Chapter 10: Muscle Tissue
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Transcript of Chapter 10: Muscle Tissue
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Chapter 10:Muscle Tissue
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Muscle Tissue
•A primary tissue type, divided into:• Cardiac muscle• Smooth muscle• Skeletal muscle• Attached to bones• Allows us to move• Contains CT, nerves and blood vessels
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Functions of Skeletal Muscles
1.
2.
3.
4.
5.
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CT Organization – 3 layers
Surrounds entire muscle Separates muscle from
surrounding tissues Connected to deep fascia
1. Epimysium
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1. Perimysium Divides the skeletal
muscle into a series of compartments
Each compartment contains a bundle of muscle fibers:
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1. Endomysium
Surrounds individual skeletal muscle fibers
Interconnects adjacent muscle fibers
Satellite Cells -
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At the end of a muscle: All 3 layers come
together to form a:
Both attach skeletal muscles to bones
Tendon fibers extend into the bone matrix
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Microanatomy of Skeletal Muscle Fibers
• Skeletal muscle cells are called fibers• Enormous• Multinucleate• Myoblasts fuse during development to form
individual skeletal muscle fibers
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Microanatomy of Skeletal Muscle Fibers
• Sarcolemma – cell membrane of muscle fiber▫ Surround sarcoplasm▫ Change in the
transmembrane potential is the start of a contraction
• Transverse Tubules – continuous with sarcolemma and extends into the sarcoplasm▫ form passageways through
muscle fibers▫ Filled with extracellular
fluid▫ Action potentials
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Microanatomy of Skeletal Muscle Fibers
• Myofibrils – cylindrical structures encircled by T tubules▫ As long as the cell▫ Made of myofilaments
Thin filaments - actin Thick filaments – myosin
▫ Responsible for muscle fiber contraction
▫ Mitochondria and glycogen
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Microanatomy of Skeletal Muscle Fibers
• Sarcoplasmic Reticulum – similar to ER of other cells▫ Forms network around each
myofibril
• Terminal cisternae – expanded chambers of SR on either side of a T tubule▫ Ca+2 ions storage
• Triad – pair of terminal cisternae plus a T tubule▫ Separate fluids
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Microanatomy of Skeletal Muscle Fibers
• Sarcomere – repeating contractile units that make up myofibrils▫ Smallest functional unit in muscle fibers▫ Muscle contraction▫ Made up of: thick and thin filaments, stabilizing proteins and
regulating proteins▫ Striated
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Microanatomy of Skeletal Muscle Fibers
• A bands – dark bands at center of sarcomere▫ Thick filaments (myosin)▫ Contains:
M line – center of A band, connects each thick filament together
H zone – lighter region on either side of M line, contains thick filaments
Zone of overlap – thick and thin filaments overlap one another
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Microanatomy of Skeletal Muscle Fibers
• I bands – light bands on both sides of A band▫ Thin filaments (actin) ▫ Contains:
Z lines – boundary between adjacent sarcomeres
Titin – protein that aligns thick and thin filaments
▫Extends from thick filaments
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Level 1: Skeletal Muscle
Level 2: Muscle Fascicle
Level 3: Muscle Fiber
Level 4: Myofibril
Level 5: Sarcomere
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Muscle Contraction• Sliding Filament Theory
▫ Caused by interactions of thick and thin filaments▫ Triggered by free Ca2+ in sarcoplasm
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Muscle Contraction• Thin Filaments – made of 4 proteins:
▫ F actin – 2 twisted strands of G actin, contain active sites for the binding of myosin
▫ Nebulin – holds 2 strands of G actin together▫ Tropomyosin – covers G actin active sites to prevent
actin/myosin interactions▫ Troponin – holds tropomyosin to G actin AND contains a site
for the binding of Ca2+
Holds until Ca2+ binds to the active site Contraction can only occur if position changes
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Muscle Contraction• Thick Filaments – consist of a pair of myosin subunits
wrapped around each other▫ Tail – binds to other myosin molecules▫ Head – 2 subunits, project towards nearest thin filament
▫ During muscle contractions myosin heads pivot towards thin filaments, forming cross-bridges with G actin active sites
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Muscle Contraction
• Sliding Filament Theory▫ Thin filaments slide
towards M line – shortening
▫ A band remains the same, but the Z lines move closer together
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Muscle Contraction• Neuromuscular Junction - NMJ
▫ Where the action potential starts▫ Each branch ends at a synaptic terminal, which contains
mitochondria and Acetylcholine Neurotransmitter that alters the permeability of the sarcolemma
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Muscle Contraction• Synaptic cleft –
• Motor end plate –
▫ Both contain AChE – breaks down Ach
• Action potential travels along the nerve axon and ends at the synaptic terminal, which changes the permeability
• ACh is released
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Muscle Contraction• ACh diffuses across the synaptic cleft
and binds to ACh receptors on motor end plate
• Increase in membrane permeability to sodium ions that rush into the sarcoplasm
▫ Keeps going until AChE removes all ACh
• Travels along sarcolemma to T tubules and leads to excitation-contraction coupling -▫ Action potential leads to contraction▫ Triads release Ca2+
▫ Triggers muscle contractions
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Muscle Contraction at Sarcomere
1. Exposure of active sites▫ Calcium ions bind to
troponin, changing its position and shifting tropomyosin away from active sites
2. Attachment of cross-bridges▫ Myosin heads bind to
active sites
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Muscle Contraction at Sarcomere
3. Pivoting▫ Power stroke
4. Detachment of cross-bridges▫ ATP binds to myosin head,
link is broken▫ Attach to another active site
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Muscle Contraction at Sarcomere
Muscle Contraction
at Sarcomere
5. Reactivation of myosin▫ ATP to ADP and phosphate▫ Cycle is repeated
• All sarcomeres contract at the same time
• Contraction duration depends on:
▫ Duration of neural stimulus▫ Amount of free Ca2+ ions
in sarcoplasm▫ Availability of ATP
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Muscle Contraction• 1. At NMJ, ACh is released and
binds to receptors on sarcolemma
• 2. Change in transmembrane potential results in action potential that spreads across entire surface of cell and T tubules
• 3. SR releases stored calcium ions, increasing Ca2+ around sarcomeres
• 4. Calcium ions bind to troponin, which exposes active sites on thin filaments and cross-bridges form
• 5. Contraction begins as repeated cycles of cross-bridge formation and detachment happen
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Muscle Contraction
• 6. ACh is broken down by AChE and action potential ends
• 7. SR reabsorbs calcium ions and concentration in sarcoplasm decreases
• 8. Active sites are re-covered
• 9. Contraction ends
• 10. Muscle relaxation – sarcomeres remain uncontracted
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Rigor Mortis
• Stop in blood circulation causes skeletal muscles to be deprived of oxygen and nutrients –
• SR becomes unable to pump calcium ions out of sarcoplasm
• Extra calcium ions trigger a sustained contraction▫ Cross-bridges form, but cannot detach
• Lasts 15-25 hours after death
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2 Types of Muscle Tension• Isotonic Contraction
▫ Skeletal muscle changes length resulting in motion▫ If muscle tension > resistance: muscle shortens (concentric
contraction)▫ If muscle tension < resistance: muscle lengthens (eccentric
contraction)
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2 Types of Muscle Contraction
• Isometric Contraction▫ Muscle develops tension, but does not shorten
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Resistance and Speed of Contraction • Inversely related• The heavier the resistance on a muscle:
▫ the longer it takes for shortening to begin▫ the less the muscle will shorten
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Muscle Relaxation
• After contraction, a muscle fiber returns to resting length by:▫ Elastic forces
The pull of elastic elements (tendons and ligaments) Expands the sarcomeres to resting length
▫ Opposing muscle contractions Reverse the direction of the original motion The work of opposing skeletal muscle pairs
▫ Gravity Can take the place of opposing muscle contraction to
return a muscle to its resting state
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ATP and Muscle Contraction• Muscle contraction uses a lot of ATP • Muscles store enough energy to start contraction, but
must manufacture more ATP▫ Generates ATP at the same rate that it is used
• ATP and CP ▫ ATP – active energy model (aerobic and anaerobic)▫ Creatine Phosphate (CP) – storage molecule for excess
ATP in resting muscle
▫ ATP – 2 seconds▫ CP – 15 seconds▫ Glycogen – 130 seconds (anaerobic) and 40 mins (aerobic)▫ Fats
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ATP and Muscle Contraction •At rest:
▫Cells use fatty acids to create CP, ATP and glycogen – rebuilding their storages (beta oxidation)
•Moderate Activity:▫Cells use fatty acids or glucose and oxygen to
produce ATP (aerobic respiration) Muscle wont fatigue until all energy is used up Marathon runners
•Peak Activity▫Cells use oxygen faster than it is supplied
Aerobic resp only provides 1/3 of needed ATP Anaerobic resp provides the rest – lactic acid
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Muscle Metabolism
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Muscle Fatigue• When muscles can no longer perform a required
activity, they are fatigued• Results of Muscle Fatigue:
Depletion of metabolic reserves Damage to sarcolemma and SR Low pH (lactic acid) Muscle exhaustion and pain
• The Recovery Period The time required after exertion for muscles to return to
normal Oxygen becomes available Mitochondrial activity resumes
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Muscle Fatigue
• The Cori Cycle The removal and recycling of lactic acid by the liver Liver converts lactic acid to pyruvic acid Glucose is released to recharge muscle glycogen reserves
Oxygen Debt – after exercise: Body needs more oxygen than usual to normalize metabolic activityHeavy breathing
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3 Types of Skeletal Fibers • Fast Fibers:
▫ Contract quickly▫ High CP▫ Large diameter, huge glycogen reserves and few
mitochondria▫ Strong contractions, but fatigue quickly▫ White meat – chicken breast
• Slow Fibers▫ Slow to contract and slow to fatigue▫ Low CP▫ Small diameter, but a lot of mitochondria▫ High oxygen supply▫ Contain myoglobin (red pigment, binds to oxygen)▫ Dark meat – chicken legs
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3 Types of Muscle Fibers• Intermediate Fibers
▫ Mid-sized▫ Low myoglobin▫ More capillaries than fast fibers, slower to fatigue▫ Table 10-3, page 298 ▫ Human Muscles
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• Muscle Hypertrophy - muscle Growth from heavy training▫ increases diameter of
muscle fibers▫ increases number of
myofibrils▫ increases mitochondria,
glycogen reserves
• Muscle Atrophy – lack of muscle activity▫ Reduced in muscle size,
tone and power
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Physical Conditioning
• Anaerobic Endurance ▫ Uses fast fibers, fatigues quickly with strenuous
activities 50 m dash, weightlifting
▫ Improved by frequent, brief, intensive workouts – interval training
• Aerobic Endurance – supported by mitochondria▫ Prolonged activity – uses a lot of oxygen and
nutrients Marathon running
▫ Improved by repetitive and cardiovascular training
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Cardiac Muscle Tissue
• Striated tissue• Smaller cells with single nucleus• Short T-tubules and sarcoplasm
▫No triads or terminal cisternae• All aerobic
▫High in myoglobin and mitochondria
• Intercalated discs
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Smooth Muscle• Blood vessels, reproductive and digestive systems, etc• Different arrangement of actin and myosin• Non-striated
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Characteristics of Skeletal, Cardiac, and Smooth Muscle