Module 9 - Muscle Physiology Flashcards
1
Q
What are the 3 types of muscle?
A
Skeletal, cardiac, and smooth
2
Q
What does contraction of muscles allow the body to do?
A
Purposeful movement
Manipulation of external objects
Propulsion of contents through hollow internal organs
Emptying the contents of certain organs to the external environment
3
Q
What are the 2 different ways that muscles can be categorized?
A
Striated - skeletal and cardiac muscle
Unstriated - smooth muscle
Voluntary - skeletal muscle
Involuntary - cardiac and smooth muscle
4
Q
Where in the body is cardiac muscle found?
A
Only in the heart
5
Q
What muscles make up the muscular system?
A
Skeletal muscle
6
Q
What muscle appears throughout the body systems as components of hollow organs and tubes?
A
Smooth muscle
7
Q
Describe the size and shape of a single muscle fiber
A
Relatively large, elongated, and cylinder shaped, measuring about 10-100 micrometers in diameter and up to 0.76 m long
8
Q
What is another term for a single skeletal muscle cell?
A
Muscle fibre
9
Q
Why do muscle fibers have multiple nuclei?
A
• Made from the fusion of a hundred or more small mesodermal cells called myoblasts, this results in myocytes having an equal number of nuclei
10
Q
What do skeletal muscle fibers have many of as compared to other cells in the body?
A
nuclei and mitochondria
11
Q
What is the contractile element of skeletal muscle?
A
Myofibrils
12
Q
What are myofibrils?
A
- Contractile elements of skeletal muscles
- About 2 µm in diameter and extend the entire length of the muscle fiber
- Have striations that give skeletal muscle its striped appearance
13
Q
What are filaments?
A
- Small protein structures located within the myofibrils
- Directly involved in contractile process
- 2 thin filaments for every thick filament, and they overlap
14
Q
Describe thin filaments
A
- About 8 nm in diameter and 1-2 µm long
* Composed mostly of the protein actin
15
Q
Describe thick filaments
A
- About 16 nm in diameter and 1-2 µm long
* Composed mostly of the protein myosin
16
Q
Describe the level of organization of a skeletal muscle
A
Whole muscle -> muscle fiber -> myofibril -> thick and thin filaments -> myosin and actin
17
Q
What is a bundle of muscle fiber bundle covered by a layer of perimysium called?
A
Muscle fascicle
18
Q
Describe the connective tissue covering the muscle fibers together. How do these connect the muscles to bone? What is their main function?
A
• 3 connective tissues that form a continuous interconnected network
o Endomysium (inner layer), perimysium, epimysium (outer)
• At both ends of the muscles, the connective tissues become the tendons
• The tendons are continuous with the periosteum and deeper collage fibers enter the bone to blend with the collagen of the bone extracellular matrix
• This continuous run of connective tissue is what makes muscles so strong
• Provides tension for stabilization and/or movement
19
Q
Describe the endomysium
A
- Thin wrapping of mostly reticular fiber surrounding each muscle fiber
- Helps bind muscle tissues together while allowing them to move over each other
- Carries small blood vessels to provide muscle fibers with nutrients
20
Q
Describe the perimysium
A
• Dense irregular connective tissue covering the muscle fascicles
o Mostly collagen with some elastin
• Allows for some degree of motion between neighboring muscle fascicles
• Transmits blood vessels
21
Q
Describe the epimysium
A
• Thicker covering of dense irregular connective tissue that covers the periphery of the muscle
o Mostly collagen with some elastin
• Binds all muscle fascicles together to form the muscle body
22
Q
What does the myofibrils of a muscle fiber look like under an electron microscope?
A
Alternating dark bands (A bands) and light bands (I bands)
Bands line up parallel to one another to produce the striated or striped appearance of skeletal muscle
23
Q
What is a functional unit?
A
The smallest component that can perform all the functions of the organ
24
Q
How does a muscle fiber grow in length as a person grows?
A
By adding new sarcomeres on the ends of the myofibrils; the same works for increasing flexibility :)
25
What is the basic functional unit of skeletal muscle?
sarcomere
26
What is a sarcomere?
* Basic functional units of a myofibril
* Filaments are arranged in the compartments of sarcomeres rather than running the length of the muscle fiber
* Consists of the area between 2 Z discs
27
What is a Z disc?
Also called a Z line
• Narrow plate shaped region of dense protein material
• Connects the sarcomeres of a muscle fiber
28
What is the A band?
• Darker middle section of the sarcomere
• Thick filaments span the entire length of A band
• Zone of overlap
o Located towards each end of the A band
o Where thick and thin filaments lie side by side
29
What is the I band?
* Lighter, less dense area
* Contains thin filaments but no thick filaments
* Z disc passes through each I band
30
What is the H band?
Also called the H zone
• Narrow part in the center of each A band
• Contains thick filaments but no thin filaments
31
What is the M line?
* Center of the H band
* Where supporting proteins that hold thick filaments together
* Named for the middle
32
What is a good memory trick for remembering which lines are which in a sarcomere?
Z is the final alphabet: Z lines represents the end of sarcomere
M for middle: M line represents the midline of sarcomere
I is a thin letter: I band has only thin filaments
H is a thick letter: H zone has only thick filaments
A is a hybrid of “I” and “H”: A band has both thin and thick filaments (remains constant during contraction)
33
What is titin?
• Third most plentiful protein in skeletal muscles (after acting and myosin)
• Is large in size, about 50 times larger than an average-sized protein
• Goes half the length of the sarcomere, from the Z disc to the M line
• Helps stabilize the position of the thick filament
• Very elastic in area from Z disc to thick filament
o Accounts for much of the elasticity and extensibility of myofibrils
• May help prevent overextension of sarcomeres
• Maintains central location of A bands
34
Three dimensionally, how are thick and thin filaments arranged around each other?
```
Thin filaments arranged hexagonally around the thick filaments
Cross bridges (myosin heads) project from the thick filaments in all 6 directions
Each thin filament is surrounded by 3 thick filaments
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35
What are the 2 contractile proteins and what is their main function?
Myosin and actin
| • Generate force during contraction
36
What are the two regulatory proteins and what is their main function?
Troponin
Tropomyosin
• Help switch the contraction process on and off
37
Describe the myosin protein including the various functional parts of it
Myosin
• Main component of thick filaments
• Functions as motor protein in all three types of muscle tissue
o Motor proteins convert ATP into mechanical energy of motion
• About 300 molecules of myosin form a single thick filament in skeletal muscle
o Each molecule shaped like two golf clubs twisted together
Myosin tail
• Twisted golf club handles
• Point toward the M line
• Neighboring molecules lie parallel to one another forming the shaft of thick filament
Myosin head
• Also called cross bridges
• Golf club heads
• Projects outward from the shaft in a spiraling fashion
• Each head extends toward one of the six thin filaments that surrounds each thick filament
• Actin binding site
o Protein on the thin filaments
• ATP-binding site
o Functions as an ATPase
Enzyme that hydrolyzes ATP to generate energy for muscle contraction
38
Describe the actin protein including the various functional parts of it
• Main protein of thin filaments
• Individual spherical molecules join to form an actin filament that is twisted into a helix
• Each is a myosin-binding site where a myosin head can attach
o Binding results in contraction
39
Describe the tropomyosin protein including the various functional parts of it
• Part of thin filaments
• Threadlike proteins that lie end to end alongside the groove of the actin spiral
• Cover the myosin-binding sites on actin when the muscle is relaxed
o Blocks myosin-actin binding that would lead to contraction
40
Describe the troponin protein including the various functional parts of it
• Part of thin filaments
• Complex made of 3 polypeptide units
o One binds to tropomyosin
o One binds to actin
o Can bind with calcium
• When not bound to calcium, it holds the tropomyosin strands in place
• When calcium binds to troponin, it changes shape and moves the tropomyosin away from the myosin-binding sites on actin
o This allows myosin to bind to actin and the contraction can begin
41
Compare the relationship of myofibrils and a muscle fiber with the relationship between muscle fibers and a whole muscle
A muscle fiber is composed of myofibrils that extend the entire length of the muscle fiber; in general, the larger-diameter muscle fibers have a greater number of myofibrils.
A whole muscle is composed of muscle fibers that extend the entire length of the muscle; in general, the larger-diameter muscles have more muscle fibers.
42
Describe the relationship between actin, tropomyosin, and troponin in a relaxed muscle fiber
The regulatory protein, troponin, binds to both actin and tropomyosin. In the relaxed state, troponin assumes a conformation that causes tropomyosin to cover the myosin cross-bridge binding sites on the actin molecules
43
What is excitation-contraction coupling?
• Series of events linking muscle excitation (via action potential) to muscle contraction
44
Describe T tubules
• Also called a transverse tubule
• Tunnel from the surface toward the center of each muscle fiber
• Filed with interstitial fluid as they are open to the outside of the fiber
• Muscle action potentials spread along the sarcolemma and through the T tubules spreading quickly throughout the muscle fiber
o Ensures that all parts of the fiber are excited nearly simultaneously
45
What is the receptor located on the T tubule? What triggers this receptor?
Dihydropyridine Receptor
• 4 subunits in the same pattern as the foot proteins of the sarcoplasmic reticulum
• Voltage gated receptor
• When an action potential moves down the T tubule, these receptors are triggered
46
Describe the sarcoplasmic reticulum. What part of it makes a close tie with the T tubule? What happens when they are triggered?
Sarcoplasmic Reticulum
• Fluid filed system of membranous sacs that encircle each myofibril like a net
o Not continuous, segments for each A band and each I band
• Similar to smooth endoplasmic reticulum in non-muscle cells
• Stores calcium ions when muscle is relaxed
Terminal Cisterns
• Also called lateral sac
• Dilated end sacs of sarcoplasmic reticulum
• Butt up against the T tubules from both sides, separated by a slight gap
• When triggered, calcium released from here into the sarcoplasm, triggering muscle contractions
47
What are the calcium channels from the sarcoplasmic reticulum and how do they function?
Foot Proteins
• Also called Ca2+ release channel or ryanodine receptor
• 4 subunits in the same pattern as the dihydropyridine receptors of the T tubules
• Serve as a calcium channel and are locked in the open position by ryanodine protein
• Spans the gap between the terminal cistern and T tubule
• Half are fused together with dihydropyridine receptors of T tubules
o When dihydropyridine receptors open, the connected foot proteins release calcium
o Triggers opening of the other half not connected to foot proteins to release calcium
48
What is the area where one T tubule has a terminal cisterna of the sarcoplasmic reticulum on either side?
triad
49
What are concentric contractions?
* Whole muscle contracts, shortening, generating force
| * This is due to all muscle fibers (and therefore all sarcomeres of each fiber) contract simultaneously
50
Describe the sliding filament mechanism and how this changes the various zones and bands of the sarcomere
* Thin filaments on each side of a sarcomere slide inward over the thick filaments shortening the overall length of the sarcomere
* H zone and I band narrow
* Thick (A band) and thin filaments do not change in length
51
How does calcium function in muscle contractions?
During contraction, calcium binding to troponin pulling the tropomyosin away exposing the actin binding sites
52
How does the cross bridge go from active to the relaxed state?
• The link between the first myosin and actin molecules breaks and the cross-bridge returns to its original shape only after another molecule of ATP is attached at the binding site
o ATPase breaks down ATP into ADP and P1 to put the myosin head back to the resting state
53
How does the power stroke work?
• Myosin-actin binding occurs, and the cross bridge (head of myosin) changes shape, bending it inward toward the center of the sarcomere
o Pulls the thin filament it is attached to inwards to the M line
o ADP and P1 are released by the myosin head
54
What prevents the thick and thin filaments from sliding backwards when the cross bridge separates at the end of a cycle?
• All 6 surrounding thin filaments are pulled in simultaneously
o Some cross bridges are stroking, others are returning to their original position
55
What is rigor mortis and how does this occur?
* Stiffness after death that begins about 3-4 hours after death and completes in about 12 hours
* Due to calcium levels rising in the sarcoplasm, the power stroke is engaged but because there is not ATP to bind again, it does not release back to its resting state
56
What is allowed to happen because the muscle contraction is much longer than the action potential only being 1-2 msec?
o This allows body to produce muscle contractions of variable strength
57
What are the 3 time periods of a contraction between the action potential and the complete relaxation of the muscle fiber? About how long does each period last?
Latent period
o 0.5 msec delay between action potential stimulation and onset of contraction
Contraction time
o Time from onset of contraction until peak tension develops
o Lasts about 40-120 msec depending on the type of fiber
Fast-twitch (FT) muscle fibers reach peak faster
Slow-twitch (ST) muscle fibres reach peak slower
Each muscle group has a combination of both and affects overall time
Relaxation Time
• Time from peak tension until relaxation is complete
• Lasts about 50-200 msec
o Depends on fiber type just like contraction
o FT relaxes faster than ST fibers
Due to ST having lower density of SR and slower calcium reuptake
58
How does relaxation occur as the action potential ends?
• When action potential stops
o Acetylcholinesterase (AChE) removes ACh from the neuromuscular junction
o T tubules no longer release calcium
• Sarcoplasm has a Ca2+-ATPase pump
o Transports calcium from cytosol and concentrates in in the lateral sacs
o Requires input of energy
• Troponin-tropomyosin complex resume blocking the binding sites
• No longer bound to the thick filaments, the thin filaments return passively to their resting position
• Muscle fiber is now relaxed
59
Illustrate the relationship between the thick and thin filaments in a relaxed sarcomere and in a contracted sarcomere
* Thin filaments on each side of a sarcomere slide inward over the thick filaments shortening the overall length of the sarcomere
* H zone and I band narrow
* Thick (A band) and thin filaments do not change in length
60
Describe the role of the dihydropyridine and ryanodine receptors in the process of excitation-contraction coupling
Dihydropyridine receptors serve as voltage-gated sensors that are activated by an action potential as it propagates along a T tubule.
Activated dihydropyridine receptors trigger the opening of calcium release channels (ryanodine receptors) in the adjacent lateral sacs of the sarcoplasmic reticulum, thereby permitting calcium release from the lateral sacs (terminal cisterns)
The released calcium repositions the troponin-tropomyosin complex so that actin and the myosin cross bridges can interact to accomplish contraction
61
Describe the cross-bridge cycle, and illustrate whether ATP, ADP, or APD and P1 are bound to the myosin head during the various stages of the cycle
ATP binds to the myosin head and causes the head to detach from the actin molecule
During the cocking of the myosin head, ATP is hydrolyzed to ADP and P1
When the myosin head binds to actin, P1 is released from the head during the power stroke
ADP is released from the myosin head after the power stroke
62
How many actin binding sites are revealed with a single molecule of calcium?
1 molecule of calcium, moves a single troponin-tropomyosin complex, each covering 7 cross bridge binding sites
63
Describe motor unit recruitment
* Number of muscle fibers contracting within a muscle
* Completed by contracting more motor units to achieve the required strength
* Used more by larger, more proximal muscles
* Used when submaximal forces are required
* The greater the number of fibers contracting, the greater the total muscle tension
64
What is a motor unit?
* One motor neuron and all of the muscle fibers it innervates
* Each muscle consists of a number of intermingled motor units
* One motor unit activation results in weak contraction of the whole muscle
* The finer the movements, the smaller the motor unit (as few as a dozen fibers)
* Powerful muscles may have 2000 fibers per motor unit
65
Describe the size principle of the motor unit
* The larger the motor unit, the harder it is to activate
* Need a greater firing frequency to recruit the larger neurons that feed the larger motor units
* Generally, the order or recruitment is smallest motor units to largest
66
How does motor unit recruitment help avoid fatigue when lifting submaximal loads?
• During weak or moderate activities, motor units most resistant to fatigue are recruited first
o Accomplished by each muscle containing types of fibers differing metabolically
Asynchronous Recruitment of Motor Units
• Body alternates motor unit activity to give motor units that have been active an opportunity for rest
• Used for sustained contractions involving only a portion of motor units, such as maintaining muscle tone
• Staggers shift for smooth transition
67
What is the frequency of stimulation in a muscle?
* Firing frequency of each contracting fiber
* Used more by smaller, more distal muscles
* Used when forces required are coming close to maximum force
68
What is a twitch in a skeletal muscle?
* A brief, weak contraction produced by a single action potential
* Too weak to be useful and does not do anything alone
* If fiber completely relaxed before being stimulated again, the same twitch will occur
* Due to fast calcium reuptake, some troponin-tropomyosin complexes will slide back to block the binding sites so not all bind, so this twitch is not maximal strength for the fiber
69
Describe twitch summation
* A second action potential occurs before the fiber relaxes, and they add together to produce a greater tension than produced by a single action potential
* This repeated stimulation replenishes the calcium stores increasing the number of binding sites available for participation in power stroke cycles thus increasing tension
* Submaximal force requires 8-12 Hz, which is 8-12 stimulations per second
* Moderate force requires 30 Hz, which is 30 stimulations per second
* Maximal force about 60 Hz, which is 60 stimulations per second
70
What is tetanus as it pertains to muscle contractions?
* Smooth, sustained contraction of maximal strength
* Muscle fiber stimulated so rapidly it does not relax at all between stimulations
* Ensures maximum number of binding sites remain uncovered for cross-bridge cycling bringing and keeping tension at peak
* Believed to require 100 Hz, so about 100 stimulations per second
71
What is the length-tension relationship?
* The amount of tension that is produced by a muscle as a feature of its length
* Explained by sliding filament mechanism
72
What is the optimal length of a muscle?
• Length at which maximal force can be achieved on subsequent tetanic contraction
• Maximum tension developed when thin filament optimally overlaps thick filament
• This ensures maximum number of cross-bridge binding sites are accessible to myosin molecules for binding and bending
• In the body, muscles are positioned so their relaxed length is about their optimal length
o Skeleton prevents significant stretching or shortening of muscle (< 30%)
73
What happens when the length of a muscle is greater than its optimal length?
* Thin filaments pulled out from between thick filaments
* Decreases number of sites available a cross-bridges don’t match up and therefore go unused and subsequently results in less tension that can develop
>70% Longer Than Optimal Length
• Muscles stretched about 70% longer than optimal length cannot contract
• Thin filaments are completely pulled away from thick filaments preventing any cross-bridge activity
74
What happens when the length of a muscle is less than its optimal length?
Less Than Optimal Length
• Less tension due to
o Thin filaments overlapping limiting cross-bridge interactions
o Thick filaments forced against Z lines preventing movement
<80% of Optimal Length
• Calcium release is inhibited for unknown reasons
• Prevents troponin-tropomyosin complex from moving away from binding sites
• Results in even few sites available for cross-bridge binding
75
What is the contractile component of muscles?
• Sarcomeres, as a result of the sliding filaments
76
What are the series-elastic components of muscle?
```
• Noncontractile tissue
• Have a certain degree of passive elasticity
• Stretched when sarcomeres shorten
• Transmits tension to the bone
• Includes
o Connective tissue sheaths and tendon
o Other muscle components such as titin
```
77
Describe the origin and insertion points of a muscle
Origin
• The more stationary part of the skeleton the muscle is attached to
Insertion
• The end of the muscle attached to the skeletal part that moves
78
Describe the means by which the strength of a contraction of a skeletal muscle can be changed to generate overall force
Greater strength of contraction can be achieved through motor unit recruitment, twitch summation, positioning the muscle at its optimal length, the absence of fatigue, and hypertrophy of a muscle (strength training)
79
Describe the role of calcium in twitch summation
In twitch summation, the level of cytosolic calcium is increased by repeated release of calcium from the lateral sacs. In addition, with repeated excitation of a skeletal muscle cell, insufficient time is available between action potentials for the sarcoplasmic reticulum to pump all the released calcium back into the lateral sacs. The sustained, elevated cytosolic calcium leads to prolonged exposure of myosin cross-bridge binding sites for interaction with actin and thus greater power stroke opportunities
80
Explain why a muscle that has been stretched beyond its optimal length is unable to generate as much tension
When a skeletal muscle is stretched beyond its optimal length, fewer actin sites are available for cross-bridge cycling, which reduces tension generation
81
How are force and velocity for shortening related?
Inversely
82
How are force and velocity for lengthening related?
directly
83
What is work?
Force x distance
84
What is force?
The muscle tension required to overcome the load
85
In an isometric contraction, what is the muscle contractions efficiency?
Because no object is moved, there is no distance and therefore the work produced is zero
100% of the energy consumed by the muscle during the contraction is converted to heat
86
In an isotonic contraction, what is the muscle contractions efficiency?
About 25% is realized as external work, with the remaining 75% being converted to heat
87
What is an isotonic contraction?
* Contraction in which enough muscle fibers are contracting to shorten the muscle against the load
* A dynamic muscle contraction as force continues to change throughout the lift
* Includes eye muscles and the tongue that are not attached to bone at one end
88
What are the 2 types of isotonic contractions?
Concentric Contraction
• Muscle shortens as it produces a constant tension and overcomes the load it is moving
• Allows you to pick something up
• The heavier the object, the lower the velocity each muscle fiber shortens
o Takes cross-bridges longer to stroke against a greater force
Eccentric Contraction
• Muscle produces a constant tension but lengthens as it gives in to the load it is moving
• Allows you to put something down in a controlled way
• Sarcomere lengthens as actin filaments are pulled apart and Z lines move farther from the center
• The heavier the object, the more the muscle is contracting to resist the stretch, which results in greater speed the muscle lengthens
o Cross bridges break and is associated with muscle damage called
Delayed onset muscle soreness
89
What is an isometric contraction?
* Number of fibers contracting and generating a force are equal to the opposite force of the load so the muscle does not change length
* Activated sarcomeres still shorten, which stretches the series-elastic component to exert a constant force
* Is a static (not in motion) contraction at the force exerted and muscle length remain unchanged
* Occurs when the load is greater than you are capable of lifting
* Used for posture
* Holding an object still
* Includes sphincters preventing movement
90
What is a lever and what is this served by in the body?
* Rigid structure that can move around a fixed point
| * Served by bones in the body
91
What is a fulcrum and what is this served by in the body?
* The fixed point in a lever system
| * Served by joints in the body
92
What is effort in a muscle movement?
* Force that causes the movement
| * Exerted by the muscular contraction
93
What is a load in regards to muscle movement?
* Also called resistance
* Force that opposes the movement
* Typically, the weight of the body part that is moved
* Also includes weight of any object that the moving body part is also moving
94
What is the power arm in regards to muscle movement
• Portion of a lever between the fulcrum and the point where an upward force is applied
95
What is the load arm in regards to a muscle movement
• Portion of a lever between the fulcrum and the point where a downward force is applied
96
Let's look at the elbow joint with a 5 kg ball in the hand.
The distance from the elbow to the muscle insertion is 5 cm and the distance from the elbow to the hand is 35 cm.
What is the lever, fulcrum, load, power arm and load arm?
Lever - forearm
Fulcrum - elbow
Load - 5 kg ball
Power arm - 5 cm
Load arm - 35 cm
97
Let's look at the elbow joint with a 5 kg ball in the hand.
The distance from the elbow to the muscle insertion is 5 cm and the distance from the elbow to the hand is 35 cm.
How much farther will the ball move than the distance the muscle shortens?
How much is the velocity of the ball vs the muscle shortening?
7 cm farther
| 7 times faster
98
Let's look at the elbow joint with a 5 kg ball in the hand.
The distance from the elbow to the muscle insertion is 5 cm and the distance from the elbow to the hand is 35 cm.
Describe the mechanical disadvantage the body is working under
• Skeletal muscles typically work at mechanical disadvantage
• Power arm x upward force must equal load arm x downward force to lift
o In the picture, it would be 5 cm x 35 kg of force = 35 cm x 5 kg
o So muscle has to exert a force 7 times as great as the load
99
What do we know about muscle fibers within a motor unit and between motor units?
* Muscle fibers within a single motor unit are homogeneous in contractile and metabolic function
* Muscle fibers between motor units can differ
100
Describe slow-twitch fibers
• Contract and relax slower than fast-twitch fibers
• Innervated by α2 motor neurons which are smaller neurons
o Lower activation threshold
o Slower conduction velocity of the motor unit
• Have the slow form of ATPase
101
Describe fast twitch fibers
• Innervated by α1 motor neurons which are larger neurons
o Higher activation threshold
o Faster conduction velocity of the motor unit
• Have the fast form of ATPase
• Have maximum shortening velocity (about 10 times faster than slow twitch)
102
Describe the basics of oxidative metabolic properties
• Aerobic
• Can be slow twitch or fast twitch
• Contain abundance of mitochondria that house the enzymes for oxidative phosphorylation
• Rich capillary supply to keep up with needed oxygen supply
• High myoglobin content to help with oxygen needs and provides bright red colouring
o They are also called red fibers
103
Describe the basics of glycolytic metabolic properties
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• Anaerobic
• Fast twitch only
• Specialized for glycolysis
o Fewer mitochondria
o High content of glycolytic enzymes
o Large stores of glycogen
• Fewer capillaries and lower myoglobin count due to less need for oxygen
o They are also called white fibers due to pale colour
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104
While most muscles contain a mixture of all 3 skeletal muscle types, how is the percentage determined?
by type of activity of the muscle
105
Describe type I fibers of skeletal muscle
• Also called slow oxidative
o Found predominantly in muscles maintaining low intensity contractions for long periods of time without fatigue, such as back and legs
• Slow twitch fibers
o Small muscle diameter
o Slow twitch and relaxation time
o Poor sarcoplasmic reticulum development
• Oxidative
o High mitochondrial density
High Krebs cycle enzyme activity
o High myoglobin content so dark red in colour
o High capillary: fiber ratio
• Not predominantly glycolytic
o Low glycogen stores and low glycolytic enzyme activity
• Low phosphocreatine stores and low creatine kinase activity
• Many small motor units per muscle
• High sensitivity for recruitment
• Fatigue-resistant
• Intermediate Z line development
106
Describe type IIa fibers of skeletal muscle
• Also called fast oxidative glycolytic, FOG, or FTa
• Fast twitch fibers
o Largest muscle diameter of the 3 types
o Fast twitch time and intermediate relaxation time
o Intermediate sarcoplasmic reticulum development
• Oxidative
o High mitochondrial density
Medium Krebs cycle enzyme activity
o Medium myoglobin content so dark red in colour
o High capillary: fiber ratio
• Glycolytic
o Intermediate glycogen stores and intermediate glycolytic enzyme activity
• High phosphocreatine stores and medium creatine kinase activity
• Fewer but larger motor units per muscle
• Intermediate sensitivity for recruitment
• Fatigable
• Wide Z line development
107
Describe type IIx fibers of skeletal muscle
• Also called fast glycolytic, FG, FTb, or type IIb
• Found predominantly in muscles adapted for rapid, forceful movements, like the arm
• Fast twitch fibers
o Large muscle diameter
o Fast twitch time and fast relaxation time
o High sarcoplasmic reticulum development
• Glycolytic
o High glycogen stores and high glycolytic enzyme activity
• Not predominantly oxidative
o Low mitochondrial density
Low Krebs cycle enzyme activity
o Low myoglobin content so pale red in colour
o Low capillary: fiber ratio
• High phosphocreatine stores and high creatine kinase activity
• Fewer but larger motor units per muscle
• High sensitivity for recruitment
• Most fatigable
• Narrow Z line development
108
How can muscles improve their oxidative capacity?
• Regular aerobic exercise induces metabolic changes in oxidative fibers
o Also accomplished by HIIT training
• Increase in mitochondria and capillary supply
• Muscles use oxygen more efficiently and therefore perform longer before fatigued
• No change in size
• May even atrophy with prolonged periods of endurance training
109
Describe muscle hypertrophy
* Regular resistance training or weightlifting
* Increase in muscle mass
* Addition of contractile proteins to existing myofibrils
110
Describe muscle atrophy including types and common causes
• Loss of muscle mass
• Sometimes called sarcopenia
o But this generally reflects reduced muscle regenerative ability
• Disuse atrophy
o Occurs when nerve supply is intact, but muscle is not used (casts, bedridden etc.)
o May be age related, in which case resistance training is a corrective measure
• Denervation atrophy
o Occurs after loss of nerve supply
• Other factors may be hormonal, immunological, and nutritional
111
Describe the interconversion of the 3 types of skeletal muscle
• Muscle fiber types within a motor unit are of the same type of fiber
• Slow and fast fibers are not interconvertible
• Fast fibers are on a continuum from highly glycolytic to more oxidative and this can adjust based on exercise
o Chronic resistance training increases type IIa while type IIx decreases
112
Describe how muscles can repair themselves after injury
• Muscles are incapable of mitosis
• Satellite cells
o Inactive muscle-specific stem cells
o Located close to muscle surface
o Damaged muscle fiber releases factors activating the satellite cells
o Satellite cells divide to become myoblasts and eventually differentiate to a mature muscle fiber
• Provides limited repair ability
113
Describe the differences between muscle fibers in a turkey drumstick (slow-oxidative) and muscle fibers in turkey breast meat (fast-glycolytic)
The leg (drumstick) muscles of a turkey consist primarily of red muscle fibers (Type I), which have a large number of mitochondria, high levels of myoglobin, low glycogen content, and relatively few glycolytic enzymes. The turkey's legs are built for endurance, not for speed or power.
In contrast, the turkey's breast muscles, composed primarily of white muscle fibers (Type II), have relatively few mitochondria, low levels of myoglobin, high glycogen content, and an abundance of glycolytic enzymes. The breast muscles are built for speed and power, but lack endurance (so they can only fly for a short distance)
114
Discuss the relative contributions of creatine phosphate, glycolysis, and oxidative phosphorylation to the production of ATP during running of a marathon
ATP comes from all these sources during the course of the race, but oxidative phosphorylation is responsible for generating the largest amount of ATP expended during this event
115
What are extrafusal fibers and what are they innervated by?
Ordinary muscle fibers with sarcomeres down the entire length
Innervated by the alpha motor neurons
116
What are the 3 main areas that provide neural input for skeletal motor control?
Afferent neurons (including reflexes)
Primary motor cortex
Brain stem
117
Describe how afferent neurons provide neuronal information to control skeletal movement?
* Interneurons at the level of the spinal cord
* Includes reflexes
* Carries information to the brain stem nuclei
* Carries information to sensory areas of the cortex via the thalamus
118
Describe how the primary motor cortex provides neuronal information to control skeletal movement?
• Corticospinal (pyramidal) motor system
o Originates in cell bodies called pyramidal cells within primary motor cortex
o Terminate on motor neurons
Sometimes on interneurons that terminate on motor neurons
o Important for fine muscle movement
119
Describe how the brain stem provides neuronal information to control skeletal movement?
• Multineuronal (extrapyramidal) motor system
o Synapses that involve many regions of the brain including
Cortex
• Premotor and supplementary motor areas
Cerebellum
• Works with primary motor cortex to plan motor commands
Basal nuclei
o Final link in brainstem, especially reticular formation
o Included connections between motor cortex and
Thalamus
Premotor and supplementary motor areas
• Only primary motor cortex and brain stem directly influence motor neurons
o Everything else is indirect, sending information to primary motor cortex and brain
120
Describe spastic paralysis
* Characterized by increased muscle tone and augmented limb reflexes
* Inhibitory system originating in the brain stem is disrupted and muscles become hyperactive due to unopposed activity of excitatory inputs
121
Describe flaccid paralysis
• Muscles are relaxed and unable to voluntarily contract muscles
• Loss of excitatory input
• Destruction of motor neurons
o Damage to either cell bodies or efferent fibers
o Causes flaccid paralysis and lack of reflex responsiveness in affected muscles
122
Describe hemiplegia
* Paralysis of one side of the body
* Caused by damage to the primary motor cortex on one side of the brain
* Can be caused by stroke
123
Describe tetraplegia and paraplegia
* Paralysis of all 4 limbs/paralysis of the legs respectively
* Disruption of all descending pathways producing flaccid paralysis below the level of the damaged region
* Can be caused by traumatic severance of the spinal cord
124
Describe the result of a damaged cerebellum or basal nuclei
• Results in uncoordinated, clumsy activity and inappropriate patterns of movement
125
Describe the result of damage to the higher-cortical regions
• Results in inability to establish appropriate motor commands and accomplish desired goals
126
Describe the muscle spindle structure
• Monitors changes in muscle length
• Distributed throughout the fleshy part (belly) of the skeletal muscle
• Intrafusal fibers
o Collections of specialized muscle fibers that make up the spindle
o Lie within connective tissue capsules parallel to the extrafusal (ordinary) muscle fibers
o Has a noncontractile central portion
o Contractile elements limited to the ends
• Gamma motor neuron
o Efferent neuron that innervates intrafusal fibers
• Primary (annulospiral) endings
o Afferent neuron
o Wrapped around central portion of intrafusal fibers
o Detect changes in length of fibers
o Detects speed of fiber stretching
• Secondary (flower-spray) endings
o Afferent neuron
o Clustered at the end segments of many intrafusal fibers
o Sensitive only to length changes
127
Describe the Golgi tendon organ
• Monitors changes in muscle tension
• Afferent fiber endings entwined within connective tissue fibers of tendons
• Muscle contraction pull on tendons, this tension is then transmitted to the bone the tendon is attached to, stimulating action potential in the Golgi tendon organ
o Frequency of firing directly relates to tension developed
• Information used for unconscious smooth execution of movement
• You are consciously aware of this tension though
128
Describe the cell structure of smooth muscle tissue
• Non striated as there is no sarcomere pattern
• Elongated spindle shape
o 50-400 µm long
o 2-10 µm wide
• Single nucleus
• No T tubules
o Much smaller cell means calcium influx can reach the inside of the cell without assistance
• Poorly developed sarcoplasmic reticulum
o Only contains a small source of calcium
• Do not extend the length of the whole muscle and no myofibrils formed
• Groups of muscles typically arranged in sheets
129
What are dense bodies?
Located in smooth muscle tissue
o Contain same protein as Z lines
o Scattered throughout smooth muscle and attached to plasma membrane
o Held in place by intermediate filaments
130
How do the actin filaments in smooth muscle compare to those in skeletal muscle?
o Anchored to dense bodies
o 10-15 per myosin filament (rather than the 2 in skeletal muscle)
o Does not contain troponin
o Tropomyosin present but does not block the cross-bridges
131
How do the myosin filaments in smooth muscle compare to those in skeletal muscle?
o Larger than in skeletal muscle
132
How are the myosin-actin contractile units oriented in smooth muscle
• Myosin-actin contractile units oriented slightly diagonally in diamond shaped lattice
o Contraction makes the cell shorter and wider than at rest
133
What are light chains in smooth muscle?
o Lightweight chains attached to myosin heads
| o Only once phosphorylated can a cross bridge occur
134
What is calmodulin?
o Intracellular protein similar to troponin and found in most cells
o Binds with calcium
135
What is myosin light chain kinase?
o Protein activated by the calcium-calmodulin complex
| o This phosphorylates the myosin light chain
136
What are the calcium sources of smooth muscle?
o Voltage-gated dihydropyridine receptors in plasma membrane which function as calcium channels
Causes calcium to rush in from the ECF
Also causes release of what calcium the SR holds
137
How does smooth muscle relax?
• Calcium is pumped out actively into
o ECF
o Sarcoplasmic reticulum
• Myosin is dephosphorylated resulting in inability to bind to actin
138
What is multiunit smooth muscle? How is it stimulated? Where is it located in the body?
```
• Multiple units that function independently and stimulated separately much like motor units of skeletal muscle
• Neurogenic
o Nerve produced contraction
o Supplied by autonomic nervous system
• Locations
o Walls of large blood vessels
o Small airways to the lungs
o Ciliary muscles of the eye to allow for accommodation
o Iris
o Arrector pili
```
139
What is single unit smooth muscle? Where is it located in the body?
• Once excited, potential is linked by gap junctions, so the entire group contracts together
• Functional syncytium
o The interconnected group of smooth muscle cells that contract together
o Most are contractile cells
o A few are self-excitable cells
• Allows a pushing motion
• Locations
o Walls of hollow organs or viscera
I.e., digestive, reproductive, and urinary tracks
Small blood vessels
140
What is an interconnected group of smooth muscle cells that contract together called?
• Functional syncytium
141
Which smooth muscle cells are myogenic? What does this mean?
• Self-excitable
• Cluster of specialized cells within a functional syncytium capable of spontaneous electrical activity
o Not capable of contraction
• Do not maintain constant resting potential
142
What are the two types of myogenic activity?
Pacemaker potential and slow wave potential
143
Describe pacemaker potential
* After repolarizing, membrane potential depolarizes to threshold, creating a new action potential
* Results in cyclical self-generating action potentials
144
Describe slow wave potentials
* Continuous waves in depolarizing and repolarizing, which alone does not reach threshold
* If the membrane potential is altered at the onset of its depolarizing swing, it will reach threshold at the peak of the depolarizing wave causing a burst of action potentials to occur
* Membrane potential altered by neural and local factors
145
How is tension regulated in single unit smooth muscle?
* Graded tension is accomplished with varying amounts of calcium allowed in, the more calcium, the tighter the contraction
* Tone, low level of tension, is maintained by sufficient amounts of calcium even in the absence of action potentials
146
How does the autonomic nervous system influence single unit smooth muscle?
• Each muscle cell can be influenced by more than one type of neurotransmitter
• Each autonomic terminal can influence more than one muscle cell
• Neurotransmitter receptors located throughout entire membrane
• Autonomic neurotransmitters can
o Influence contractile activity by altering the calcium present in the cell
o Does not initiate contraction though
147
Other than neurotransmitters, what other factors modify permeability of calcium channels of the plasma membrane, SR, or both, of smooth muscle?
o Mechanical stretch
o Certain hormones
o Local metabolites
o Specific drugs
148
Describe the tension-length relationship of smooth muscle
* Resting length is much shorter than its lo
* Muscle fiber can still reach near-maximum tension over a larger range of lengths than skeletal muscle
* Thin filaments continue to overlap the much longer thick filament even when stretched out so that cross-bridge cycling and tension can still take place
* Allows for considerable stretch and still maintain tension as well as contract to empty
* High connective tissue content prevents overstretching
149
Describe the stress relaxation response
* When smooth muscle is suddenly stretched, there is a sudden increase in tension before it relaxes back to original tone
* Theory is that it takes longer for cross bridges to detach and slide back to an unstrained position
150
Describe the latch phenomenon
* Cross bridges latch onto thin filaments for a longer period of time
* Enables muscle to maintain tension with less ATP use than skeletal muscle
151
Describe how smooth muscle contracts
• Light chains
o Lightweight chains attached to myosin heads
o Only once phosphorylated can a cross bridge occur
• Calmodulin
o Intracellular protein similar to troponin and found in most cells
o Binds with calcium
• Myosin light chain kinase
o Protein activated by the calcium-calmodulin complex
o This phosphorylates the myosin light chain
• Calcium sources
o Voltage-gated dihydropyridine receptors in plasma membrane which function as calcium channels
Causes calcium to rush in from the ECF
Also causes release of what calcium the SR holds
152
For skeletal muscle contractions, calcium binds to
a) actin
b) nebulin
c) troponin
d) tropomyosin
C
153
Which of the following applies to the length-tension relationship for skeletal muscles at rest?
a) less than optimal length
b) optimal length
c) greater than optimal length
d) always at optimal length
B
154
Direct input to alpha motor neurons comes from the
a) brain stem
b) hypothalamus
c) cerebellum
d) basal nuclei
A
155
Which muscle type does not have T tubules?
a) skeletal muscle
b) cardiac muscle
c) smooth muscle
d) they all have T tubules
C
156
With respect to the lines and bands seen in skeletal muscle myofibrils, which one of the following represents the area in which proteins hold the thick filaments together?
a) A band
b) I band
c) M line
d) A line
C
157
Describe the levels of organization in a skeletal muscle
Whole muscle -> muscle fiber -> myofibril -> thick and thin filaments -> myosin and actin
158
What is the functional unit of skeletal muscle?
Sarcomere
159
Describe the composition of thick and thin filaments
Thick filaments - myosin
Thin filaments - actin
160
Explain twitch summation
A second action potential stimulates a muscle fiber before it has relaxed, resulting in a contraction greater than the first stimulus alone
161
In smooth muscle, what are dense bodies?
Contain the same protein as found in skeletal muscle Z lines
162
Skeletal muscle is
a) striated and voluntary
b) striated and involuntary
c) non-striated and voluntary
d) non-striated and involuntary
A
163
Activation of dihydropyridine receptors in skeletal muscle does what?
a) increases cellular cAMP
b) increases cellular ATP
c) releases calcium from the sarcoplasmic reticulum
d) directly initiates cross-bridge cycling
C
164
When skeletal muscle is contracted, which one of the following skeletal muscle bands does NOT shorten?
a) H zone
b) I band
c) A band
d) Z line
C
165
Slow twitch skeletal muscle fibers are
a) large diameter, high mitochondrial density
b) small diameter, high mitochondrial density
c) large diameter, fatigable
d) small diameter, fatigable
B
166
Muscle fatigue can be caused by a decrease in which of the following?
a) ADP
b) ATP
c) accumulation of extracellular potassium
d) lactic acid
B
167
Describe the process of tetanus
A muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between stimuli. Results in smooth, sustained contraction of maximal strength
168
What role does ATP play in cross-bridge cycling?
ATP binds to myosin cross bridge, causing detachment from actin, during which the ATP is hydrolyzed to ADP and inorganic phosphate. In the presence of calcium, the actin again binds with the myosin cross bridge. Cross bridge undergoes a power stroke, during which ADP and phosphate detach. Process repeats if ATP again binds to myosin cross bridge
169
Compare and contrast isotonic and isometric contractions
Isotonic - muscle fiber tension remains constant, length changes
Isometric - muscle fiber tension changes, length remains constant
170
Describe the roles of muscle spindles and Golgi tendon organs in muscle function
Spindles are sensors located among the muscle fibers responding to changes in muscle length
Golgi tendon organs are sensors located within the tendons that responds to changes in muscle tension
171
Differentiate between end-plate potentials in skeletal muscles and pacemaker potentials in smooth muscles
Skeletal muscle depends on end-plate potentials, which are triggered by external stimuli (acetylcholine), for excitation
Some smooth muscle cells have gradually depolarizing membrane potentials. If sufficient depolarization occurs, an action potential is triggered and can be passed to other smooth muscle cells for coordinated contraction
172
Which one of the following adaptations would you expect to see in skeletal muscle following regular aerobic exercise?
a) increased number of muscle fibers
b) increased mass of existing muscle fibers
c) increased number of Golgi tendon organs
d) increased amount of mitochondria
D
173
Rigor mortis occurs when there is not enough
a) ADP
b) ATP
c) calcium
d) inorganic phosphate
B
174
If you were to see the sarcomeres of a contracting muscles and were to observe that there was no change in the distance between Z lines, what kind of contraction would you say occurred?
a) concentric
b) eccentric
c) isometric
d) isotonic
C
175
Which one of the following conditions would be most associated with the disruption of an inhibitory system that originates within the brain stem?
a) flaccid paralysis
b) spastic paralysis
c) hemiplegia
d) paraplegia
B
176
What would you see in a smooth muscle that was excited when it had no calcium in the extracellular fluid?
a) decreased contraction
b) normal contraction
c) increased contraction
d) enhanced pacemaker potentials
A
177
Why does regular aerobic exercise provide more cardiovascular benefit than weight training does?
(hint: the heart responds to the demands placed on it in a way similar to that of skeletal muscle)
By placing increased demands on the heart to sustain increased delivery of oxygen and nutrients to working skeletal muscles, regular aerobic exercise induces changes in cardiac muscle enabling it to use oxygen more efficiently, such as increasing capillary blood supply.
Intense exercise of short duration does not induce cardiac efficiency as this type of exercise relies on anaerobic glycolysis for ATP formation, no demands are placed on the heart for increased blood delivery to the muscles
178
If the biceps muscle of a child inserts 4 cm from the elbow and the length of the arm from the elbow to the hand is 28 cm, how much force must the biceps generate in order for the child to life an 8kg stack of books with one hand?
Power arm (4cm):load arm (28cm) is a lever ratio of 1:7
7x 8kg = 56 kg of force needed
179
Put yourself in the position of the scientists who discovered the sliding filament mechanism of muscle contraction, by considering what molecular changes must be involved to account for the observed alterations in the banding pattern during contraction. If you were comparing a relaxed and a contracted muscle fiber under an electron microscope, how could you determine that the thin filaments do not change in length during muscle contraction? You cannot see or measure a single thin filament at this magnification.
(hint: what landmark in the banding pattern represents each end of the thin filament? If these landmarks are the same distance apart in a relaxed and contracted fiber, then the thin filaments must not change in length.
Distance from Z band to H band distance; this distance remains unchanged between contraction and relaxation
180
What type of off-the-snow training would you recommend for a competitive downhill skier versus a competitive cross-country skier? What adaptive skeletal muscle changes would you hope to accomplish in the athletes in each case?
Downhill - anaerobic, short-duration, high-intensity resistance training by promoting hypertrophy of fast glycolytic fibers. This adapts muscle to activities requiring intense strength for brief periods
cross-country - aerobic exercise will induce metabolic changes within oxidative fivers enabling more efficient oxygen use, increasing mitochondria and capillary supply; better for prolonged activity
181
When the bladder is filled and the micturition reflex is initiated, the nervous supply to the bladder promotes contraction of the bladder and relaxation of the external urethral sphincter, a ring of muscle that guards the exit from the bladder. If the time is inopportune for bladder emptying when the micturition reflex is initiated, the external urethral sphincter can be voluntarily tightened to prevent urination even though the bladder is contraction. Using your knowledge of the muscle types and their innervation, of what types of muscle are the bladder and the external urethral sphincters composed, and what branch of the efferent division of the peripheral nervous system supplies each of these muscles?
External urethral sphincter is skeletal muscle, which is innervated by the voluntary controlled somatic nervous system.
Bladder is smooth muscle, innervated by the autonomic nervous system.
182
Jason is waiting impatiently for the doctor to finish removing the cast from his leg, which Jason broke the last day of school 6 weeks ago. Summer vacation is half over, and he hasn't been able to swim, play baseball, or participate in any of his favorite sports. When the case is finally off, Jason's excitement gives way to concern when he sees that the injured limb is noticeably smaller in diameter than his normal leg. What explains this reduction in size? How can the leg be restored to its normal size and functional ability?
Muscles in immobilized leg have undergone disuse atrophy. Physician or physical therapist can prescribe regular resistance-type exercises that specifically use the atrophied muscles to help restore them to their normal size
