1
Q
what does motion result from?
A
the alternating contraction and relaxation of muscles, which make up 40–50% of total adult body weight
2
Q
what is the primary function of muscles?
A
the transformation of chemical energy into mechanical energy to generate force, perform work, and produce movement
3
Q
what is myology?
A
the scientific study of muscles
4
Q
how can the activity of skeletal muscles be consciously controlled?
A
by neurons (nerve cells) that are part of the somatic (voluntary) division of the nervous system
5
Q
what is autorhythmicity?
A
The ability to repeatedly generate spontaneous nerve impulses
6
Q
how do muscular tissues produce body movements?
A
Movements of the whole body such as walking and running, and localized movements such as grasping a pencil, keyboarding, or nodding the head rely on the integrated functioning of skeletal muscles, bones, and joints
7
Q
how do muscular tissues stabilize body positions?
A
- Skeletal muscle contractions stabilize joints and help maintain body positions, such as standing or sitting
- Postural muscles contract continuously when you are awake (ex. sustained contractions of your neck muscles hold your head upright)
8
Q
how do muscular tissues store and move substances within the body?
A
- sphincters prevent outflow of the contents of a hollow organ
- Cardiac muscle contractions in the wall of the heart pump blood through the blood vessels of the body, contraction and relaxation adjust blood vessel diameter and regulate blood flow rate
- Smooth muscle contractions move food and substances through digestive canal via peristalsis
- Skeletal muscle contractions promote the flow of lymph plasma and aid the return of blood in veins to the heart
9
Q
how do muscular tissues generate heat?
A
thermogenesis, muscular tissue generate heat as they contract
- involuntary contractions of skeletal muscles (shivering) increases rate of heat production
10
Q
what are the special properties muscular tissues have that enable it to function and contribute to homeostasis?
A
- electrical excitability
- contractility
- extensibility
- elasticity
11
Q
what is electrical excitability?
A
the ability to respond to certain stimuli by producing electrical signals called action potentials
12
Q
what main types of stimuli trigger muscle action potentials?
A
- autorhythmic electrical signals arising in the muscular tissue itself, as in the heart’s natural pacemaker
- chemical stimuli, such as neurotransmitters released by neurons, hormones distributed by the blood, or even local changes in pH
13
Q
what is contractility?
A
- the ability of muscular tissue to contract forcefully when stimulated by a nerve impulse.
- skeletal muscle generates tension when pulling on its attachment points when it contracts
14
Q
what is extensibility?
A
- the ability of muscular tissue to stretch, within limits, without being damaged
15
Q
what is the function of connective tissue within the muscles?
A
- limits the range of extensibility and keeps muscles within the contractile range of the muscle cells
- surrounds and protects muscular tissue
16
Q
what is elasticity?
A
the ability of muscular tissue to return to its original length and shape after contraction or extension
17
Q
what are myocytes?
A
muscle fibers, composed the skeletal muscles
18
Q
what does skeletal muscle consist of?
A
- myocytes (muscle fibers)
- connective tissue
- blood vessels
- nerves
19
Q
what is the function of subcutaneous tissue in context of muscles?
A
- separates muscle from skin
- provides a pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles
- protects muscles from physical trauma due to adipose tissue
20
Q
what are fascia?
A
dense sheets or broad bands of irregular connective tissue that line the body wall and limbs and support and surround muscles and other organs of the body
21
Q
what is the function of fascia?
A
- holds muscles with similar functions together
- allows free movement of muscles
- carries nerves, blood vessels, and lymphatic vessels
- fills spaces between muscles
22
Q
what are the layers of connective tissue that extend from the fascia to protect and lengthen skeletal muscles?
A
- epimysium
- perimysium
- endomysium
23
Q
what is the epimysium?
A
the outer layer, encircling the entire muscle
- consists of dense irregular connective tissue
24
Q
what is the perimysium?
A
a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called muscle fascicles
25
what are muscle fascicles?
groups of 10-100 or more muscle fibres surrounded by perimysium
26
what is fibromyalgia?
a chronic, painful, nonarticular rheumatic disorder that **affects the fibrous connective tissue components of muscles, tendons, and ligaments**
- pain that results from gentle pressure at specific “tender points”, severe fatigue, poor sleep, headaches, depression, irritable bowel syndrome, and inability to carry out their daily activities, pain, tenderness, and stiffness of muscles, tendons, and surrounding soft tissues
- treatment: stress reduction, regular exercise, application of heat, gentle massage, physical therapy, medication for pain, and a low-dose antidepressant to help improve sleep
27
what is endomysium?
Invagination of the perimysium separating each individual muscle fiber, mostly reticular fibres
28
what is a tendon?
A white fibrous cord of dense regular connective tissue that attaches muscle to bone
29
what is an aponeurosis?
A **sheetlike** tendon joining one muscle with another or with bone, broad flat sheet
30
what are somatic motor neurons?
neurons that stimulate skeletal muscle to contract
31
why are blood capillaries plentiful in muscular tissue?
- bring in oxygen and nutrients and remove heat and the waste products of muscle metabolism
- muscle fiber synthesizes and uses considerable ATP, which requires oxygen, glucose, fatty acids, and other substances to be delivered to muscle fiber
32
what are myoblasts?
mesodermal cells that fuse into skeletal muscle fibers during embryonic development
33
when do muscle fibers lose their ability to undergo cell division?
when fusion of the myoblasts occurs
34
what is the sarcolemma?
the plasma membrane of a muscle fiber
35
what are transverse tubules?
tiny tube-shaped invaginations of the sarcolemma that tunnel in from the surface toward the center of each muscle fiber
36
why are transverse tubules filled with interstitial fluid?
T tubules are open to the outside of the fiber
37
what is the function of transverse tubules?
- muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber
- ensures that an action potential excites all parts of the muscle fiber at essentially the same instant
38
what is the sarcoplasm?
the cytoplasm of a muscle fiber
- includes substantial amount of glycogen and myoglobin
39
what is myoglobin?
red-coloured, iron-containing protein found only in sarcoplasm of muscles
- binds oxygen molecules that diffuse into muscle fibers from interstitial fluid
- releases oxygen when it is needed by the mitochondria for ATP production
40
where are mitochondria placed in muscle fibers?
mitochondria lie in rows throughout the muscle fiber, **strategically close to the contractile muscle proteins that use ATP during contraction** so that ATP can be produced quickly as needed
41
what is muscular hypertrophy?
enlargement of existing muscle fibers
- due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles
- results from very forceful, repetitive muscular activity, such as strength training
- capable of more forceful contractions
- growth hormone, testosterone promotes enlargement of skeletal muscle fibers
42
what are satellite cells?
myoblasts that persist in mature skeletal cells
- retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers
43
what is fibrosis in muscular tissue?
the replacement of muscle fibers by fibrous scar tissue when the number of new skeletal muscle fibers that can be formed by satellite cells is not enough to compensate for significant skeletal muscle damage or degeneration
44
what is muscular atrophy?
a decrease in size of individual muscle fibers because of progressive loss of myofibrils
45
what is disuse atrophy?
Atrophy that occurs because muscles are not used
46
what is denervation atrophy?
muscle decreases in size as nerve supply is disrupted or cut
- Over a period of 6 months to 2 years, the muscle shrinks to about one-fourth its original size, and its fibers are **irreversibly** replaced by **fibrous connective tissue**
47
Which structure in muscle fibers release calcium ions to trigger muscle contraction?
the sarcoplasmic reticulum
48
what are myofibrils?
the contractile organelles of skeletal muscle
- their prominent striations make the entire skeletal muscle fiber appear striated
- are about 2 µm in diameter and extend the entire length of a muscle fiber
49
what is the sarcoplasmic reticulum?
fluid-filled system of membranous sacs that encircles each myofibril
50
what are terminal cisternae?
Dilated end sacs of the sarcoplasmic reticulum that butt against a transverse tubule from both sides
51
what is a triad in muscle fibers?
terminal cistern | T tubule | terminal cistern
52
what is the function of the sarcoplasmic reticulum?
- release of Ca2+ from the terminal cisterns of the sarcoplasmic reticulum triggers muscle contraction
- the sarcoplasmic reticulum stores Ca2+ in a relaxed muscle fiber
53
what are myofilaments?
smaller protein structures within myofibrils
54
what proteins are thin filaments composed of?
actin
55
what proteins are thick filaments composed of?
myosin
56
what are sarcomeres?
the basic contractile unit in a striated muscle fiber extending from one Z disc to the next Z disc
57
what are Z discs?
Narrow, plate-shaped regions of dense protein material that separate one sarcomere from the next
58
what is the A band of a sarcomere?
- darker middle part of the sarcomere
- extends the entire length of the thick filaments
59
what is the zone of overlap in sarcomeres?
where the thick and thin filaments lie side by side, found towards each end of the A band
60
what is the I band in sarcomeres?
a lighter, less dense area that contains the rest of the thin filaments but no thick filaments
- a Z disc passes through the center of each I band
61
what creates the striations that can be seen in both myofibrils and in whole skeletal and cardiac muscle fibers?
The alternating dark A bands and light I bands`
62
what is the H band in sarcomeres?
a narrow band in the center of each A band contains thick but not thin filaments
63
what is the M line in sarcomeres?
center of H band where supporting proteins that hold the thick filaments together are found
64
what kinds of proteins are myofibrils made of?
1. contractile proteins
2. regulatory proteins
3. structural proteins
65
what do the contractile proteins do in myofibrils?
generate force during contraction
66
what do the regulatory proteins do in myofibrils?
help switch the contraction process on and off
67
what do the structural proteins do in myofibrils?
- keep the thick and thin filaments in the proper alignment
- give the myofibril elasticity and extensibility
- link the myofibrils to the sarcolemma and extracellular matrix
68
what is myosin?
a contractile protein, the main component of thick filaments and functions as a motor protein in all three types of muscle tissue
69
what are motor proteins?
Motor proteins pull various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion, that is, the production of force
70
what are the binding sites that are found on a myosin head?
1. actin-binding site
2. ATP-binding site
71
what enzyme does the ATP-binding site of the myosin function as?
ATPase, hydrolyzes/catabolizes ATP to generate energy for muscle contraction
72
what is actin?
A contractile protein that is part of thin filaments in muscle fibers
73
Which proteins connect into the Z disc? Which proteins are present in the A band? In the I band?
Z disc - Actin and titin
A bands - myosin, actin, troponin, tropomyosin, and titin
I bands - actin, troponin, tropomyosin, and titin
74
what is the function of tropomyosin?
regulatory protein, covers the myosin-binding sites on actin, blocking myosin from binding to actin in relaxed muscles
75
what is the function of troponin?
holds tropomyosin strands in place
- undergoes conformational change when it binds to Ca2+, moving tropomyosin away from the myosin-binding sites on actin
76
what is the function of structural proteins?
contribute to the alignment, stability, elasticity, and extensibility of myofibrils
77
what are some of the key structural proteins in myofibrils?
- titin
- α-actinin
- myomesin
- nebulin
- dystrophin
78
what is the function of titin?
connects a Z disc to the M line of the sarcomere, thereby helping stabilize the position of the thick filament
- accounts for much of the elasticity and extensibility of myofibrils
- helps the sarcomere return to its resting length after a muscle has contracted or been stretched
- may help prevent overextension of sarcomeres
- maintains the central location of the A bands
79
what is the function of α-actinin?
binds to actin molecules of the thin filament and to titin
80
what is the function of myomesin?
forms the M line, binds to titin and connect adjacent thick filaments to one another
- myomesin holds the thick filaments in alignment at the M line
81
what is the function of nebulin?
long, nonelastic protein wrapped around the entire length of each thin filament
- helps anchor the thin filaments to the Z discs and regulates the length of thin filaments during development
82
what is the function of dystrophin?
- links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached to proteins in the connective tissue extracellular matrix that surrounds muscle fibers
- thought to reinforce the sarcolemma and help transmit the tension generated by the sarcomeres to the tendons
83
what is the sliding filament mechanism?
skeletal muscle shortens during contraction because the thick and thin filaments **slide** past one another
84
how does muscle contraction occur according to the sliding filament mechanism?
- myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line
- the thin filaments slide inward and meet at the center of a sarcomere
- As the thin filaments slide inward, the I band and H zone narrow and eventually disappear altogether when the muscle is maximally contracted
- the width of the A band and the individual lengths of the thick and thin filaments remain unchanged - Since the thin filaments on each side of the sarcomere are attached to Z discs, when the thin filaments slide inward, the Z discs come closer together, and the sarcomere shortens
- Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle
85
What happens to the I band and H zone as muscle contracts? Do the lengths of the thick and thin filaments change?
The I bands and H zones disappear during muscle contraction; the lengths of the thin and thick filaments do not change
86
What are the steps of the contraction style?
1. ATP hydrolysis
2. Attachment of myosin to actin
3. Power stroke
4. Detachment of myosin from actin
87
when does the contraction cycle begin?
when troponin, upon Ca2+ binds to it, moves tropomyosin away from the myosin-binding sites on actin and freeing them
88
what happens in ATP hydrolysis of the contraction cycle?
- ATP that binds to ATP-binding site of myosin is hydrolyzed into ADP and Pi, energy generated is stoted in the myosin head for later use
- the energized myosin head is "cocked" like a stretched spring (perpendicular to the thick and thin filaments, the proper orientation to bind to an actin molecule)
- ADP and Pi are still attached to myosin head
89
what happens in the "attachment of myosin to actin" step of the contraction cycle?
energized myosin head attaches to the myosin-binding site on actin and releases Pi
90
how much actin molecules can bind to one myosin molecule?
only one, each myosin only has one actin-binding site
91
what is a cross-bridge?
myosin head is attached to an actin molecule
92
what happens in the "power stroke" step of the contraction cycle?
- myosin head pivots, changing its position from a 90° angle to a 45° angle relative to the thick and thin filaments
- power stroke occurs: as the myosin head changes to its new position, it pulls the thin filament past the thick filament toward the center of the sarcomere, generating tension
- ADP is released from the myosin head
93
what happens in the "detachment of myosin from actin" step of the contraction cycle?
- cross-bridge remains firmly attached to actin until it binds to another ATP
- myosin head detaches from actin as ATP binds
94
What would happen if ATP suddenly were not available after the sarcomere had started to shorten?
If ATP were not available, the cross-bridges would not be able to detach from actin. The muscles would remain in a state of rigidity, as occurs in rigor mortis.
95
what is excitation-contraction coupling?
The sequence of events that links excitation (a muscle action potential) to contraction (sliding of the filaments) that occurs at the triads of the skeletal muscle fiber
96
where are voltage-gated Ca2+ channels located on a triad?
on the T tubule membrane, arranged in tetrads
97
what is the function of the voltage-gated Ca2+ channels?
to serve as voltage sensors that trigger the opening of the Ca2+ release channels
98
where are Ca2+ release channels located in the triad?
on the terminal cisternal membrane of the sarcoplasmic reticulum
99
how are the Ca2+ channels in the triad configured when a skeletal muscle fiber is at rest?
the part of the Ca2+ release channel that extends into the sarcoplasm is blocked by a given cluster of voltage-gated Ca2+ channels, preventing Ca2+ from leaving the SR
100
how are the Ca2+ channels in the triad configured when a skeletal muscle fiber is excited?
when an action potential travels along the T tubule, the voltage-gated Ca2+ channels detect the change in voltage and undergo a conformational change that causes the Ca2+ release channels to open
- copious amounts of Ca2+ flow out of the SR into the sarcoplasm around the thick and thin filaments
- Ca2+ concentration in the sarcoplasm rises tenfold
- The released calcium ions combine with troponin and start the contraction cycle
101
what are Ca2+-ATPase pumps
proteins located on the terminal cisternal membrane of the SR that use ATP to constantly transport Ca2+ from the sarcoplasm back into the SR
102
what is calsequestrin?
proteins that bind to Ca2+, allowing even more Ca2+ to be sequestered (stored) within the SR
103
what is electrodiagnostic medicine?
a branch of medicine concerned with the diagnosis of neuromuscular disorders
104
what do nerve conduction velocity tests measure?
the speed of nerve impulses conducted through nerves outside the brain and spinal cord; for example, those of your limbs
- involve stimulating a nerve with an electrical impulse applied to the skin and recording the response from a muscle (contraction) or another portion of a nerve through patches placed on the skin
- used to diagnose conditions such as carpal tunnel syndrome, herniated discs, and sciatica
105
what is an electromyography?
- a very thin needle, which serves as a recording device, is placed through the skin into a muscle
- The needle is connected by a wire to the screen of a device (an oscilloscope)
- Resting muscle produces no electrical activity as there are no muscle action potentials
- The more forceful the contraction, the higher the level of electrical activity
- the client is asked to contract a muscle and the activity is recorded on the screen and may also be detected audibly with a speaker
- used to diagnose disorders such as muscular dystrophy, myasthenia gravis, and amyotrophic lateral sclerosis
106
what is the length-tension relationship for skeletal muscle?
the forcefulness of muscle contraction depends on the length of the sarcomeres within a muscle before contraction begins
107
when can a muscle fiber develop maximum tension?
when zone of overlap in each sarcomere is optimal, close to resting length in most muscles
- when the zone of overlap between a thick and thin filament extends from the edge of the H zone to one end of a thick filament
108
What are the three functions of ATP in muscle contraction?
(1) Its hydrolysis by an ATPase activates the myosin head so it can bind to actin and rotate
(2) its binding to myosin causes detachment from actin after the power stroke
(3) it powers the pumps that transport Ca2+ from the sarcoplasm back into the sarcoplasmic reticulum
109
what is rigor mortis?
State of partial contraction of muscles after death due to lack of ATP; myosin heads (crossbridges) remain attached to actin, thus preventing relaxation
- begins 3–4 hours after death and lasts about 24 hours; then it disappears as proteolytic enzymes from lysosomes digest the cross-bridges
110
what happens when the sarcomeres of a muscle fiber are stretched to a longer length?
the zone of overlap shortens, and fewer myosin heads can make contact with thin filaments. Therefore, the tension the fiber can produce decreases.
111
when is there no overlap between thick and thin filaments?
When a skeletal muscle fiber is stretched to 170% of its optimal length
- because none of the myosin heads can bind to thin filaments, the muscle fiber cannot contract, and tension is zero
112
what happens when the sarcomere lengths become increasingly shorter than the optimum?
the tension that can develop again decreases
- thick filaments crumple as they are compressed by the Z discs, resulting in fewer myosin heads contacting thin filaments
113
what holds resting muscle fiber length to its optimum?
by firm attachments of skeletal muscles to bones via their tendons and to other inelastic tissues
114
Why is tension maximal at a sarcomere length of 2.2 µm?
A sarcomere length of 2.2 µm gives a generous zone of overlap between the parts of the thick filaments that have myosin heads and the thin filaments without the overlap being so extensive that sarcomere shortening is limited.
115
what is a neuromuscular junction?
The synapse between a somatic motor neuron and a skeletal muscle fiber where muscle action potentials arise
116
what is a synapse?
a region where communication occurs between two neurons, or between a neuron and a target cell—between a somatic motor neuron and a muscle fiber
117
what is a synpatic cleft?
The narrow gap at a chemical synapse that separates the axon terminal of one neuron from another neuron or muscle fiber and across which a neurotransmitter diffuses to affect the postsynaptic cell
118
what are neurotransmitters?
One of a variety of molecules within axon terminals that are released into the synaptic cleft in response to a nerve impulse and that change the membrane potential of the postsynaptic neuron
119
what is acetylcholine?
A neurotransmitter liberated by many peripheral nervous system neurons and some central nervous system neurons
- It is excitatory at neuromuscular junctions but inhibitory at some other synapses
120
what is an axon terminal?
*telodendria*
Terminal branch of an axon where synaptic vesicles undergo exocytosis to release neurotransmitter molecules
121
what is a synaptic end bulb?
Expanded distal end of an axon terminal that contains synaptic vesicles
122
what is a motor end plate?
Region of the sarcolemma of a muscle fiber opposite the synaptic end bulbs that includes acetylcholine receptors, which bind ACh released by synaptic end bulbs of somatic motor neurons
123
where are acetylcholine receptors abundant within the motor end plates?
abundant in junctional folds, deep grooves in the motor end plate that provide a large surface area for ACh
124
what are the steps a nerve action potential elicits a muscle action potential?
1. release of ACh
2. activation of ACh receptors
3. production of muscle action potential
4. termination of ACh activity
125
what happens in the "release of acetylcholine"?
- arrival of nerve action potential at synaptic end bulbs stimulates voltage-gated channels to open
- Ca2+ flows inward through the open channels, Ca2+ are more concentrated in ECF
- Ca2+ in turn stimulates the synaptic vesicles to undergo exocytosis
- the synaptic vesicles fuse with the motor neuron’s plasma membrane, liberating ACh into the synaptic cleft
- ACh then diffuses across the synaptic cleft between the motor neuron and the motor end plate
126
what happens in "activation of ACh receptors"?
- Binding of two molecules of ACh to the receptor on the motor end plate opens an ion channel in the ACh receptor
- small cations, most importantly Na+, can flow across the membrane once channel is open
127
what happens in "production of muscle action potential"?
- inflow of Na+ (down its electrochemical gradient) makes the inside of the muscle fiber more positively charged
- change in the membrane potential triggers a muscle action potential
- muscle action potential then propagates along the sarcolemma into the system of T tubules
- sarcoplasmic reticulum releasex its stored Ca2+ into the sarcoplasm, and the muscle fiber subsequently contracts
128
what happens in "termination of ACh activity"?
- effect of ACh binding lasts only briefly because ACh is rapidly broken down by **acetylcholinesterase** located on the extracellular side of the motor end plate membrane
- AChE breaks down ACh into acetyl and choline
129
What part of the sarcolemma contains acetylcholine receptors?
The part of the sarcolemma that contains acetylcholine receptors is the motor end plate.
130
how many NMJs does a skeletal muscle fiber have?
only one, usually located near the midpoint of the fiber which permits nearly simultaneous activation (and thus contraction) of all parts of the muscle fiber
131
How does Botulinum toxin block certain events at NMJ?
- blocks exocytosis of synaptic vesicles at the NMJ
- ACh is not released, and muscle contraction does not occur
132
how can one die from botulinum toxin?
Breathing stops due to paralysis of respiratory muscles, including the diaphragm
133
how is botox used?
- Injections of Botox into the affected muscles can help patients who have crossed eyes, uncontrollable blinking, or spasms of the vocal cords that interfere with speech
- It is also used to alleviate chronic back pain due to muscle spasms in the lumbar region and as a cosmetic treatment to relax muscles that cause facial wrinkles
134
how does curare block certain events at NMJ?
causes muscle paralysis by binding to and blocking ACh receptors
135
what are anticholinesterase agents?
family of chemicals that have the property of slowing the enzymatic activity of acetylcholinesterase, thus slowing removal of ACh from the synaptic cleft, can strengthen weak muscle contractions
(ex. Neostigmine)
136
what are the three ways muscle fibers can produce ATP?
1. Creatine Phosphate
2. Anaerobic Glycolysis
3. Aerobic Respiration
137
why do muscle fibers produce more ATP than they need for resting metabolism while they are relaxed?
Most of the excess ATP is used to synthesize creatine phosphate
138
what enzyme catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine?
Creatine Kinase
139
what is creatine?
a small, amino acid–like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers
140
what does creatine kinase do when contraction begins and ADP levels start to rise?
CK catalyzes the transfer of a high-energy phosphate group from creatine phosphate back to ADP
(ADP+Pi=ATP)
141
why is creatine phosphate the first source of energy when muscle contraction begins?
direct phosphorylation reaction catalyzed by CK quickly generates new ATP molecules, occurs very rapidly
142
how much energy does creatine phosphate and ATP provide?
Together, stores of creatine phosphate and ATP provide enough energy for muscles to contract maximally for about **15 seconds**
143
what are some risks of creatine supplementation?
- ingesting extra creatine decreases the body’s own synthesis of creatine, not known whether natural synthesis recovers after long-term creatine supplementation
- creatine supplementation can cause dehydration and may cause kidney dysfunction
144
Where does creatine phosphate synthesis?
in the cytosol of muscle fibers
145
what happens to pyruvate originally produced from glycolysis during heavy exercise?
- not enough oxygen is available to skeletal muscle fibers, anaerobic conditions
- pyruvate converted to lactic acid
146
what is the yield of each glucose catabolized via anaerobic glycolysis?
- 2 lactic acid molecules
- 2 ATP
147
where does lactic acid go after anaerobic glycolysis?
- most diffuse out of skeletal muscle into blood where liver converts them back to glucose, reducing blood acidity
148
where does lactic acid go after anaerobic glycolysis is performed at a rapid rate?
- lactic acid can accumulate in active skeletal muscle fibers and in bloodstream
- buildup is thought to be responsible for muscle soreness during exercise
149
how much energy does anaerobic glycolysis provide?
- provides 2 mins maximal muscle activity
150
what are the two sources of oxygen muscular tissues have?
1. oxygen that diffuses into muscle fibers from blood
2. oxygen released by myoglobin within muscle fibers
151
how much energy does aerobic respiration provide?
30-32 ATP
- several minutes - an hour or more of activity
152
what is muscle fatigue?
inability of a muscle to maintain force of contraction after prolonged activity
153
what is central fatigue?
- feelings of tiredness and the desire to cease activity
- caused by changes in CNS
- protective mechanism to stop a person from exercising before muscles become damaged
154
what are some mechanisms that contribute to muscle fatigue?
- inadequate release of Ca2+ from SR
- insufficient oxygen
- depletion of glycogen
- buildup of lactic acid and ADP
- failure of action potentials in motor neuron to release enough ACh
155
what is oxygen debt?
the added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise
156
why does heavy breathing continue for a while even after muscle contraction has stopped?
oxygen debt, oxygen is added at a higher amount than resting oxygen consumption after prolonged periods of muscle contraction
157
what is the function of oxygen debt?
- to convert lactic acid back into glycogen stores in the liver
- to resynthesize creatine phosphate and ATP in muscle fibers
- to replace O2 removed from myoglobin
158
what is a motor unit?
unit that consists of a somatic motor neuron plus all of the skeletal muscle fibers it stimulates
(a single somatic motor neuron makes contact with an average of 150 skeletal muscle fibers, and all of the muscle fibers in one motor unit contract in unison)
159
whole muscles that control **precise movements** consist of **many small motor units**
Because all of the muscle fibers of a motor unit contract and relax together, the total strength of a contraction depends, in part, on the size of the motor units and the number that are activated at a given time
160
skeletal muscles responsible for **large-scale and powerful movements**, such as the biceps brachii muscle in the arm and the gastrocnemius muscle in the calf of the leg, have as **many** as 2000 to 3000 muscle fibers in some **motor units**
Because all of the muscle fibers of a motor unit contract and relax together, the total strength of a contraction depends, in part, on the size of the motor units and the number that are activated at a given time
161
What is the effect of the size of a motor unit on its strength of contraction?
motor units having many muscle fibers are capable of more forceful contractions than those having only a few fibers
162
what is a twitch contraction?
the brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron
163
what is the latent period?
a brief delay occurs between application of the stimulus and the beginning of contraction
- muscle action potential sweeps over the sarcolemma and calcium ions are released from the SR
164
what happens in the contraction period?
- Ca2+ binds to troponin
- myosin-binding sites on actin are exposed, and cross-bridges form
- peak tension develops in the muscle fiber
165
what happens in the relaxation period?
- Ca2+ is actively transported back into the SR
- myosin-binding sites are covered by tropomyosin
- myosin heads detach from actin
- tension in the muscle fiber decreases
166
Describe the duration of the latent, contraction, and relaxation periods in fast-twitch fibers compared to slow-twitch fibers
- fast-twitch fibers that move the eyes have contraction periods as brief as 10 msec and equally brief relaxation periods
- slow-twitch fibers that move the legs have contraction and relaxation periods of about 100 msec each
167
what is the refractory period?
period of lost excitability when a muscle fiber receives enough stimulation to contract and cannot respond for a time
168
when two stimuli are applied one immediately after the other, why does the muscle respond to the first stimulus but not the second?
because of the refractory period
169
describe the duration of the refractory period in skeletal vs. cardiac muscle
- skeletal muscle has a short refractory period of about 1 msec
- cardiac muscle has a longer refractory period of about 250 msec
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what is wave summation?
phenomenon in which a stimulus that occurs after the refractory period of the first stimulus is over but before the skeletal muscle fiber has relaxed causes larger contractions
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what is unfused tetanus?
sustained but wavering contraction that occurs when a skeletal muscle fiber is stimulated at a rate of 20 to 30 times per second and can only partially relax between stimuli
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what is fused tetanus?
a sustained contraction in which individual twitches cannot be detected, occurs when a skeletal muscle fiber is stimulated at a higher rate of 80 to 100 times per second and does not relax at all
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why do wave summation and both kinds of tetanus occur?
additional Ca2+ is released from the sarcoplasmic reticulum by subsequent stimuli while the levels of Ca2+ in the sarcoplasm are still elevated from the first stimulus
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how do tendons and connective tissues around muscle fibers affect wave summation?
- during wave summation, elastic components are not given much time to spring back between contractions and remain taut
- elastic components do not require much stretching before the beginning of the next muscular contraction
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what is motor unit recruitment?
The process in which the number of active motor units increases
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describe the pattern of motor unit activity that delays muscle fatigue and allows contraction of a whole muscle to be sustained for long periods
- the different motor units of an entire muscle are not stimulated to contract in unison
- while some motor units are contracting, others are relaxed
- the weakest motor units are recruited first, with progressively stronger motor units added if the task requires more force
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how is recruitment a factor in producing smooth movement rather than a series of jerks?
- precise movements are brought about by small changes in muscle contraction
- the small muscles that produce precise movements are made up of small motor units
- when a motor unit is recruited or turned off, only slight changes occur in muscle tension
- in contrast, large motor units are active when a large amount of tension is needed and precision is less important
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what is anaerobic training?
activities such as weight lifting that rely more on anaerobic production of ATP through glycolysis
- stimulate synthesis of muscle proteins and result, over time, in increased muscle size
- diet should include an adequate amount of proteins
- protein intake will allow the body to synthesize muscle proteins and to increase muscle mass
- builds muscle strength for short-term feats
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what is aerobic training?
regular, repeated activities such as jogging or aerobic dancing that increase the supply of oxygen-rich blood available to skeletal muscles for aerobic respiration
- builds endurance for prolonged activities
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what is interval training?
workout regimen that incorporates both anaerobic and aerobic training
(ex. alternating sprints with jogging)
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what is muscle tone?
a small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units that skeletal muscle exhibits even at rest
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what is muscle tone established by?
neurons in the brain and spinal cord that excite the muscle’s motor neurons
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when does a muscle become flaccid?
when the motor neurons serving a skeletal muscle are damaged or cut
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how is muscle tone sustained?
small groups of motor units are alternatively active and inactive in a constantly shifting pattern
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what is the function of muscle tone?
- keeps skeletal muscles firm, but it does not result in a force strong enough to produce movement
- especially important in smooth muscle tissues
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what is hypotonia?
decreased or lost muscle tone
- Flaccid muscles are loose and appear flattened rather than rounded
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what are some characteristics of flaccid paralysis?
- loss of muscle tone
- loss or reduction of tendon reflexes
- atrophy and degeneration of muscles
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what is hypertonia?
increased muscle tone and is expressed in two ways: spasticity or rigidity
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what is spasticity?
increased muscle tone (stiffness) associated with an increase in tendon reflexes and pathological reflexes
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what is spastic paralysis?
partial paralysis in which the muscles exhibit spasticity
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what is rigidity?
increased muscle tone in which reflexes are not affected, as occurs in tetanus
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what occurs during isotonic contraction?
the tension (force of contraction) developed in the muscle remains almost constant while the muscle changes its length
- used for body movements and for moving objects
- concentric/eccentric
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what is a concentric isotonic contraction?
the length of a muscle decreases during a contraction, tension remains constant
- muscle **shortens** and pulls on another structure, such as a tendon, to produce movement and to reduce the angle at a joint
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what happens in eccentric isotonic contraction?
the length of a muscle increases during a contraction, tension remains constant
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which type of isotonic contraction produces more muscle damaged?
eccentric isotonic contractions
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what is an isometric contraction?
the muscle does not change length but tension increases greatly
- does not result in movement, but energy is still expended
- important for maintaining posture and for supporting objects in a fixed position
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important for maintaining posture and for supporting objects in a fixed position
Holding your head upright without movement involves mainly isometric contractions
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what are slow oxidative fibers?
- appear **dark red**
- HIGH of myoglobin
- HIGH of blood capillaries
- LARGE mitochondria
- generate ATP mainly by **aerobic respiration**
- **slow** speed of contraction
- resistant to fatigue
- capable of prolonged, sustained, contractions for **many hours**
- adapted for maintaining posture and for aerobic, endurance-type activities
- running, jogging
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what are fast oxidative-glycolytic fibers?
- largest fibers
- appear **dark red**
- HIGH myoglobin
- HIGH blood capillaries
- generate considerable ATP by **aerobic respiration**
- moderately high resistance to fatigue
- HIGH lvls intracellular glycogen
- MED glycogen stores
- can generate ATP through anaerobic glycolysis
- **fast** speed of contraction
- walking, sprinting
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what are fast glycolytic fibers?
- appear **white**
- LOW myoglobin
- LOW blood capillaries
- LOW mitochondria
- HIGH glycogen
- generate ATP mainly by **glycolysis**
- contract **strongly and quickly**
- adapted for intense movements of **short** duration
- weightlifting
- throwing a ball
- fatigue quickly
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why does improvement in flexibility occur slowly?
- When a relaxed muscle is physically stretched, its ability to lengthen is limited by connective tissue structures, such as fasciae
- To see an improvement in flexibility, stretching exercises must be performed regularly—daily, if possible—for many weeks.
- regular stretching gradually lengthens these structures but occurs slowly
202
what are anabolic steroids?
synthetic variations of testosterone, the sex hormone responsible for the development of male sexual characteristics
- increases muscle size by increasing protein synthesis in muscles, which increases strength
- irreversible effects to body
- can cause liver cancer
- can couse body not to make any more testosterone
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why does stretching cold muscles possibly cause injury?
- Tissues stretch best when slow, gentle force is applied at elevated tissue temperatures
- Exercise heats muscle more deeply and thoroughly than external measures
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what is strength training?
the process of exercising with progressively heavier resistance for the purpose of strengthening the musculoskeletal system
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what are the benefits of strength training?
- stronger muscles
- increases bone strength by increasing the deposition of bone minerals
- raises resting metabolic rate
- helps prevent back injury
- reduces stress and fatigue
- builds exercise tolerance, takes longer before lactic acid builds up in muscles
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what is the function of intercalated discs?
- connect the ends of cardiac muscle fibers to one another
- desmosomes hold the fibers together
- gap junctions allow muscle action potentials to spread from one cardiac muscle fiber to another
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why does cardiac muscle tissue remain contracted 10 to 15 times longer than skeletal muscle tissue?
- due to prolonged delivery of Ca2+ into the sarcoplasm
- Ca2+ enters the sarcoplasm both from the sarcoplasmic reticulum and from the interstitial fluid that bathes the fibers.
- the channels that allow inflow of Ca2+ from interstitial fluid stay open for a relatively long time
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Is cardiac muscle tissue stimulated by acetylcholine?
No, cardiac muscle tissue contracts when stimulated by its own **autorhythmic muscle fibers**
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why do many athletes have enlarged hearts?
*physiological enlarged heart*
cardiac muscle fibers can undergo hypertrophy in response to an increased workload
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how do cardiac muscle tissues get their energy?
- through aerobic respiration
- mitochondria in cardiac muscle fibers are larger and more numerous than in skeletal muscle fibers
- requires a constant supply of oxygen
- can also use lactic acid from skeletal fibres to make ATP (during exercise)
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what is visceral smooth muscle tissue?
autorhythmic smooth muscle, most common type of smooth muscle tisue
- found in the skin and in tubular arrangements that form part of the walls of small arteries and veins and of hollow organs such as the stomach, intestines, uterus, and urinary bladder
- fibers connect to one another by gap junctions, forming a network through which muscle action potentials can spread
- one fiber and its neighbours contract in unison, as a single unit
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what is multi-unit smooth muscle tissue?
- consists of individual fibers, each with its own motor neuron terminals and with few gap junctions between neighboring fibers
- stimulation of one multi-unit fiber causes contraction of that fibre only
- found in the walls of large arteries, in airways to the lungs, in the arrector muscles of the hair that attach to hair follicles, in the muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye
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why does smooth muscle tissue not appear striated?
- the various filaments have no regular pattern of overlap
- lack T tubules
- only have small amount of SR for storage of Ca2+
- **caveolae** contain extracellular Ca2+
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what are dense bodies?
structures that thin filaments attach to and function similarly to Z discs
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Which type of smooth muscle is more like cardiac muscle than skeletal muscle, with respect to both its structure and function?
Visceral smooth muscle is more like cardiac muscle; both contain gap junctions, which allow action potentials to spread from each cell to its neighbors.
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what are some important physiological differences smooth muscle tissue exhibit from cardiac and skeletal muscle tissue?
- contraction starts more slowly and lasts much longer
- can both shorten and stretch to a greater extent
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why does smooth muscle contract slowly?
- no T tubules in smooth muscle
- takes longer for Ca2+ to reach the filaments in center of fiber and trigger contraction
- calmodulin and myosin light chain kinase work slowly
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what is calmodulin?
regulatory protein that binds to Ca2+ in sarcoplasm (like troponin)
- activates myosin light chain kinase that allows for contraction to occur in smooth muscle
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what is myosin light chain kinase?
enzyme that is activated by calmodulin and uses ATP to add a Pi to myosin heads, allowing for contraction to occur
- works slowly
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what is smooth muscle tone?
continued partial contraction in smooth muscle tissue due to prolonged presence of Ca2+ in the cytosol
- important for maintaining steady pressure in vessels and canals that smooth muscle tissue line
221
how does growth of muscle occur after birth?
mainly due to hypertrophy, enlargement of existing muscle cells
222
which muscle tissue retains their capacity for division and can grow by hyperplasia?
smooth muscle tissue
- pericytes/stem cells allow for hyperplasia of smooth muscle
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which germ layer gives rise to muscles?
the mesoderm
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what are somites?
Block of mesodermal cells in a developing embryo that is distinguished into a **myotome** (which forms most of the skeletal muscles), **dermomyotome** (which forms the subcutaneous tissue, dermis, and all skeletal muscles of the trunk and limbs), and **sclerotome** (which forms the vertebrae and ribs).
225
between what ages does muscle loss start?
between 30 and 50
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what percentage of muscle is lost between 30 and 50?
10%
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what percentage of muscles is lost between 50 and 80?
40%
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what are consequences of muscle loss due to aging?
- decrease in maximal strength
- slowing of muscle reflexes
- loss of flexibility
- SO fibers increase
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what is myopathy?
a disease or disorder of the skeletal muscle tissue itself.
230
what may lead to abnormalities of skeletal muscle function?
- disease
- damage to components of motor unit
- somatic motor neurons
- neuromuscular junctions
- muscle fibers
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what is myasthenia gravis?
- an autoimmune disease that causes chronic, progressive damage of the neuromuscular junction
- immune system inappropriately produces antibodies that bind to and block some ACh receptors, thereby decreasing the number of functional ACh receptors at the motor end plates of skeletal muscles
- muscles become increasingly weaker, fatigue more easily, may eventually cease to function
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what is muscular dystrophy?
- a group of inherited muscle-destroying diseases that cause progressive degeneration of skeletal muscle fibers
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what is a spasm?
a sudden involuntary contraction of a single muscle in a large group of muscles
- if painful, it becomes a cramp
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what is the reason cramps occur?
inadequate blood flow to muscles, overuse of a muscle, dehydration, injury, holding a position for prolonged periods, and low blood levels of electrolytes, such as potassium
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what is a tic?
a spasmodic twitching made involuntarily by muscles that are ordinarily under voluntary control. Twitching of the eyelid and facial muscles are examples of tics.
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what is a tremor?
a rhythmic, involuntary, purposeless contraction that produces a quivering or shaking movement
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what is a fasciculation?
an involuntary, brief twitch of an entire motor unit that is visible under the skin; it occurs irregularly and is not associated with movement of the affected muscle.
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what is a fibrillation?
a spontaneous contraction of a single muscle fiber that is not visible under the skin but can be recorded by electromyography. Fibrillations may signal destruction of motor neurons.
239
Weightlifter Jamal has been practicing many hours a day, and his muscles have gotten noticeably bigger. He tells you that his muscle cells are “multiplying like crazy and making him get stronger and stronger.” Do you believe his explanation? Why or why not?
Muscle cells lose their ability to undergo cell division after birth. Therefore, the increase in size is not due to an increase in the number of muscle cells but rather is due to enlargement of the existing muscle fibers (hypertrophy). This enlargement can occur from forceful, repetitive muscular activity. It will cause the muscle fibers to increase their production of internal structures such as mitochondria and myofibrils and produce an increase in the muscle fiber diameter.
240
Chicken breasts are composed of “white meat,” whereas chicken legs are composed of “dark meat.” The breasts and legs of migrating ducks are dark meat. The breasts of both chickens and ducks are used in flying. How can you explain the differences in the color of the meat (muscles)? How are they adapted for their particular functions?
The “dark meat” of both chickens and ducks is composed primarily of slow oxidative (SO) muscle fibers. These fibers contain large amounts of myoglobin and capillaries, which accounts for their dark color. In addition, these fibers contain large numbers of mitochondria and generate ATP by aerobic respiration. SO fibers are resistant to fatigue and can produce sustained contractions for many hours. The legs of chickens and ducks are used for support, walking, and swimming (in ducks), all activities in which endurance is needed. In addition, migrating ducks require SO fibers in their breasts to enable them to have enough energy to fly for extremely long distances while migrating. There may be some fast oxidative–glycolytic (FOG) fibers in the dark meat. FOG fibers also contain large amounts of myoglobin and capillaries, contributing to the dark color. They can use aerobic or anaerobic cellular respiration to generate ATP and have high-to-moderate resistance to fatigue. These fibers would be good for the occasional “sprint” that ducks and chickens undergo to escape dangerous situations. In contrast, the white meat of a chicken breast is composed primarily of fast glycolytic (FG) fibers. FG fibers have lower amounts of myoglobin and capillaries that give the meat its white color. There are also few mitochondria in FG fibers, so these fibers generate ATP mainly by glycolysis. These fibers contract strongly and quickly and are adapted for intense anaerobic movements of short duration. Chickens occasionally use their breasts for flying extremely short distances, usually to escape prey or perceived danger, so FG fibers are appropriate for their breast muscle.
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Polio is a disease caused by a virus that can attack the somatic motor neurons in the central nervous system. Individuals who suffer from polio can develop muscle weakness and atrophy. In a certain percentage of cases, the individuals may die due to respiratory paralysis. Relate your knowledge of how muscle fibers function to the symptoms exhibited by infected individuals.
Destruction of the somatic motor neurons to skeletal muscle fibers will result in a loss of stimulation to the skeletal muscles. When not stimulated on a regular basis, a muscle begins to lose its muscle tone. Through lack of use, the muscle fibers will weaken, begin to decrease in size, and can be replaced by fibrous connective tissue, resulting in a type of denervation atrophy. A lack of stimulation of the breathing muscles (especially the diaphragm) from motor neurons can result in inability of the breathing muscles to contract, thus causing respiratory paralysis and possibly death of the individual from respiratory failure.
