Control of Muscle ForceSummationControl of Muscle ForceMotor UnitRecruitmentMuscle ModelIsometric ContractionIsotonic ContractionForce-Velocity CurveMuscles can produce powerCellular EnergeticsFatigueFatiguePi accumulation is correlated with development of fatigue, as is lactic acid accumulation (drop in pH)Muscle Biopsy31P-Magnetic Resonance SpectroscopyPostulated Mechanisms of Pi Effect on ForceIs steady-state locomotion the only strategy?Intermittent ExerciseForaging animalsBenefits of moving intermittentlyIntegration of locomotor behavior and muscle stimulationMetabolite Changes Correlate with PerformanceCyclic ContractionsLecture 13, 09 Oct 2003Chapter 10MusclesVertebrate PhysiologyECOL 437University of ArizonaFall 2003instr: Kevin Boninet.a.: Bret PaschVertebrate Physiology 4371. Muscles (Ch10)2. AnnouncementsSherwood, 1997Review of ECCoupling andMuscleContraction(Also a nicesummary onp. 387 ofyour text)Vander et al., 2001Cross Bridges and Force ProductionATP required for the (3)dissociation of actin andmyosin (else rigor mortis)Myosin acts as an ATPase,hydrolyzing ATP to ADP + Pi(4) (Energy of ATP hydrolysis“cocks” the myosin head)Myosin releases ADPand Pi (very slowly (2)unless bound to actin)ATP binds to myosinActomyosin complex forms(= crossbridge) (1)Cross bridgestronger when Pireleased, thenmyosin headrotatesMovement about 1000 nmCycle repeats until Ca++ resequestered or run out of energyATP hydrolysisControl of Muscle Force• Two primary factors can be adjusted to increasewhole-muscle force:– the force developed by each contracting fiber(summation)– the number of muscle fibers contracting within amuscle (recruitment)SummationIncrease force by decreasing time between individualaction potentials (increase rate of stimulation)See textFig. 10-28It’s all about CALCIUMControl of Muscle Force• Two primary factors can be adjusted to increasewhole-muscle force:– the force developed by each contracting fiber(summation)– the number of muscle fibers contracting within amuscle (recruitment)Motor UnitMotor unit =motor neuron and all of themuscle fibers it innervatesAP in motor neuron causesall innervated fibers tocontract simultaneouslyRecruitmentMuscle fibersMotor NeuronsEach muscle consistsof many intermingledmotor unitsIncrease force by addingmore motor unitsMuscle ModelParallelElasticComponent(sarcolemma,connectivetissue withinmuscle)Contractile Unit(sarcomeres)Series ElasticComponent(tendon, connectivetissue linking musclefibers to tendon,titin, Z-linematerial, cross-bridge links)BoneBoneMuscle&TendonSee textFig. 10-26aRandall et al., 2002Isometric ContractionSee textFig. 10-12andFig. 10-27iso = samemetric = lengthRandall et al., 2002Isotonic ContractionSee textFig. 10-12andFig. 10-26biso = sametonic = tensionPurely isotoniccontractionRandall et al., 2002Force-Velocity CurveSee textFig. 10-13Greatest force during isometric contractionGreatest velocity when muscle is unloadedRandall et al., 2002Muscles can produce powerSee textFig. 10-31Muscle fiber types vary in their mechanical properties(see text Table 10-1)Power = force * velocityMaximum power outputis found at intermediateforce and velocity (~40%)Randall et al., 2002Different Muscle Fiber-TypesRandall et al., 2002Cellular EnergeticsActin + Myosin crossbridge movementATP ADP + PiPCr + ADP Cr + ATPGlucose 2 Lactate + 2 ATPImmediate energy sourceNon-oxidative energy sourceOxidative energy sourceGlucose + O2 CO2 + H2O + 36 ATPEnergy systems differ in their rate of and capacity for producing ATPMyosin ATPaseCa2+ ATPaseCa2+ pumpedback into SRSee text Fig. 10-2975%25%Fatigue can result from many factors including;-decreased motivation -failure of neuromuscular transmission-accumulation of metabolic end-products-dehydrationCause of fatigue depends on intensity & duration of exerciseFatigue• Continuous exercise at moderate speedsresults in net accumulation of PiFatiguePCr + ADP + H+ Cr + ATPATP + H2O ADP + Pi + H+ + energyExercise also produces net accumulation of lactic acidCorrelation vs. CausationPi accumulation is correlated withdevelopment of fatigue, as is lactic acidaccumulation (drop in pH)Wilson et al., 1988Muscle Biopsy• prepare homogenate & perform enzymatic analysis ofhomogenate (e.g., creatine phosphate, ATP, Pi, lactate, glucose,glycogen)– Pros: low cost per assay– Cons: many samples required for time course31P-Magnetic Resonance Spectroscopy• Intact muscle (e.g., creatine phosphate, ATP, Pi, pH)– Pros: multiple time points for each preparation– Cons: high cost per preparationpH can be determinedfrom position of Pi peak31P-Magnetic Resonance SpectroscopyPCrATPTimePirestrecoveryRat muscleKushmerick & Meyer, 1985Postulated Mechanisms ofPi Effect on Force• Reduced cross-bridge force development• Reduced Ca2+ release from sarcoplasmicreticulum• Reduced Ca2+ sensitivity of myofilamentsCooke & Pate, 1985; Allen & Westerblad, 2001; Westerblad et al. 2002Decreased pH (e.g., lactic acid) does not seem to havemuch effect on contractility - but may cause pain!Is steady-state locomotion the only strategy?Ray HarryhausenIntermittent ExerciseWhen a heavy (i.e. supramaximal)workload is divided into short exercise andpause periods, the heavy workload is“transformed to a submaximal load oncirculation and respiration and (can) bewell tolerated.”Astrand et al., 1960Foraging animalsDistanceTimeCruiseSaltatoryAmbushO’Brien et al., 1990Justin W. Mooredesertusa.comJ.L. GingerichBenefits of moving intermittently• Reduced attack rate by predators– predators more likely to attack moving prey• Increased detection of prey and/or predators• Remove potential conflict between running & breathing• Increased endurance (range)– pauses permit net clearance of metabolic end-productsLima & Dill, 1990O’Brien et al., 1990; McAdam & Kramer, 1998Weinstein & Full, 1992Carrier, 1987Integration of locomotor behaviorand muscle stimulationContinuous locomotionIntermittent locomotionTimeExercise ExercisePause PauseTimeone strideMetabolite Changes Correlate with Performance04812165101520250Lactate(µmol g leg-1)ArgininePhosphate(µmol g leg-1)120 sec30 secRest E P Rest E PRest E PRest E PWeinstein & Full, 2000***Cyclic ContractionsIn cyclic motions muscle contractions are not purely isometric or isotonic.Instead, muscles shorten and lengthenduring each cycle.How much work does a muscle do duringone