Bài giảng Biology - Chapter 49: Sensory and Motor Mechanisms

Chapter 49Sensory and Motor MechanismsOverview: Sensing and ActingBats use sonar to detect their preyMoths, a common prey for batsCan detect the bat’s sonar and attempt to fleeFigure 49.1Both of these organismsHave complex sensory systems that facilitate their survivalThe structures that make up these systemsHave been transformed by evolution into diverse mechanisms that sense various stimuli and generate the appropriate physical movementConcept 49.1: Sensory receptors transduce stimulus energy and transmit signals to the central nervous systemSensations are action potentialsThat reach the brain via sensory neuronsOnce the brain is aware of sensationsIt interprets them, giving the perception of stimuliSensations and perceptionsBegin with sensory reception, the detection of stimuli by sensory receptorsExteroreceptorsDetect stimuli coming from the outside of the bodyInteroreceptorsDetect internal stimuliFunctions Performed by Sensory ReceptorsAll stimuli represent forms of energySensation involves converting this energyInto a change in the membrane potential of sensory receptorsSensory receptors perform four functions in this processSensory transduction, amplification, transmission, and integrationTwo types of sensory receptors exhibit these functionsA stretch receptor in a crayfishFigure 49.2a(a) Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends, muscles and dendritesstretch, producing a receptor potential in the stretch receptor. The receptor potential triggers action potentials in the axon of the stretchreceptor. A stronger stretch producesa larger receptor potential and higherrequency of action potentials. MuscleDendritesStretchreceptorAxonMembranepotential (mV)–50–700–7001234567Time (sec)Action potentialsReceptor potentialWeakmuscle stretch–50–700–7001234567Time (sec)Strongmuscle stretchA hair cell found in vertebratesof action potentials in the sensory neuron. Bending in the other direction has the opposite effects. Thus, hair cells respond to the direction of motion as well as to its strength and speed.s(b) Vertebrate hair cells have specialized cilia or microvilli (“hairs”) that bend when sur-rounding fluid moves. Each hair cell releases an excitatory neurotransmitter at a synapsewith a sensory neuron, which conducts action potentials to the CNS. Bending in one direction depolarizes the hair cell, causing it to release more neurotransmitter and increasing frequency–50–700–7001234567Time (sec)Action potentialsNo fluidmovement–50–700–7001234567Time (sec)Receptor potentialFluid moving inone direction–50–700–7001234567Time (sec)Fluid moving in other directionMembranepotential (mV)Membranepotential (mV)Membranepotential (mV)“Hairs” ofhair cellNeuro-trans-mitter at synapseAxonLessneuro-trans-mitterMoreneuro-trans-mitterFigure 49.2bSensory TransductionSensory transduction is the conversion of stimulus energyInto a change in the membrane potential of a sensory receptorThis change in the membrane potentialIs known as a receptor potentialMany sensory receptors are extremely sensitiveWith the ability to detect the smallest physical unit of stimulus possibleAmplificationAmplification is the strengthening of stimulus energyBy cells in sensory pathwaysTransmissionAfter energy in a stimulus has been transduced into a receptor potentialSome sensory cells generate action potentials, which are transmitted to the CNSSensory cells without axons Release neurotransmitters at synapses with sensory neuronsIntegrationThe integration of sensory informationBegins as soon as the information is receivedOccurs at all levels of the nervous systemSome receptor potentialsAre integrated through summationAnother type of integration is sensory adaptationA decrease in responsiveness during continued stimulationTypes of Sensory ReceptorsBased on the energy they transduce, sensory receptors fall into five categoriesMechanoreceptorsChemoreceptorsElectromagnetic receptorsThermoreceptorsPain receptorsMechanoreceptorsMechanoreceptors sense physical deformationCaused by stimuli such as pressure, stretch, motion, and soundThe mammalian sense of touchRelies on mechanoreceptors that are the dendrites of sensory neuronsFigure 49.3HeatLight touchPainColdHairNerveConnective tissueHair movementStrong pressureDermisEpidermisChemoreceptorsChemoreceptors includeGeneral receptors that transmit information about the total solute concentration of a solutionSpecific receptors that respond to individual kinds of moleculesTwo of the most sensitive and specific chemoreceptors knownAre present in the antennae of the male silkworm mothFigure 49.40.1 mmElectromagnetic ReceptorsElectromagnetic receptors detect various forms of electromagnetic energySuch as visible light, electricity, and magnetismSome snakes have very sensitive infrared receptorsThat detect body heat of prey against a colder backgroundFigure 49.5a(a) This rattlesnake and other pit vipers have a pair of infrared receptors,one between each eye and nostril. The organs are sensitive enoughto detect the infrared radiation emitted by a warm mouse a meter away. The snake moves its head from side to side until the radiation is detected equally by the two receptors, indicating that the mouse is straight ahead.Many mammals appear to use the Earth’s magnetic field linesTo orient themselves as they migrateFigure 49.5b(b) Some migrating animals, such as these beluga whales, apparentlysense Earth’s magnetic field and use the information, along with other cues, for orientation.ThermoreceptorsThermoreceptors, which respond to heat or coldHelp regulate body temperature by signaling both surface and body core temperaturePain ReceptorsIn humans, pain receptors, also called nociceptorsAre a class of naked dendrites in the epidermisRespond to excess heat, pressure, or specific classes of chemicals released from damaged or inflamed tissuesConcept 49.2: The mechanoreceptors involved with hearing and equilibrium detect settling particles or moving fluidHearing and the perception of body equilibriumAre related in most animalsSensing Gravity and Sound in InvertebratesMost invertebrates have sensory organs called statocystsThat contain mechanoreceptors and function in their sense of equilibriumFigure 49.6Ciliatedreceptor cellsCiliaStatolithSensory nerve fibersMany arthropods sense sounds with body hairs that vibrateOr with localized “ears” consisting of a tympanic membrane and receptor cellsFigure 49.71 mmTympanicmembraneHearing and Equilibrium in MammalsIn most terrestrial vertebratesThe sensory organs for hearing and equilibrium are closely associated in the earExploring the structure of the human earFigure 49.8PinnaAuditory canalEustachian tubeTympanicmembraneStapesIncusMalleusSkullbonesSemicircularcanalsAuditory nerve,to brainCochleaTympanicmembraneOvalwindowEustachian tubeRoundwindowVestibular canalTympanic canalAuditory nerveBoneCochlear ductHair cellsTectorialmembraneBasilarmembraneTo auditorynerveAxons of sensory neurons1Overview of ear structure2The middle ear and inner ear4The organ of Corti3The cochleaOrgan of CortiOuter earMiddleearInner earHearingVibrating objects create percussion waves in the airThat cause the tympanic membrane to vibrateThe three bones of the middle earTransmit the vibrations to the oval window on the cochleaThese vibrations create pressure waves in the fluid in the cochleaThat travel through the vestibular canal and ultimately strike the round windowFigure 49.9CochleaStapesOval windowApexAxons ofsensoryneuronsRoundwindowBasilar membraneTympaniccanalBaseVestibularcanalPerilymphThe pressure waves in the vestibular canal Cause the basilar membrane to vibrate up and down causing its hair cells to bendThe bending of the hair cells depolarizes their membranesSending action potentials that travel via the auditory nerve to the brainThe cochlea can distinguish pitchBecause the basilar membrane is not uniform along its lengthCochlea(uncoiled)BasilarmembraneApex(wide and flexible)Base(narrow and stiff)500 Hz(low pitch)1 kHz2 kHz4 kHz8 kHz16 kHz(high pitch)Frequency producing maximum vibrationFigure 49.10Each region of the basilar membrane vibrates most vigorouslyAt a particular frequency and leads to excitation of a specific auditory area of the cerebral cortexEquilibriumSeveral of the organs of the inner earDetect body position and balanceThe utricle, saccule, and semicircular canals in the inner earFunction in balance and equilibriumFigure 49.11The semicircular canals, arranged in three spatial planes, detect angular movements of the head.Body movementNervefibersEach canal has at its base a swelling called an ampulla, containing a cluster of hair cells.When the head changes its rateof rotation, inertia prevents endolymph in the semicircular canals from moving with the head, so the endolymph presses against the cupula, bending the hairs.The utricle and saccule tell the brain which way is up and inform it of the body’s position or linear acceleration.The hairs of the hair cells project into a gelatinous cap called the cupula.Bending of the hairs increases the frequency of action potentials in sensory neurons in direct proportion to the amount of rotational acceleration.VestibuleUtricleSacculeVestibular nerveFlowof endolymphFlowof endolymphCupulaHairsHaircellHearing and Equilibrium in Other VertebratesLike other vertebrates, fishes and amphibiansAlso have inner ears located near the brainMost fishes and aquatic amphibiansAlso have a lateral line system along both sides of their bodyThe lateral line system contains mechanoreceptorsWith hair cells that respond to water movementFigure 49.12Nerve fiberSupporting cellCupulaSensoryhairsHair cellSegmental muscles of body wallLateral nerveScaleEpidermisLateral line canalNeuromastOpening of lateralline canalLaterallineConcept 49.3: The senses of taste and smell are closely related in most animalsThe perceptions of gustation (taste) and olfaction (smell)Are both dependent on chemoreceptors that detect specific chemicals in the environmentThe taste receptors of insects are located within sensory hairs called sensillaWhich are located on the feet and in mouthpartsFigure 49.13EXPERIMENT Insects taste using gustatory sensilla (hairs) on their feet and mouthparts. Each sensillum contains four chemoreceptors with dendrites that extend to a pore at the tip of the sensillum. To study the sensitivity of each chemoreceptor, researchers immobilized a blowfly (Phormia regina) by attaching it to a rod with wax. They then inserted the tip of a microelectrode into one sensillum to record action potentials in the chemoreceptors, while they used a pipette to touch the pore with various test substances.Number of action potentialsin first second of responseCONCLUSION Any natural food probably stimulates multiple chemoreceptors. By integrating sensations, the insect’s brain can apparently distinguish a very large number of tastes.To brainChemo-receptorsPore at tipPipette containingtest substanceTo voltagerecorderSensillumMicroelectrode50301000.5 MNaClMeat0.5 MSucroseHoneyStimulusChemoreceptorsRESULTS Each chemoreceptor is especially sensitive to a particular class of substance, but this specificity is relative; each cell can respond to some extent to a broad range of different chemical stimuli.Taste in HumansThe receptor cells for taste in humansAre modified epithelial cells organized into taste budsFive taste perceptions involve several signal transduction mechanismsSweet, sour, salty, bitter, and umami (elicited by glutamate)Taste poreSugar moleculeSensoryreceptorcellsSensoryneuronTaste budTongueG proteinAdenylyl cyclase—Ca2+ATPcAMPProteinkinase ASugarSugarreceptorSENSORYRECEPTORCELLSynapticvesicleK+NeurotransmitterSensory neuronTransduction in taste receptorsOccurs by several mechanismsFigure 49.144 The decrease in the membrane’s permeability to K+ depolarizes the membrane.5 Depolarization opens voltage-gated calcium ion (Ca2+) channels, and Ca2+ diffuses into the receptor cell.6 The increased Ca2+ concentration causes synaptic vesicles to release neurotransmitter.3 Activated protein kinase A closes K+ channels in the membrane.2 Binding initiates a signal transduction pathway involving cyclic AMP and protein kinase A.1 A sugar molecule binds to a receptor protein on the sensory receptor cell.Smell in HumansOlfactory receptor cellsAre neurons that line the upper portion of the nasal cavityWhen odorant molecules bind to specific receptorsA signal transduction pathway is triggered, sending action potentials to the brainBrainNasal cavityOdorantOdorantreceptorsPlasmamembraneOdorantCiliaChemoreceptorEpithelial cellBoneOlfactory bulbAction potentialsMucusFigure 49.15Concept 49.4: Similar mechanisms underlie vision throughout the animal kingdomMany types of light detectorsHave evolved in the animal kingdom and may be homologousVision in InvertebratesMost invertebratesHave some sort of light-detecting organLightLight shining from the front is detectedPhotoreceptorVisual pigmentOcellusNerve to brainScreening pigmentLight shining from behind is blockedby the screening pigmentOne of the simplest is the eye cup of planariansWhich provides information about light intensity and direction but does not form imagesFigure 49.16Two major types of image-forming eyes have evolved in invertebratesThe compound eye and the single-lens eyeCompound eyes are found in insects and crustaceansAnd consist of up to several thousand light detectors called ommatidiaFigure 49.17a–bCorneaCrystallineconeRhabdomPhotoreceptorAxonsOmmatidiumLens2 mm(a) The faceted eyes on the  head of a fly, photographed with  a stereomicroscope. (b) The cornea and crystalline cone of  each ommatidium function as a lens that focuses light on the  rhabdom, a stack of pigmented plates inside a circle of  photoreceptors. The rhabdom  traps light and guides it to  photoreceptors. The image  formed by a compound eye is a  mosaic of dots produced by different  intensities of light entering the  many ommatidia from different angles.Single-lens eyesAre found in some jellies, polychaetes, spiders, and many molluscsWork on a camera-like principleThe Vertebrate Visual SystemThe eyes of vertebrates are camera-likeBut they evolved independently and differ from the single-lens eyes of invertebratesStructure of the EyeThe main parts of the vertebrate eye areThe sclera, which includes the corneaThe choroid, a pigmented layerThe conjunctiva, that covers the outer surface of the scleraThe iris, which regulates the pupilThe retina, which contains photoreceptorsThe lens, which focuses light on the retinaThe structure of the vertebrate eyeFigure 49.18Ciliary bodyIrisSuspensoryligamentCorneaPupilAqueoushumorLensVitreous humorOptic disk(blind spot)Central artery andvein of the retinaOpticnerveFovea (centerof visual field)RetinaChoroidScleraHumans and other mammalsFocus light by changing the shape of the lensFigure 49.19a–bLens (flatter)Lens (rounder)CiliarymuscleSuspensoryligamentsChoroidRetinaFront view of lensand ciliary muscleCiliary muscles contract, pullingborder of choroid toward lensSuspensory ligaments relaxLens becomes thicker and rounder, focusing on near objects(a) Near vision (accommodation)(b) Distance visionCiliary muscles relax, and border of choroid moves away from lensSuspensory ligaments pull against lensLens becomes flatter, focusing on distant objectsThe human retina contains two types of photoreceptorsRods are sensitive to light but do not distinguish colorsCones distinguish colors but are not as sensitiveSensory Transduction in the EyeEach rod or cone in the vertebrate retinaContains visual pigments that consist of a light-absorbing molecule called retinal bonded to a protein called opsinRods contain the pigment rhodopsinWhich changes shape when it absorbs lightFigure 49.20a, bRodOutersegmentCell bodySynapticterminalDisksInsideof disk(a) Rods contain the visual pigment rhodopsin, which is embedded in a stack of membranous disks in the rod’s outer segment. Rhodopsin consists of the light-absorbing molecule retinal bonded to opsin, a protein. Opsin has seven  helices that span the disk membrane.(b) Retinal exists as two isomers. Absorption of light converts the cis isomer to the trans isomer, which causes opsin to change its conformation (shape). After a few minutes, retinal detaches from opsin. In the dark, enzymes convert retinal back to its cis form, which recombines with opsin to form rhodopsin.RetinalOpsinRhodopsinCytosolHCCH2CCH2CCHCH3CH3HCCCH3HCH3CCCCCCCHHHHOHH3CHCCH2CCH2CCHCH3CH3HCCCH3HCH3CCCCHHCH3HCCCHOCH3trans isomercis isomerEnzymesLightProcessing Visual InformationThe processing of visual informationBegins in the retina itselfAbsorption of light by retinalTriggers a signal transduction pathwayFigure 49.21EXTRACELLULARFLUIDMembranepotential (mV)0– 40– 70Dark Light– Hyper- polarizationTimeNa+Na+cGMPCYTOSOLGMPPlasmamembraneINSIDE OF DISKPDEActive rhodopsinLightInactive rhodopsinTransducinDisk membrane2 Active rhodopsin in turn activates a G protein called transducin.3 Transducinactivates theenzyme phos-phodiesterae(PDE).4 Activated PDE detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP.5 The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases, and the rod hyperpolarizes.1 Light isomerizes retinal, which activates rhodopsin. In the dark, both rods and conesRelease the neurotransmitter glutamate into the synapses with neurons called bipolar cells, which are either hyperpolarized or depolarizedIn the light, rods and cones hyperpolarizeShutting off their release of glutamateThe bipolar cellsAre then either depolarized or hyperpolarizedFigure 49.22Dark ResponsesRhodopsin inactiveNa+ channels openRod depolarizedGlutamatereleasedBipolar cell eitherdepolarized orhyperpolarized,depending onglutamate receptorsLight ResponsesRhodopsin activeNa+ channels closedRod hyperpolarizedNo glutamatereleasedBipolar cell eitherhyperpolarized ordepolarized, depending on glutamate receptorsThree other types of neurons contribute to information processing in the retinaGanglion cells, horizontal cells, and amacrine cellsFigure 49.23OpticnervefibersGanglioncellBipolarcellHorizontalcellAmacrinecellPigmentedepitheliumNeuronsConeRodPhotoreceptorsRetinaRetinaOptic nerveTobrainSignals from rods and conesTravel from bipolar cells to ganglion cellsThe axons of ganglion cells are part of the optic nerveThat transmit information to the brainFigure 49.24LeftvisualfieldRightvisualfieldLefteyeRighteyeOptic nerveOptic chiasmLateralgeniculatenucleusPrimaryvisual cortexMost ganglion cell axons lead to the lateral geniculate nuclei of the thalamusWhich relays information to the primary visual cortexSeveral integrating centers in the cerebral cortexAre active in creating visual perceptionsConcept 49.5: Animal skeletons function in support, protection, and movementThe various types of animal movementsAll result from muscles working against some type of skeletonTypes of SkeletonsThe three main functions of a skeleton areSupport, protection, and movementThe three main types of skeletons areHydrostatic skeletons, exoskeletons, and endoskeletonsHydrostatic SkeletonsA hydrostatic skeletonConsists of fluid held under pressure in a closed body compartmentThis is the main type of skeletonIn most cnidarians, flatworms, nematodes, and annelidsAnnelids use their hydrostatic skeleton for peristalsisA type of movement on land produced by rhythmic waves of muscle contractionsFigure 49.25a–c(a) Body segments at the head and just in front of the rear are short and thick (longitudinal muscles contracted; circular muscles relaxed) and anchored to the ground by bristles. The other segments are thin and elongated (circular muscles contracted; longitudinal muscles relaxed.)(b) The head has moved forward because circular muscles in the head segments have contracted. Segments behind the head and at the rear are now thick and anchored, thus preventing the worm from slipping backward.(c) The head segments are thick again and anchored in their new positions. The rear segments have released their hold on the ground and have been pulled forward.Longitudinalmuscle relaxed(extended)CircularmusclecontractedCircularmusclerelaxedLongitudinalmusclecontractedHeadBristlesExoskeletonsAn exoskeleton is a hard encasementDeposited on the surface of an animalExoskeletonsAre found in most molluscs and arthropodsEndoskeletonsAn endoskeleton consists of hard supporting elementsSuch as bones, buried within the soft tissue of an animalEndoskeletonsAre found in sponges, echinoderms, and chordatesThe mammalian skeleton is built from more than 200 bonesSome fused together and others connected at joints by ligaments that allow freedom of movementThe human skeletonFigure 49.261 Ball-and-socket joints, where the humerus contactsthe shoulder girdle and where the femur contacts thepelvic girdle, enable us to rotate our arms andlegs and move them in several planes.2 Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.3 Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.keyAxial skeletonAppendicularskeletonSkullShouldergirdleClavicleScapulaSternumRibHumerusVertebraRadiusUlnaPelvicgirdleCarpalsPhalangesMetacarpalsFemurPatellaTibiaFibulaTarsalsMetatarsalsPhalanges1Examplesof joints23Head ofhumerusScapulaHumerusUlnaUlnaRadiusPhysical Support on LandIn addition to the skeletonMuscles and tendons help support large land vertebratesConcept 49.6: Muscles move skeletal parts by contractingThe action of a muscleIs always to contractSkeletal muscles are attached to the skeleton in antagonistic pairsWith each member of the pair working against each otherFigure 49.27HumanGrasshopperBicepscontractsTricepsrelaxesForearmflexesBicepsrelaxesTricepscontractsForearmextendsExtensormusclerelaxesFlexormusclecontractsTibiaflexesExtensormusclecontractsFlexormusclerelaxesTibiaextendsVertebrate Skeletal MuscleVertebrate skeletal muscleIs characterized by a hierarchy of smaller and smaller unitsFigure 49.28MuscleBundle ofmuscle fibersSingle muscle fiber(cell)Plasma membraneMyofibrilLightbandDark bandZ lineSarcomereTEM0.5 mI bandA bandI bandM lineThickfilaments(myosin)Thinfilaments(actin)H zoneSarcomereZ lineZ lineNucleiA skeletal muscle consists of a bundle of long fibersRunning parallel to the length of the muscleA muscle fiberIs itself a bundle of smaller myofibrils arranged longitudinallyThe myofibrils are composed to two kinds of myofilamentsThin filaments, consisting of two strands of actin and one strand of regulatory proteinThick filaments, staggered arrays of myosin moleculesSkeletal muscle is also called striated muscleBecause the regular arrangement of the myofilaments creates a pattern of light and dark bandsEach repeating unit is a sarcomereBordered by Z linesThe areas that contain the myofilmentsAre the I band, A band, and H zoneThe Sliding-Filament Model of Muscle ContractionAccording to the sliding-filament model of muscle contractionThe filaments slide past each other longitudinally, producing more overlap between the thin and thick filamentsAs a result of this slidingThe I band and the H zone shrinkFigure 49.29a–c(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.(b) Contracting muscle fiber. During contraction, the thick andthin filaments slide past each other, reducing the width of theI bands and H zone and shortening the sarcomere.(c) Fully contracted muscle fiber. In a fully contracted musclefiber, the sarcomere is shorter still. The thin filaments overlap,eliminating the H zone. The I bands disappear as the ends ofthe thick filaments contact the Z lines.0.5 mZHASarcomereThe sliding of filaments is based onThe interaction between the actin and myosin molecules of the thick and thin filamentsThe “head” of a myosin molecule binds to an actin filamentForming a cross-bridge and pulling the thin filament toward the center of the sarcomereMyosin-actin interactions underlying muscle fiber contractionFigure 49.30Thick filamentThin filamentsThin filamentATPATPADPADPADPP iP iP iCross-bridgeMyosin head (low-energy configuration)Myosin head (high-energy configuration)+Myosin head (low-energy configuration)Thin filament moves toward center of sarcomere.Thick filamentActinCross-bridge binding site1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration. P1 The myosin head binds toactin, forming a cross-bridge. 34 Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament. P5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.The Role of Calcium and Regulatory ProteinsA skeletal muscle fiber contractsOnly when stimulated by a motor neuronWhen a muscle is at restThe myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosinFigure 49.31aActinTropomyosinCa2+-binding sitesTroponin complex(a) Myosin-binding sites blockedFor a muscle fiber to contractThe myosin-binding sites must be uncoveredThis occurs when calcium ions (Ca2+)Bind to another set of regulatory proteins, the troponin complexFigure 49.31bCa2+Myosin-binding site(b) Myosin-binding sites exposedThe stimulus leading to the contraction of a skeletal muscle fiberIs an action potential in a motor neuron that makes a synapse with the muscle fiberFigure 49.32Motorneuron axonMitochondrionSynapticterminalT tubuleSarcoplasmicreticulumMyofibrilPlasma membraneof muscle fiberSarcomereCa2+ releasedfrom sarcoplasmicreticulumThe synaptic terminal of the motor neuronReleases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potentialAction potentials travel to the interior of the muscle fiberAlong infoldings of the plasma membrane called transverse (T) tubulesThe action potential along the T tubulesCauses the sarcoplasmic reticulum to release Ca2+The Ca2+ binds to the troponin-tropomyosin complex on the thin filamentsExposing the myosin-binding sites and allowing the cross-bridge cycle to proceedAChSynapticterminalof motorneuronSynaptic cleftT TUBULEPLASMA MEMBRANESRADPCYTOSOLCa2Ca2P2Cytosolic Ca2+ is removed by active transport into SR after action potential ends.6Review of contraction in a skeletal muscle fiberFigure 49.33Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.1Action potential is propa-gated along plasmamembrane and downT tubules.2Action potentialtriggers Ca2+release from sarco-plasmic reticulum(SR).3Myosin cross-bridges alternately attachto actin and detach, pulling actinfilaments toward center of sarcomere;ATP powers sliding of filaments.5Calcium ions bind to troponin;troponin changes shape,removing blocking actionof tropomyosin; myosin-bindingsites exposed.4Tropomyosin blockage of myosin-binding sites is restored; contractionends, and muscle fiber relaxes. 7Neural Control of Muscle TensionContraction of a whole muscle is gradedWhich means that we can voluntarily alter the extent and strength of its contractionThere are two basic mechanisms by which the nervous system produces graded contractions of whole musclesBy varying the number of fibers that contractBy varying the rate at which muscle fibers are stimulatedIn a vertebrate skeletal muscleEach branched muscle fiber is innervated by only one motor neuronEach motor neuronMay synapse with multiple muscle fibersFigure 49.34Spinal cordNerveMotor neuroncell bodyMotorunit 1Motorunit 2Motor neuronaxonMuscleTendonSynaptic terminalsMuscle fibersA motor unitConsists of a single motor neuron and all the muscle fibers it controlsRecruitment of multiple motor neuronsResults in stronger contractionsA twitchResults from a single action potential in a motor neuronMore rapidly delivered action potentialsProduce a graded contraction by summationFigure 49.35ActionpotentialPair ofactionpotentialsSeries of action potentials at high frequencyTimeTensionSingletwitchSummation of two twitchesTetanusTetanus is a state of smooth and sustained contractionProduced when motor neurons deliver a volley of action potentialsTypes of Muscle FibersSkeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolyticBased on their contraction speed and major pathway for producing ATPTypes of skeletal musclesOther Types of MuscleCardiac muscle, found only in the heartConsists of striated cells that are electrically connected by intercalated discsCan generate action potentials without neural inputIn smooth muscle, found mainly in the walls of hollow organsThe contractions are relatively slow and may be initiated by the muscles themselvesIn addition, contractions may be caused byStimulation from neurons in the autonomic nervous systemConcept 49.7: Locomotion requires energy to overcome friction and gravityMovement is a hallmark of all animalsAnd usually necessary for finding food or evading predatorsLocomotionIs active travel from place to placeSwimmingOvercoming frictionIs a major problem for swimmersOvercoming gravity is less of a problem for swimmersThan for animals that move on land or flyLocomotion on LandWalking, running, hopping, or crawling on landRequires an animal to support itself and move against gravityDiverse adaptations for traveling on landHave evolved in various vertebratesFigure 49.36FlyingFlight requires that wings develop enough liftTo overcome the downward force of gravityCONCLUSIONFor animals of a given body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.FlyingRunning Swimming10–3103106110–1101021Body mass(g)Energy cost (J/Kg/m)CONCLUSIONThis graph compares the energy cost, in joules per kilogram of body mass per meter traveled, for animals specialized for running, flying, and swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.RESULTSPhysiologists typically determine an animal’s rate of energy use during locomotion by measuring its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a  wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that collects the air the bird exhales as it flies.EXPERIMENTThe energy cost of locomotionDepends on the mode of locomotion and the environmentFigure 49.37Comparing Costs of LocomotionAnimals that are specialized for swimmingExpend less energy per meter traveled than equivalently sized animals specialized for flying or running