·288PART VI NERVOUS SYSTEMaresult,thepostsynapticneuronisactivatedandthimpulse is conducted from one neuron to the others.II.Reflex and Reflex ArcsMicrotubulesBasic components of reflex arcMitochondriaAreflex arc is the basic functional unit of thenervous systemTherefex archasfivebasiccompoVesiclesnents:a receptor respond to stimuli and producePresynapticnervous impulse, @ a sensory neuron transmits themembraneimpulse to the CNS, where the axons usually syn-apse with interneurons which synapse with motorPostsynapticSynaptic cleftmembraneneurons in the CNS; @ a motor neuron carry actionpotentials out of the CNS and through the PNS to Fig.VI-1-6Structures of the synapseeffector organs, such as muscles or glands, respond toarrives at thepresynaptic element, the neurotransmit-the action potentials (Fig. VI-1-7) . The response pro-terdiffuses across the synaptic cleft and bind to theduced by the reflex arc is called a reflex.receptor molecules in the postsynaptic membrane.AsDorsal rootDorsal rootganglionSensory(receptor)Peripheral nerveMotorresponse(musclefiber)Ventral rootSpinal cordFig. VI-1-7A schematic monosynaptic reflex arcis structurally the most differentiated, consisting oIII.Terminologyaxon, myelin and sheath of Schwann (Fig. VI-1-8).Myelinated fibers conduct more rapidly than un-myelinated ones. Speed of conduction is proportionalto the diameter of the fiber and the thickness of the1.Nerve fibermyelin sheath.The myelin sheath may be regarded asinsulation, whilethe extracellular space at the nodesA nerve fiber in the central nervous system con-of Ranvier and the periaxonal space provide readysists of the axon and the surrounding myelin sheathavenues for ionicdiffusion.or of the axon only in the case of unmyelinated fibers.In the peripheral nervous systembothmyelinatedand2. Nucleus and grey matterunmyelinated fibers have, in addition, an outer deli-cate nucleated membrane, the sheath of Schwann ornurilemmal sheath. The peripheral myelinated fiberAnucleus is a aggregation of neuronal cell bodies扫描全能王创建
Chapter1 General Description · 289 ·the cortex.The medullary substance or medulla is addsiarsadfcentral core of white matter beneath the cortex of theeariousinizadshmmypecerebrumand cerebellum.mmalh5.GanglionAxonfromanotherneruonIn the peripheral nervous system (PNS),the cellbodies are grouped together to form the ganglia. Sen--SynapseDendnitessory ganglion cells in dorsal roots of spinal nervesPerikaryonNissbodiesand some cranial nerves give off both central and pe-Axonhillockripheral processes, and do not have synapses on theirInitial segmentofaxoncell bodies, whilst ganglionic neurons of the visceralMyelin sheathOligodendrocytenervous system receive synaptic contacts from vari-SAxonous sources.Nodeof RanvierCENTRALNERVOUS SYSTEMPERIPHERALNERVOUSSYSTEM6.NerveSchwann cellIn the peripheral nervous system, a nerve is aCollateral branchbundle of nervefibers togetherwith supporting connective tissue.Most of nerves have a whitish appear-ance because of their myelin content Thereare threeconnective tissue sheaths in peripheral nerves.Theyare, from inside out, the endoneurium,perineuriumMotorend-platesand epineurium.Each nerve fiber is surrounded byareenforcing sheath of delicate connective tissue, theFig. V-1-8 Schematic drawing of a Nissl stainedendoneurium.It is continuous with the more abundantmotorneuronconnective tissue of the perineurium,which envelopsThe neuronal cell bodies and dendrites may formboth small and largebundles of fibers within a periph-more extensive layers or masses in the central ner-eral nerve trunk and divides the nerve into fascicles.vous system collectively called grey matter.The greyThe epineurium is the outermost sheath. This dense,matter looks grey because it consists of masses ofcollagenous layer forms an external connective tissuenerve cell bodies that contain pigment and organelles.ensheathment for all peripheral nerves (Fig. VI-1-9)Neuronal dendrites and synaptic activity are mostlyconfined to nuclei and areas of grey matter.Epineurium(aroundnerve and betweenfascicles)3. Tracts and white matterPerineurium(aroundeachBundles of nerve fibers in the central nervous sys-fascicle)tmoumraiInthbrainandspinalordconentationstrasEndoneuriumconsttutethewhitematter,socalledbecauseth(within fascicles)axons areofenensheathed inmyelin whichglistenswhite in the fresh state.Fig. VI-1-9The connective tissue sheaths in aCortex and medullary sub-4transversely sectioned nervestanceZhangjing（张长静）LiuZhiyu（刘执玉）The continuous sheet of grey matter coveringthesurfacoftherebumandrebelumialled扫描全能王创建