Ch 6 Contraction of Skeletal Muscle Flashcards
1
Q
What percentage of the body is composed of skeletal muscle?
A
About 40% of the body is skeletal muscle
Additionally, approximately 10% is smooth and cardiac muscle.
2
Q
What is the diameter range of skeletal muscle fibers?
A
10 to 80 micrometers
3
Q
What is the sarcolemma?
A
A thin membrane enclosing a skeletal muscle fiber
4
Q
What does the sarcolemma consist of?
A
- Plasma membrane
- Outer coat of polysaccharide material
- Thin collagen fibrils
5
Q
How many nerve endings usually innervate each skeletal muscle fiber?
A
Usually only one nerve ending
6
Q
What are myofibrils composed of?
A
- Actin filaments
- Myosin filaments
7
Q
What is the composition of a myofibril?
A
About 1500 myosin filaments and 3000 actin filaments
8
Q
What are I bands and A bands in muscle fibers?
A
- I bands: contain only actin filaments, isotropic to polarized light
- A bands: contain myosin filaments and overlapping actin filaments, anisotropic to polarized light
9
Q
What are cross-bridges?
A
Projections from the sides of myosin filaments that interact with actin filaments
10
Q
What is a sarcomere?
A
The portion of the myofibril between two successive Z disks
11
Q
What role do titin molecules play in muscle contraction?
A
They keep the myosin and actin filaments in place
12
Q
What is sarcoplasm?
A
The intracellular fluid between myofibrils, containing potassium, magnesium, phosphate, and protein enzymes
13
Q
What is the sarcoplasmic reticulum?
A
A specialized endoplasmic reticulum of skeletal muscle that regulates calcium storage and release
14
Q
What initiates muscle contraction?
A
An action potential traveling along a motor nerve
15
Q
What neurotransmitter is released at the muscle fiber membrane?
A
Acetylcholine
16
Q
Fill in the blank: The action potential causes the sarcoplasmic reticulum to release large quantities of _______.
A
calcium ions
17
Q
What causes the actin and myosin filaments to slide alongside each other during contraction?
A
Attractive forces initiated by calcium ions
18
Q
What energy source is required for muscle contraction?
A
ATP (Adenosine triphosphate)
19
Q
What is the molecular weight of a myosin molecule?
A
About 480,000
20
Q
What are the components of a myosin molecule?
A
- Two heavy chains
- Four light chains
21
Q
What is the tail of the myosin molecule formed by?
A
Two heavy chains wrapped spirally around each other
22
Q
What happens to calcium ions after muscle contraction?
A
They are pumped back into the sarcoplasmic reticulum
23
Q
What is the structure at one end of each myosin molecule called?
A
Myosin head
The myosin head is a globular polypeptide structure that plays a crucial role in muscle contraction.
24
Q
How many light chains are part of each myosin head?
A
Four light chains
There are two light chains associated with each myosin head, totaling four light chains for the two heads.
25
What is the length of each myosin filament?
Approximately 1.6 micrometers
## Footnote
The myosin filament maintains a uniform length of about 1.6 micrometers.
26
What are the protruding arms and heads of myosin called?
Cross-bridges
## Footnote
The cross-bridges are flexible structures that extend from the myosin filament and interact with actin during contraction.
27
What enzyme activity does the myosin head exhibit?
Adenosine triphosphatase (ATPase) activity
## Footnote
This activity allows the myosin head to cleave ATP and use its energy for muscle contraction.
28
What are actin filaments composed of?
Actin, tropomyosin, and troponin
## Footnote
The actin filament consists of a double-stranded F-actin molecule, with tropomyosin and troponin playing critical roles in contraction regulation.
29
What is the molecular weight of each tropomyosin molecule?
70,000
## Footnote
Tropomyosin molecules are significant components of the actin filament that help regulate contraction.
30
What role does troponin play in muscle contraction?
It helps control muscle contraction by binding to actin, tropomyosin, and calcium ions
## Footnote
Troponin consists of three subunits: troponin I (binds actin), troponin T (binds tropomyosin), and troponin C (binds calcium).
31
What is the 'walk-along' theory of contraction?
A theory describing how myosin heads pull actin filaments during contraction
## Footnote
This theory suggests that myosin heads attach to active sites on actin, tilt to pull the filament, and then detach to bind to another site.
32
What initiates the contraction process in muscles?
Calcium ions
## Footnote
Calcium ions bind to troponin, causing a conformational change that uncovers active sites on actin.
33
What is the Fenn effect?
The phenomenon where more ATP is cleaved during greater muscle work
## Footnote
This effect explains the relationship between muscle activity and energy consumption.
34
What happens to ADP and phosphate during the power stroke of contraction?
They are released and a new ATP binds
## Footnote
The release of ADP allows the myosin head to detach from the actin filament and reset for the next contraction cycle.
35
What is the effect of sarcomere length on muscle contraction?
The amount of actin-myosin overlap determines the tension developed
## Footnote
Optimal sarcomere length (around 2.0 to 2.2 micrometers) allows for maximum force generation during contraction.
36
Fill in the blank: The active sites on the actin filament are covered by the _______ complex in a relaxed muscle.
troponin-tropomyosin
## Footnote
This complex inhibits the binding of myosin to actin until calcium ions are present.
37
True or False: Each cross-bridge operates independently during muscle contraction.
True
## Footnote
This independence allows for a continuous contraction cycle where multiple cross-bridges can attach and pull simultaneously.
38
What is the staggered arrangement of active sites on the F-actin strands?
About every 2.7 nanometers
## Footnote
This staggered arrangement facilitates the interaction between actin and myosin during contraction.
39
What happens to the actin filament at a sarcomere length of about 2 micrometers?
The actin filament overlaps all the cross-bridges of the myosin filament but has not yet reached the center of the myosin filament.
40
What occurs as the sarcomere length decreases from 2 micrometers to about 1.65 micrometers?
The strength of contraction decreases rapidly as the two Z disks abut the ends of the myosin filaments.
41
What happens to the strength of contraction as the sarcomere contracts to its shortest length?
The strength of contraction approaches zero.
42
What does the curve in Figure 6-10 represent?
The tension of the intact whole muscle before and during muscle contraction.
43
At what sarcomere length does a muscle contract with approximate maximum force?
At a sarcomere length of about 2 micrometers.
44
What effect does stretching a muscle beyond its normal length have on active tension?
Active tension decreases as the muscle is stretched beyond its normal length.
45
What is the relationship between load and velocity of contraction in skeletal muscle?
The velocity of contraction decreases progressively as the load increases.
46
What happens when the load equals the maximum force the muscle can exert?
The velocity of contraction becomes zero, and no contraction results.
47
What does work output during muscle contraction depend on?
Work output depends on the load and the distance of movement against the load.
48
What is the equation for work output during muscle contraction?
W = L × D, where W is work output, L is load, and D is distance.
49
What are the three sources of energy for muscle contraction?
* Phosphocreatine
* Glycolysis
* Oxidative metabolism
50
How long can the combined energy of ATP and phosphocreatine sustain maximal muscle contraction?
For only 5 to 8 seconds.
51
What is glycolysis?
The breakdown of glycogen stored in muscle cells to liberate energy.
52
What is the main energy source for sustained long-term contraction?
Oxidative metabolism.
53
What percentage of energy input to muscle is converted into work?
Less than 25%.
54
What are isometric contractions?
Contractions where the muscle does not shorten.
55
What are isotonic contractions?
Contractions where the muscle shortens but the tension remains constant.
56
What is the duration of isometric contraction for the ocular muscle?
Less than 1/50 second.
57
What characterizes slow muscle fibers?
* Smaller size
* Innervated by smaller nerve fibers
* Extensive blood vessel system
* Increased numbers of mitochondria
* Large amounts of myoglobin
58
What characterizes fast muscle fibers?
* Larger size for greater strength
* Faster contraction speed
59
What is the appearance of slow muscle fibers due to myoglobin?
Reddish appearance, hence the name red muscle.
60
What is the primary function of the soleus muscle?
Slow contraction for continual, long-term support of the body against gravity.
61
What is the velocity of contraction at which maximum efficiency occurs?
About 30% of maximum velocity.
62
What is the appearance of slow muscle fibers due to myoglobin?
Reddish appearance
## Footnote
Myoglobin enhances oxygen transport to mitochondria
63
What are the characteristics of fast fibers (Type II, White Muscle)?
* Large for great strength of contraction
* Extensive sarcoplasmic reticulum for rapid calcium release
* Large amounts of glycolytic enzymes
* Less extensive blood supply
* Fewer mitochondria
## Footnote
Fast fibers are also known as white muscle due to low myoglobin content
64
What is a motor unit?
All the muscle fibers innervated by a single nerve fiber
## Footnote
Includes a motor neuron and the group of skeletal muscle fibers it innervates
65
How does the number of muscle fibers innervated by a motor neuron vary?
Depends on the type of muscle
## Footnote
Small muscles have more nerve fibers for fewer muscle fibers; large muscles have more muscle fibers per motor unit
66
What is the average number of muscle fibers in a motor unit?
About 80 to 100 muscle fibers
## Footnote
This varies significantly based on muscle type and function
67
What is summation in muscle contraction?
Adding together individual twitch contractions to increase intensity
## Footnote
Can occur via multiple fiber summation or frequency summation
68
What is tetanization?
Complete fusion of muscle contractions to appear smooth and continuous
## Footnote
Occurs when calcium ions are maintained in the muscle sarcoplasm
69
What is the staircase effect (Treppe)?
Initial strength of contraction increases to a plateau after successive muscle twitches
## Footnote
Believed to be caused by increasing calcium ions in the cytosol
70
What causes muscle tone?
Low rate of nerve impulses from the spinal cord
## Footnote
Controlled partly by signals from the brain and muscle spindles
71
What leads to muscle fatigue during prolonged contraction?
Depletion of muscle glycogen and impaired contractile processes
## Footnote
Also affected by nerve signal transmission through the neuromuscular junction
72
What is coactivation of muscles?
Simultaneous contraction of agonist and antagonist muscles on opposite sides of joints
## Footnote
Controlled by motor control centers in the brain and spinal cord
73
What is muscle hypertrophy?
Increase of total mass of a muscle
## Footnote
Results from an increase in actin and myosin filaments in muscle fibers
74
What is muscle atrophy?
Decrease of total mass of a muscle
## Footnote
Often occurs when a muscle remains unused for extended periods
75
What is the primary cause of muscle contractile protein synthesis during hypertrophy?
Forceful contraction
## Footnote
Leads to greater numbers of actin and myosin filaments
76
What is the size principle in muscle contraction?
Smaller motor units are activated before larger ones
## Footnote
Allows gradations of muscle force during contractions
77
What is the maximum strength of tetanic contraction at normal muscle length?
3 to 4 kg/cm2 of muscle or 50 pounds/inch2
## Footnote
Can result in significant tension applied to tendons
78
What happens to muscle fibers during denervation atrophy?
Replacement with fibrous tissue and loss of contractile properties
## Footnote
Can lead to contractures if not managed properly
79
What factors determine the analysis of lever systems in the body?
* Point of muscle insertion
* Distance from the fulcrum
* Length of the lever arm
* Position of the lever
## Footnote
Important for understanding different types of muscle movements
80
What is kinesiology?
Study of different types of muscles, lever systems, and their movements
## Footnote
Important component of human physiology
81
What is the primary goal of therapy for atrophying muscles?
To keep atrophying muscles stretched and prevent contractures
## Footnote
Contractures can develop due to prolonged muscle disuse.
82
What is the ATP-dependent ubiquitin-proteasome pathway?
A pathway that degrades damaged or unneeded proteins in muscles undergoing atrophy
## Footnote
This process is critical for regulating protein levels in muscle cells.
83
What occurs when some nerve fibers to a muscle are destroyed in poliomyelitis?
Remaining nerve fibers branch off to form new axons that innervate many paralyzed muscle fibers
## Footnote
This results in large motor units called macromotor units.
84
What happens to muscle fibers when they are stretched beyond normal length?
New sarcomeres are added at the ends of the muscle fibers
## Footnote
This process contributes to muscle hypertrophy.
85
What is rigor mortis?
A state of muscle contracture and rigidity that occurs several hours after death
## Footnote
Rigor mortis results from the depletion of ATP needed for muscle relaxation.
86
What is fiber hyperplasia?
An increase in the actual number of muscle fibers under rare conditions of extreme force generation
## Footnote
This occurs in addition to fiber hypertrophy.
87
What is Duchenne muscular dystrophy (DMD)?
An inherited disorder causing progressive muscle weakness, affecting only males
## Footnote
DMD is caused by a mutation in the dystrophin gene.
88
How does muscle denervation affect muscle size?
It leads to rapid atrophy as the muscle loses its nerve supply
## Footnote
Atrophy begins almost immediately after denervation.
89
What are the symptoms of Duchenne muscular dystrophy?
Muscle weakness starting in early childhood, leading to wheelchair dependence by age 12
## Footnote
Patients often die of respiratory failure before age 30.
90
What characterizes Becker muscular dystrophy (BMD)?
A milder form of muscular dystrophy with later onset and longer survival compared to DMD
## Footnote
BMD is also caused by mutations in the dystrophin gene.
91
What is the role of dystrophin in muscle cells?
Links actins to proteins in the muscle cell membrane, providing stability
## Footnote
It forms an interface between the intracellular contractile apparatus and extracellular matrix.
92
What happens to muscle fibers during the final stage of denervation atrophy?
Most muscle fibers are destroyed and replaced by fibrous and fatty tissue
## Footnote
This stage is characterized by significant loss of muscle function.
93
What is the estimated prevalence of DMD and BMD in males aged 5 to 24 years?
1 in every 5,600 to 7,700 males
## Footnote
This statistic highlights the rarity of these conditions.
94
What is the consequence of a lack of dystrophin in muscle fibers?
Increased membrane permeability to calcium, leading to muscle fiber breakdown
## Footnote
This is associated with altered intracellular calcium handling.
