Name Answer Key
1. What is the main function of skeletal muscles?
Movement of the body.
2. List the three types of contractile cells in the body.
Skeletal muscle cells, cardiac muscle cells, and smooth muscle cells.
3-5. Give the shapes for each of the three muscle types.
Skeletal muscles are elongated, cardiac muscles are branched, and smooth muscles are spindle-shaped.
6-11. Describe the striation pattern and nucleation of each type of muscle.
Cardiac muscle cells have visible striations and single, centrally located nuclei. Skeletal muscle cells have visible striations, and multiple peripheral nuclei. Smooth muscle cells have no visible striations and single, centrally-located nuclei.
12. What structures attach skeletal muscle to bones? Tendons
13. What are bundles of skeletal muscle called? Fascicles
14-15. The connective tissue which immediately surrounds a muscle is called _epimysium_ and the connective tissue around the fascicles is called _perimysium_.
16. What is the function of endomysium? It electrically insulates muscle cells from one another.
17-24. Match these terms to their description:
_ Terminal cisternae_ a. Sac-like regions of the sarcoplasmic reticulum that contain calcium ions.
_ Mitochondria___ b. Sites of ATP synthesis.
_ Sarcolemma____ c. Plasma membrane of the muscle cell.
_ Muscle fibers___ d. Alternative name for skeletal muscle cells.
_ Sarcoplasmic reticulum_ e. Interconnecting tubules of endoplasmic reticulum that surround each myofibril.
_ Triad__________ f. A group of one T tubule lying between two adjacent terminal cisternae.
_ T-tubules______ g. Invaginations of the sarcolemma that projecting deep into the cell.
_ Myofibril______ h. Contains the contractile filaments within the skeletal muscle cell.
25. What are the names for the two types of filament in a myofibril?
Thin and thick filaments.
26. What creates the skeletal muscle cell's striated appearance?
The arrangement of thick and thin myofilaments, which form light and dark alternating bands.
27-29. Match the following:
A band I band H zone
____I band____ a. Contains only thin filaments.
____H zone____ b. Contains only thick filaments.
____A band____ c. Contains both thin and thick filaments.
30-31. 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.
32. What is the region of the myofibril between two Z lines that is the contractile unit of a muscle cell called? Sarcomere
33. Arrange the following from smallest structure to largest structure by ranking them with a number (1 for smallest, 5 for largest):
Muscle cell or muscle fiber 3
Whole skeletal muscle 5
Contraction of Motor Units
34. What are the groups of muscle cells called, within a muscle, that are innervated by one motor neuron and contracted together and stimulated by that motor neuron? Motor units.
35. What important factors determine the strength of contraction of a muscle? The size of the motor unit being stimulated and the number of motor units being stimulated.
36. What is a motor unit composed of? A motor neuron and all of the muscle cell which are stimulated by that motor neuron.
37. What is the junction between the branch of an neuron and a muscle cell called? A neuromuscular junction.
38. When a motor neuron fires, how many muscle cells are stimulated? All of the muscle cells that form junctions with that one neuron. The number of muscle cells in a motor unit can vary.
39. What is the relationship between the strength of the muscle contraction and the number of motor units which are stimulated? The stronger the muscle contraction, the more motor units are stimulated.
40. What is recruitment? The stimulation of additional motor units for increased strength of muscular contraction.
41. What is the general function of motor units that contain only a few muscle cells?
Precise muscle movements.
42. What two factors affect the strength and degree of muscle movement? The number of motor units firing and the number of muscle cells per motor unit.
43. Describe the motor units in large muscles which exhibit gross movements. Large muscles which exhibit gross movements have large motor units in which a single neuron is connected to a large number of muscle cells.
44. What did the virtual professor mean when she said "small precise muscles that cause fine movements, like those of the eye, have a huge number of motor fibers controlling them, comparatively speaking"? If a muscle has motor units which contain only a few muscle cells per motor neuron, then there would have to be quite a few motor neurons going to that particular muscle.
45. Tell if the following muscles would have small or large motor units:
a. muscles of the fingers Small
b. biceps brachii muscle Large
c. gastrocnemius muscle Large
d. muscles of the throat involved in speech Small
46. Muscle A contains 12,000 muscle cells and 30 motor neurons which innervate these cells. Muscle B contains 2,000 muscle cells and 400 motor neurons which innervate these cells.
a. On average, how many muscle cells are there in each motor unit for both of these muscles?
Muscle A: 400 muscle cells per motor unit
Muscle B: 5 muscle cells per motor unit
b. Which of these muscles would you expect to contract with precise control? Muscle B
47. What is muscle tone? Random, minute, asynchronous motor unit contractions which provide a nearly constant state of low-level tension and resistance to stretch in a muscle, even when the muscle is in a relaxed state.
48. What causes muscle tone? Nerve impulses are constantly being sent to skeletal muscles.
49. What happens to a muscle if all the motor neurons to that muscle are cut? The muscle loses its muscle tone, elongates, and becomes flaccid.
50. Why is it necessary for muscle to have mechanisms to make ATP quickly? Because there is very little ATP stored in muscles.
51. Why is the reaction that releases energy from ATP termed a "hydrolysis" reaction? Because water is used up in the reaction to split the terminal phosphate group off with water.
52. Give a specific example of a hydrolytic enzyme that hydrolyzes ATP into ADP and Pi in muscle cells. The myosin head (cross bridge) will function as a hydrolytic enzyme when it hydrolyzes ATP into ADP and Pi. The energy from the ATP is transferred into the myosin head, which pops up and becomes ready to bind to actin.
53. Why is the production of ATP from ADP termed a "dehydration synthesis" reaction? It is called "dehydration" because water is removed during the reaction. It's called "synthesis" because a bigger molecule is synthesized from two smaller ones.
54. Muscle cells have only enough ATP to last for _ a few seconds _ during an active contraction period.
55. Why are white muscle fibers also called "fast-twitch glycolytic fibers?" They use glycolysis, which synthesizes ATP quickly, resulting in fast contractions, hence "fast-twitch". They use mainly glycolysis for metabolism, hence "glycolytic".
56. Why are red muscle fibers also called "slow-twitch oxidative fibers?" They use Krebs cycle and oxidative phosphorylation, which synthesizes ATP relatively slowly compared to glycolysis, resulting in prolonged contractions, hence "slow-twitch". The Krebs cycle and oxidative phosphorylation requires oxygen, hence "oxidative."
The Neuromuscular Junction
57-64. Match the following terms to their description:
___ T-Tubule ____________ a. Invaginations of the sarcolemma penetrating deep into the interior of the muscle cell.
__ Synaptic Cleft _________ b. The space between the axon terminal and the motor end plate.
__ Axon terminal _________ c. The swollen distal end of the motor neuron axon.
__ Sarcolemma __________ d. The muscle cell membrane.
__ Synaptic Vesicles _______ e. Structures within the axon terminal that contain the neurotransmitter acetylcholine.
__ Sarcomere ____________ f. The contractile unit of the muscle cell that extends from one Z line to the next.
__ Terminal Cisternae & Sarcoplasmic Reticulum __ g. Structures within skeletal muscle cells that serve as reservoirs of calcium ions.
__ Motor End Plate _______ h. A folded region of the sarcolemma at the neuromuscular junction.
65-69. List the following events in the order they occur:
__3__ a. The motor end plate is depolarized.
__5__ b. The sarcomeres contract.
__2__ c. Acetyl choline is released from the axon terminal into the synaptic cleft.
__4__ d. The depolarization triggers an action potential which propagates along the sarcolemma and the T tubules.
__1__ e. An action potential arrives at the axon terminal
70-79. Place the following events in their proper sequence:
__4__ a. Acetyl choline is released into the synaptic cleft.
__8__ b. Action potential propagates along the sarcolemma and down the T Tubules.
__3__ c. Synaptic vesicles fuse to membrane of axon terminal.
__6__ d. Motor end plate becomes depolarized.
__7__ e. Action potential is initiated on the sarcolemma.
__1__ f. Action potential arrives at the axon terminal.
__9__ g. Calcium ions are released from the terminal cisternae.
__5__ h. Acetylcholine binds to receptor sites on the motor end plate.
_10__ i. The muscle cell contracts.
__2__ j. Calcium ions enter the axon terminal.