Y khoa, y dược - The central nervous system: Part C

12 The Central Nervous System: Part CFunctional Brain SystemsNetworks of neurons that work together and span wide areas of the brainLimbic systemReticular formationLimbic SystemStructures on the medial aspects of cerebral hemispheres and diencephalonIncludes parts of the diencephalon and some cerebral structures that encircle the brain stem Figure 12.18Corpus callosumSeptum pellucidumOlfactory bulbDiencephalic structuresof the limbic system•Anterior thalamic nuclei (flanking 3rd ventricle)•Hypothalamus•Mammillary body Fiber tractsconnecting limbic system structures•Fornix•Anterior commissureCerebral struc-tures of the limbic system•Cingulate gyrus•Septal nuclei•Amygdala•Hippocampus•Dentate gyrus•Parahippocampal gyrusLimbic SystemEmotional or affective brainAmygdala—recognizes angry or fearful facial expressions, assesses danger, and elicits the fear responseCingulate gyrus—plays a role in expressing emotions via gestures, and resolves mental conflictPuts emotional responses to odorsExample: skunks smell badLimbic System: Emotion and CognitionThe limbic system interacts with the prefrontal lobes, therefore:We can react emotionally to things we consciously understand to be happeningWe are consciously aware of emotional richness in our livesHippocampus and amygdala—play a role in memoryReticular FormationThree broad columns along the length of the brain stemRaphe nucleiMedial (large cell) group of nucleiLateral (small cell) group of nucleiHas far-flung axonal connections with hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cordReticular Formation: RAS and Motor FunctionRAS (reticular activating system) Sends impulses to the cerebral cortex to keep it conscious and alertFilters out repetitive and weak stimuli (~99% of all stimuli!)Severe injury results in permanent unconsciousness (coma)Reticular Formation: RAS and Motor FunctionMotor functionHelps control coarse limb movementsReticular autonomic centers regulate visceral motor functionsVasomotorCardiacRespiratory centersFigure 12.19VisualimpulsesReticular formationAscending generalsensory tracts(touch, pain, temperature)Descendingmotor projectionsto spinal cordAuditoryimpulsesRadiationsto cerebralcortexElectroencephalogram (EEG)Records electrical activity that accompanies brain functionMeasures electrical potential differences between various cortical areasFigure 12.20a(a) Scalp electrodes are used to record brain waveactivity (EEG). Brain WavesPatterns of neuronal electrical activity Generated by synaptic activity in the cortexEach person’s brain waves are uniqueCan be grouped into four classes based on frequency measured as Hertz (Hz)Types of Brain WavesAlpha waves (8–13 Hz)—regular and rhythmic, low-amplitude, synchronous waves indicating an “idling” brainBeta waves (14–30 Hz)—rhythmic, less regular waves occurring when mentally alert Theta waves (4–7 Hz)—more irregular; common in children and uncommon in adultsDelta waves (4 Hz or less)—high-amplitude waves seen in deep sleep and when reticular activating system is damped, or during anesthesia; may indicate brain damageFigure 12.20bAlpha waves—awake but relaxedBeta waves—awake, alertTheta waves—common in childrenDelta waves—deep sleep(b) Brain waves shown in EEGs fall intofour general classes.1-second intervalBrain Waves: State of the BrainChange with age, sensory stimuli, brain disease, and the chemical state of the bodyEEGs used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesionsA flat EEG (no electrical activity) is clinical evidence of deathEpilepsyA victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerkingEpilepsy is not associated with intellectual impairmentsEpilepsy occurs in 1% of the populationEpileptic SeizuresAbsence seizures, or petit malMild seizures seen in young children where the expression goes blankTonic-clonic (grand mal) seizuresVictim loses consciousness, bones are often broken due to intense contractions, may experience loss of bowel and bladder control, and severe biting of the tongueControl of EpilepsyAnticonvulsive drugsVagus nerve stimulators implanted under the skin of the chest can keep electrical activity of the brain from becoming chaoticConsciousnessConscious perception of sensationVoluntary initiation and control of movementCapabilities associated with higher mental processing (memory, logic, judgment, etc.)Loss of consciousness (e.g., fainting or syncopy) is a signal that brain function is impairedConsciousnessClinically defined on a continuum that grades behavior in response to stimuliAlertnessDrowsiness (lethargy)StuporComaSleepState of partial unconsciousness from which a person can be aroused by stimulationTwo major types of sleep (defined by EEG patterns)Nonrapid eye movement (NREM)Rapid eye movement (REM)SleepFirst two stages of NREM occur during the first 30–45 minutes of sleepFourth stage is achieved in about 90 minutes, and then REM sleep begins abruptlyFigure 12.21aAwake(a) Typical EEG patternsREM: Skeletal muscles (except ocular muscles and diaphragm) are actively inhibited; most dreaming occurs.NREM stage 1:Relaxation begins; EEG shows alpha waves, arousal is easy.NREM stage 2: IrregularEEG with sleep spindles (short high- amplitude bursts); arousal is more difficult.NREM stage 3: Sleep deepens; theta and delta waves appear; vital signs decline.NREM stage 4: EEG is dominated by delta waves; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur.Sleep PatternsAlternating cycles of sleep and wakefulness reflect a natural circadian (24-hour) rhythmRAS activity is inhibited during, but RAS also mediates, dreaming sleepThe suprachiasmatic and preoptic nuclei of the hypothalamus time the sleep cycleA typical sleep pattern alternates between REM and NREM sleepFigure 12.21b(b) Typical progression of an adult through onenight’s sleep stages AwakeREMStage 1Stage 2NonREMStage 3Stage 4Time (hrs)Importance of SleepSlow-wave sleep (NREM stages 3 and 4) is presumed to be the restorative stagePeople deprived of REM sleep become moody and depressedREM sleep may be a reverse learning process where superfluous information is purged from the brainDaily sleep requirements decline with ageStage 4 sleep declines steadily and may disappear after age 60Sleep DisordersNarcolepsy Lapsing abruptly into sleep from the awake stateInsomnia Chronic inability to obtain the amount or quality of sleep neededSleep apnea Temporary cessation of breathing during sleepLanguageLanguage implementation systemBasal nucleiBroca’s area and Wernicke’s area (in the association cortex on the left side)Analyzes incoming word sounds Produces outgoing word sounds and grammatical structuresCorresponding areas on the right side are involved with nonverbal language componentsMemoryStorage and retrieval of informationTwo stages of storageShort-term memory (STM, or working memory)—temporary holding of information; limited to seven or eight pieces of informationLong-term memory (LTM) has limitless capacityFigure 12.22Outside stimuliGeneral and special sensory receptorsData transferinfluenced by:ExcitementRehearsalAssociation ofold and new dataLong-termmemory(LTM)Data permanentlylost Afferent inputsRetrievalForgetForgetData selectedfor transferAutomaticmemoryData unretrievableTemporary storage(buffer) in cerebral cortexShort-termmemory (STM)Transfer from STM to LTMFactors that affect transfer from STM to LTMEmotional state—best if alert, motivated, surprised, and arousedRehearsal—repetition and practice Association—tying new information with old memories Automatic memory—subconscious information stored in LTMCategories of MemoryDeclarative memory (factual knowledge) Explicit informationRelated to our conscious thoughts and our language abilityStored in LTM with context in which it was learnedCategories of MemoryNondeclarative memory Less conscious or unconsciousAcquired through experience and repetition Best remembered by doing; hard to unlearnIncludes procedural (skills) memory, motor memory, and emotional memoryBrain Structures Involved in Declarative MemoryHippocampus and surrounding temporal lobes function in consolidation and access to memory ACh from basal forebrain is necessary for memory formation and retrievalFigure 12.23aSmellBasal forebrainPrefrontal cortexTasteThalamusTouchHearingVisionHippocampusThalamusPrefrontalcortexBasalforebrainAssociationcortexSensoryinputAChAChMedial temporal lobe(hippocampus, etc.)(a) Declarativememory circuitsBrain Structures Involved in Nondeclarative MemoryProcedural memoryBasal nuclei relay sensory and motor inputs to the thalamus and premotor cortexDopamine from substantia nigra is necessary Motor memory—cerebellumEmotional memory—amygdalaFigure 12.23bDopamineThalamusPremotorcortexSubstantianigraAssociationcortexBasalnucleiSensory andmotor inputsPremotorcortexThalamusSubstantia nigraBasal nuclei(b) Procedural (skills) memory circuitsMolecular Basis of MemoryDuring learning:Altered mRNA is synthesized and moved to axons and dendritesDendritic spines change shapeExtracellular proteins are deposited at synapses involved in LTMNumber and size of presynaptic terminals may increaseMore neurotransmitter is released by presynaptic neuronsMolecular Basis of MemoryIncrease in synaptic strength (long-term potentiation, or LTP) is crucialNeurotransmitter (glutamate) binds to NMDA receptors, opening calcium channels in postsynaptic terminalMolecular Basis of MemoryCalcium influx triggers enzymes that modify proteins of the postsynaptic terminal and presynaptic terminal (via release of retrograde messengers) Enzymes trigger postsynaptic gene activation for synthesis of synaptic proteins, in presence of CREB (cAMP response-element binding protein) and BDNF (brain-derived neurotrophic factor)Protection of the BrainBone (skull)Membranes (meninges)Watery cushion (cerebrospinal fluid)Blood-brain barrierMeningesCover and protect the CNSProtect blood vessels and enclose venous sinusesContain cerebrospinal fluid (CSF)Form partitions in the skullMeningesThree layersDura materArachnoid materPia materFigure 12.24Skin of scalpPeriosteumFalx cerebri(in longitudinalfissure only)Blood vesselArachnoid villusPia materArachnoid materDuramater MeningealPeriostealBone of skullSuperiorsagittal sinus Subduralspace Subarachnoidspace Dura MaterStrongest meninxTwo layers of fibrous connective tissue (around the brain) separate to form dural sinusesDura MaterDural septa limit excessive movement of the brainFalx cerebri—in the longitudinal fissure; attached to crista galliFalx cerebelli—along the vermis of the cerebellumTentorium cerebelli—horizontal dural fold over cerebellum and in the transverse fissureFigure 12.25aFalx cerebriSuperiorsagittal sinusStraightsinusCrista galliof theethmoid bonePituitarygland Falxcerebelli (a) Dural septaTentoriumcerebelliArachnoid MaterMiddle layer with weblike extensionsSeparated from the dura mater by the subdural spaceSubarachnoid space contains CSF and blood vesselsArachnoid villi protrude into the superior sagittal sinus and permit CSF reabsorptionFigure 12.24Skin of scalpPeriosteumFalx cerebri(in longitudinalfissure only)Blood vesselArachnoid villusPia materArachnoid materDuramater MeningealPeriostealBone of skullSuperiorsagittal sinus Subduralspace Subarachnoidspace Pia MaterLayer of delicate vascularized connective tissue that clings tightly to the brainCerebrospinal Fluid (CSF)CompositionWatery solution Less protein and different ion concentrations than plasmaConstant volumeCerebrospinal Fluid (CSF)FunctionsGives buoyancy to the CNS organsProtects the CNS from blows and other traumaNourishes the brain and carries chemical signals Figure 12.26aSuperiorsagittal sinusArachnoid villusSubarachnoid spaceArachnoid materMeningeal dura materPeriosteal dura materRight lateral ventricle(deep to cut)Choroid plexusof fourth ventricle Central canalof spinal cordChoroidplexus InterventricularforamenThird ventricleCerebral aqueductLateral apertureFourth ventricleMedian aperture(a) CSF circulation CSF is produced by thechoroid plexus of eachventricle. 1 CSF flows through theventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord.2 CSF flows through thesubarachnoid space. 3 CSF is absorbed into the dural venoussinuses via the arachnoid villi. 41234Choroid PlexusesProduce CSF at a constant rate Hang from the roof of each ventricleClusters of capillaries enclosed by pia mater and a layer of ependymal cellsEpendymal cells use ion pumps to control the composition of the CSF and help cleanse CSF by removing wastesFigure 12.26bEpendymalcells CapillaryConnectivetissue ofpia materWastes andunnecessarysolutes absorbedSectionof choroidplexus(b) CSF formation by choroid plexusesCavity ofventricleCSF forms as a filtratecontaining glucose, oxygen, vitamins, and ions(Na+, Cl–, Mg2+, etc.)Blood-Brain BarrierHelps maintain a stable environment for the brain Separates neurons from some bloodborne substancesBlood-Brain BarrierCompositionContinuous endothelium of capillary wallsBasal laminaFeet of astrocytesProvide signal to endothelium for the formation of tight junctionsFigure 11.3a(a) Astrocytes are the most abundantCNS neuroglia.CapillaryNeuronAstrocyteBlood-Brain Barrier: FunctionsSelective barrierAllows nutrients to move by facilitated diffusionAllows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics Absent in some areas, e.g., vomiting center and the hypothalamus, where it is necessary to monitor the chemical composition of the bloodHomeostatic Imbalances of the BrainTraumatic brain injuriesConcussion—temporary alteration in functionContusion—permanent damageSubdural or subarachnoid hemorrhage—may force brain stem through the foramen magnum, resulting in deathCerebral edema—swelling of the brain associated with traumatic head injuryHomeostatic Imbalances of the BrainCerebrovascular accidents (CVAs)(strokes)Blood circulation is blocked and brain tissue dies, e.g., blockage of a cerebral artery by a blood clotTypically leads to hemiplegia, or sensory and speed deficitsTransient ischemic attacks (TIAs)—temporary episodes of reversible cerebral ischemiaTissue plasminogen activator (TPA) is the only approved treatment for strokeHomeostatic Imbalances of the BrainDegenerative brain disordersAlzheimer’s disease (AD): a progressive degenerative disease of the brain that results in dementiaParkinson’s disease: degeneration of the dopamine-releasing neurons of the substantia nigraHuntington’s disease: a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei and cerebral cortex