What is the definition of a "motor unit"?

a. a single motor neuron plus all the muscle fibers it innervates

b. a single muscle fiber plus all the motor neurons that innervate it

c. all of the motor neurons supplying a single muscle

d. a pair of antagonistic muscle

e. all of the muscles that affect the movement of any given joint

a. a single motor neuron plus all the muscle fibers it innervates

Which of these occurs during an isometric contraction of a skeletal muscle?

a. The I bands shorten and the A bands stay the same length

b. The thick and thin filaments slide past each other

c. Sarcomere length does not change significantly

d. The A bands shorten and the I bands stay the same length

e. Cross-bridges lock onto actin, similar to what occurs in rigoris mortis

c. Sarcomere length does not change significantly

What is the best description of a tetanic contraction in a skeletal muscle cell?

a. A single action potential in the motor neuron causes a sustained contraction

b. Multiple action potentials in the motor neuron cause a sustained contraction

c. The action potential in the muscle cell is prolonged to last as long as the contraction

d. Repeated action potentials from the motor neuron summate into a sustained depolarization of the motor end plate, causing a sustained contraction

e. A very large amplitude action potential in the motor neuron causes a very strong contraction in the skeletal muscle cell

b. Multiple action potentials in the motor neuron cause a sustained contraction

The optimal length (L_o) of a skeletal muscle cell is

a. the length at which the muscle can generate its maximum tetanic tension

b. the shortest length the muscle can achieve while attached to bone, because the amount of overlap between thick and thin filaments is maximal then

c. generally the length attained when the joint it crosses is fully extended

d. the only length at which any tension can be generated by the muscle cell

e. the length at which thin filaments from opposite sides of a sarcomere overlap

a. the length at which the muscle can generate its maximum tetanic tension

Compared to type 1 (slow oxidative) skeletal muscle fibers, how are type 2X (fast-glycolytic) skeletal muscle fibers different?

a. type 2x fibers have more abundant mitochondria

b. Type 2X fibers fatigue more readily

c. Type 2X fibers have more abundant myoglobin

d. Type 2X fibers are smaller in diameter

e. Type 2X motor units contain fewer fibers per alpha motor neuron

b. Type 2X fibers fatigue more readily

Which of these would increase the tension generated in a skeletal muscle?

a. Increasing the frequency of firing in alpha motor neurons innervating the muscle

b. Stretching the muscle to very long lengths

c. Resting the muscle for several weeks

d. Switching off fast-glycolytic motor units and activating an equal number of slow-oxidative motor units

e. Increasing the amplitude of action potentials in the alpha motor neurons that innervate the muscle

a. Increasing the frequency of firing in alpha motor neurons innervating the muscle

Which is a TRUE statement about smooth muscle?

a. smooth muscle is striated

b. smooth muscle does not have thick and thin filaments

c. smooth muscle does not use troponin-tropomyosin to regulate cross-bridge activity

d. changes in cytosolic calcium do not regulate cross-bridge activity in smooth muscle

e. the myosin in smooth muscle requires phosphorylation before it can bind to ATP

c. smooth muscle does not use troponin-tropomyosin to regulate cross-bridge activity

Which of the following does NOT correctly describe cardiac muscle cells?

a. they are arranged in layers surrounding hollow cavities in the heart

b. they are much shorter than skeletal muscle fibers and generally have a single nucleus

c. their membranes are depolarized initially by the influx of sodium ions

d. they depolarize prior to contraction

e. they contract in the absence of external calcium

e. they contract in the absence of external calcium

The prolonged electrical depolarization of cardiac muscle cells that occurs during contraction is due primarily to the persistent influx of what ion?

a. sodium

b. calcium

c. nitrate

d. chloride

e. potassium

b. calcium

Which best describes a role of L-type Ca²⁺ channels in cardiac muscle cells?

a. they function exactly the same in the cardiac muscle cells as they do in skeletal muscle

b. they are directly responsible for making cardiac muscle fatigue-resistant

c. they cause the absolute refractory period to be very brief

d. they are responsible for preventing tetanic contractions

e. they act as non-conducting voltage sensors that mediate excitation-contraction coupling

d. they are responsible for preventing tetanic contractions

Myasthenia gravis is a collection of muscle disorders that cause muscle fatigue and weakness. What is the major mechanism?

a. overproduction of acetylcholinesterase

b. autoimmune destruction of nicotinic acetylcholine receptors

c. demyelination of axons in motor pathways

d. autoimmune destruction of L-type Ca2+ channels

e. a tumor that overproduces acetylcholine

b. autoimmune destruction of nicotinic acetylcholine receptors

What is the function of the transverse tubules in a skeletal muscle fiber?

a. They store the calcium ions that are the main source of activation for the cross-bridge cycle

b. They form the Z lines that mark the end of each sarcomere

c. They allow action potentials to propagate deep into the center of skeletal muscle cells

d. They manufacture and store ATP

e. They run parallel with the myofibrils and have abundant Ca2+ ATPase proteins for pumping Ca2+ back into the sarcoplasmic reticulum

c. They allow action potentials to propagate deep into the center of skeletal muscle cells

Which of the following statements regarding skeletal muscle contraction is true?

a. during a lengthening contraction, the tension exerted by the muscle exceeds the load on the muscle

b. in every isotonic muscle contraction, the length of the muscle remains constant

c. during every muscle contraction, muscle fibers change length

d. during every muscle contraction, tension is developed in the muscle

e. cross-bridge cycle faster during isometric contractions than during isotonic contractions

d. during every muscle contraction, tension is developed in the muscle

Which of the following statements regarding myosin in skeletal muscle is TRUE?

a. myosin cross-bridge heads contain two-binding sites, one for actin and one for tropomyosin

b. myosin is an ATPase

c. the rate of ATP hydrolysis by myosin is the same in all types of skeletal muscle

d. all of the myosin cross-bridge heads in a thick filament are oriented and rotate in the same direction

e. troponin covers the binding site on myosin molecules until Ca2+ binds to troponin to remove it from its blocking position

b. myosin is an ATPase

Which of these is a major difference between smooth muscle and skeletal muscle?

a. myosin is the main regulatory protein in smooth muscle

b. myosin is the main regulatory protein in skeletal muscle

c. skeletal muscle usually exhibits spontaneous activity, while smooth muscle cannot contract spontaneously

d. only skeletal muscle requires increased calcium ion concentration in the cytosol for contraction

e. only skeletal muscle has both actin and myosin

a. myosin is the main regulatory protein in smooth muscle

Which is a TRUE statement about excitation-contraction coupling?

a. in skeletal muscle cells, it requires the influx of extracellular calcium ions

b. in smooth muscle cells, it must be preceded by an action potential in the cell membrane

c. in all kinds of muscle it requires the entry of calcium from the extracellular fluid

d. calcium-induced calcium release plays a role in cardiac muscle cells, as well as in some smooth muscle cells

e. in skeletal muscle cells, excitation coupling begins when an action potential propagates along the sarcoplasmic reticulum membrane

d. calcium-induced calcium release plays a role in cardiac muscle cells, as well as in some smooth muscle cells

Which of the following statements regarding contraction in skeletal and smooth muscle is TRUE?

a. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding directly to myosin.

b. In skeletal muscle, calcium ions bind to a regulatory protein on thin filaments; in smooth muscle, calcium ions bind to a regulatory protein on thick filaments.

c. In skeletal muscle, calcium initiates contraction by binding to myosin light-chain kinase, while in smooth muscle calcium initiates contraction by binding directly to tropomyosin.

d. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding to calmodulin.

e. All of the choices are true.

d. In skeletal muscle, calcium initiates contraction by binding to troponin, while in smooth muscle calcium initiates contraction by binding to calmodulin.

Which of the following choices lists a correct sequence of events following the depolarization of transverse tubules in excitation-contraction coupling in cardiac muscle?

a. Calcium release into cytosol, calcium ion influx through sarcolemma, actin and myosin attach, thin filaments slide toward the middle of sarcomeres

b. Actin and myosin attach, thin filaments slide toward the middle of sarcomeres, calcium release into cytosol

c. Calcium release into cytosol, actin and myosin attach, calcium ion influx through sarcolemma, thin myofilaments slide toward the middle of sarcomeres

d. Calcium release into cytosol, actin and myosin attach, thin filaments slide toward the middle of sarcomeres, calcium ion influx through sarcolemma

e. Calcium ion influx through sarcolemma, calcium release into cytosol, actin and myosin attach, thin myofilaments slide toward the middle of sarcomeres

e. Calcium ion influx through sarcolemma, calcium release into cytosol, actin and myosin attach, thin myofilaments slide toward the middle of sarcomeres

Sarah is a sprinter who specializes in quick and powerful bursts of speed followed by periods of rest. Priya is a marathon runner who specializes in long, steady runs. Compared to Priya, Sarah is likely to have

a. leg muscles with a larger diameter.

b. leg muscles with a smaller diameter.

c. hypertrophy of type I muscle fibers.

d. a higher density of capillaries in her leg muscles.

e. lower concentrations of glycolytic enzymes in her leg muscles.

a. leg muscles with a larger diameter.

In a laboratory experiment you have treated a muscle cell with a calcium channel blocker. The membrane still depolarizes, but the concentration of calcium never increases in the cytosol. Which of the following describes what will happen next?

a. Only one cross-bridge cycle will occur but no second cycle.

b. Binding of myosin to actin will take place.

c. Tropomyosin will continue to cover the myosin binding sites on actin and no cross-bridges will form.

d. A single twitch in skeletal muscle but no sustained contraction.

e. Contraction will occur, but the muscle will be stuck in the contracted state and unable to relax.

c. Tropomyosin will continue to cover the myosin binding sites on actin and no cross-bridges will form.

Tetanus toxin inhibits acetylcholinesterase activity. What will a person suffering from tetanus toxin poisoning look like?

a. All muscles of the body might exhibit frequent small twitches.

b. All muscles of the body might present a tense, fully contracted state.

c. All muscles of the body might present a limp, relaxed state known as flaccid paralysis.

d. All muscles of the body will be completely normal in function.

b. All muscles of the body might present a tense, fully contracted state.