Y khoa, y dược - Muscles and muscle tissue: Part B

9Muscles and Muscle Tissue: Part BReview Principles of Muscle MechanicsSame principles apply to contraction of a single fiber and a whole muscleContraction produces tension, the force exerted on the load or object to be movedReview Principles of Muscle MechanicsContraction does not always shorten a muscle:Isometric contraction: no shortening; muscle tension increases but does not exceed the load Isotonic contraction: muscle changes in length and moves the loadIsotonic ContractionsIsotonic contractions are either concentric or eccentric:Concentric contractions: the muscle shortens and does workEccentric contractions: the muscle lengthens as it contracts and does work Review Principles of Muscle MechanicsForce and duration of contraction vary in response to stimuli of different frequencies and intensitiesMotor Unit: The Nerve-Muscle Functional UnitMotor unit = a motor neuron and all (four to several hundred) muscle fibers it suppliesFigure 9.13aSpinal cordMotor neuroncell bodyMuscleNerveMotorunit 1Motorunit 2MusclefibersMotorneuronaxonAxon terminals atneuromuscular junctions Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.Motor UnitSmall motor units in muscles that control fine movements (fingers, eyes) Large motor units in large weight-bearing muscles (thighs, hips) Motor UnitMuscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscleMotor units in a muscle usually contract asynchronously; helps prevent fatigueMuscle TwitchResponse of a muscle to a single, brief threshold stimulusSimplest contraction observable in the lab (recorded as a myogram)Muscle TwitchThree phases of a twitch:Latent period: events of excitation-contraction couplingPeriod of contraction: cross bridge formation; tension increasesPeriod of relaxation: Ca2+ reentry into the SR; tension declines to zeroFigure 9.14aLatentperiodSinglestimulusPeriod ofcontractionPeriod ofrelaxation(a) Myogram showing the three phases of an isometric twitchMuscle Twitch ComparisonsDifferent strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles Figure 9.14bLatent periodExtraocular muscle (lateral rectus)GastrocnemiusSoleusSinglestimulus(b) Comparison of the relative duration of twitch responses of three muscles Graded Muscle ResponsesVariations in the degree of muscle contractionRequired for proper control of skeletal movementResponses are graded by:Changing the frequency of stimulationChanging the strength of the stimulusResponse to Change in Stimulus FrequencyA single stimulus results in a single contractile response—a muscle twitchFigure 9.15aContractionRelaxationStimulusSingle stimulussingle twitch A single stimulus is delivered. The muscle contracts and relaxesResponse to Change in Stimulus FrequencyIncrease frequency of stimulus (muscle does not have time to completely relax between stimuli)Ca2+ release stimulates further contraction  temporal (wave) summationFurther increase in stimulus frequency  unfused (incomplete) tetanusFigure 9.15bStimuliPartial relaxationLow stimulation frequencyunfused (incomplete) tetanus(b) If another stimulus is applied before the muscle relaxes completely, then more tension results. This is temporal (or wave) summation and results in unfused (or incomplete) tetanus.Response to Change in Stimulus FrequencyIf stimuli are given quickly enough, fused (complete) tetany resultsFigure 9.15cStimuliHigh stimulation frequencyfused (complete) tetanus(c) At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus.Response to Change in Stimulus StrengthThreshold stimulus: stimulus strength at which the first observable muscle contraction occursMuscle contracts more vigorously as stimulus strength is increased above thresholdContraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into actionFigure 9.16Stimulus strengthProportion of motor units excitedStrength of muscle contractionMaximal contractionMaximalstimulusThresholdstimulusResponse to Change in Stimulus StrengthSize principle: motor units with larger and larger fibers are recruited as stimulus intensity increasesFigure 9.17Motorunit 1Recruited(smallfibers)Motorunit 2recruited(mediumfibers)Motorunit 3recruited(largefibers)Muscle ToneConstant, slightly contracted state of all musclesDue to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in musclesKeeps muscles firm, healthy, and ready to respond Isotonic ContractionsMuscle changes in length and moves the loadIsotonic contractions are either concentric or eccentric:Concentric contractions: the muscle shortens and does workEccentric contractions: the muscle contracts as it lengthensFigure 9.18aIsometric ContractionsThe load is greater than the tension the muscle is able to developTension increases to the muscle’s capacity, but the muscle neither shortens nor lengthensFigure 9.18bMuscle Metabolism: Energy for ContractionATP is the only source used directly for contractile activitiesAvailable stores of ATP are depleted in 4–6 secondsMuscle Metabolism: Energy for ContractionATP is regenerated by:Direct phosphorylation of ADP by creatine phosphate (CP)Aerobic respirationAnaerobic pathway (glycolysis) Figure 9.19aCoupled reaction of creatinephosphate (CP) and ADPEnergy source: CP(a) Direct phosphorylationOxygen use: NoneProducts: 1 ATP per CP, creatineDuration of energy provision:15 secondsCreatinekinaseADPCPCreatineATPAerobic PathwayProduces 95% of ATP during rest and light to moderate exerciseFuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acidsEnergy source: glucose; pyruvic acid;free fatty acids from adipose tissue;amino acids from protein catabolism(c) Aerobic pathwayAerobic cellular respirationOxygen use: RequiredProducts: 32 ATP per glucose, CO2, H2ODuration of energy provision: Hours Glucose (fromglycogen breakdown ordelivered from blood)32O2O2H2OCO2Pyruvic acidFattyacidsAminoacidsAerobic respirationin mitochondriaAerobic respirationin mitochondriaATPnet gain perglucoseAnaerobic PathwayAt 70% of maximum contractile activity:Bulging muscles compress blood vesselsOxygen delivery is impairedPyruvic acid is converted into lactic acidAnaerobic PathwayLactic acid:Diffuses into the bloodstreamUsed as fuel by the liver, kidneys, and heartConverted back into pyruvic acid by the liverEnergy source: glucoseGlycolysis and lactic acid formation(b) Anaerobic pathwayOxygen use: NoneProducts: 2 ATP per glucose, lactic acidDuration of energy provision:60 seconds, or slightly moreGlucose (fromglycogen breakdown ordelivered from blood)Glycolysisin cytosolPyruvic acidReleasedto bloodnet gain2Lactic acidO2O2ATPFigure 9.20Short-duration exerciseProlonged-durationexerciseATP stored inmuscles isused first.ATP is formedfrom creatinePhosphateand ADP.Glycogen stored in muscles is brokendown to glucose, which is oxidized togenerate ATP.ATP is generated bybreakdown of severalnutrient energy fuels byaerobic pathway. Thispathway uses oxygenreleased from myoglobinor delivered in the bloodby hemoglobin. When itends, the oxygen deficit ispaid back.Muscle FatiguePhysiological inability to contractOccurs when:Ionic imbalances (K+, Ca2+, Pi) interfere with E-C couplingProlonged exercise damages the SR and interferes with Ca2+ regulation and releaseTotal lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions)Oxygen DeficitExtra O2 needed after exercise for:Replenishment ofOxygen reserves Glycogen stores ATP and CP reserves Conversion of lactic acid to pyruvic acid, glucose, and glycogenHeat Production During Muscle Activity~ 40% of the energy released in muscle activity is useful as workRemaining energy (60%) given off as heat Dangerous heat levels are prevented by radiation of heat from the skin and sweating