tropomyosin is displaced.
Ca2+ flows through DHPR; Ca2+ binds to troponin; tropomyosin is displaced.
RYRs open; Ca2+ binds to troponin; tropomyosin is displaced.
RYRs open; Ca2+ binds to troponin; tropomyosin is displaced
A biotech company is developing a new drug for the treatment of high blood pressure. Based on in vitro studies, this new drug activates myosin light chain phosphatase. The plan is to test this drug in human volunteers next month. What acute side effect would you anticipate?
It may enhance the latch state of smooth muscle.
It may decrease fat stores in smooth muscle.
It may increase glucose uptake by skeletal muscle.
It may relax airway smooth muscle.
It may relax airway smooth muscle
Muscle fibers and the motor neurons that innervate them are organized into motor units. Which of the following is not a characteristic of motor units?
Fatigue resistance depends on the motor unit type.
All muscle fibers within a motor unit are of the same type.
The size of a motor unit is defined by its number of muscle fibers.
All motor neurons innervate the same number of muscle fibers.
All motor neurons innervate the same number of muscle fibers
Which of the foIlowing will increase the force of a skeletal muscle contraction?
Correct answer:
Increase the number of active motor units.
Decrease the frequency of stimulation.
Increase the binding of calcium to tropomyosin.
Shorten the length of the muscle
Increase the number of active motor units
Imagine a situation in which all ATP in a skeletal muscle fiber disappears instantaneously in the middle of a contraction. Which of the following do you expect?
The skeletal muscle fiber repolarizes immediately.
Calcium release into the sarcoplasmic reticulum stops.
Calcium reuptake into the sarcoplasmic reticulum stops.
The skeletal muscle fiber relaxes immediately.
Calcium reuptake into the sarcoplasmic reticulum stops
Which of the following statements about skeletal muscle is false?
(a) Muscle fibers are made up of myofibrils.
(b) Muscle fibers are multinucleated.
(c) Muscle fibers are organized into motor units.
(d) Thin filaments are composed mainly of myosin.
Thin filaments are composed mainly of myosin
The striations in skeletal muscle represent its organization into ____________ made up of
____________ arranged in series.
(a) myofilaments — tendons
(b) myofibrils — sarcomeres
(c) sarcomeres — myosin
(d) myofilaments — myosin
myofibrils – sarcomeres
The following are proteins present in the thin filaments except:
(a) tropomyosin
(b) troponin
(c) titin
Titin
The cross-bridge cycle describes the steps for:
(a) the conversion of ATP to mechanical work by actomyosin.
(b) the binding of tropomyosin to troponin in the thick filaments.
(c) the depolarization of the sarcolemma and calcium reuptake.
(d) the flow of calcium from the SR into the cytosol.
The conversion of ATP to mechanical work by actomyosin
Skeletal muscle relaxes when ______ is removed from the cytosol.
(a) ATP
(b) Phosphocreatine
(c) Calcium
(d) Sodium
Calcium
The following are events that take place during the latent period between membrane
depolarization and force development:
(a) troponin moves to block actin binding sites.
(b) the sarcoplasmic reticulum actively pumps calcium to the cytosol.
(c) calcium binds to tropomyosin.
(d) myosin heads bind to actin.
myosin heads bind to actin
In skeletal muscle, excitation contraction coupling requires the following steps, except:
(a) dihydropyridine receptors bind covalently to the ryanodine receptors.
(b) calcium diffuses from the SR to the myofilaments.
(c) tropomyosin shifts position to expose myosin binding sites on actin.
(d) myosin heads bind to actin on thin filaments.
dihydropyridine receptors bind covalently to the ryanodine receptors
The sarcoplasmic reticulum is a useful intracellular calcium store because:
(a) it is a large compartment that allows diffusion of calcium from the cytosol.
(b) it can efficiently pump calcium to the cytosol during contractions.
(c) it depolarizes at the same time as the sarcolemma.
(d) it maximizes the amount of stored calcium by binding it to calsequestrin.
it maximizes the amount of stored calcium by binding it to calsequestrin
The motor units in a muscle used for very fine motor control will be:
(a) large and fatigable.
(b) small.
(c) fast.
(d) recruited all day.
small
Myasthenia gravis and botulism disrupt neuromuscular communication by different
mechanisms: myasthenia is an autoimmune disease that induces the destruction of
endplate acetylcholine receptors. Botulinum toxin (Botox) prevents neurotransmitter release
from the presynaptic terminal. Based on this information, what electrophysiological
consequence do they have in common?
(a) They block action potentials in the motor neuron.
(b) They prevent calcium influx into the presynaptic terminal.
(c) They inhibit the development of endplate potentials.
(d) They block calcium efflux through the dihydropyridine receptors.
They inhibit the development of endplate potentials
Sarcomere length influences ____ by changing the ____ between ____.
(a) velocity of contraction — myosin ATPase — beginning and end of contraction
(b) thick filaments — overlap — myofibrils
(c) force — overlap — thick and thin filaments
(d) fatigue — distance — thick filaments
force — overlap — thick and thin filaments
The force produced by a skeletal muscle during a contraction can be regulated by the
following mechanisms, except:
(a) changes in stimulation frequency.
(b) changes in actin ATPase activity.
(c) variable recruitment of its motor units.
(d) optimization of its operating length.
changes in ATPase activity
____ and ____ are metabolic pathways for ATP production, and ____
is the main ATP buffer:
(a) glycolysis; oxidative phosphorylation; glycogen
(b) creatine phophate; glucose; glycogen
(c) oxidative phosphorylation; glycolysis; creatine phosphate
(d) glycogen; lipid; glucose
oxidative phosphorylation — glycolysis — creatine phosphate
The contraction of a smooth muscle cell ends when the following occurs:
(a) myosin light chain kinase activity decreases.
(b) calmodulin binds calcium.
(c) myosin light chain phosphatase activity decreases.
(d) myosin light chain kinase is activated.
myosin light chain kinase activity decreases
One important difference between smooth muscle and skeletal muscle contraction is that:
(a) smooth muscle contraction is not regulated by calcium.
(b) smooth muscle contraction is always preceded by depolarization.
(c) actin and myosin are only present in skeletal muscle.
(d) smooth muscle does not have sarcomeres.
smooth muscle does not have sarcomeres
Single unit smooth muscles are characterized by:
(a) extensive sarcoplasmic reticulum.
(b) abundant gap junctions between the cells.
(c) inability to sustain muscle tone.
(d) the presence of transverse tubules.
abundant gap junctions between the cells
