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Flashcards in Skeletal Muscle Deck (13):

In skeletal muscle, which of the following events occurs before depolarization of the T tubules in the
mechanism of excitation–contraction coupling?
A. Binding of actin and myosin
B. Binding of Ca2+ to troponin C
C. Depolarization of the sarcolemmal membrane
D. Opening of Ca2+ release channels on the sarcoplasmic reticulum (SR)
E. Uptake of Ca2+ into the SR by Ca2+
- ATPase

Answer: C
Explanation: In the mechanism of excitation–contraction coupling, excitation always precedes
contraction. Excitation refers to the electrical activation of the muscle cell, which begins with an action
potential (depolarization) in the sarcolemmal membrane (muscle plasma membrane) that spreads to the
T tubules. Depolarization of the T tubules then leads to the release of Ca2+ from the nearby sarcoplasmic
reticulum (SR), followed by an increase in intracellular Ca2+ concentration, binding of Ca2+ to troponin C,
and then contraction.


A 66-year-old man who lives alone has a severe myocardial infarction and dies during the night. The
medical examiner’s office is called the following morning and describes the man’s body as being in rigor
mortis. This state of rigor mortis is due to most likely due to which of the following?
A. Absence of ATP preventing detachment of the myosin heads from actin
B. A lack of action potentials in motor neurons
C. Failure of tropomyosin and troponin to move away from the actin binding sites on myosin
D. Inhibition of Ca2+ entry from the extracellular fluid and sarcoplasmic reticulum
E. Increased intracellular Ca2+concentration

Answer: A
Explanation. Rigor is a state of permanent contraction that occurs in skeletal muscle when adenosine
triphosphate (ATP) levels are depleted. With no ATP bound, myosin remains attached to actin and the
cross-bridge cycle cannot continue. If there were no action potentials in motor neurons, the muscle
fibers they innervate would not contract at all, since action potentials are required for release of Ca2+
from the sarcoplasmic reticulum (SR). When intracellular Ca
2+ concentration increases, Ca2+ binds
troponin C, permitting the cross-bridge cycle to occur. Decreases in intracellular Ca2+ concentration
cause relaxation.


During the process of excitation-contraction coupling in skeletal muscle, which of the following
stimulates calcium release from the sarcoplasmic reticulum?
A. Activation of troponin C
B. An increase in intracellular calcium concentration
C. Inositol trisphosphate (IP3)
D. Membrane depolarization
E. Protein Kinase A

Answer: D
Explanation: Depolarization of the t-tubules in skeletal muscle fibers causes the calcium release
channels of the sarcoplasmic reticulum (SR) to open, allowing calcium to enter the cytoplasm.
Depolarization activates the voltage-sensitive dihydropyridine receptors (DHPR) on the T-tubular
membrane. The DHPR are linked to calcium release channels in the SR membrane called ryanodine
receptors (RYR). The AP in the T-tubule causes a conformational change in the DHPR which mechanically
opens RYR in the SR membrane. Once these channels open, calcium is released from the SR into the


Which of the following characterizes events which occur during the latent period of an isometric
twitch in skeletal muscle?
A. Ca2+ binds to troponin C.
B. Dihydropyridine receptors open and conduct Ca2+ into the SR.
C. Myosin hydrolyzes ATP and releases from actin.
D. Ryanodine receptors open and Ca2+ enters the cytoplasm.
E. Tropomyosin moves to block myosin-binding sites on actin.

A muscle twitch is divided into three phases: 1) the latent period; 2) the contraction phase; 3) the
relaxation phase. A brief delay occurs between application of the stimulus (black arrow) and the
beginning of contraction (contraction phase, increase in tension). The delay, which lasts about 2 msec, is
termed the latent period. During the latent period, the muscle action potential sweeps over the
sarcolemma and calcium ions are released from the sarcoplasmic reticulum. The second phase, the
contraction period, lasts 10–100 msec. During this time, Ca2+ binds to troponin C, myosin‐binding sites
on actin are exposed, and cross‐bridges form. Peak tension develops in the muscle fiber. During the
third phase, the relaxation period, also lasting 10–100 msec, Ca2+ is actively transported back into the
sarcoplasmic reticulum, myosin‐binding sites are covered by tropomyosin, myosin heads detach from
actin, and tension in the muscle fiber decreases. The actual duration of these periods depends on the
type of skeletal muscle fiber. Some fibers, such as the fast‐twitch fibers that move the eyes, have
contraction periods as brief as 10 msec and equally brief relaxation periods. Others, such as the slow‐
twitch fibers that move the legs, have contraction and relaxation periods of about 100 msec each.


A 28-year-old female qualifies to run in the New York marathon. She undertakes an endurance
training regimen designed to improve marathon performance. Which of the following properties is
greater in type I (slow-oxidative) compared to type IIb (fast-glycolytic) muscle fibers, thereby promoting
distance running success?
A. Glycogen content
B. Myosin ATPase activity
C. Glycolytic capacity
D. Oxidative capacity
E. Speed of contraction

Answer: D.
Explanation: Skeletal muscle is a heterogeneous tissue made up of 3 different fiber types – type I (slow
oxidative), type IIa (fast oxidative) and type IIb (fast glycolytic). Compared to type IIb, type I fibers also
have less fatigability, decreased force of contraction, and decreased sp


When the cell membrane of a skeletal muscle is depolarized, ryanodine receptors (RYRs) change
configuration and permit flow of Ca2+ through which of the following mechanisms?
A. Actively, from the extracellular fluid to the cytoplasm.
B. Actively, from the cytoplasm to the sarcoplasmic reticulum.
C. Actively, from the sarcoplasmic reticulum to the cytoplasm.
D. Passively, from the extracellular fluid to the cytoplasm.
E. Passively, from the sarcoplasmic reticulum to the cytoplasm.

Answer: E.
Explanation: Dihydropyridine receptors (DHPR) are present on the t-tubular membrane. They are
arranged in rows along the t-tubule and, directly opposite them in the adjacent sarcoplasmic reticulum
membrane are rows of Ca2+ release channels called Ryanodine Receptors (RYR) (type 1 RYR1 in skeletal
muscle). AP in the t-tubule causes a conformational change in the DHPR and this mechanically opens
RYR, permitting Ca2+ to be released passively from the SR into the muscle fiber cytoplasm.


A healthy 32-year-old man lifts weights regularly as part of his workout. In one of his bicep muscle
fibers at rest, the length of the I band is 1.0 μm and the A band is 1.5 μm. Contraction of that muscle
fiber results in a 10% shortening of the length of the sarcomere. What is the length of the A band after
the shortening produced by muscle contraction?
A. 0.45 μm
B. 1.00 μm
C. 1.35 μm
D. 1.50 μm
E. 1.90 μm

Answer: D.
Explanation: The A band, the region of the sarcomere where thick And thin filaments overlap. It
encompasses the width of the thick filament and this does not change during contraction.


Which of the following best characterizes a skeletal muscle sarcomere at rest (not contracting).
A. ADP+P are bound to myosin.
B. ATP is bound to actin.
C. Calcium is bound to troponin C.
D. Myosin and actin are attached.
E. Myosin is inactivated.

Answer: A.
Explanation: At rest, there is no calcium in the cytoplasm to bind to troponin C. The myosin binding sites
on actin are covered by tropomyosin and myosin and actin are unable to interact. Myosin is bound to
ADP+P. The myosin head is energized and ready to bind to actin once calcium levels in the cytoplasm
increase. In skeletal muscle, myosin is constitutively active and always ready to bind to actin. It is unlike
smooth muscle in this regard, where myosin light chains must be phosphorylated by myosin light chain
kinase in order for myosin to bind to actin. Actin does not bind ATP. ATP binds to ATP binding sites in the
myosin head. When ATP binds to myosin, myosin and actin detach from each other.


Which of the following statements best describes a motor unit?
A. All of the fibers of a particular fiber type in a given muscle.
B. All of the muscles that contract to complete a particular body movement.
C. A particular muscle and all of its synergistic and antagonistic muscles.
D. A small group of connecting muscle fibers.
E. A single motor neuron and all of the muscle fibers that it innervates.
F. A group of muscle fibers and all of the motor neurons that innervate them.

Answer: E.
Explanation: A motor unit is defined as a single motor neuron and all the muscle fibers that it
innervates. Muscle fibers are typically innervated by one motor neuron and one motor neuron
innervates > 1 muscle fiber. A single muscle typically has many motor units. Typically, the different
motor units of an entire muscle are not stimulated to contract in unison. While some motor units are
contracting, others are relaxed. This pattern of motor unit activity delays muscle fatigue and allows
contraction of a whole muscle to be sustained for long periods. The weakest motor units are recruited
first, with progressively stronger motor units added if the task requires more force. The process in which
the number of active motor units increases is called motor unit recruitment.


A 25-year old female works out on a daily basis. She combines strength training and cardio in her
routine. She uses 20 lb. dumbbells to do bicep curls. Which of the following signifies the isometric
contraction of the muscle during a bicep curl?
A. Bringing the dumbbell down.
B. Holding the weight stationary after the lift is complete.
C. Lifting the dumbbell up.
D. When the muscle is in the relaxed state.
E. When the weight is released and it drops.

Answer: B
Explanation: During isometric contraction, the length of the muscle does not change, nor is any
movement (velocity of shortening = 0) or joint motion involved (work = 0) (e.g. trying to lift a car).
During a bicep curl, isometric contraction corresponds to the time where the weight is held stationary.
Bringing the weight up or down involves changing the length of the muscle. This is isotonic contraction.
When the muscle is in the relaxed state, it is not contracting. When the weight is released, the muscle is
not contracting.


In skeletal muscle at rest, myosin cross-bridges are prevented from binding to actin molecules by
which of the following?
A. Calmodulin.
B. Myosin phosphatase.
C. Titin.
D. Troponin.
E. Tropomyosin.

Answer: E.
Explanation: When skeletal muscle fibers are at rest, tropomyosin inhibits the interaction between actin
and myosin. The inhibition is removed when Ca2+ binds to troponin C. The binding of Ca2+ to troponin
causes troponin to undergo a conformational change, during which tropomyosin is moved from its
resting position to a position where it no longer blocks the interaction between the myosin and actin. In
smooth muscle, calcium binds calmodulin activating myosin light chain kinase (MLCK) which
phosphorylates myosin light chains. This is necessary for cross-bridge formation in smooth muscle.
Myosin phosphatase (also called myosin light chain phosphatase, MLCP) dephosphorylates myosin light
chains permitting relaxation.


Repeated stimulation of a skeletal muscle fiber causes a sustained contraction (tetanus).
Accumulation of which of the following in intracellular fluid is responsible for tetanus?
A. Adenosine triphosphate (ATP)
B. Calmodulin
C. Ca2+
D. Cl−
E. K
F. Mg2+
G. Na+
H. Troponin

Answer: C
Explanation: A single action potential in a skeletal muscle fiber briefly releases enough Ca2+ to saturate
troponin, and all the myosin-binding sites on the thin filaments are therefore initially available.
However, the binding of energized cross-bridges to these sites (step 1 of the cross-bridge cycle) takes
time, whereas the Ca2+ released into the cytosol begins to be pumped back into the sarcoplasmic
reticulum almost immediately. Thus, after a single action potential, the Ca2+ concentration begins to
decrease and the troponin–tropomyosin complex re-blocks many binding sites before cross-bridges
have had time to attach to them. This means that a single action potential results in a single twitch.
In contrast, during a tetanic contraction, the successive action potentials each release Ca2+ from the
sarcoplasmic reticulum before all the Ca2+ from the previous action potential has been pumped back into
the sarcoplasmic reticulum. This results in a persistent elevation of cytosolic Ca2+concentration. Under
these conditions, more binding sites remain available and many more cross-bridges become bound to
the thin filaments.


Which of the following statements best describes how is the length of a skeletal muscle cell in vivo
relates to the force it can generate?
A. The longer a skeletal muscle cell is when it begins to contract, the stronger the force generation
will be.
B. The shorter a skeletal muscle cell is when it begins to contract, the stronger the force generation
will be.
C. The tension in a skeletal muscle cell is greatest when contractions occur at either very short or
very long lengths.
D. Skeletal muscle cells generate the most force when the contraction occurs at an intermediate
E. Skeletal muscle cells generate the same amount of force, regardless of their length.

Answer: D.
Explanation: When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-
bridges. The thin filaments slide over the thick filaments as the heads pull the actin, and this results in
sarcomere shortening, creating the tension of the muscle contraction. The cross-bridges can only form
where thin and thick filaments overlap. Therefore, the length of the sarcomere has a direct influence on
the force generated when the sarcomere shortens. This is called the length-tension relationship. The
ideal length of a sarcomere to produce maximal tension occurs at 80 percent to 120 percent of its
resting length (~2-2.2 m). This length maximizes the overlap of actin-binding sites and myosin heads.
As the sarcomeres of a muscle fiber are stretched to a longer length, the zone of overlap shortens, and
fewer myosin heads can make contact with thin filaments. Therefore, the tension the fiber can produce
decreases. When a skeletal muscle fiber is stretched to >3 m (>170%) of its optimal length, there is no
overlap between the thick and thin filaments. Because none of the myosin heads can bind to thin
filaments, the muscle fiber cannot contract, and tension is zero. As sarcomere lengths become
increasingly shorter than the optimum, the tension that can develop again decreases. This is because
thick filaments crumple as they are compressed by the Z discs, resulting in fewer myosin heads making
contact with thin filaments. Normally, resting muscle fiber length is held very close to the optimum
length by firm attachments of skeletal muscle to bones (via their tendons) and to other inelastic tissues