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

12 The Central Nervous System: Part D The Spinal Cord: Embryonic DevelopmentBy week 6, there are two clusters of neuroblastsAlar plate—will become interneurons; axons form white matter of cordBasal plate—will become motor neurons; axons will grow to effectorsNeural crest cells form the dorsal root ganglia sensory neurons; axons grow into the dorsal aspect of the cordFigure 12.28WhitematterNeural tubecellsCentralcavityAlar plate:interneuronsDorsal root ganglion: sensoryneurons from neural crestBasal plate:motor neuronsSpinal CordLocationBegins at the foramen magnum Ends as conus medullaris at L1 vertebraFunctionsProvides two-way communication to and from the brainContains spinal reflex centersSpinal Cord: ProtectionBone, meninges, and CSFCushion of fat and a network of veins in the epidural space between the vertebrae and spinal dura mater CSF in subarachnoid spaceSpinal Cord: ProtectionDenticulate ligaments: extensions of pia mater that secure cord to dura materFilum terminale: fibrous extension from conus medullaris; anchors the spinal cord to the coccyxFigure 12.30LigamentumflavumSupra-spinousligamentLumbar punctureneedle enteringsubarachnoidspaceFilumterminaleInter-vertebraldiscT12L5Cauda equinain subarachnoidspaceDuramaterL5L4S1ArachnoidmatterFigure 12.29aCervicalenlargementDura andarachnoidmaterLumbarenlargementConusmedullarisCaudaequinaFilumterminaleCervicalspinal nervesLumbarspinal nervesSacralspinal nervesThoracicspinal nerves(a) The spinal cord and its nerve roots, with the bony vertebral arches removed. The dura mater and arachnoid mater are cut open and reflected laterally.Spinal CordSpinal nerves31 pairs Cervical and lumbar enlargementsThe nerves serving the upper and lower limbs emerge hereCauda equinaThe collection of nerve roots at the inferior end of the vertebral canalCross-Sectional AnatomyTwo lengthwise grooves divide cord into right and left halves Ventral (anterior) median fissure Dorsal (posterior) median sulcus Gray commissure—connects masses of gray matter; encloses central canalFigure 12.31a(a) Cross section of spinal cord and vertebraEpidural space(contains fat)Pia materSpinalmeninges Arachnoidmater Dura materBone ofvertebraSubdural spaceSubarachnoidspace(contains CSF)Dorsal rootganglionBodyof vertebraFigure 12.31b(b) The spinal cord and its meningeal coveringsDorsal funiculusDorsal median sulcusCentral canalVentral medianfissure Pia materArachnoid materSpinal dura materGraycommissure Dorsal hornGraymatterLateral hornVentral hornVentral funiculusLateral funiculusWhitecolumnsDorsal rootganglionDorsal root(fans out into dorsal rootlets)Ventral root(derived from severalventral rootlets)Spinal nerveGray MatterDorsal horns—interneurons that receive somatic and visceral sensory inputVentral horns—somatic motor neurons whose axons exit the cord via ventral rootsLateral horns (only in thoracic and lumbar regions) –sympathetic neuronsDorsal root (spinal) gangia—contain cell bodies of sensory neuronsFigure 12.32Somaticsensoryneuron Dorsal root (sensory)Dorsal root ganglionVisceralsensory neuronSomaticmotor neuron Spinal nerveVentral root(motor) Ventral horn(motor neurons)Dorsal horn (interneurons)Visceralmotorneuron Interneurons receiving input from somatic sensory neuronsInterneurons receiving input from visceral sensory neuronsVisceral motor (autonomic) neuronsSomatic motor neuronsWhite Matter Consists mostly of ascending (sensory) and descending (motor) tractsTransverse tracts (commissural fibers) cross from one side to the otherTracts are located in three white columns (funiculi on each side—dorsal (posterior), lateral, and ventral (anterior)Each spinal tract is composed of axons with similar functionsPathway GeneralizationsPathways decussate (cross over)Most consist of two or three neurons (a relay)Most exhibit somatotopy (precise spatial relationships)Pathways are paired symmetrically (one on each side of the spinal cord or brain)Figure 12.33Ascending tractsDescending tractsFasciculus gracilisDorsalwhitecolumnFasciculus cuneatusDorsalspinocerebellar tractLateralspinothalamic tract Ventral spinothalamictractVentral whitecommissure Lateralcorticospinal tract Lateralreticulospinal tractVentral corticospinaltract Medialreticulospinal tractRubrospinaltract Vestibulospinal tractTectospinal tractVentralspinocerebellartractAscending PathwaysConsist of three neuronsFirst-order neuronConducts impulses from cutaneous receptors and proprioceptorsBranches diffusely as it enters the spinal cord or medullaSynapses with second-order neuron Ascending PathwaysSecond-order neuronInterneuronCell body in dorsal horn of spinal cord or medullary nucleiAxons extend to thalamus or cerebellumAscending PathwaysThird-order neuronInterneuronCell body in thalamus Axon extends to somatosensory cortexAscending PathwaysTwo pathways transmit somatosensory information to the sensory cortex via the thalamusDorsal column-medial lemniscal pathwaysSpinothalamic pathwaysSpinocerebellar tracts terminate in the cerebellum Dorsal Column-Medial Lemniscal PathwaysTransmit input to the somatosensory cortex for discriminative touch and vibrationsComposed of the paired fasciculus cuneatus and fasciculus gracilis in the spinal cord and the medial lemniscus in the brain (medulla to thalamus)Figure 12.34a (2 of 2)Medulla oblongataFasciculus cuneatus(axon of first-order sensory neuron)Fasciculus gracilis(axon of first-order sensory neuron)Axon offirst-orderneuronMuscle spindle(proprioceptor)Joint stretchreceptor(proprioceptor)Cervical spinal cordTouchreceptorMedial lemniscus (tract)(axons of second-order neurons)Dorsalspinocerebellartract (axons ofsecond-orderneurons)Nucleus gracilisNucleus cuneatusLumbar spinal cord(a)Spinocerebellarpathway Dorsal column–mediallemniscal pathway Figure 12.34a (1 of 2)PrimarysomatosensorycortexAxons of third-orderneuronsThalamusCerebrumMidbrainCerebellumPons(a)Spinocerebellarpathway Dorsal column–mediallemniscal pathway Anterolateral PathwaysLateral and ventral spinothalamic tracts Transmit pain, temperature, and coarse touch impulses within the lateral spinothalamic tractFigure 12.34b (2 of 2)Axons of first-orderneuronsTemperaturereceptorsLateralspinothalamictract (axons ofsecond-orderneurons)Pain receptorsMedulla oblongataCervical spinal cordLumbar spinal cord(b)Spinothalamic pathwayFigure 12.34b (1 of 2)PrimarysomatosensorycortexAxons of third-orderneuronsThalamusCerebrumMidbrainCerebellumPons(b)Spinothalamic pathwaySpinocerebellar TractsVentral and dorsal tractsConvey information about muscle or tendon stretch to the cerebellumFigure 12.34a (2 of 2)Medulla oblongataFasciculus cuneatus(axon of first-order sensory neuron)Fasciculus gracilis(axon of first-order sensory neuron)Axon offirst-orderneuronMuscle spindle(proprioceptor)Joint stretchreceptor(proprioceptor)Cervical spinal cordTouchreceptorMedial lemniscus (tract)(axons of second-order neurons)Dorsalspinocerebellartract (axons ofsecond-orderneurons)Nucleus gracilisNucleus cuneatusLumbar spinal cord(a)Spinocerebellarpathway Dorsal column–mediallemniscal pathway Figure 12.34a (1 of 2)PrimarysomatosensorycortexAxons of third-orderneuronsThalamusCerebrumMidbrainCerebellumPons(a)Spinocerebellarpathway Dorsal column–mediallemniscal pathway Descending Pathways and TractsDeliver efferent impulses from the brain to the spinal cord Direct pathways—pyramidal tractsIndirect pathways—all othersDescending Pathways and TractsInvolve two neurons:Upper motor neuronsPyramidal cells in primary motor cortexLower motor neuronsVentral horn motor neuronsInnervate skeletal musclesThe Direct (Pyramidal) SystemImpulses from pyramidal neurons in the precentral gyri pass through the pyramidal (corticospinal)l tracts Axons synapse with interneurons or ventral horn motor neurons The direct pathway regulates fast and fine (skilled) movementsFigure 12.35a (1 of 2)Primary motor cortexInternal capsuleCerebralpeduncleMidbrainCerebellumCerebrumPons(a)Pyramidal cells(upper motor neurons)Pyramidal (lateral and ventral corticospinal) pathwaysFigure 12.35a (2 of 2)Medulla oblongataCervical spinal cordSkeletalmuscle PyramidsDecussationof pyramidLateralcorticospinaltractVentralcorticospinaltractLumbar spinal cordSomatic motor neurons(lower motor neurons)(a)Pyramidal (lateral and ventral corticospinal) pathwaysIndirect (Extrapyramidal) SystemIncludes the brain stem motor nuclei, and all motor pathways except pyramidal pathwaysAlso called the multineuronal pathwaysIndirect (Extrapyramidal) SystemThese pathways are complex and multisynaptic, and regulate:Axial muscles that maintain balance and postureMuscles controlling coarse movements Head, neck, and eye movements that follow objectsIndirect (Extrapyramidal) SystemReticulospinal and vestibulospinal tracts—maintain balanceRubrospinal tracts—control flexor musclesSuperior colliculi and tectospinal tracts mediate head movements in response to visual stimuliFigure 12.35b (1 of 2)MidbrainCerebellumCerebrumRed nucleusPonsRubrospinal tract(b)Figure 12.35b (2 of 2)Medulla oblongataCervical spinal cordRubrospinal tractRubrospinal tract(b)Spinal Cord TraumaFunctional lossesParasthesiasSensory lossParalysisLoss of motor functionSpinal Cord TraumaFlaccid paralysis—severe damage to the ventral root or ventral horn cellsImpulses do not reach muscles; there is no voluntary or involuntary control of musclesMuscles atrophySpinal Cord TraumaSpastic paralysis—damage to upper motor neurons of the primary motor cortex Spinal neurons remain intact; muscles are stimulated by reflex activityNo voluntary control of musclesSpinal Cord TraumaTransectionCross sectioning of the spinal cord at any levelResults in total motor and sensory loss in regions inferior to the cutParaplegia—transection between T1 and L1Quadriplegia—transection in the cervical regionPoliomyelitisDestruction of the ventral horn motor neurons by the poliovirusMuscles atrophyDeath may occur due to paralysis of respiratory muscles or cardiac arrestSurvivors often develop postpolio syndrome many years later, as neurons are lost Amyotrophic Lateral Sclerosis (ALS)Also called Lou Gehrig’s diseaseInvolves progressive destruction of ventral horn motor neurons and fibers of the pyramidal tractSymptoms—loss of the ability to speak, swallow, and breatheDeath typically occurs within five yearsLinked to glutamate excitotoxicity, attack by the immune system, or bothDevelopmental Aspects of the CNSCNS is established during the first month of developmentGender-specific areas appear in both brain and spinal cord, depending on presence or absence of fetal testosteroneMaternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the developing CNSSmoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damageDevelopmental Aspects of the CNSThe hypothalamus is one of the last areas of the CNS to developVisual cortex develops slowly over the first 11 weeksNeuromuscular coordination progresses in superior-to-inferior and proximal-to-distal directions along with myelination Developmental Aspects of the CNSAge brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80sShrinkage of brain accelerates in old ageExcessive use of alcohol causes signs of senility unrelated to the aging process