11) The muscle spindle is
A) an actin-myosin complex.
B) a troponin-tropomyosin complex.
C) axons wound around muscle fibers.
D) a group of dendrite-encircled muscle fibers.
E) a muscle cell that makes up a muscle group.

Answer: D

43) The contraction of skeletal muscles is based on
A) actin filaments coiling up to become shorter.
B) myosin filaments coiling up to become shorter.
C) actin and myosin filaments both coiling up to become shorter.
D) actin cross-bridges binding to myosin and then flexing.
E) myosin cross-bridges binding to actin and then flexing.

Answer: E

44) Compared to oxidative skeletal muscle fibers, those classified as glycolytic typically have
A) a higher concentration of myoglobin.
B) a higher density of mitochondria.
C) a darker visual appearance.
D) a smaller diameter.
E) less resistance to fatigue.

Answer: E

45) Myasthenia gravis is a form of muscle paralysis in which
A) motor neurons lose their myelination and the ability to rapidly fire action potentials.
B) acetylcholine receptors are destroyed by an overactive immune system.
C) ATP production becomes uncoupled from mitochondrial electron transport.
D) the spinal cord is infected with a virus that attacks muscle stretch receptors.
E) troponin molecules become unable to bind calcium ions

Answer: B

46) A skeletal muscle deprived of adequate ATP supplies will
A) immediately relax.
B) release all actin-myosin bonds.
C) enter a state where actin and myosin are unable to separate.
D) fire many more action potentials than usual and enter a state of "rigor."
E) sequester all free calcium ions into the sarcoplasmic reticulum.

Answer: C

47) Most of the ATP supplies for a skeletal muscle undergoing 1 hour of sustained exercise come from
A) creatine phosphate.
B) glycolysis.
C) substrate phosphorylation.
D) oxidative phosphorylation.
E) de novo synthesis.

D

48) The calcium ions released into the cytosol during excitation of skeletal muscle bind to
A) troponin.
B) tropomyosin.
C) actin.
D) myosin.
E) transverse tubules.

A

49) The "motor unit" in vertebrate skeletal muscle refers to
A) one actin binding site and its myosin partner.
B) one sarcomere and all of its actin and myosin filaments.
C) one myofibril and all of its sarcomeres.
D) one motor neuron and all of the muscle fibers on which it has synapses.
E) an entire muscle.

D

51) Calcium ions initiate sliding of filaments in skeletal muscles by
A) breaking the actin-myosin cross-bridges.
B) binding to the troponin complex, which then relocates tropomyosin.
C) transmitting action potentials across the neuromuscular junction.
D) spreading action potentials through the T tubules.
E) reestablishing the resting membrane potential following an action potential.

B

50) The muscles of a recently deceased human can remain in a contracted state, termed rigor mortis, for several hours, due to the lack of
A) phosphorylated myosin.
B) ATP needed to break actin-myosin bonds.
C) calcium ions needed to bind to troponin.
D) oxygen supplies needed for myoglobin.
E) sodium ions needed to fire action potentials.

B

52) Muscle cells are stimulated by neurotransmitters released from the synaptic terminals of
A) T tubules.
B) motor neuron axons.
C) sensory neuron axons.
D) motor neuron dendrites.
E) sensory neuron dendrites.

B

53) In a relaxed skeletal muscle, actin is not chemically bound to
A) myosin.
B) troponin.
C) tropomyosin.
D) Z lines

A

54) Skeletal muscle contraction begins when calcium ions bind to
A) energized cross-bridges.
B) myosin.
C) actin.
D) tropomyosin.
E) troponin.

Answer: E

55) A skeletal muscle with abnormally low levels of calcium ions would be impaired in
A) ATP hydrolysis.
B) the initiation of an action potential.
C) maintaining its resting membrane potential.
D) initiating contraction.
E) its ability to sustain glycolysis.

D

56) Which of the following is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber?

1. Tropomyosin shifts and unblocks the cross-bridge binding sites.
2. Calcium is released and binds to the troponin complex.
3. Transverse tubules depolarize the sarcoplasmic reticulum.
4. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.
5. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane.

A) 1 → 2 → 3 → 4 → 5
B) 2 → 1 → 3 → 5 → 4
C) 2 → 3 → 4 → 1 → 5
D) 5 → 3 → 1 → 2 → 4
E) 5 → 3 → 2 → 1 → 4

E

57) The lumen of the transverse tubules of skeletal muscles contains
A) extracellular fluid.
B) cytosol.
C) actin.
D) myosin.
E) sarcomeres.

A

58) Sustained muscle contraction without relaxation between successive stimuli is called
A) tonus.
B) fused tetanus.
C) an all-or-none response.
D) fatigue.
E) a spasm.

B

59) Skeletal, cardiac, and smooth muscle all have
A) A bands and I bands.
B) transverse tubules.
C) gap junctions.
D) motor units.
E) thick and thin filaments.

E

60) Calcium ions regulate contraction of smooth muscle cells by binding to
A) troponin.
B) tropomyosin.
C) actin.
D) myosin.
E) calmodulin.

E

61) Action potentials in the heart move from one contractile cell to the next via
A) chemical synapses using acetylcholine.
B) chemical synapses using norepinephrine.
C) electrical synapses using gap junctions.
D) myelinated motor neurons.
E) non-myelinated motor neurons.

C

65) A ball-and-socket joint connects
A) the radius to the ulna.
B) the radius to the humerus.
C) the ulna to the humerus.
D) the humerus to the scapula.
E) the radius to the scapula.

D

73) The structure pictured in the figure is found in
A) skeletal muscles and smooth muscles.
B) cardiac muscles and skeletal muscles.
C) smooth muscles and cardiac muscles.
D) smooth muscles, skeletal muscles, and cardiac muscles.
E) smooth muscles.

b

74) Myosin filaments without actin overlap are in which section of the figure?
A) A
B) B
C) C
D) D
E) E

D

75) Overlapping actin and myosin filaments are found in which section of the figure?
A) A
B) B
C) C
D) D
E) E

B

79) During the contraction of a vertebrate skeletal muscle fiber, calcium ions
A) break cross-bridges by acting as a cofactor in the hydrolysis of ATP.
B) bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.
C) transmit action potentials from the motor neuron to the muscle fiber.
D) spread action potentials through the T tubules.
E) re-establish the polarization of the plasma membrane following an action potential.

B