2Single contraction characteristics of the three muscle types
3Skeletal Muscle Striated (sarcomere is functional unit); multinucleate Voluntary (activated by a motor neurons)Each fiber is a cell; many fibers to a muscleDifferent types of fibersEach fiber has a nerve connection
5Typing based on succinate dehydrogenase activity Type I = low activityType IIa = moderate activityType IIb = high activityTyping based on speed of myosin (myosin ATPase activityType I = low activityType II = high activity
6Basic Structure of a Skeletal Muscle Fiber Sarcolemma = cell membraneFiber (cell) > myofibril (bundles of actin and myosin) > myofilaments (actin and myosin)
7Functional unit of a myofibril is the sarcomere Major filamentous proteins are actin and myosin, z proteins (in the z disk)Many structural proteins; alpha actinin, myomesin, C protein, titin, nebulinCytoskeletal proteins; desmin, vimentin, filaminsarcomere
9Myosin (thick) filaments Each filament contains about 400 individual myosin moleculesBundled so that half of the molecules have their head facing one direction, the other half, the opposite directionMyosin heads are in a staggered arrangement along the filament
10Myosin is composed of 6 polypeptide chains, 2 H or heavy chains, and two L or light chains Myosin head has ATPase activityMyosin has a hinge region where the molecule is flexibleThe myosin head has a high affinity for g actinIn smooth muscle, light chains regulate myosin action; in cardiac and skeletal muscle, light chains partially determine the speed of the myosin ATPase activity
11Actin Filaments (thin filaments) Monomers of globular or g actin combine to form long fibers of f actin. Two f actin molecules twist around one another to form a single thin filamentAll actin myofilaments are anchored to the proteins of the z diskEach g actin molecule has a binding site for the myosin head
12ActinTropomyosin – covers active sites on g actin moleculesTroponin – regulates tropomyosin; three subunits – troponin c, troponin I, and troponin mTroponin c has a binding site for calcium and is bound to the other two subunitsTroponin I keeps the tropomyosin over the myosin binding sites on G actin (inhibits actin/myosin binding)Troponin m anchors the three subunits to the tropomyosin molecule
16Excitation-contraction coupling Motor neuron releases Ach onto the surface of the skeletal muscle fiber. The fiber’s nicotinic receptors are activated, opening K+ and Na+ channels. The cell membrane is depolarized.The action potential moves away from the motor end plate in all directions, including down the t-tubule system.Receptors in the t-tubules called dihydropyridine receptors (DHP) are activated by the change in voltage. They are connected to the ryanodine receptors in the lateral sacks of the sarcoplasmic reticulum (SR).The ryanodine receptors are opened by the change in conformation of the DHP receptors and Ca2+ is released from the SR.Ca2+ diffuses across the myofilaments.The Ca2+ binds to troponin C, causing it to change conformation, pulling on troponin I, which in turn pulls on tropomyosin. With the altered conformation in tropomyosin, the myosin binding sites on g actin molecules are exposed.Myosin heads can now bind to g actin molecules and cross-bridge cycling begins, shortening the sarcomere by pulling on the actin filaments and drawing the z disks closer together
20Cross-bridge cycling Why? Continues as long as nerve depolarizes muscle membrane; also dependent on fuel supply as myosin power stroke is dependent on ATPPulls z lines closer together; shortens sarcomereMyosin heads do not pull in a synchronized manner; random rowingWhy?
21RelaxationWhat must happen for a muscle to relax:_
22Factors Affecting Strength of Muscle Contraction Energy supplyAnaerobic glycolysis; aerobic glycolysis; FA metabolism; phosphocreatine metabolismATP + creatine phosphocreatine + ADP ATP + creatineMuscle length
24Frequency of stimulation Summation; tetanusWhy does the force of contraction increase? What factors are changing at the cellular level?
25Frequency of stimulus determines the overall cellular [Ca2+] Frequency of stimulus determines the overall cellular [Ca2+]. The more Ca2+, the greater the binding to troponin. The greater the binding, the more movement of tropomyosin. The more active sites exposed, the greater the number of myosin heads that can bind. The more myosin heads performing a powerstroke, the greater the strength of contraction.
26Number of motor units activated (recruitment) Slow twitch, fatigue resistant activated first; weak stimulus activates themFast twitch, fatigue-resistant (intermediate) activated as stimulus intensity increasesFast twitch, glycolytic activated at stimuli of highest intensityWhat will happen if you maximally contract a muscle for an extended period of time? Why?
27Types of ContractionsIsotonic = same stretch; moves a load; as muscle contracts, it shortensConcentric = muscle shortens as it moves a loadEccentric = muscle lengthens as it moves a loadIsometric = same length; load is not moved; muscle contracts but doesn’t shorten
28Smooth muscle Small, not striated Involuntary Found in hollow organs; other organs important to homeostasisCan be activated by stretch, neurons, hormonesLittle SR; caveolaeMyosin has heads down entire lengthSlow ATPase activityRegulated by myosin light chainsDense bodies anchor actinActin has no troponin or tropomyosin components