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63 Cards in this Set
- Front
- Back
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Muscle Tissue
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one of four primary tissues, divided into 3 types
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What are the three types of muscle tissue?
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skeletal muscle, cardiac muscle and smooth muscle
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skeletal muscle
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moves body by pulling on bones striated, ~ voluntary
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cardiac muscle
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pumps blood throughout body straiated, involuntary
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smooth muscle
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pushes fluids & solids along digestive tract & regulates diameters of small arteries non-striated, involuntary
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Skeletal Muscle Functions
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1. Produce skeletal muscle
2. Maintain body posture & position 3. Support soft tissues 4. Guard body openings (e.g. digestive tract - voluntary control over swallowing) 5. Maintain body temperature (muscle contractions use energy, heat released) 6. Store nutrient reserves (when diet is low in proteins & calories; muscle protein is broken down) |
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Skeletal Muscle Fibers
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skeletal muscle cells = muscle fibers Different from "typical" cells:
• Large cells (large diameter & can span between 2 tendons → up to 12 inches) • Multinucleate - 1 cell has 100s of nuclei ∙ genes in nuclei control production of enzymes/proteins (more copies = faster protein production) |
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sarcolemma
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cell membrane of muscle cell
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sarcoplasm
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cytoplasm of muscle fiber
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Transverse tubules
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narrow tubes continuous with sarcolemma; extend into sarcoplasm
• filled with extracellular fluid • transmit action potential through cell • allow entire muscle fiber to contract simultaneously |
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myofibrils
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lengthwise subdivisions within muscle fiber Made up of bundles of protein filaments (myofilaments)
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What are the two types of myofilaments?
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thin filaments & thick filaments
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thin filaments
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made up of protein actin
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thick filaments
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made up of myosin
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When myofibrils shorten they will cause ……
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muscle fiber contraction
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Skeletal Muscle Structures Hierarchy of structure
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Muscle → Fascicle → Muscle fiber → Myofibril → Myofilament
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Fascicle
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bundles of muscle cells
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Muscle Fibers
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multinucleated, elongated cell
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Myofibril
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bundle of overlapping myofilaments
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Myofilament
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protein filaments (thick and thin filaments)
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sarcoplasmic reticulum
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membranous structure surrounding each myofibril
• helps transmit action potential to myofibril • similar in structure to smooth endoplasmic reticulum • forms expanded chambers (terminal cisternae) attached to T tubules |
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cisternae
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concentrate Ca²+ (via ion pumps)
~40,000 times more calcium in cisternae than sarcoplasm • stored Ca²+ is released into sarcomeres to begin muscle contraction |
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sarcomeres
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contractile unit
• contractile units of muscle • structural units of myofibrils • form visible patterns within myofibrils |
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Muscle striations
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striped or striated pattern within myofibrils: alternating dark, thick filaments (A Bands) and light, thin filaments (I Bands)
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What kinds of bands do sarcomeres have? Identify which is thick and thin filaments
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A Bands - dark, thick filaments I Bands - light, thin filaments
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M line
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center of A band; at midline os sarcomere - connects thick filaments
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Z lines
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centers of I bands; at 2 ends of sarcomere
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Zones of overlap
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where thick and thin filaments overlap
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H zone
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area around M line; has thick filaments but no thin filaments
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Titin
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strands of elastic protein - extend from tips of thick filaments to Z line Function: stabalize the filaments → recoils after stretching during muscle contraction
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What is the function of Titin?
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stabilize the filaments →recoils after stretching during muscle contraction
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muscle contraction
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Transverse tubules encircle the sarcomere near zones of overlap Ca²+ released by sarcoplasmic reticulum causes thin and thick filaments to interact
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Thick filaments
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Myosin subunits twisted around filament
• tail - binds to other myosin molecules • head - made of 2 globular protein subunits; projects toward nearest thin filament |
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During muscle contraction, myosin heads:
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• interact with actin filaments, forming cross-bridges
• pivot, producing motion |
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Thin filaments
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contain 3 major thin filament proteins
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What are the three major thin filament proteins?
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actin, tropomyosin and troponin
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actin
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2 twisted strands of globular polypeptide subunits (G actin) form long actin filaments (F actin)
• "actin sites" on G actin strands bind to myosin |
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tropomyosin
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double stranded; covers active sites on actin to prevent actin-myosin interaction
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troponin
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globular protein; binds tropomyosin to G actin; controlled by Ca²+ via a Ca²+ binding side
• resting conditions = low Ca²+ concentration |
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Sliding Filament Model
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during muscle contraction → thin filaments of sarcomere slide toward M line between thick filaments
• width of A band stays the same • I band and H zone shorten • Z lines move closer together |
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Thick and Thin filaments ____________ in length; ……
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Thick and Thin filaments do not change in length; they slide past one another to shorten muscle fiber
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What are the 5 Steps of the Crossbridge Cycle?
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1. Exposure of active sites
2. Formation of cross-bridges 3. Pivoting of myosin heads 4. Detachment of cross-bridges 5. Reactivation of myosin |
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What occurs during Step 1 of the Crossbridge Cycle?
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a) resting sarcomere - tropomyosin covers active sites on thin filaments (preventing myosin binding)
b) active site exposure = Ca²+ binds to troponin; rotates & swings tropomyosin away from active site • resting sarcomere - tropomyosin covers active sites on thin filaments (preventing myosin binding) • Ca²+ released → binds to troponin • weakens bond between troponin-tropomyosine complex • troponin rotates & swings tropomyosin away from active site |
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What occurs during Step 2 of the Crossbridge Cycle?
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• active sites are exposed
• "energized" myosin heads bind to active sites forming cross bridges |
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What occurs during Step 3 of the Crossbridge Cycle?
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resting position - myosin points away from M line
• myosin goes back to "ready" position (~like a spring in a mousetrap - ready to spring forward) • myosin becomes energized by breaking down ATP into ADP & Phosphate • energy is "stored" in energized head as potential energy • after cross bridge formation - stored energy is released • myosin head pivots toward M line = power stroke • ADP and Pi are released • myosin head - pulls on actin filament sliding it towards M line |
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What occurs during Step 4 of the Crossbridge Cycle?
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• link between myosin & actin is broken when ATP binds to myosin heads
• active site is now exposed and ready to form another cross bridge |
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What occurs during Step 5 of the Crossbridge Cycle?
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• ATPase in free myosin head hydrolyzes ATP into ADP + P
• energy released in process = moves myosin head back to "energized" position • energy is stored in myosin head as potential energy |
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The Entire Contraction Cycle can be repeated as long as:
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• duration of neural stimulus
• calcium ion concentration is high • ATP is available |
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What do you think would happen if the body suddenly ran out of ATP?
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1. ATP energizes the myosin heads and without ATP you can't energize the myosin heads
2. ATP breaks the crossbridge |
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Regulation of Muscle Contraction: Structures
Neuromuscular junction |
connection between motor neuron & muscle cell
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Regulation of Muscle Contraction: Structures
presynaptic cell (motor neuron) |
delivers AP to axon terminal
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Regulation of Muscle Contraction: Structures
ACh secretes across _________ |
synaptic cleft
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Regulation of Muscle Contraction: Structures
ACh binds to ACh receptors on ______ |
postsynaptic cell (muscle cell)
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Regulation of Muscle Contraction: Structures
Motor end plate |
region of sarcolemma (highly folded; many ACh receptors)
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Regulation of Muscle Contraction: Structures
T Tubules |
conduct APs deep into muscle fiber
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Excitation-Contraction Coupling: Structures
DHP receptors |
functions as "voltage sensors"
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Excitation-Contraction Coupling: Structures
Sarcoplasmic Reticulum |
stores & releases Ca²+ to myofilaments
closely associated with T tubules |
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Excitation-Contraction Coupling: Structures
Ryanodine receptors |
acts as Ca²+ channels
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Excitation-Contraction Coupling: Structures
Ca²+ pumps |
actively transport Ca²+ back into SR into cytosol
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Relaxed state
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tropomyosin covers actin's myosin binding site
• prevents crossbridge formation |
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Contraction state
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Ca²+ binds to troponin → causes conformational change in troponin
• troponin shifts tropomyosin's position • myosin binding site is exposed |
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What are the steps of Excitation in Excitation-Contraction Coupling?
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1. Action potentials travels down motor neuron to axon terminal through voltage-gated Ca²+ channels open causing Ca²+ to enter cell
2. Vesicles in axon terminal fuse with membrane → exocytosis of Acetylcholine into synaptic cleft 3. Acetylcholine diffuses across the synaptic cleft → binds to receptors on motor end plate → Na+ rushes in muscle fiber 4. Action potential travels across sarcolemma → and down the T tubules 5. DHP receptors "sense" voltage change → shape change occurs → open ryanodine/Ca²+ channels 6. Ca²+ exits SR → enters sarcoplasm |
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What are the steps of Contraction in Excitation-Contraction Coupling?
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1. Ca²+ binds to troponin → shape change → moves tropomyosin away from active site (on actin)
2. Myosin binds to active site = crossbridge formation 3. Myosin head pivots → pull thin filaments toward M line = power stroke - sarcomere shortens 4. ATP binds to ATPase on myosin head = breaks crossbridge 5. ATP →(ATPase)→ ADP + Pi → causes myosin to return to "energized" position 6. Ca²+ is actively transported back into SR → troponin/tropomyosin cover active site |
