BIO 3324 1nd Edition Lecture 12Outline of Last LectureI. vision lightII. photo pigmentIII. visual transductionIV. nervesV. PNSVI. ReceptorsVII. musclesOutline of Current Lecture I. ATPII. Power strokeIII. AutonomicIV. Control of movementV. visceraATP-powered crossbridging• Myosin has an ATPase site• When ATP binds it is broken down to ADP and Pi• Energy is used to configure myosin in a receptive position for binding actin• If myosin contacts actin, ADP and Pi are released and there is a power stroke• Actin and myosin remain linked until ATP is added to the system These notes represent a detailed interpretation of the professor’s lecture. Grade Buddy is best Used as a supplement to your own notes, not as a substitute.The power stroke• Results from cross bridge activity between actin and myosin• 2 myosin heads act independently of one another• When a head contacts actin, it changes shape pulling actin towards the M line• Myosin detaches, springs back into position to bind the next actin• All six surrounding thin filaments are pulled in unison, but asynchronouslyRelaxation• SR has Ca2+-ATPase pump which actively pumps Ca2+ from the cytosol into the SR• Condition of no Action Potential– No Ca2+ release– Ca2+ pumped out of the cytosol– Actin and myosin cannot interactPutting it all together.Number shows neuromuscular junction. One action potential results in depolarization within context of the cell. Action potential depolarizes the crust. Calcium is big molecule it allows contraction to occur. Difference between non-contracted and contracted is availability of calcium. Job of triple myosin is to block myosin. Tropomisin acts as a chaperone. (U can’t touch each other). What are the steps of a power stroke? Calcium allows action and myosin to come together. Relationship between myosin and actin are in those heads and only interactdoing power stroke. Contractile activity v. electrical activity• Latent period – time delay between stimulation & the beginning of the contraction (APis long gone)• Contraction time – onset of contraction to the peak of tension (twitch)• Relaxation – roughly equivalent to contraction time; time from peak of tension to restATP is essential for muscle metabolism• There are limited stores of ATP in muscles, so muscles must use 3 pathways to supply additional ATP during contractions– Transfer phosphate from creatine phosphate to ATP– Oxidative phosphorylation (citric acid cycle and electron transfer)– GlycolysisCreatine Phosphate – first energy storehouse• Highly reversible reaction• As ATP reserves increase, phosphates will be transferred to creatine• Gets burned early and quicklyOxidative Phosphorylation• Aerobic energy production• Requires O2• Glucose (or fatty acids) converted into 36 ATP• Relatively slow and dependent on nutrients being delivered to the muscles• Myoglobin in the muscle fibers increases rate of O2 transfer from the blood to muscleGlycolysis• Anaerobic energy production• Some glucose is stored in the muscle as glycogen• The last resort• Glucose is converted into 2 pyruvate yielding two ATP– In presence of O2, pyruvate is used by oxidative phosphorylation– In absence of O2, pyruvate converted into lactic acid• Can occur rapidlyFatigue• Peripheral fatigue – exercising muscle can no longer respond to stimulation with the same degree of contractile activity– Defense mechanism to protect muscle from reaching zero ATP production• Central fatigue – CNS no longer activates the motor neurons supplying the working musclesThree types of skeletal muscle fibers• Slow-oxidative (type I) – red muscle• Fast-oxidative (type IIa) – red muscle• Fast-glycolytic (type IIx) – white muscle• Fast v. slow: fast have higher ATPase activity and (quicker tension development) & pump Ca2+ into SR faster thus get a faster twitch• Oxidative v. glycolytic: reliance on oxidative phosphorylation vs. glycolysis for ATP. Oxidative phosphorylation yields more ATP and thus fatigues slower than glycolysisOptimal muscle length• Maximal force that can be achieved on a subsequent tetanic contraction• Found at rest• Maximal number of cross bridges are accessible• Passively stretched – fewer cross bridges are accessible• Passively contracted – three issues– Thin filaments are overlapping (mechanical)– Thick filaments are forced against the Z line (mechanical)– Less Ca2+ is released (unknown why) & the ability for Ca2+ to bind troponin is reduced (unknown why)Frequency of stimulation• Twitch summation – temporal summation of twitches• Tetanus – smooth, sustained contraction at maximal strength resulting from rapid successive action potentials• What is happening to Ca2+ levels?• What is the role of neural system in all this?– It controls the number of motor neurons stimulated and the frequency of their stimulationSkeletal muscle mechanics• Muscles are groups of muscle fibers bound together by connective tissue• Tendons are formed from the connective tissue to attach muscle to bone• An AP in a muscle fiber produces a twitch which is too weak & short to do anything useful...• …but if twitches can be used cooperatively and additively, then there is contraction• Thus, contractions can vary in strength– Number of fibers involved– The amount of tension developed by each fiberMotor units• All the fibers that a motor neuron innervates• Recall: a motor neuron splits and innervates several muscle fibers & a single muscle fiber is only innervated by 1 motor neuron• Motor units are dispersed throughout a muscle, so a stimulation of a motor unit will result in an evenly distributed contraction = weak• The number of fibers in a motor unit is dependent on the type of action performed by that muscle– Delicate activity – few muscle fibers– Coarse activity – many muscle fibersFatigue• Inability to maintain muscle tension• To alleviate fatigue:– The body uses asynchronous recruitment of motor units– And different muscle types have different metabolic activities• Fatigue resistant muscles are usually recruited first• Rapidly fatiguing muscles are recruited lastTwo primary types of contraction:• Isotonic – muscle tension remains constant as muscle length changes– Concentric – muscle is shortening with tension– Eccentric – muscle is lengthening with tension• Isometric – muscle doesn’t change length,