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