Bài giảng Biology - Chapter 48: Nervous Systems

Chapter 48Nervous SystemsOverview: Command and Control Center The human brainContains an estimated 100 billion nerve cells, or neuronsEach neuronMay communicate with thousands of other neuronsFunctional magnetic resonance imagingIs a technology that can reconstruct a three-dimensional map of brain activityFigure 48.1The results of brain imaging and other research methodsReveal that groups of neurons function in specialized circuits dedicated to different tasksConcept 48.1: Nervous systems consist of circuits of neurons and supporting cellsAll animals except spongesHave some type of nervous systemWhat distinguishes the nervous systems of different animal groupsIs how the neurons are organized into circuitsOrganization of Nervous SystemsThe simplest animals with nervous systems, the cnidariansHave neurons arranged in nerve netsFigure 48.2aNerve net(a) Hydra (cnidarian)Sea stars have a nerve net in each armConnected by radial nerves to a central nerve ringFigure 48.2bNerveringRadialnerve(b) Sea star (echinoderm)In relatively simple cephalized animals, such as flatwormsA central nervous system (CNS) is evidentFigure 48.2cEyespotBrainNerve cordTransversenerve(c) Planarian (flatworm)Annelids and arthropodsHave segmentally arranged clusters of neurons called gangliaThese ganglia connect to the CNSAnd make up a peripheral nervous system (PNS)BrainVentral nervecordSegmentalganglionBrainVentralnerve cordSegmentalgangliaFigure 48.2d, e(d) Leech (annelid)(e) Insect (arthropod)Anteriornerve ringLongitudinalnerve cordsGangliaBrainGangliaFigure 48.2f, g (f) Chiton (mollusc)(g) Squid (mollusc)Nervous systems in molluscsCorrelate with the animals’ lifestylesSessile molluscs have simple systemsWhile more complex molluscs have more sophisticated systemsIn vertebratesThe central nervous system consists of a brain and dorsal spinal cordThe PNS connects to the CNSFigure 48.2hBrainSpinalcord(dorsalnervecord)Sensoryganglion(h) Salamander (chordate)Information ProcessingNervous systems process information in three stagesSensory input, integration, and motor outputFigure 48.3 SensorEffectorMotor outputIntegrationSensory inputPeripheral nervoussystem (PNS)Central nervoussystem (CNS)Sensory neurons transmit information from sensors That detect external stimuli and internal conditionsSensory information is sent to the CNSWhere interneurons integrate the informationMotor output leaves the CNS via motor neuronsWhich communicate with effector cellsThe three stages of information processingAre illustrated in the knee-jerk reflexFigure 48.4 Sensory neurons from the quadriceps also communicatewith interneurons in the spinal cord. The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.456The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle.1 Sensors detecta sudden stretch in the quadriceps.2 Sensory neuronsconvey the information to the spinal cord.3QuadricepsmuscleHamstringmuscleSpinal cord(cross section)Gray matterWhite matterCell body of sensory neuronin dorsal root ganglionSensory neuronMotor neuronInterneuronNeuron StructureMost of a neuron’s organellesAre located in the cell bodyFigure 48.5DendritesCell bodyNucleusAxon hillockAxonSignal directionSynapseMyelin sheathSynapticterminalsPresynaptic cellPostsynaptic cellMost neurons have dendritesHighly branched extensions that receive signals from other neuronsThe axon is typically a much longer extensionThat transmits signals to other cells at synapsesThat may be covered with a myelin sheathNeurons have a wide variety of shapesThat reflect their input and output interactionsFigure 48.6a–cAxonCell bodyDendrites(a) Sensory neuron(b) Interneurons(c) Motor neuronSupporting Cells (Glia)Glia are supporting cellsThat are essential for the structural integrity of the nervous system and for the normal functioning of neuronsIn the CNS, astrocytesProvide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmittersFigure 48.750 µmOligodendrocytes (in the CNS) and Schwann cells (in the PNS)Are glia that form the myelin sheaths around the axons of many vertebrate neuronsMyelin sheathNodes of RanvierSchwanncellSchwanncellNucleus of Schwann cellAxonLayers of myelinNode of Ranvier0.1 µmAxonFigure 48.8Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuronAcross its plasma membrane, every cell has a voltage Called a membrane potentialThe inside of a cell is negativeRelative to the outsideThe membrane potential of a cell can be measuredFigure 48.9APPLICATIONElectrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells.TECHNIQUEA microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.MicroelectrodeReferenceelectrodeVoltage recorder–70 mVThe Resting PotentialThe resting potentialIs the membrane potential of a neuron that is not transmitting signalsIn all neurons, the resting potentialDepends on the ionic gradients that exist across the plasma membraneCYTOSOLEXTRACELLULARFLUID[Na+]15 mM[K+]150 mM[Cl–]10 mM[A–]100 mM[Na+]150 mM[K+]5 mM[Cl–]120 mM–––––+++++PlasmamembraneFigure 48.10 The concentration of Na+ is higher in the extracellular fluid than in the cytosolWhile the opposite is true for K+By modeling a mammalian neuron with an artificial membraneWe can gain a better understanding of the resting potential of a neuronFigure 48.11a, bInner chamberOuter chamberInner chamberOuter chamber–92 mV+62 mVArtificialmembranePotassiumchannelK+Cl–150 mMKCL150 mMNaCl15 mMNaCl5 mMKCLCl–Na+Sodium channel+ –+ –+ –+ –+ –+ –(a) Membrane selectively permeable to K+(b) Membrane selectively permeable to Na+A neuron that is not transmitting signalsContains many open K+ channels and fewer open Na+ channels in its plasma membraneThe diffusion of K+ and Na+ through these channelsLeads to a separation of charges across the membrane, producing the resting potentialGated Ion ChannelsGated ion channels open or closeIn response to membrane stretch or the binding of a specific ligandIn response to a change in the membrane potentialConcept 48.3: Action potentials are the signals conducted by axonsIf a cell has gated ion channelsIts membrane potential may change in response to stimuli that open or close those channelsSome stimuli trigger a hyperpolarizationAn increase in the magnitude of the membrane potentialFigure 48.12a+500–50–100Time (msec)0 1 2 3 4 5ThresholdRestingpotentialHyperpolarizationsMembrane potential (mV)Stimuli(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization.Other stimuli trigger a depolarizationA reduction in the magnitude of the membrane potentialFigure 48.12b+500–50–100Time (msec)0 1 2 3 4 5ThresholdRestingpotentialDepolarizationsMembrane potential (mV)Stimuli(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization.Hyperpolarization and depolarizationAre both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulusProduction of Action PotentialsIn most neurons, depolarizationsAre graded only up to a certain membrane voltage, called the thresholdA stimulus strong enough to produce a depolarization that reaches the thresholdTriggers a different type of response, called an action potentialFigure 48.12c+500–50–100Time (msec)0 1 2 3  4 5 6ThresholdRestingpotentialMembrane potential (mV)Stronger depolarizing stimulusActionpotential(c) Action potential triggered by a depolarization that reaches the threshold.An action potentialIs a brief all-or-none depolarization of a neuron’s plasma membraneIs the type of signal that carries information along axonsBoth voltage-gated Na+ channels and voltage-gated K+ channelsAre involved in the production of an action potentialWhen a stimulus depolarizes the membraneNa+ channels open, allowing Na+ to diffuse into the cellAs the action potential subsidesK+ channels open, and K+ flows out of the cellA refractory period follows the action potentialDuring which a second action potential cannot be initiatedThe generation of an action potential–  –  –  –  –  –  –  –+  +  +  +  +  +  +  ++  ++  ++  +–  ––  ––  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  –+  +–  ––  –+  +–  –+  +–  –+  +–  –+  +Na+Na+K+Na+Na+K+Na+Na+K+Na+K+K+Na+Na+51Resting state2Depolarization3Rising phase of the action potential4Falling phase of the action potentialUndershoot123451SodiumchannelActionpotentialResting potentialTimePlasma membraneExtracellular fluidActivationgatesPotassiumchannelInactivationgateThresholdMembrane potential (mV)+500–50–100ThresholdCytosolFigure 48.13Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respectto the outside.The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative. A stimulus opens theactivation gates on some Na+ channels. Na+influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained. Both gates of the Na+ channelsare closed, but the activation gates on some K+ channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state.Conduction of Action PotentialsAn action potential can travel long distancesBy regenerating itself along the axonFigure 48.14–+–+++++–+–++++++–+–+++++–+–+++++–+–––––+–+–––––––––––––––––++++++++––––++++––––––––++++––––++++Na+Na+Na+ActionpotentialActionpotentialActionpotentialK+K+K+AxonAn action potential is generated as Na+ flows inward across the membrane at one location.12The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.K+At the site where the action potential is generated, usually the axon hillockAn electrical current depolarizes the neighboring region of the axon membraneConduction SpeedThe speed of an action potentialIncreases with the diameter of an axonIn vertebrates, axons are myelinatedAlso causing the speed of an action potential to increaseAction potentials in myelinated axonsJump between the nodes of Ranvier in a process called saltatory conductionCell bodySchwann cellMyelin sheathAxonDepolarized region(node of Ranvier)++ +++ +++ +++– –– –– –––––––Figure 48.15Concept 48.4: Neurons communicate with other cells at synapsesIn an electrical synapseElectrical current flows directly from one cell to another via a gap junctionThe vast majority of synapses Are chemical synapsesIn a chemical synapse, a presynaptic neuron Releases chemical neurotransmitters, which are stored in the synaptic terminalFigure 48.16PostsynapticneuronSynapticterminalof presynapticneurons5 µmWhen an action potential reaches a terminalThe final result is the release of neurotransmitters into the synaptic cleftFigure 48.17PresynapticcellPostsynaptic cellSynaptic vesiclescontainingneurotransmitterPresynapticmembranePostsynaptic membraneVoltage-gatedCa2+ channelSynaptic cleftLigand-gatedion channelsNa+K+Ligand-gatedion channelPostsynaptic membraneNeuro-transmitter1Ca2+23456Direct Synaptic TransmissionThe process of direct synaptic transmissionInvolves the binding of neurotransmitters to ligand-gated ion channelsNeurotransmitter binding Causes the ion channels to open, generating a postsynaptic potentialPostsynaptic potentials fall into two categoriesExcitatory postsynaptic potentials (EPSPs)Inhibitory postsynaptic potentials (IPSPs)After its release, the neurotransmitter Diffuses out of the synaptic cleftMay be taken up by surrounding cells and degraded by enzymesSummation of Postsynaptic PotentialsUnlike action potentialsPostsynaptic potentials are graded and do not regenerate themselvesSince most neurons have many synapses on their dendrites and cell bodyA single EPSP is usually too small to trigger an action potential in a postsynaptic neuronFigure 48.18aE1E1RestingpotentialThreshold of axon ofpostsynaptic neuron(a) Subthreshold, nosummationTerminal branch of presynaptic neuronPostsynaptic neuronE10–70Membrane potential (mV)If two EPSPs are produced in rapid successionAn effect called temporal summation occursFigure 48.18bE1E1Actionpotential(b) Temporal summationE1AxonhillockIn spatial summationEPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add togetherFigure 48.18cE1 + E2Actionpotential(c) Spatial summationE1E2Through summationAn IPSP can counter the effect of an EPSPFigure 48.18dE1E1 + II(d) Spatial summationof EPSP and IPSPE1IIndirect Synaptic TransmissionIn indirect synaptic transmissionA neurotransmitter binds to a receptor that is not part of an ion channelThis binding activates a signal transduction pathwayInvolving a second messenger in the postsynaptic cell, producing a slowly developing but long-lasting effectNeurotransmittersThe same neurotransmitterCan produce different effects in different types of cellsMajor neurotransmittersTable 48.1AcetylcholineAcetylcholineIs one of the most common neurotransmitters in both vertebrates and invertebratesCan be inhibitory or excitatoryBiogenic AminesBiogenic amines Include epinephrine, norepinephrine, dopamine, and serotoninAre active in the CNS and PNSAmino Acids and PeptidesVarious amino acids and peptidesAre active in the brainGasesGases such as nitric oxide and carbon monoxideAre local regulators in the PNSConcept 48.5: The vertebrate nervous system is regionally specializedIn all vertebrates, the nervous systemShows a high degree of cephalization and distinct CNS and PNS componentsFigure 48.19Central nervoussystem (CNS)Peripheral nervoussystem (PNS)BrainSpinal cordCranialnervesGangliaoutsideCNSSpinalnervesThe brain provides the integrative powerThat underlies the complex behavior of vertebratesThe spinal cord integrates simple responses to certain kinds of stimuliAnd conveys information to and from the brainThe central canal of the spinal cord and the four ventricles of the brainAre hollow, since they are derived from the dorsal embryonic nerve cordGray matterWhitematterVentriclesFigure 48.20 The Peripheral Nervous SystemThe PNS transmits information to and from the CNSAnd plays a large role in regulating a vertebrate’s movement and internal environmentThe cranial nerves originate in the brainAnd terminate mostly in organs of the head and upper bodyThe spinal nerves originate in the spinal cordAnd extend to parts of the body below the headThe PNS can be divided into two functional componentsThe somatic nervous system and the autonomic nervous systemPeripheralnervous systemSomaticnervoussystemAutonomicnervoussystemSympatheticdivisionParasympatheticdivisionEntericdivisionFigure 48.21The somatic nervous systemCarries signals to skeletal musclesThe autonomic nervous systemRegulates the internal environment, in an involuntary mannerIs divided into the sympathetic, parasympathetic, and enteric divisions The sympathetic and parasympathetic divisionsHave antagonistic effects on target organsParasympathetic divisionSympathetic divisionAction on target organs:Action on target organs:Location ofpreganglionic neurons:brainstem and sacralsegments of spinal cordNeurotransmitterreleased bypreganglionic neurons:acetylcholineLocation ofpostganglionic neurons:in ganglia close to orwithin target organsNeurotransmitterreleased bypostganglionic neurons:acetylcholineConstricts pupilof eyeStimulates salivarygland secretionConstrictsbronchi in lungsSlows heartStimulates activityof stomach andintestinesStimulates activityof pancreasStimulatesgallbladderPromotes emptyingof bladderPromotes erectionof genitaliaCervicalThoracicLumbarSynapseSympatheticgangliaDilates pupilof eyeInhibits salivary gland secretionRelaxes bronchiin lungsAccelerates heartInhibits activity of stomach and intestinesInhibits activityof pancreasStimulates glucoserelease from liver;inhibits gallbladderStimulatesadrenal medullaInhibits emptyingof bladderPromotes ejaculation and vaginal contractionsSacralLocation ofpreganglionic neurons:thoracic and lumbarsegments of spinal cordNeurotransmitterreleased bypreganglionic neurons:acetylcholineLocation ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near spinal cordNeurotransmitterreleased bypostganglionic neurons:norepinephrineFigure 48.22The sympathetic divisionCorrelates with the “fight-or-flight” responseThe parasympathetic divisionPromotes a return to self-maintenance functionsThe enteric divisionControls the activity of the digestive tract, pancreas, and gallbladderEmbryonic Development of the BrainIn all vertebratesThe brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrainFigure 48.23aForebrainMidbrainHindbrainMidbrainHindbrainForebrain(a) Embryo at one monthEmbryonic brain regionsBy the fifth week of human embryonic developmentFive brain regions have formed from the three embryonic regionsFigure 48.23bTelencephalonDiencephalonMesencephalonMetencephalonMyelencephalon(b) Embryo at five weeksMesencephalonMetencephalonMyelencephalonSpinal cordDiencephalonTelencephalonEmbryonic brain regionsAs a human brain develops furtherThe most profound change occurs in the forebrain, which gives rise to the cerebrumFigure 48.23cBrain structures present in adultCerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)Diencephalon (thalamus, hypothalamus, epithalamus)Midbrain (part of brainstem)Pons (part of brainstem), cerebellumMedulla oblongata (part of brainstem)(c) AdultCerebral hemisphereDiencephalon:HypothalamusThalamusPineal gland(part of epithalamus)Brainstem:MidbrainPonsMedullaoblongataCerebellumCentral canalSpinal cordPituitaryglandThe BrainstemThe brainstem consists of three partsThe medulla oblongata, the pons, and the midbrainThe medulla oblongataContains centers that control several visceral functionsThe ponsAlso participates in visceral functionsThe midbrainContains centers for the receipt and integration of several types of sensory informationArousal and SleepA diffuse network of neurons called the reticular formationIs present in the core of the brainstemFigure 48.24EyeReticular formationInput from touch, pain, and temperature receptorsInput from earsA part of the reticular formation, the reticular activating system (RAS)Regulates sleep and arousalThe CerebellumThe cerebellumIs important for coordination and error checking during motor, perceptual, and cognitive functionsThe cerebellum Is also involved in learning and remembering motor skillsThe DiencephalonThe embryonic diencephalon develops into three adult brain regionsThe epithalamus, thalamus, and hypothalamusThe epithalamusIncludes the pineal gland and the choroid plexusThe thalamusIs the main input center for sensory information going to the cerebrum and the main output center for motor information leaving the cerebrumThe hypothalamus regulatesHomeostasisBasic survival behaviors such as feeding, fighting, fleeing, and reproducingCircadian RhythmsThe hypothalamus also regulates circadian rhythmsSuch as the sleep/wake cycleAnimals usually have a biological clockWhich is a pair of suprachiasmatic nuclei (SCN) found in the hypothalamusBiological clocks usually require external cuesTo remain synchronized with environmental cyclesFigure 48.25 In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and endsat dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captivesquirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness.The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating andwhen it was still.EXPERIMENTLightDarkLight20151051(a) 12 hr light-12 hr dark cycle(b) Constant darkness121620244812121620244812Time of day (hr)Time of day (hr) When the squirrelswere exposed to a regular light/darkcycle, their wheel-turning activity (indicated by the dark bars) occurredat roughly the same time every day.However, when they were kept inconstant darkness, their activity phasebegan about 21 minutes later each day.RESULTS The northern flying squirrel’s internal clock can run in constant darkness, but it does so onits own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.CONCLUSIONDarkDays of experimentThe CerebrumThe cerebrum Develops from the embryonic telencephalonThe cerebrum has right and left cerebral hemispheresThat each consist of cerebral cortex overlying white matter and basal nucleiLeft cerebralhemisphereCorpuscallosumNeocortexRight cerebralhemisphereBasalnucleiFigure 48.26The basal nucleiAre important centers for planning and learning movement sequencesIn mammalsThe cerebral cortex has a convoluted surface called the neocortexIn humans, the largest and most complex part of the brain Is the cerebral cortex, where sensory information is analyzed, motor commands are issued, and language is generated A thick band of axons, the corpus callosumProvides communication between the right and left cerebral corticesConcept 48.6: The cerebral cortex controls voluntary movement and cognitive functionsEach side of the cerebral cortex has four lobesFrontal, parietal, temporal, and occipitalFrontal lobeTemporal lobeOccipital lobeParietal lobeFrontalassociationareaSpeechSmellHearingAuditoryassociationareaVisionVisualassociationareaSomatosensoryassociationareaReadingSpeechTasteSomatosensory cortexMotor cortexFigure 48.27Each of the lobesContains primary sensory areas and association areasInformation Processing in the Cerebral CortexSpecific types of sensory inputEnter the primary sensory areasAdjacent association areasProcess particular features in the sensory input and integrate information from different sensory areasIn the somatosensory cortex and motor cortexNeurons are distributed according to the part of the body that generates sensory input or receives motor inputFigure 48.28 TongueJawLipsFaceEyeBrowNeckThumbFingersHandWristForearmElbowShoulderTrunkHipKneePrimarymotor cortexAbdominalorgansPharynxTongueTeethGumsJawLipsFaceNoseEyeFingersHandForearmElbowUpper armTrunkHipLegThumbNeckHeadGenitaliaPrimarysomatosensory cortexToesParietal lobeFrontal lobeLateralization of Cortical FunctionDuring brain development, in a process called lateralizationCompeting functions segregate and displace each other in the cortex of the left and right cerebral hemispheresThe left hemisphereBecomes more adept at language, math, logical operations, and the processing of serial sequencesThe right hemisphereIs stronger at pattern recognition, nonverbal thinking, and emotional processingLanguage and SpeechStudies of brain activityHave mapped specific areas of the brain responsible for language and speechFigure 48.29HearingwordsSeeingwordsSpeakingwordsGeneratingwordsMaxMinPortions of the frontal lobe, Broca’s area and Wernicke’s areaAre essential for the generation and understanding of languageEmotionsThe limbic system Is a ring of structures around the brainstemFigure 48.30HypothalamusThalamusPrefrontal cortexOlfactorybulbAmygdalaHippocampusThis limbic system includes three parts of the cerebral cortexThe amygdala, hippocampus, and olfactory bulbThese structures interact with the neocortex to mediate primary emotions And attach emotional “feelings” to survival-related functionsStructures of the limbic system form in early developmentAnd provide a foundation for emotional memory, associating emotions with particular events or experiences Memory and LearningThe frontal lobesAre a site of short-term memoryInteract with the hippocampus and amygdala to consolidate long-term memory Many sensory and motor association areas of the cerebral cortexAre involved in storing and retrieving words and images Cellular Mechanisms of LearningExperiments on invertebratesHave revealed the cellular basis of some types of learningFigure 48.31a, b(a) Touching the siphon triggers a reflex thatcauses the gill to withdraw. If the tail isshocked just before the siphon is touched,the withdrawal reflex is stronger. Thisstrengthening of the reflex is a simple formof learning called sensitization.(b) Sensitization involves interneurons thatmake synapses on the synaptic terminals ofthe siphon sensory neurons. When the tailis shocked, the interneurons releaseserotonin, which activates a signaltransduction pathway that closes K+channels in the synaptic terminals ofthe siphon sensory neurons. As a result,action potentials in the siphon sensoryneurons produce a prolongeddepolarization of the terminals. That allowsmore Ca2+ to diffuse into the terminals, which causes the terminals to release more of their excitatory neurotransmitter onto the gill motor neurons. In response, the motor neuronsgenerate action potentials at a higher frequency,producing a more forceful gill withdrawal. SiphonMantleGillTailHeadGill withdrawal pathwayTouchingthe siphonShockingthe tailTail sensoryneuronInterneuronSensitization pathwaySiphon sensoryneuronGill motorneuronGillIn the vertebrate brain, a form of learning called long-term potentiation (LTP)Involves an increase in the strength of synaptic transmissionFigure 48.32 PRESYNAPTIC NEURONNOGlutamateNMDAreceptorSignal transduction pathwaysNOCa2+AMPA receptorPOSTSYNAPTIC NEURON Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.5 Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).6 The presynapticneuron releases glutamate.1 Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.2 Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.3 Ca2+ diffuses into thepostsynaptic neuron.4 NO diffuses into thepresynaptic neuron, causing it to release more glutamate.7PConsciousnessModern brain-imaging techniques Suggest that consciousness may be an emergent property of the brain that is based on activity in many areas of the cortexConcept 48.7: CNS injuries and diseases are the focus of much researchUnlike the PNS, the mammalian CNSCannot repair itself when damaged or assaulted by diseaseCurrent research on nerve cell development and stem cellsMay one day make it possible for physicians to repair or replace damaged neuronsNerve Cell DevelopmentSignal molecules direct an axon’s growth By binding to receptors on the plasma membrane of the growth coneThis receptor binding triggers a signal transduction pathwayWhich may cause an axon to grow toward or away from the source of the signalFigure 48.33a, bMidline ofspinal cordDeveloping axonof interneuronGrowthconeNetrin-1receptorNetrin-1FloorplateCelladhesionmoleculesSlitreceptorSlitDeveloping axon of motor neuronNetrin-1receptorSlitreceptorSlitNetrin-11 Growth toward the floor plate.Cells in the floor plate of thespinal cord release Netrin-1, whichdiffuses away from the floor plateand binds to receptors on thegrowth cone of a developinginterneuron axon. Binding stimulatesaxon growth toward the floor plate.2 Growth across the mid-line.Once the axon reaches thefloor plate, cell adhesion moleculeson the axon bind to complementarymolecules on floor plate cells,directing the growth of the axonacross the midline.3 No turning back. Now the axon synthesizes receptors that bind to Slit, a repulsion protein re- leased by floor plate cells. This prevents the axon from growing back across the midline.Netrin-1 and Slit, produced by cellsof the floor plate, bind to receptorson the axons of motor neurons. Inthis case, both proteins act to repelthe axon, directing the motor neuronto grow away from the spinal cord.(a) Growth of an interneuron axon toward and across the midline of the spinal cord(diagrammed here in cross section)(b) Growth of a motor neuron axon awayfrom the midline of the spinal cordThe genes and basic events involved in axon guidanceAre similar in invertebrates and vertebratesKnowledge of these events may be applied one dayTo stimulate axonal regrowth following CNS damageNeural Stem CellsThe adult human brainContains stem cells that can differentiate into mature neuronsFigure 48.3410 mThe induction of stem cell differentiation and the transplantation of cultured stem cellsAre potential methods for replacing neurons lost to trauma or diseaseDiseases and Disorders of the Nervous SystemMental illnesses and neurological disordersTake an enormous toll on society, in both the patient’s loss of a productive life and the high cost of long-term health care SchizophreniaAbout 1% of the world’s populationSuffers from schizophreniaSchizophrenia is characterized byHallucinations, delusions, blunted emotions, and many other symptomsAvailable treatments have focused onBrain pathways that use dopamine as a neurotransmitterDepressionTwo broad forms of depressive illness are knownBipolar disorder and major depressionBipolar disorder is characterized byManic (high-mood) and depressive (low-mood) phasesIn major depressionPatients have a persistent low moodTreatments for these types of depression includeA variety of drugs such as Prozac and lithium Alzheimer’s DiseaseAlzheimer’s disease (AD)Is a mental deterioration characterized by confusion, memory loss, and other symptomsAD is caused by the formation ofNeurofibrillary tangles and senile plaques in the brainFigure 48.35Senile plaqueNeurofibrillary tangle20 mA successful treatment for AD in humansMay hinge on early detection of senile plaquesParkinson’s DiseaseParkinson’s disease is a motor disorderCaused by the death of dopamine-secreting neurons in the substantia nigraCharacterized by difficulty in initiating movements, slowness of movement, and rigidityThere is no cure for Parkinson’s diseaseAlthough various approaches are used to manage the symptoms