Y khoa, y dược - Fundamentals of the nervous system and nervous tissue: Aart A

11 Fundamentals of the Nervous System and Nervous Tissue: Part AFunctions of the Nervous SystemSensory inputInformation gathered by sensory receptors about internal and external changes IntegrationInterpretation of sensory inputMotor outputActivation of effector organs (muscles and glands) produces a responseFigure 11.1Sensory inputMotor outputIntegrationDivisions of the Nervous SystemCentral nervous system (CNS) Brain and spinal cordIntegration and command center Peripheral nervous system (PNS)Paired spinal and cranial nerves carry messages to and from the CNSPeripheral Nervous System (PNS)Two functional divisionsSensory (afferent) divisionSomatic afferent fibers—convey impulses from skin, skeletal muscles, and joints Visceral afferent fibers—convey impulses from visceral organs Motor (efferent) division Transmits impulses from the CNS to effector organsMotor Division of PNSSomatic (voluntary) nervous systemConscious control of skeletal musclesMotor Division of PNSAutonomic (involuntary) nervous system (ANS)Visceral motor nerve fibersRegulates smooth muscle, cardiac muscle, and glandsTwo functional subdivisionsSympatheticParasympatheticFigure 11.2Central nervous system (CNS)Brain and spinal cordIntegrative and control centersPeripheral nervous system (PNS)Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body ParasympatheticdivisionConserves energyPromotes house-keeping functionsduring restMotor (efferent) divisionMotor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNSSomatic nervoussystemSomatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal musclesSympathetic divisionMobilizes bodysystems during activity Autonomic nervoussystem (ANS)Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glandsStructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNSSomatic sensoryfiber Visceral sensory fiberMotor fiber of somatic nervous systemSkinStomachSkeletalmuscleHeartBladderParasympathetic motor fiber of ANSSympathetic motor fiber of ANSHistology of Nervous TissueTwo principal cell typesNeurons—excitable cells that transmit electrical signalsHistology of Nervous TissueNeuroglia (glial cells)—supporting cells:Astrocytes (CNS)Microglia (CNS)Ependymal cells (CNS)Oligodendrocytes (CNS)Satellite cells (PNS)Schwann cells (PNS)AstrocytesMost abundant, versatile, and highly branched glial cellsCling to neurons, synaptic endings, and capillariesSupport and brace neuronsAstrocytesHelp determine capillary permeabilityGuide migration of young neuronsControl the chemical environmentParticipate in information processing in the brainFigure 11.3a(a) Astrocytes are the most abundantCNS neuroglia.CapillaryNeuronAstrocyteMicrogliaSmall, ovoid cells with thorny processesMigrate toward injured neuronsPhagocytize microorganisms and neuronal debrisFigure 11.3b(b) Microglial cells are defensive cells inthe CNS.NeuronMicroglialcellEpendymal CellsRange in shape from squamous to columnarMay be ciliatedLine the central cavities of the brain and spinal columnSeparate the CNS interstitial fluid from the cerebrospinal fluid in the cavitiesFigure 11.3cBrain orspinal cordtissueEpendymalcellsFluid-filled cavity(c) Ependymal cells line cerebrospinalfluid-filled cavities.OligodendrocytesBranched cellsProcesses wrap CNS nerve fibers, forming insulating myelin sheaths Figure 11.3d(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.NervefibersMyelin sheathProcess ofoligodendrocyteSatellite Cells and Schwann Cells Satellite cellsSurround neuron cell bodies in the PNSSchwann cells (neurolemmocytes)Surround peripheral nerve fibers and form myelin sheathsVital to regeneration of damaged peripheral nerve fibersFigure 11.3e(e) Satellite cells and Schwann cells (whichform myelin) surround neurons in the PNS.Schwann cells(forming myelin sheath)Cell body of neuronSatellitecellsNerve fiberNeurons (Nerve Cells)Special characteristics:Long-lived ( 100 years or more)Amitotic—with few exceptionsHigh metabolic rate—depends on continuous supply of oxygen and glucosePlasma membrane functions in:Electrical signaling Cell-to-cell interactions during development Cell Body (Perikaryon or Soma)Biosynthetic center of a neuronSpherical nucleus with nucleolusWell-developed Golgi apparatusRough ER called Nissl bodies (chromatophilic substance) Cell Body (Perikaryon or Soma)Network of neurofibrils (neurofilaments) Axon hillock—cone-shaped area from which axon arisesClusters of cell bodies are called nuclei in the CNS, ganglia in the PNSFigure 11.4bDendrites(receptive regions)Cell body(biosynthetic centerand receptive region)NucleolusNucleusNissl bodiesAxon(impulse generatingand conducting region)Axon hillockNeurilemmaTerminalbranches Node of RanvierImpulsedirectionSchwann cell(one inter-node)Axonterminals(secretoryregion)(b)ProcessesDendrites and axonsBundles of processes are called Tracts in the CNSNerves in the PNSDendritesShort, tapering, and diffusely branched Receptive (input) region of a neuronConvey electrical signals toward the cell body as graded potentials The AxonOne axon per cell arising from the axon hillockLong axons (nerve fibers)Occasional branches (axon collaterals)The AxonNumerous terminal branches (telodendria)Knoblike axon terminals (synaptic knobs or boutons) Secretory region of neuronRelease neurotransmitters to excite or inhibit other cellsAxons: FunctionConducting region of a neuronGenerates and transmits nerve impulses (action potentials) away from the cell bodyAxons: FunctionMolecules and organelles are moved along axons by motor molecules in two directions:Anterograde—toward axonal terminal Examples: mitochondria, membrane components, enzymesRetrograde—toward the cell body Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxinsFigure 11.4bDendrites(receptive regions)Cell body(biosynthetic centerand receptive region)NucleolusNucleusNissl bodiesAxon(impulse generatingand conducting region)Axon hillockNeurilemmaTerminalbranches Node of RanvierImpulsedirectionSchwann cell(one inter-node)Axonterminals(secretoryregion)(b)Myelin SheathSegmented protein-lipoid sheath around most long or large-diameter axonsIt functions to:Protect and electrically insulate the axonIncrease speed of nerve impulse transmissionMyelin Sheaths in the PNS Schwann cells wraps many times around the axon Myelin sheath—concentric layers of Schwann cell membrane Neurilemma—peripheral bulge of Schwann cell cytoplasmMyelin Sheaths in the PNS Nodes of Ranvier Myelin sheath gaps between adjacent Schwann cellsSites where axon collaterals can emergeFigure 11.5a(a) Myelination of a nervefiber (axon)Schwann cellcytoplasmAxonNeurilemmaMyelin sheathSchwann cellnucleusSchwann cellplasma membrane 123 A Schwann cellenvelopes an axon. The Schwann cell thenrotates around the axon, wrapping its plasma membrane loosely around it in successive layers. The Schwann cellcytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.Unmyelinated AxonsThin nerve fibers are unmyelinatedOne Schwann cell may incompletely enclose 15 or more unmyelinated axonsMyelin Sheaths in the CNSFormed by processes of oligodendrocytes, not the whole cellsNodes of Ranvier are presentNo neurilemmaThinnest fibers are unmyelinated Figure 11.3d(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.NervefibersMyelin sheathProcess ofoligodendrocyteWhite Matter and Gray MatterWhite matterDense collections of myelinated fibers Gray matterMostly neuron cell bodies and unmyelinated fibersStructural Classification of NeuronsThree types:Multipolar—1 axon and several dendritesMost abundantMotor neurons and interneuronsBipolar—1 axon and 1 dendriteRare, e.g., retinal neuronsStructural Classification of NeuronsUnipolar (pseudounipolar)—single, short process that has two branches:Peripheral process—more distal branch, often associated with a sensory receptorCentral process—branch entering the CNSTable 11.1 (1 of 3)Table 11.1 (2 of 3)Functional Classification of NeuronsThree types: Sensory (afferent)Transmit impulses from sensory receptors toward the CNSMotor (efferent)Carry impulses from the CNS to effectorsFunctional Classification of NeuronsInterneurons (association neurons)Shuttle signals through CNS pathways; most are entirely within the CNSTable 11.1 (3 of 3)