FSU BSC 2085 - Lesson 21: The Muscular System – Muscle Contraction

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BSC2085 Final Exam Study Guide 1 Lesson 21: The Muscular System – Muscle Contraction Skeletal Muscle Contraction • Process of contraction: o Neural stimulation of sarcolemma  Causes excitation-contraction coupling o Muscle fiber contraction  Caused by thick & thin (actin & myosin) filament interactions o Tension production • Action potential that travels along the sarcolemma & down the T tubule cause release of Ca2+ o Ca2+ release allows for actin-myosin interaction Control of Skeletal Muscle Activity • Neuromuscular Junction (NMJ): Special intercellular connection between the nervous system & skeletal muscle fiber (where the motor neuron interacts with the muscle fiber) o Action potential reaches the axon terminal of motor neuron – causes release of acetylcholine into synaptic cleft o Acetylcholine binds chemically-gated Na+ channels on muscle, which opens them  Na+ influx depolarizes the muscle fiber o Action potentials generated in muscle fiber along inner surface of the sarcolemmaBSC2085 Final Exam Study Guide 2 • Excitation-Contraction Coupling o Action potential reaches a triad (T tubule + terminal cisternae of sarcoplasmic reticulum)  First step = releasing Ca2+ from terminal cisternae of the sarcoplasmic reticulum  Triggering contraction IF: • Myosin heads are in “primed” (high energy) position o Used ATP energy to get into this position o ATP hydrolyzed to ADP + Pi on myosin head • Contraction Cycle: o 1. Contraction cycle beginsBSC2085 Final Exam Study Guide 3  Begins with arrival of Ca2+ within the zone of overlap o 2. Active site exposure  Ca2+ ions bind to troponin  Troponin removes tropomyosin from the active sites on actin  This allows interaction between actin’s active sites & the energized myosin heads o 3. Cross-bridge formation  Energized myosin heads bind to active sites on actin, forming cross-bridges o 4. Myosin head pivoting  Energy released as myosin head pivots toward the M line • Power stroke  ADP & phosphate group are releasedBSC2085 Final Exam Study Guide 4 o 5. Cross-bridge detachment  Another ATP binds to myosin head  Link between myosin & actin broken  Active site exposed & able to form another cross-bridge o 6. Myosin reactivation  Free myosin head splits (hydrolyzes) ATP into ADP & P  Energy released puts myosin head back into the high energy position o Notes:  ATP binding site exists on myosin  Multiple myosin heads interact with the thin filament at a time  As long as calcium is present for troponin to remove tropomyosin, muscles can contractBSC2085 Final Exam Study Guide 5 Skeletal Muscle Contraction and Relaxation • Fiber shortening o Sarcomeres shorten  muscles pull together  tension produced o Muscle shortening can occur at both ends of muscle or just one end  Depends on how the muscle is attached at the ends  Muscle usually held in position at origin  Insertion end moves toward fixed end • Contraction duration depends on… o Duration of neural stimulus  How much ACh is released by the motor neuron o Number of free calcium ions in sarcoplasm o Availability of ATP • Relaxation occurs when… o Ca2+ concentrations fall  Ca2+ detaches from troponin  Active sites are re-covered by tropomyosin • Rigor Mortis: A fixed muscular contraction after death o Occurs 2-7 hours after death o Ion pumps cease to function – run out of ATP o Calcium builds up in the sarcoplasm o End of rigor mortis occurs 1-6 days after death  Proteins that hold everything together break down Summary • Skeletal muscle fibers shorten as thin filaments (actin) slide between thick filaments (myosin)BSC2085 Final Exam Study Guide 6 • Free Ca2+ in the sarcoplasm triggers contraction • SR releases Ca2+ when a motor neuron stimulates the muscle fiber • Contraction is an active process o Acetylcholine released by motor neuron • Relaxation & return to resting length are passive o Can occur once motor neuron stops releasing acetylcholine Tension Production and Contraction Types • Muscle fiber is either contracted OR relaxed • Depends on: o Number of pivoting cross-bridges o Fiber’s resting length at the time of stimulation o Frequency of stimulation  More interactions  greater tension generated • Length-tension relationships o Number of pivoting cross-bridges depends on:  Amount of overlap between thick & thin fibers  Optimum overlap produces greatest amount of tension • Too much or too little reduces efficiency  Normal resting sarcomere length = 75% to 130% of optimal lengthBSC2085 Final Exam Study Guide 7 o Frequency of stimulation  Single neural stimulation produces a single contraction (twitch)  Lasts 7-100 msec  Sustained muscular contractions require MANY repeated stimuli (multiple twitches) • Characteristics of muscle twitches o 1. Latent period  The action potential moves through the sarcolemma  Causes Ca2+ release from sarcoplasmic reticulum o 2. Contraction phase  Calcium ions bind to troponin  Tension builds to peak o 3. Relaxation phase  Ca2+ levels fall  Active sites covered  Tension falls to resting levelsBSC2085 Final Exam Study Guide 8 • Treppe: A stair-step increase in twitch tension o Caused by repeated stimulations immediately after relaxation phase  Stimulus frequency = less than 50 per second o Causes a series of contractions with increasing tension o Increase in tension caused by gradual increase in Ca2+ concentration in sarcoplasm  Ca2+ pumps not fast enough to pump released Ca2+ back into SR • Wave Summation: Increasing tension or summation of twitches o Repeated stimulations before the end of relaxation phase  Doesn’t wait for the muscle to relax  Stimulus frequency = more than 50 per second o Causes increasing tension or summation of twitches • Incomplete Tetanus: When a muscle is producing almost peak tension during rapid cycles of contraction and relaxation • Complete Tetanus: Twitches reach maximum tension o If stimulation frequency is high enough, muscle never begins to relax & is in continuous contractionBSC2085 Final Exam Study Guide 9 • Tetanus “Lockjaw”: Caused by toxin from Clostridium tetani bacterium that causes overactivity of skeletal muscle motor neurons o Results in muscle stiffness,


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FSU BSC 2085 - Lesson 21: The Muscular System – Muscle Contraction

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