Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 49.5: Animal skeletons function insupport, protection, and movement• The various animal movements result frommuscles working against a skeletonCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsTypes of Skeletons• The three main functions of a skeleton aresupport, protection, and movement• The three main types of skeletons are hydrostaticskeletons, exoskeletons, and endoskeletonsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsHydrostatic Skeletons• A hydrostatic skeleton consists of fluid held underpressure in a closed body compartment• This is the main type of skeleton in mostcnidarians, flatworms, nematodes, and annelids• Annelids use their hydrostatic skeleton forperistalsis, a type of movement on land producedby rhythmic waves of muscle contractionsLE 49-25Longitudinalmuscle relaxed(extended)BristlesHeadCircularmusclecontractedLongitudinalmuscle contractedCircularmusclerelaxedCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsExoskeletons• An exoskeleton is a hard encasement depositedon the surface of an animal• Exoskeletons are found in most molluscs andarthropodsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsEndoskeletons• An endoskeleton consists of hard supportingelements, such as bones, buried in soft tissue• Endoskeletons are found in sponges,echinoderms, and chordatesCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• A mammalian skeleton has more than 200 bones• Some are fused; others are connected at joints byligaments that allow freedom of movementLE 49-26_6ShouldergirdleScapulaClavicleSternumSkullAppendicularskeletonAxial skeletonKeyRibHumerusVertebraRadiusExamplesof jointsFibulaUlnaTibiaPelvicgirdleCarpalsPhalangesMetacarpalsFemurPatellaTarsalsMetatarsalsPhalangesUlnaPivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.UlnaHinge joints, such as between the humerusand the head of the ulna, restrict movement toa single plane.HumerusBall-and-socket joints, where the humeruscontacts the shoulder girdle and where the femurcontacts the pelvic girdle, enable us to rotate ourarms and legs and move them in several planes.Head ofhumerusScapulaRadiusCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsPhysical Support on Land• In addition to the skeleton, muscles and tendonshelp support large land vertebratesTension and CompressionCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 49.6: Muscles move skeletal parts bycontracting• The action of a muscle is always to contract• Skeletal muscles are attached in antagonisticpairs, with each member of the pair workingagainst each otherLE 49-27BicepscontractsHumanTricepsrelaxesForearmflexesBicepsrelaxesTricepscontractsForearmextendsExtensormusclerelaxesFlexormusclecontractsGrasshopperExtensormusclecontractsFlexormusclerelaxesTibiaextendsTibiaflexesCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsVertebrate Skeletal Muscle• Vertebrate skeletal muscle is characterized by ahierarchy of smaller and smaller units• A skeletal muscle consists of a bundle of longfibers running parallel to the length of the muscle• A muscle fiber is itself a bundle of smallermyofibrils arranged longitudinallyCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The myofibrils are composed to two kinds ofmyofilaments:– Thin filaments consist of two strands of actinand one strand of regulatory protein– Thick filaments are staggered arrays of myosinmoleculesCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Skeletal muscle is also called striated musclebecause the regular arrangement of myofilamentscreates a pattern of light and dark bands• Each unit is a sarcomere, bordered by Z lines• Areas that contain the myofilments are the I band,A band, and H zoneLE 49-28Bundle ofmuscle fibersSingle muscle fiber(cell)Plasma membraneNucleiMuscleMyofibrilDark bandSarcomereZ lineLightbandI bandTEMA band I band0.5 µmM lineThick filaments(myosin)SarcomereH zoneZ lineThin filaments(actin)Z lineCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Sliding-Filament Model of Muscle Contraction• According to the sliding-filament model, filamentsslide past each other longitudinally, producingmore overlap between thin and thick filaments• As a result of sliding, the I band and H zone shrinkLE 49-29Sarcomere0.5 µmZHARelaxed muscle fiberIContracting muscle fiberFully contracted muscle fiberCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The sliding of filaments is based on interactionbetween actin and myosin molecules of the thickand thin filaments• The “head” of a myosin molecule binds to an actinfilament, forming a cross-bridge and pulling thethin filament toward the center of the sarcomereLE 49-30–4Thin filamentsThick filamentThin filamentThick filamentMyosin head (low-energyconfiguration)Cross-bridgebinding siteMyosin head (high-energy configuration)ActinCross-bridgeMyosin head (low-energy configuration)Thin filament movestoward center of sacomere.Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Role of Calcium and Regulatory Proteins• A skeletal muscle fiber contracts only whenstimulated by a motor neuron• When a muscle is at rest, myosin-binding sites onthe thin filament are blocked by the regulatoryprotein tropomyosinLE 49-31Myosin-binding sites blocked.Myosin-binding sites exposed.TropomyosinCa2+-binding sitesActin Troponin complexMyosin-binding siteCa2+Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• For a muscle fiber to contract, myosin-bindingsites must be uncovered• This occurs when calcium ions (Ca2+) bind to a setof regulatory proteins, the troponin complexCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The stimulus leading to contraction of a musclefiber is an action potential in a motor neuron thatmakes a synapse with the muscle fiberLE 49-32Ca2+ releasedfrom sarcoplasmicreticulumMitochondrionMotorneuron axonSynapticterminalT tubuleSarcoplasmicreticulumMyofibrilPlasma membraneof muscle fiberSarcomereCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The synaptic terminal of the motor
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