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BIOL 1117 (81)

Muscles: Behavior of Skeletal Muscle Fibers and Whole Muscles

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Northeastern University
BIOL 1117
Christopher Richardson

L19-Muscle Cells 10/23/13 • Contraction (steps 10 and 11) o The muscle fibers develop tension and may shorten o myosinATPase enzyme in myosin head hydrolyzes anATP molecule (10) o energy released, activates the head “cocking” it into an extended position (10)  This activation is endergonic o activated myosin head binds to actin active site forming a myosin - actin cross-bridge (11) • Contraction (steps 12) o myosin head releases P, which releases energy (12) i  This energy is used to strengthen the actin-myosin bond (endergonic) to generate a stronger force during the rotation of myosin head o flexes tugging thin filament along with it, which pulls thin past the thick filament (power stroke) (12)  Head shifts from unstable, high energy extend out position to low energy, bent position (exergonic) • Contraction (step 13) o Myosin binds to new ATP and then myosin releases actin and process is repeated (13)  By hydrolyzing theATP and recocking or extending the heat again (recovery stroke)  attach to a new active site farther down  each stroke utilizes one molecule ofATP  If no ATP is available in cell, then actin and myosin cross bridges stay formed and the muscle locks (with death, rigor mortis) o To recap: OneATP molecule is needed to release the myosin head from actin but not using energy to do this while energy from hydrolysis of that ATP molecule is used to extend out the myosin head o myosin heads all do not stroke at once (they contract in sequence); when one myosin releases an actin, many other heads on the same thick filament are still bound so the thin filament will not slide back o Even though the muscle contracts and can shorten, the myofilaments do not become shorter; instead the thin slides over the thick , creating more overlap • Relaxation (steps 14 and 15) o Allows muscle fiber to relax and return to its resting length o Stop nerve stimulation and stopACh release (14) o AsACh leaves receptors,AChE breaks downACh (15) o Synaptic knob reabsorbs the fragments (15) • Relaxation+2step 16) +2 o Ca pumped back into SR cisternae; Ca binds to calsequestrin; stored until stimulation occurs again o Because active transport requires ATP,ATP is needed for muscle relaxation as well as muscle contraction. • Relaxation (steps 17 and 18) o Ca removed from troponin is also pumped back into SR (17) o tropomyosin reblocks the active sites and prevents myosin binding (18) o Muscle fiber stops producing or maintaining any tension o muscle fiber returns to its resting length due two forces: o recoil of elastic components lengthen the muscle back out o Elastic filament is like a recoiling spring which springs back from a compressed position pulling the thin away from the thick while elastic recoil of stretched tendon will pull contracted muscle back out o contraction of antagonistic muscles lengthen opposing muscle; for ex, contraction of triceps stretches out the biceps • Length-Tension Relationship o Length – Tension Relationship: the amount of tension generated by a muscle and the force of contraction depends on how stretched or contracted it was before it was stimulated (at rest) o Tension is force exerted on an object by muscle while Load is the force exerted on the muscle by the object; tension and load are opposing forces o if overly contracted at rest, a weak contraction results  actin filaments from each side collide  Myosin filaments crumple against Z discs o if too stretched at rest, a weak contraction results  little overlap between thin and thick filaments  Some of the myosin heads cannot form cross bridges with actin o optimum resting length produces greatest force or tension when muscle contracts  When the overlap between thick and thin filaments allows the greatest number of myosin to actin cross bridges to form, the tension produced by the muscle is maximal and the muscle length is called optimal length  muscle tone: central nervous system continually monitors and adjusts the length of the resting muscle, and maintains a state of partial contraction called muscle tone  This helps keep different opposing muscles at optimal length as needed  a stretched relaxed muscle will tend to return to equilibr
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