Muscle Questions
1. What is the main function of skeletal muscles? movement
2. List the three types of contractile cells of the body. skeletal, smooth, cardiac
3. Compare the shapes of the three types of contractile cells.
smooth = spindle-shaped
skeletal = elongated, cylindrical
cardiac = branched
4. Compare the visible appearance of the three types of contractile cells.
smooth = no visible striations; one centrally located nucleus
cardiac = visible striations; one centrally located nucleus; intercalated discs
skeletal = visible striations; multiple peripheral nuclei
5. What is the name of the structure that attaches skeletal muscles to bones? tendon
6. Bundles of skeletal muscle cells are called fascicles.
7. The connective tissue which immediately surrounds a muscle is called epimysium and the connective tissue
around the fascicles is called perimysium.
8. What is the function of endomysium? electrically isolates one muscle fiber from the next
9. Describe the following features associated with skeletal muscles.
a. Sarcolemma - plasma membrane
b. Muscle fibers - muscle cell
c. Mitochondria - organelle that synthesizes ATP during aerobic respiration
d. Sarcoplasmic reticulum - specialized smooth ER; stores calcuim
e. Myofibril - organelle responsible for contraction
10. What are the names for the two types of filament in a myofibril? thick (myosin) and thin (actin)
11. What creates the skeletal muscle cell's striated appearance? The arrangement of thick and thin
myofilaments, which form light and dark alternating bands.
12. Perpendicular to the myofilaments are the Z lines and the M lines. The Z lines connect the thin filaments
and the M lines connect the thick filaments.
13. The region of the myofibril between two Z lines that is the contractile unit of a muscle cell is called a
sarcomere.
14. Arrange the following from smallest structure to largest structure:
a. Muscle cell or muscle fiber
b. Fascicle
c. Myofilaments
d. Whole skeletal muscle
e. Myofibril
Myofilaments, Myofibril, Muscle cell or muscle fiber, Fascicle, Whole skeletal muscle
15. What happens at the neuromuscular junction when the action potential arrives at the axon terminal? vesicles
release neurotrasmitter (acetylcholine) into synaptic cleft
16. What happens to acetylcholine after it is released into the synaptic cleft? binds to receptor on sarcolemma
17. What happens after the acetylcholine binds to the acetylcholine receptor on the motor end plate
(sarcolemma)? changes initiated in membrane - becomes permeable to sodium ions leading to change in
polarity of cell membrane
18. What happens to the acetylcholine after it diffuses away from its receptor on the motor end plate? quickly
broken down
19. What happens when calcium ions are present in the cytosol of the muscle cell? sarcomeres contract - Ca2+
binds to troponin which moves tropomyosin revealing myosin binding sites
20. According to the sliding filament theory, when a muscle cell contracts, the thin filaments slide past the thick
filaments and the sarcomere shortens.
21. List the six most important chemicals involved in muscle contraction.
myosin
actin
tropomyosin
troponin
ATP
calcium ions
22. Where is myosin found in skeletal muscle cells? thick filament of sarcomere
23. What are the two parts to a myosin molecule? tail and heads
24. Which part moves providing the power stroke for muscle contraction? myosin heads
25. Which part of the myosin molecule has a hinge which allows vertical movement so that the cross-bridge can
bind to actin? heads
26. What two important binding sites are found on the cross bridges (heads) of myosin? ATP and Actin
27. What three protein molecules are the thin filaments made of?
actin
tropomyosin
troponin
28. Each subunit on actin contains binding sites for myosin head.
29. What is the function of tropomyosin? block myosin binding sites on actin - prevents continuous contraction
(tetnus)
30. What is the function of troponin? move tropomyosin, revealing myosin binding sites
31. What causes the tropomyosin to move away from the myosin binding sites on the actin? calcium binds to
troponin causing a conformation (shape) change; shape change moves tropomyosin off binding sites
32. Which of the following will attach to myosin?
actin
tropomyosin
troponin
ATP
calcium ions
33. Which of the following will attach to actin?
actin
tropomyosin
troponin
ATP
calcium ions
34. Which of the following will attach to troponin?
myosin
tropomyosin
actin
ATP
calcium ions
35. What causes the release of calcium ions into the cytosol from the terminal cisternae? arrival of action
potential (nerve impulse)
36. What causes the myosin binding sites on actin to be exposed? calcium binds to troponin causing a
conformation (shape) change; shape change moves tropomyosin off binding sites
37. What happens after the tropomyosin moves over, exposing the binding sites on the actin? myosin heads
form cross-bridges
38. What is it called when the cross bridge flexes, pulling the filament inward toward the center of the
sarcomere? power stroke
39. What causes the myosin heads (cross bridges) to disconnect from the actin? binding of ATP
40. What causes the myosin cross bridges to go from their tilted state to their upright, high energy state? transfer
of energy from ATP
41. What is required to move the calcium ions from the cytosol back into the sarcoplasmic reticulum? the action
potential ending
42. List the following steps in the order they would occur in a single cross bridge cycle.
a. ATP binds to the cross bridge and the cross bridge disconnecting from actin.
b. Myosin bind to actin.
c. Calcium ions are transported back into the sarcoplasmic reticulum.
d. Presence of calcium ions in the cytosol trigger the exposure of binding sites on actin.
e. The power stroke occurs.
f. ATP is hydrolyzed, leading to the re-energizing and repositioning of the cross bridge.
d, b, e, a, f, c
43. During the contraction of a muscle cell, what is happening to
a. the length of the sarcomere? shortens
b. the position of the Z lines with respect to one another? closer together
c. the length of the thin filament? does not change
d. the length of the thick filament? does not change
e. the width of the H zone? shortens
44. Compare skeletal, smooth, and cardiac muscles as to their body location, microscopic anatomy, regulation
of contraction, speed of contraction, and rhythmicity.
a. Body location:�skeletal muscle is attached to bones or to skin (some facial muscles); cardiac muscle is
located in the walls of the heart; smooth muscle is found in the walls of hollow visceral organs (other
than the heart).
b. Microscopic anatomy:�skeletal muscle consists of very long, cylindrical, multinucleated cells with very
obvious striations; cardiac muscle consists of branching chains of cells that are uninucleated and possess
striations; smooth muscle consists of single fusiform uninucleated cells that lack striations.
c. Regulation of contraction:�skeletal muscle is voluntary via nervous system controls, but this normal
voluntary control can be overridden by involuntary reflex arcs (as explained in later chapters); cardiac
muscle is involuntary via the heart pacemaker, nervous system controls, and hormones; smooth muscle
is involuntary via nervous system controls, hormones, other chemicals, and stretching.
d. Speed of contraction:�skeletal muscle is slow to fast; cardiac muscle is slow; smooth muscle is the
slowest.
e. Rhythmicity:�skeletal muscle is arrhythmic; cardiac muscle is rhythmic; smooth muscle is sometimes
rhythmic.
45. List the seven criteria that are used in naming muscles and give an example of each.
a. Direction of the muscle fibers (e.g., external oblique)
b. Relative size of the muscle (e.g., maximus, minimus, longus)
c. Location of the muscle (e.g., temporalis, frontalis)
d. Number of origins (e.g., biceps, triceps, quadriceps)
e. Location of the muscle's origin and insertion (e.g., the sternocleidomastoid muscle has its origin on the
sternum [sterno] and clavicle [cleido] and inserts on the mastoid process of the temporal bone)
f. Shape of the muscle (e.g., the deltoid muscle is roughly triangular�deltoid means "triangular")
g. Action of the muscle (e.g., the adductor muscles of the thigh all bring about its adduction, and the
extensor muscles of the wrist all extend the wrist)
46. Discuss the importance of calcium in skeletal muscle contraction.
a. Calcium is necessary for myosin heads to attach to binding sites on actin filaments. As the action
potential travels into the muscle cell, it stimulates the sarcoplasmic reticulum surrounding each
myofibril to release its stored calcium into the sarcoplasm. The calcium triggers the binding of myosin
heads to actin filaments and the initiation of the sliding of filaments.
47. Discuss the role of the myosin heads in sliding filament theory.
a. The myosin heads attach to binding sites on the actin filaments to form cross bridges and to begin the
process of sliding. The myosin heads swivel toward the center of the sarcomere, attaching and detaching
several times. The actin filaments are pulled toward the center of the sarcomere. The actin filaments
slide past the thick filaments as the Z discs are pulled closer together. As this event occurs
simultaneously in sarcomeres throughout the cell, the musclecell shortens.
48. Describe the events that occur from the time that a motor neuron releases acetylcholine at the neuromuscular
junction until muscle cell contraction occurs.
a. Acetylcholine is released, which diffuses through the synaptic cleft and attaches to receptors on the
sarcolemma. The sarcolemma permeability to sodium ions increases briefly, causing sodium ions to rush
into the muscle cell, which changes the electrical conditions of the resting sarcolemma. An action
potential is initiated and sweeps over the entire sarcolemma. Calcium ions are released from storage
areas inside the sarcoplasmic reticulum of the muscle cell. They attach to the myofilaments, which
triggers the sliding of the myofilaments and causes a muscle cell contraction.
49. Explain the steps in the sliding filament theory of muscle contraction, following the spreading of an action
potential along the sarcolemma.
a. An action potential triggers the sarcoplasmic reticulum to release calcium ions into the sarcoplasm of the
cell. The calcium ions bind to regulatory proteins on the actin filaments, changing both their shape and
their position on the actin filaments. This action allows myosin receptor sites on the thin actin filaments
to become exposed. The myosin heads attach to the myosin binding sites on the actin filaments.
Energized by ATP, the myosin heads swivel toward the center of the sarcomere, attaching and detaching
several times. In the process the thin actin filaments are pulled toward the center of the sarcomere. As
this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens. When the
action potential ends, the calcium ions are reabsorbed into the sarcoplasmic reticulum storage areas,
causing the regulatory proteins to resume their original shape and position. Since the myosin heads now
have nothing to attach to, the muscle cell relaxes and returns to its original length.
50. What is the primary functional difference between an origin and an insertion?
a. Origins are where muscles begin and are fixed (what is the one exception that we have studied?).
Insertions are the ends of muscles and are the moving location.
51. Identify the labeled features on the muscle below.
a.
b.
c.
d.
epimysium
fascicle
perimysium
muscle fiber
52. Label the regions of the sarcomere below.
a. see sheet from class
53. Identify the steps in the contraction process.
a. resting sarcomere
b. binding site exposure
c. cross bridge formation
d. power stroke
e. release of myosin head
f. recharging of myosin head