BSC2085 Final Exam Study Guide 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 Action potential that travels along the sarcolemma down the T tubule cause release of o Tension production 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 sarcolemma 1 BSC2085 Final Exam Study Guide 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 begins 2 BSC2085 Final Exam Study Guide 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 bridges Energized myosin heads bind to active sites on actin forming cross o 4 Myosin head pivoting Energy released as myosin head pivots toward the M line Power stroke ADP phosphate group are released 3 BSC2085 Final Exam Study Guide 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 contract 4 BSC2085 Final Exam Study Guide 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 myosin Skeletal muscle fibers shorten as thin filaments actin slide between thick filaments 5 BSC2085 Final Exam Study Guide 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 length 6 BSC2085 Final Exam Study Guide 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 levels 7 BSC2085 Final Exam Study Guide 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 contraction 8 BSC2085 Final Exam Study Guide Tetanus Lockjaw Caused by toxin from Clostridium tetani bacterium that causes overactivity of skeletal muscle motor neurons o Results in muscle stiffness headaches difficulty swallowing Tension production by skeletal muscles depends on o Internal tension produced by muscle fibers o External tension exerted by muscle fibers on elastic extracellular fibers o Total number of muscle fibers stimulated Motor Unit A single motor neuron all the muscle fibers it controls o Motor units in a skeletal muscle Contain hundreds of muscle fibers Contract at the same time o Recruitment Multiple Motor Unit Summation When the size OR number of motor units stimulated is increased Produces smooth motion increasing tension in a whole muscle or
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