1
Q
What is the primary characteristic regarding the control of skeletal muscle tissue contractions?
A
Skeletal muscle tissue contractions are under voluntary control.
2
Q
Name the six functions of skeletal muscle tissue.
A
They produce body movement
maintain posture and body position
support soft tissues
guard body entrances and exits (using sphincters)
maintain body temperature
store nutrients.
3
Q
How does skeletal muscle tissue help maintain body temperature?
A
Muscle contractions use energy, and some of that energy is converted to heat, which keeps the body temperature in the normal functional range.
4
Q
What components make up a skeletal muscle organ?
A
Skeletal muscles are complex organs composed primarily of skeletal muscle fibers (muscle cells) plus connective tissues, nerves, and blood vessels.
5
Q
What is the dense layer of collagen fibers that surrounds the entire skeletal muscle and separates it from surrounding tissues?
A
The epimysium.
6
Q
What is a muscle fascicle?
A
A bundle of muscle fibers.
7
Q
What connective tissue layer divides the skeletal muscle into compartments and surrounds each muscle fascicle?
A
The perimysium.
8
Q
What is the thin layer of areolar connective tissue that surrounds each individual muscle fiber?
A
The endomysium.
9
Q
What two types of structures can be formed when the collagen fibers of the connective tissue layers merge at the ends of a skeletal muscle?
A
A tendon (a bundle) or an aponeurosis (a broad sheet).
10
Q
What is the name for the plasma membrane of a skeletal muscle fiber?
A
The sarcolemma.
11
Q
What is the name for the cytoplasm of a skeletal muscle fiber?
A
The sarcoplasm.
12
Q
What are myofibrils?
A
Cylindrical structures within the sarcoplasm, consisting of bundles of protein filaments called myofilaments, which give the fiber a striated appearance.
13
Q
What are the primary protein components of thin filaments and thick filaments?
A
Thin filaments are composed primarily of actin, and thick filaments are composed primarily of myosin.
14
Q
What are sarcomeres?
A
Repeating contractile units formed by the arrangement of myofilaments within each myofibril.
15
Q
What structures mark the boundary between adjacent sarcomeres?
A
Z lines.
16
Q
What are Transverse tubules (T tubules)?
A
Narrow tubes continuous with the sarcolemma that extend into the sarcoplasm at right angles to the cell surface, forming passageways through the muscle fiber.
17
Q
What is the primary specialized function of the sarcoplasmic reticulum (SR) in a muscle fiber?
A
It is specialized for the storage and release of calcium ions (Ca 2+).
18
Q
What structure is formed by a T tubule paired with two terminal cisternae of the sarcoplasmic reticulum?
A
A triad.
19
Q
What protein holds the F-actin strand together in a thin filament?
A
Nebulin.
20
Q
What protein covers the active sites on G-actin and prevents interaction with myosin in a resting muscle?
A
Tropomyosin.
21
Q
What is the role of the third subunit of the troponin molecule?
A
It has a receptor that binds two calcium ions (Ca 2+).
the other two subunits attach to tropomyosin and g-actin
22
Q
What component of the thick filament acts as a ‘molecular spring’ and recoils after stretching, connecting the thick filament to the Z line?
A
The elastic protein titin.
23
Q
According to the sliding filament theory, what happens during muscle contraction?
A
Thin filaments slide past the thick filaments.
24
Q
When a myofibril shortens, what three changes occur in the sarcomere bands and lines?
A
(1) The H bands and I bands get smaller, (2) the zones of overlap get larger, and (3) the Z lines move closer together.
25
What term describes a cell whose inner plasma membrane surface has a slight negative charge compared to the outer surface?
The cell is polarized, and the charge separation is called the membrane potential.
26
What unique structures do neurons and skeletal muscle fibers possess that allow them to produce and carry action potentials?
They have **electrically excitable membranes** containing **voltage-gated ion channels.**
27
What is the defining characteristic of an action potential in electrically excitable membranes?
The initial depolarization and repolarization events produce an electrical impulse that is spread, or propagated, **along the plasma membrane**.
## Footnote
non electrically excitable membranes only have action potentials occur locally
28
Why does an action potential travel in only one direction?
The refractory period prevents it from propagating back in the direction from which it was initiated.
29
What causes the membrane potential to become positive (depolarization)?
When the threshold is reached, voltage-gated Na+ channels open, and positively charged sodium ions rush into the cell.
30
What causes repolarization?
Voltage-gated Na+ channels close and voltage-gated K+ channels open, causing potassium ions to move out of the cell, returning the potential toward resting levels.
31
What structure propagates an electrical impulse from the nervous system to skeletal muscle fibers?
A motor neuron.
32
What are the three main components of the neuromuscular junction (NMJ)?
The axon terminal (synaptic terminal) of a motor neuron, the motor end plate (specialized sarcolemma region), and the synaptic cleft (intervening space).
33
What is acetylcholine (ACh)?
A neurotransmitter released by a neuron to change the permeability or other properties of another cell's plasma membrane.
34
What is the function of the enzyme acetylcholinesterase (AChE)?
It breaks down ACh in the synaptic cleft and sarcolemma.
35
What process triggers the release (exocytosis) of ACh into the synaptic cleft from the axon terminal?
The arrival of an electrical impulse, or action potential, at the axon terminal.
36
What event is responsible for generating an action potential in the muscle fiber sarcolemma?
ACh binding to ACh-receptor membrane channels, causing Na+ to rush into the sarcoplasm.
37
What is the term for the event that links the action potential sweeping down the T tubule to the release of Ca 2+ from the SR?
Excitation-contraction coupling.
38
What molecule supplies the energy for a muscle fiber contraction?
ATP.
39
How does the myosin head become 'energized' before contraction?
The myosin head functions as ATPase to break down ATP into ADP and a phosphate (P), which remain bound to the head, cocking it.
40
What event allows the energized myosin heads to interact with the thin filaments?
Calcium ions (Ca 2+) bind to troponin, causing the troponin molecule to change position, rolling the tropomyosin away from the active sites on actin.
41
What is the power stroke?
The action after cross-bridges form, where the stored energy is released as the myosin heads pivot toward the M line, releasing the bound ADP and phosphate group.
42
How is the link between the myosin head and the actin active site broken (cross-bridge detachment)?
When another ATP binds to the myosin head.
43
What condition must be met for the entire contraction cycle to repeat?
Ca 2+ concentrations must remain elevated, and ATP reserves must be sufficient.
44
What relationship explains how sarcomere length affects the amount of tension a muscle fiber produces?
The sarcomere length-tension relationship.
45
When is tension produced most efficiently by a sarcomere?
When the resting length is within the optimal range, allowing the maximum number of cross-bridges to form.
46
What are the three phases of a muscle twitch?
The latent period 2ms
the contraction phase 15ms
the relaxation phase 25ms
47
What happens during the latent period of a twitch?
The action potential sweeps across the sarcolemma and the SR releases Ca 2+; the muscle fiber does not yet produce tension.
48
What happens during the relaxation phase of a twitch?
Calcium levels fall, active sites are covered by tropomyosin, and the number of active cross-bridges declines as they detach, causing tension to return to resting levels.
49
What is a motor unit?
A motor neuron and all the muscle fibers that it controls.
50
What is the relationship between the number of fibers in a motor unit and the precision of movement?
The smaller the size of the motor unit (fewer fibers), the finer, or more precise, the movement can be.
51
What is recruitment?
The smooth but steady increase in muscular tension produced by increasing the number of active motor units.
52
What phenomenon describes a progressive increase in twitch tension that occurs when a skeletal muscle fiber is stimulated immediately after the relaxation phase has ended?
Treppe.
53
What is the difference between incomplete tetanus and complete tetanus?
In incomplete tetanus, the muscle produces almost peak tension during rapid cycles of contraction and relaxation. In complete tetanus, a higher stimulation frequency eliminates the relaxation phase entirely, leading to continuous maximal tension.
54
What characterizes an isotonic contraction?
Tension rises to a constant level, and the skeletal muscle's length changes.
55
What is a concentric contraction?
An isotonic contraction where the muscle tension exceeds the load, and the muscle shortens.
56
What is an eccentric contraction?
An isotonic contraction where the peak tension developed is less than the load, and the muscle elongates (lengthens).
57
What characterizes an isometric contraction?
The tension rises, but the muscle as a whole does not change length because the tension produced never exceeds the load.
58
What two processes are primarily used by muscle fibers to generate ATP from glucose?
Glycolysis (anaerobic breakdown) and aerobic metabolism (in mitochondria).
59
At what activity level does aerobic metabolism normally meet 95 percent of ATP demands?
At resting levels.
60
What happens to pyruvate produced by glycolysis during peak exertion when oxygen is limited?
Pyruvate is converted to a three-carbon lactate molecule.
61
What primary chemical change occurs inside muscle fibers at peak activity that contributes to muscle fatigue?
Hydrogen ions (H+) and lactate build up, resulting in a decline in pH (metabolic acidosis), which alters the functional characteristics of key enzymes.
62
What is the Cori cycle?
The shuffling of lactate, released by muscle fibers into the bloodstream, to the liver, where it is recycled into glucose and then released back to the muscle cells.
63
What is the oxygen debt (or EPOC)?
The amount of oxygen required during the recovery period to restore normal, preexertion conditions, including restoring ATP, creatine phosphate, and glycogen concentrations.
64
What are the two primary reasons slow fibers are highly resistant to fatigue?
They have a dense capillary network supplying dramatically higher oxygen and they contain the red oxygen-carrying pigment myoglobin.
65
Why do fast fibers fatigue rapidly?
Their contractions use ATP in massive amounts, and they have relatively few mitochondria, relying primarily on anaerobic metabolism.
66
What color do skeletal muscles dominated by fast fibers appear, and why?
They appear pale, often called white muscles, due to low myoglobin content.
67
What is muscle hypertrophy?
The enlargement of a stimulated muscle due to muscle fibers increasing in diameter, which involves developing more mitochondria, higher enzyme/glycogen reserves, and more myofibrils.
68
What is muscle atrophy?
A reduction in muscle size, tone, and power, typically caused by lack of regular stimulation or disease, leading to muscle fibers becoming smaller and weaker.
69
What causes Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)?
They are inherited, sex-linked diseases that produce progressive muscle weakness and deterioration.
70
What causes the stiffness of rigor mortis?
As ATP reserves are exhausted after death, cross-bridges cannot detach from active sites, locking the affected muscle fibers in the contracted state, triggered by the release of Ca 2+ as the SR deteriorates.
71
What is the approximate diameter and length of a muscle fiber from a thigh muscle?
It can have a diameter of 100 μm and be as long as 30 cm (12 inches).
72
During embryonic development, what type of cells fuse to form multinucleate skeletal muscle fibers?
Myoblasts.
73
What is the approximate diameter of a myofibril?
1−2 μm.
74
How many sarcomeres are typically aligned end to end in each myofibril?
Approximately 10,000 sarcomeres.
75
What is the typical resting length of a sarcomere?
About 2 μm.
76
What is the ratio of thin filaments arranged around each thick filament in the zone of overlap within the A band?
A 6:1 ratio.
77
In a resting skeletal muscle fiber, how much greater is the total concentration of Ca 2+ within the terminal cisternae compared to the surrounding cytosol?
40,000 times that of the surrounding cytosol.
78
How many individual molecules of G-actin are typically found in the two rows forming an F-actin strand?
300−400 individual molecules.
79
How many myosin molecules are contained in a typical thick filament?
About 300 myosin molecules.
80
What are the typical resting membrane potentials for neurons and skeletal muscle fibers, respectively?
Neurons are about −70 mV, and skeletal muscle fibers are about −85 mV.
81
What is the exchange ratio maintained by sodium-potassium ion pumps to maintain the resting membrane potential?
They constantly export 3 Na+ from the cell and import 2 K+.
82
What potential is typically the threshold needed to initiate a charge reversal (in neurons)?
−55 mV.
83
At what membrane potential does depolarization typically peak?
+30 mV.
84
The opening and closing of voltage-gated ion channels generate an action potential in what duration of time?
In less than 2 msec.
85
What is the normal range of sarcomere lengths in the body, expressed as a percentage of the optimal length?
75 to 130 percent of the optimal length.
86
What is the typical duration of the latent period of a twitch in a muscle fiber?
About 2 msec.
87
What is the typical duration of the contraction phase of a twitch?
About 15 msec.
88
What is the typical duration of the relaxation phase of a twitch?
About 25 msec.
89
How many muscle fibers might a single motor neuron control in the muscles of the eye (where precise control is important)?
4−6 muscle fibers.
90
What percentage of total muscle weight may glycogen account for, allowing contractions to continue for extended periods?
1.5 percent.
91
What is the net gain of ATP molecules from glycolysis per glucose molecule?
A net gain of 2 ATP molecules.
92
What is the ATP yield for a cell for each molecule of pyruvate 'fed' into the citric acid cycle?
The cell gains 15 ATP molecules.
93
What is the relative quantity of creatine phosphate (CP) stored in a typical muscle fiber, measured in millimoles (mmol)?
20 mmol.
94
For a typical muscle fiber, how long can energy from CP alone support an isometric tetanic contraction?
15 sec.
95
During peak exertion, what approximate proportion of the needed ATP can the mitochondria provide?
Only about one-third of the ATP needed.
96
During peak activity leading to fatigue, what is the approximate pH decline within the muscle fibers?
From about 7.1 to 6.4.
97
Of the new pyruvate molecules generated from lactate in the liver during the recovery period, what percentage is used to synthesize glucose?
70 percent.
98
How much longer do slow fibers take to reach peak tension after stimulation compared to fast fibers?
Three times as long.
99
Fast fibers can reach peak twitch tension in what duration of time or less after stimulation?
0.01 second or less.
100
What are the alternative names for slow fibers?
Type 1, S (slow), red, SO (slow oxidative), slow-twitch oxidative.
101
In Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD), what is the chance that a woman carrying the defective gene will have a male child with the condition?
A 50 percent chance.
102
Rigor mortis typically begins and disappears during what timeframes after death?
It begins 2−7 hours after death and disappears after 1−6 days.
103
List the things contained in the endomysium
muscle fiber
capillaries
myosatellite cells
axons
104
Describe the H band of a sarcomere
Lighter region on both sides of M-line. Only has thick filaments
basically the middle portion of myosin between the two actin filaments
105
describe the A band of the sarcomere
the dark region that contains all the thick filaments
106
describe the I band of the sarcomere
the region with only light filaments
light in colour
107
What are the M lines?
m line is the middle of the thick filaments
108
What are the two ways that the sarcoplasmic reticulum stores Ca2+?
free floating
bound to calsequestrin (mostly this)
109
What keeps the thin filaments attached at the z line?
actinin
110
True or False: there are more K+ outside than inside the cell at rest
False! its the other way around
111
What is the primary contributor of negative charges inside the cell?
charged proteins that cannot cross the membrane to exit the cell
112
Where are the acetylcholine receptors in the motor end plate?
within little grooves called the **junctional folds**
113
What is the behaviour of motor units during continuous contraction?
asynchronous motor unit summation: motor units swap out and recover before being stimulated again. This produces less than maximal tension but helps limit fatigue to an extent
114
define muscle tone
tension produced by muscles due to **subconscious** activation of motor neurons at rest
115
List the different factors contributing to fatigue in muscles
Contraction
* Leakage of Ca 2+ back into the sarcoplasm
* Microtears in myofibrils
* Build-up of lactate and H +
Excitation
* Depletion of ACh vesicles in MN axon
terminal
* Accumulation of K + in the T-tubules
(ECF) due to repeated APs
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