Chapter 3 Neural Pathwaysas alater bundle. The axons of the second order neuThe neural pathways are the routes formed byrons in thelamina IVand Vof the posterior homschains of neurons, through which sensory awarenesstraveluponeortwo segments and cross through thereaches the cerebral cortex and a motor response isanterior white commissure to the opposite lateral andfollowed.So,theneural pathway is commonly classi-anteriorfuniculustoform thelateraland anterorspified into sensory(ascending) pathways and motor(de-nothalamic tracts. The two tracts joins togetherin thescending)pathwaysaccordingly.brainstem as the spinothalamic tract, which joins theSection 1 Sensory(As-third-order neurons in the ventral posterolateral nucle-us(VPL) of the thalamus. The axons of the third ordercending) Pathwaysneurons run to the thalamocortical tract and passthrough the posterior limb ofthe internal capsule, andSensations occurred may be divided into fourthen project to the upper and middle parts of the post-types: superficial, deep, special and visceral. Superfi-central gyrus, and the posterior part of the paracentralcial sensation is concerned with pain, touch, tempera-lobule.ture and pressure. Deep sensation includes muscle,The sensations of pain and temperature are me-joint and ligamentposition(proprioception),two-diate by the lateral spinothalamic tract and the sensa-point discrimination and vibration sensation. Specialtions of rough touch and pressure are conducted bysensation includes smell,taste,etc.Visceral sensa-anterior spinothalamic tract.tions are relayed by autonomic afferent fibers andinclude nausea, hunger, somatic pressure, blood pres-Superficial sensory pathwayofsure, etc. The sensory pathways mainly include thesuperficial and deep sensory,visual and acoustic path-head and face(Fig.VI-3-1)ways.Generally speaking, three orders of neurons areinvolved in each path:the first-order neurons(lowerThe cell bodies of the first-order neurons are lo-sensory neurons),the second-order neurons(inter-cated in the trigeminal ganglion just to receive themediate neurons) and the third-order neurons(uppersuperficial sensations of the head and face, and theirsensory neurons).central processes enter the pons.The sensations ofpain and temperature terminate in the spinal nucleusI .Superficial Sensory Path-of the trigeminal nerve, while the sensations of tactileand pressure terminate in the pontine nucleus of thewaytrigeminal nerve. The axons of second order neurons,which situate in the spinal and pontine nuclei of thetrigeminal nerve, run across to the opposite side to1. Superficial sensory pathway ofform the trigeminal lemniscus,which runs throughtrunk and limbs(Fig. VI-3-1)the pons and the midbrain and terminates in theventral posteromedial nucleus of dorsal thalamus,The first-order neurons, with their neuronal cellthe third order neurons. The axons of the third orderbodies in the spinal ganglia, carry sensations from theneurons comprise the thalamocortical tract ascendingexteroceptors located in the skin of trunk and limbsthrough the posterior limb of the internal capsule toand enter the posterolateral sulcus of the spinal cordthe inferior part of the postcentral gyrus.·350.扫描全能王创建
Chapter 3 Neural Pathways · 351 .Postcentral gyrusInternal capsuleVPLnucleusInternalcapsuleVentral posterolateral nucleusVentral posteromedial nucleusSpinothalamic tractPontine nucleus of trigeminal nTrigeminal lemniscus(Trigemino thalamictract)0Spinal tractoftrigeminalnSpinal nucleus oftrigeminalnSpinal ganglionic cellubstantia gelatinosaateralspinothalamictractSpinal ganglionic cellAnterior spinothalamictractThe superficial sensory pathwaysFig. VI-3-1Conscious deep sensory pathwayI. Deep Sensory(or the Pro-of trunk and limbsprioceptive) PathwaysIt consists of three orders of neurons (Fig. VI-3-21).Thefirst-order neurons, with their cell bodies in the spi-Twopathways are involved(Fig.V-3-2): connal ganglia, carry sensations from the muscle, tendonwperiosteum and joint of the trunk and limbs, their cen-Postion sensations ofmuscle,joint and ligamentral processes enter the spinal cord as a medial bundle(thick myelinated fibers) through the dorsal roots ofythe spinal nerve and immediately divide into long as-ayamicodinamomcending branches and short descending branches. Theuiirmshort descending branches and the motor cells in theThedpywaydadanteriorhorn constitute the spinalrfex arc,whilethelong ascending branches form the fasciculus gracils,limbs is introduced here.which transmits deep sensation from the lower half扫描全能王创建
·352·PARTVINERVOUSSYSTEMthe medial lemniscus ascending along the midlinebody(below the fifth thoracic), and fasciculus cunea-.and posterior to pyramidal tract through the medullatus, which carries deep sensation from the upper halfoblongata, pons and midbrain to the ventral postero-body. Both the fasciculus gracilis and the fasciculuslateral nucleus(VPL) of the dorsal thalamus, the thirdcuneatus run upward in the ipsilateral posterior funic-order neurons. The third order neurons send out axonsulus and terminate in the gracile and cuneate nuclei,to form the thalamocortical tract, running through thewhere the second order neurons lie.The axons of theposterior limb of the internal capsule to the cortex ofsecond order neurons ventrally round the central greythe superior and middle pars of the postcentral gyrusmatter as the internal arcuatefibers,crossto theoppo-and posterior part of the paracentral lobule.site sideof medulla and takepart in theformation ofthe decussating of medial lemniscuses, and then formPostcentralgyrusIntermal capsuleVentral posterolateralVPLnucleusnucleus(VPLnucleus)Internal capsuleSpinothalamictract(Spinal lemniscus)Trigeminal lemniscus(Trigeminothalamic tract)Medial lemniscusLateral lemnisctAnterior spinocerebellar tractSuperior cerebellar peduncleMedial lemniscCerebellar cortex-InferiorcerebellarpeduncleGracile nucleusPosterior spinocerebellar tractCuneate nucleusMedial lemniscusDecussation ofmedial lemniscusFasciculus gracilisFasciculus cuneatusSpinal ganglionic cellSpinal ganglionic cellIntermediomedial nucleusPosteriorspinocerebellar tractAnterior spinocerebellartractNucleus thoracicus(Nucleus dorsalis)Fig. V-3-2The deep sensory pathways oftrunk and limbs扫描全能王创建
Chapter 3 Neural Pathways · 353 .esionovinascicuusgailisofsuthelateral geniculate body,where the third neuronsmare located. The third order neurons give rise to thesoptic radiation (or geniculocalcarine tract) that pass-hes through the posterior limb of the intermal capsuleesgomdamusshmto the calcarine cortex in the occipital lobe.A smallportion of the optic tract continues through brachiumaleatowof superior colliculus to the pretectal area and thesuperior colliculi, thebers of the superior colliculi2.Unconscious deep sensory pathwaydescend toform the tectospinal tract that isconcernedwith optic reflex.IconsistsowoordersoneuronsThefrsThe image in the visual field image is projecteddernuronheirelbdesareadihespinto the retina in reverse and inverted form. Light raysgnglawiththeireripheralprocesenddthefrom the temporal half of the visual field project toreorseosteumad jothe nasal half of the retina and rays from the bottomofthe trunk andlimb, and central processes enterof the object strike the upper retina.thespinalcordandterminateinththoracicnucleusHemianopia refers to loss of vision for one halfposterior grey horn and accessory cuneate nucleus.of the visual filed.Any interruption of the optic nerveThe secondary neurons conducting the unconsciousmay result in complete blindness in oneeye on thedeep sensory of trunk and lower limbs locate in thesame side,an optic chiasma lesion (sometimes own-thoracic nucleus and posterior grey horn, their axons,ing to a pituitary tumor)will result in bitemporalhemianopia, characterized by blindness in the lateralascending in the lateral funiculus as the anterior ortemporal halves of the visual fields of both eyes.posterior spinocerebellar tracts which enter the cere-Blindness in the nasal half of the field vision in onebellum by way of the superior and inferior cerebellareye can be produced by damage to the uncrossed fi- peduncles respectively. The axons of the secondarybers of the corresponding side of the chiasma. Lesionneurons, which conduct the unconscious deep sensoryin one side of the optic tract,optic radiation.or opticof neck and upper limbs and lie in the accessory cu-center produces blindness in the opposite halves ofneate nucleus of medulla oblongata, enter the cerebel-both visual fields.lum by way of the inferior cerebellarpeduncles.Pupillary light reflex I. Visual Pathway and Pu-pillary Light ReflexThat light shone on the retina of one eye causesboth pupils to constrict normally is known as papil-lary light reflex. The response in the stimulated eye iscalled the direct papillary light reflex, while responseI.Visual pathwayin the other eye is called the indirect(or consensual)The visual pathway also contains three orders ofpapillary light reflex. The first and secondary orderneurons (Fig. VI-3-3).The first order neurons, bipolarneurons of visual pathway are the same of pupillarycelsthmddeaythrinasaewihlight reflex. A small number of fibers leave the op-hadllutic tract and synapse bilaterally with preganglionictemal layeroftheretinaTheaxonsof scondorderparasympathetic neurons in the accessory nucleus ofrogginellwchloculomotor nerve(Edinger-Westphal nucleus).Effer-yadent impulses pass along parasympathetic fibers of theioculomotor nerve to the orbit where they synapse inkmathe ciliary ganglion. Postganglionic fibers pass to theraWiruseyeball to supply sphincter pupillae that reduces thethersfmashaivsothretiursize of the pupil when it contracts.WihfmmrahalvsortinLight shone on the retina of one eye with dam-eminingrodhtictrattmnt扫描全能王创建
·354·PARTVINERVOUSSYSTEMaged optic nerve can not cause either eye to constrict,strict. Pupil of eye with damaged oculomotor nervebut light on the healthy one makes both pupils con-does not constrict even light is shone on either pupil.4.Homonymoushemiano1.BlindnessofoiaofrightsidelefteyeDCD5.Blindness of rightTemporal hemianopialowerquadrant,ouof botheyesOO3.Nasal hemianopia6. Blidness of rightoflefteveupperquadrant,ouOpticn.Ciliary ganglionOpticchiasmaRetinaOculomotornOptictract5Iaoateral geniculatebodyCone cell9OculomotornucleusOpticradiationE-WnucleusPretectal areaSuperior colliculusVisualareaFig.VI-3-3A diagram of the visual and pupillary light refex pathways扫描全能王创建
Chapter 3 Neural Pathways · 355 .neurons cross along the ventral border of the pontinetegmentum to form the trapezoid body, which passIV . Auditory(Acoustic) Path-through or ventrally to the medial lemniscus, andwaythen cross the raphe to the opposite side to form anascending bundle known as the lateral lemniscus thatThe first-order neurons, bipolar cells in the coterminates directly or indirectly in the medial genic-haateithharganglion whchiulate body, where the third order neurons are located.vate the spiral organ of Corti (Fig, VI-3-4).CentralAxons of some second order neurons do not crossedprocesses of these neurons form the cochlear nerve,to join in the ipsilateral lateral lemniscus.The axonswhich pass through the internal acoustic meatus ac-of the third-order neurons join the acoustic radiation,companying wihthcranialneandsynaeand end in the transverse temporal gyri through thewith the second-neurons in the ventral and dorsalposterior limb of the intermal capsule.cochiear nuclei.Axons of majority of second-orderAuditory area(center(transversetemporalgyri)Acoustic radiatiMedial geniculate bodyLateral lemniscusMedial lemniscucochlearnucleusVentral cochleearnucleusSuperior olivary nucleusMedial lemniscusTrapezoid bodyCochlearnSpiral organngliorFig. VI-3-4 The acoustic pathwaythe ipsilateral acoustic paths does not cause a hearingBecause the acoustic center on one side receivesdefect.fbersfrmthbiaterachaucleidm扫描全能王创建
·356PARTVINERVOUSSYSTEMcells of anterior hom of the spinal cord; enteringthe cerebellum via the inferior or cerebellar peduncleV . Equilibratory Pathway connecting with the reticular formation of brainstem,vagus and glossopharyngeal nuclei; connecting withThe first-order neurons of this pathway are bipo-the temporal, parietal and frontal cortex of the cerebrallar neurons situated in the vestibular ganglion, whosehemisphere. Using these information transmitted,peripheral processes innervate the cristae ampullares,the cerebellum monitors and makes corrective ad-maculae of the utricle and saccule, and whose centraljustments in the motor activities that originate in theprocesses join the vestibular nerve and terminate incerebral cortex,causing the motor system to increasethe vestibular nucleus of the pons (Fig.V-3-5).Theor decrease its impulses to specific skeletal musclesvestibular nucleus contains the second-order neuronsin order to maintain static equilibrium and dynamicwhose axons are grouped into five courses:@ joiningequilibrium.the medial longitudinal fasciculus whose fibers endThey are concerned with motor function, and py-in theoculomotor,abducent andtrochlear nuclei,andramidal and extrapyramidal systems are included.the motor cells of the anterior horm or the upper cer-vical cord; ② joining the vestibulospinal tract to theculomotornucleusSTrochlear nucleusMedial longitudinal tractAbducentnucleusVestibular nerveibular nucleusVestibulospinal tractMedial longitudinal tractAAccessorynucleuFig.VI-3-5 The equilibratory pathwayover skeletal muscles by conveying the voluntarySection 2 Motor(De-motor impulses from the motor area of the cerebrum.scending)PathwaysTwo orders neurons are always involved in this sys-tem: the upper motorneurons and lower motorneu-The motor pathways are concerned with motorrons. The upper motor neurons in the precentral gyrusfunction, and include the pyramidal and extrapyrami-and anterior part of paracentral lobule are composeddal systems.of the giant pyramidal cells (Benz cells) and otherpyramidal cells.Their axons form the descending pyI.Pyramidal Systemramidal tract, among which, the fibers ending in thecranial motor nuclei are corticonuclear tract (or corThe Pyramidal System provides voluntary controlticobulbar tract) and those terminating at the anterior扫描全能王创建
Chapter 3 Neural Pathways · 357 .msdathinalacnucleus, trochlear nucleus, motor nucleus of trigem-owermotornurons incudethecraialmorcelsoinal nerve, nucleus ambiguous, nucleus of accessoryaintmapnlthpinadnerve and superior part of the facial nucleus, and tothe contralateral hypoglossal nucleus and the in-Corticospinal tractferior part of the facial nucleus in the course ofdescending though the genu of internal capsule andIt iscomposedof axons that arisingfrom theup-the brainstem (Fig.VI-3-6).All the cranial motorpermotorneurons ihesuperiorand middle partsonuclei(lower motornuclei)mentioned above givethe precentral gyrus and anterior part of the paracen-rise to axons joining the corresponding cranialtral lobule(Fig.VI-3-6)It passes through the postenerves that provide voluntary control of the inner-nor limbofintermalcapsule,theintemediate3/5vated skeletal muscles.thecrus cerebr,thebasilar partofthe pons, andtheBecause the inferior part of the facial nucleus andventral part of the medlla oblongata. About 75%tohypoglossal nucleus receivefibers justfromthecon90% of the corticospinal tract crosses to the oppositetrallateral corticonuclear tract, any injury of the uni-side to form the pyramidal decussating at the caudallateral corticonuclear tract usually results in the pa-end of the medulla oblongata, and then continues asralysis of the contralateral glossal muscles and facialthe lateral corticospinal tract, descending in the later-muscles belowthepalpebral fissure.This isdesignedal funiculus of the spinal cord. This tract terminatesas supranuclear paralysis(Fig. VI-3-7, 8), showingin the intermeurons and lower motor neurons in thethat the nasolabial fold in the unaffected side becomesipsilateral anterior gray horns of the spinal cord.Thesmooth but the bilateral wrinkles on the foreheadother10%to25%corticospinal tractdoes not cross inexist; angle of the mouth is drawn up on the affectthe pyramidal decussation but continues as the ante-ed side; the protruded tongue deviates to unaffectedrior corticospinal tract, descending in the anterior fu-side but without atrophy of two side of the tongueniculus of the spinal cord.Thefibersof anteriorcorParalysis of the unilateral facial muscles and glossalticospinal tract cross the median plane above the midcaused by an injury of the homolateral facialmusclesthoracic level in the anterior white commissure andnerve and hypoglossal nerve is termed as infranuclearsynapse directly orindirectly with the lowermotorparalysis(Fig. VI-3-7, 8) , showing that the wrinklesneurons located in the anterior grey horn of the spinalon the forehead in the affected side are smoothed out,giverise to the motorthe ipsilateral eye can not shut down voluntarily, thecord.Thelower motor neuronsfibers that leave the spinal cord through the anterioripsilateral nasolabial fold becomes smooth;angleofroots to control the skeletal muscles. Both lateral andthe mouth is drawn up on the unaffected side; the pro-anteriorcorticospinal tracts contain afew homolateraltruded tongue deviates to affected side because of thefibers also. The trunk muscles are controlled by bilat-action of thegenioglossus in theunaffected side.eral motor cortex of the hemisphere.Injury to the upper motor neurons leads to a lossII .Extrapyramidal Systemof motorfunctions without the muscles atrophy, butwithincreasedtonicityofmusclesandtendonreexThe extrapyramidal system is a common namefftdsdeastiparalysdfor the descending pathways besides the pyramidalathooicalruchasbinknaallsystem. It is the neural pathways which regulateadprawmadrathe tonicity of the muscles, coordinate the muscularand lossoftendonreflexand tonicity ofmuscle(softactivities,maintain the normal bodypostureand pro-paralysis) are usually associated with a lesionduce habitual and rhythmic movements. For example,of lower motor neurons.writing, swimming, and running are initiated in thebeginning by the pyramidal system, but are controlled2.Corticonuclear(Conticobulbarby extrapyramidal system when the motions laterbecome habitual and rhythmic. So, pyramidal systemtractand extrapyramidal system have the coordinated andAxons of some upper motor neurons,the pyramdependent functions with each other to produce coor-dinated, precise motions. Main circuits belong to thisgivrsehllateralsothebilateralulomrsystem are as follows:扫描全能王创建
·358·PARTVINERVOUSSYSTEMCaudate nucleusLentiform nucleusInternalcapsuleDorsal thalamusCorticonucleartra6Nucleus ofoculomotoNucleus oftrochlear n.MidbrainMotor nucleus of trigeminal n.PonsNucleus of facial n.Corticospinal tractNucleus of abducent nMedulla oblongataNucleusambiguusNucleus of hypoglossal n.DecussationofpyramidLateral corticospinaltractAnteriorcorticospinaltractSpinalcoFig. VI-3-6Thepyramidal systemmotor area of the cortex.Cortico-neostriatum dorsalthalamus-cortex circuit2.Neostriatum-substantia-nigracircuitThe cortical neurons of the frontal, parietal andtemporal lobes radiate fibers to the caudate nucleusThere are reciprocal projections between the cau-and putamen, and then terminate in the globus pall-date nucleus, putamen and the substantia nigra (Fig. VI-3idus,from where the fibers go to theventroanterior9).The substantianigra has theability of production andnucleus and ventrolateral nucleus of the dorsal thala-release ofdopamine, so degeneration of substantia nigramus,and then project through the intermal capsule toresults in decrease amount of dopamine. This is con-the somatomotorareas of thecortex(Fig.VI-3-9).Thiscerned with the occurrence of Parkinson's disease.circuit has the feedback regulatory inhibition on the扫描全能王创建
Chapter 3 Neural Pathways · 359 .Infranuclear paralysis Supranuclear paralysisInfranuclear paralysis_Supranuclear paralysisFig.VI-3-7Theparalysis offacial nerveFig. VI-3-8 The paralysis of hypoglossal nerveerebralcorteCaudatenucleus-Dorsal thalamusPutamenGlobus pallidusSubthalamic nucleusRed nucleusSubstantia nigraMidbraRubrospinaltractReticularformationMedullaoblongataReticulospinaltractSpinal cord-Motorcellofanterior homTheextrapyramidal systemFig.VI-3-9扫描全能王创建