Chapter 12 – Introduction to the Nervous System

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Chapter 12 Introduction to the Nervous System. Organization Cell Types. Review. What 3 parts make up the nervous system? Brain Spinal cord Nerves. Functions of the Nervous System. - PowerPoint PPT Presentation


  • Chapter 12 Introduction to the Nervous System Organization Cell Types

  • ReviewWhat 3 parts make up the nervous system?

    Brain Spinal cordNerves


  • Functions of the Nervous SystemDetect changes (stimuli) in the internal or external environmentEvaluate the informationInitiate a change in muscles or glands

    Goal maintain homeostasis

    What does this remind you of??

  • Organization of the Nervous SystemCentral nervous system (CNS)Brain and spinal cordPeripheral nervous system (PNS)Nervous tissue in the outer regions of the nervous systemCranial nerves: originates in the brainSpinal nerves : originates from the spinal cordCentral fibers: extend from cell body towards the CNSPeripheral fibers: extend from cell body away from CNS


  • Afferent vs EfferentNervous pathways are organized into division based on the direction they carry informationAfferent division: incoming information (sensory)Efferent division: outgoing information (motor)(Efferent = Exit)

  • Somatic & Autonomic Nervous SystemsNervous pathways are also organized according to the type of effectors (organs) they regulateSomatic nervous system (SNS)Somatic sensory division (afferent)Somatic motor division (efferent)

  • Somatic & Autonomic Nervous Systems contAutonomic nervous system (ANS): Carry information to the autonomic or visceral effectors (smooth & cardiac muscles and glands)Visceral sensory division (afferent)Efferent pathwaysSympathetic division fight or flightParasympathic division rest and repair

  • Figure 12-2


  • ReviewWhat are the two main cell types in the nervous system?(Hint: we talked about this when we covered tissue types)

    Answer: neurons and glia

  • Cells of the Nervous SystemNeurons: excitable cells that conduct information

    Glia (also neuroglia or glial cells): support cells, do not conduct informationMost numerousGlia = glue

  • Types of GliaFive major types:AstrocytesMicrogliaEpendymal cellsOligodendrocytesSchwann cells

  • Astrocytes (12-3A)Star-shaped, largest, most numerousCell extension connect neurons and capillariesTransfer nutrients from blood to neuronHelp form blood-brain barrier (BBB)

  • Blood-Brain BarrierHelps maintain stable environment for normal brain functionfeet of astrocytes wrap around capillaries in brainRegulates passage of ionsWater, oxygen, CO2, glucose and alcohol pass freelyImportant for drug researchParkinsons Disease

  • Microglia (12-3B)Engulf and destroy cellular debris (phagocytosis)Enlarge during times of inflammation and degeneration

  • Ependymal cells (12-3C)Similar to epithelial cells Forms thin sheets that line the fluid-filled cavities of the brain and spinal cord Some cells help produce the fluid that fills these cavities (cerebral spinal fluid - CSF)Cilia may be present to help circulate fluid

  • Oligodendrocytes (12-3D)Hold nerve fibers togetherProduce myelin sheaths in CNS

  • Multiple Sclerosis (MS)Most common myelin disorderCharacterized by:myelin loss and destruction injury and death plaque like lesionsImpaired nerve conduction weakness, loss of coordination, vision and speech problemsRemissions & relapses Autoimmune or viral infectionWomen 20-40 yrsNo known cure

  • Multiple Sclerosis (MS)

  • Schwann cells (12-3E)Only in PNSSupport nerve fibers & form myelin sheathsSatellite cells (12-3G)Types of schwann cell that covers a neurons cell body


  • NeuronsAll neurons have 3 parts:Cell body (soma)AxonOne or more dendrites

  • Neuron AnatomySoma resembles other cellsNissl bodies part of rough ER; contain proteins necessary for nerve signal transmission & nerve regenerationDendrites branch out from soma; receptors; conduct impulse towards somaAxon process that extends from the soma at a tapered portion called the axon hillockAxon collaterals: side branchesTelodendria: distal branches of axonSynaptic knob: ends of telodendria


  • Neuron AnatomyMyelin sheaths: areas of insulation produced by Schwann cells; increases speed of nerve impulseMyelinated = white matterUnmyelinated = gray matterNodes of Ranvier: breaks in myelin sheath btwn Schwann cellsSynapse: junction btwn two neurons or btwn a neuron and an effector


  • Structural Classification of NeuronsMultipolarOne axon, several dendritesMost numerousBipolarOne axon, one dendriteLeast numerousRetina, inner ear, olfactory pathwayUnipolarAxon is a single process that branches into a central process (towards CNS) and a peripheral process (towards PNS)Dendrites at distal end of peripheral processAlways sensory neurons


  • Functional Classification of NeuronsAfferentSensoryTowards CNSEfferent MotorTowards muscles & glandsInterneuronsConnect afferent & efferent neuronsLie within CNS

  • Reflex Arc

  • Examples of Reflex ArcsIpsilateralContralateralintersegmental

  • Nerves vs TractsNerves bundles of parallel neurons held together by fibrous CT in the PNSTracts bundles of parallel neurons in the CNS

  • Nerve FibersRemember the difference between nerves and tracts?Endoneurium: surrounds each nerve fiberPerineurium: surrounds fascicles (bundles of nerve fibersEpineurium: surrounds a complete nerve (PNS) or tract (CNS)

  • Review: Gray vs White MatterWhite matter myelinated nerve fibersMyelin sheaths help increase the speed of an action potentialGray matter unmyelinated nerve fibers & cell bodiesGanglia: regions of gray matter in PNS

  • Nerve Fiber RepairNervous tissue has a limited repair capacity b/c mature neurons are incapable of cell divisionRepair can take place if soma and neurilemma remain intact

  • Steps of Nerve Fiber RepairInjuryDistal axon and myelin sheaths degeneratesRemaining neurilemma & endoneurium forms a tunnel from the injury to the effectorProteins produced in the nissl bodies help extend a new axon down the tunnel to the effector

  • Nerve ImpulsesNeurons are specialized to initiate and conduct signals nerve impulsesExhibit excitability & conductivityNerve impulse wave of electrical fluctuation that travels along the plasma membrane

  • Membrane PotentialsDifference in charges across the plasma membraneInside slightly negative Outside slightly positiveResult in a difference in electrical charges membrane potential Stored potential energyAnalogy = water behind a dam

  • Membrane PotentialsMembrane potential creates a polarized membraneMembrane has pole & + polePotential difference of a polarized membrane is measured in millivolts (mV)The sign indicates the charge of the inside of a polarized membrane

  • Resting Membrane Potential (RMP)When not conducting electrical signals, a membrane is resting-70mVRMP maintained by ionic imbalance across membraneSodium-Potassium PumpPumps 3 Na+ out for every 2 K+ pumps inCreates an electrical gradient (more positive on outside)

  • Resting Membrane Potential (RMP)

  • Local PotentialLocal potential - The slight shift away from the RMPIsolated to a particular region of the plasma membraneStimulus-gated Na+ channels open Na+ enters membrane potential to moves closer to zero (depolarization)Stimulus-gated K+ channels open K+ exits membrane potential away from zero (hyperpolarization)**Local potentials do not spread to the end of the axon**

  • Local Potentials

  • Action Potentials Definitions:Membrane potential of an active neuron (one that is conducting an impulseAction potential = nerve impulseAn electrical fluctuation that travels along the plasma membrane

  • Steps of Producing an Action Potential (table 12-1)A stimulus triggers stimulus-gated Na+ channels to open Na+ diffuses inside the cell depolarizationThreshold potential is reached (-59mV) voltage-gated Na+ channels open depolarization continuesAction potential peaks at +30mV, voltage-gated Na+ channels closeVoltage-gated K+ channels open K+ diffuses outward repolarizationBrief period of hyperpolarization (below -70mV) RMP is restored by Na+/K+ pump

  • Refractory Period

  • Refractory PeriodPeriod of time where the neuron resists restimulation (AP cannot fire)Absolute refractory period: half a millisecond after membrane reaches threshold potential Will not respond to ANY stimulusRelative refractory period: few milliseconds after absolute refractory period (during repolarization)Only respond to VERY strong stimulus

  • Refractory Period What does this mean?Greater stimulus = quicker another action potential can take placeThe magnitude of the stimulus does not affect the magnitude of the APb/c APs are all or nothingDoes cause proportional increase in frequencies of impulses

  • Conduction of an Action PotentialDuring the peak of an AP, the polarity reversesNegative outside, positive insideCauses impulse to travel from site of AP to adjacent plasma membraneNo fluctuation in AP due to all or nothing principle AP cannot travel backwards on axon due to refractory periods

  • Conduction of an Action PotentialHow does myelin sheaths affect the speed of an action potential?

    Sheaths prevent movement of ionsElectrical changes can only take place at Nodes of RanvierAPs leap from node to node (current flows under sheaths)Saltatory conduction

  • Random FactsIn nerve fibers that innervate skeletal muscle, impulses travel up to 130 m/s (300 mph)Sensory pathways from skin 0.5 m/s (
  • Types of SynapsesElectrical synapses: two cells joined end to end by gap junctionsEx: btwn cardiac muscle cells, smooth muscles cells

  • Types of SynapsesChemical synapses: use neurotransmitter to send a signal from a presynaptic cell to postsynaptic cell3 Parts:Synaptic knobSynaptic cleftPlasma membrane of postsynaptic neuron

  • Mechanisms of Synaptic TransmissionAP depolarizes synaptic knobVoltage-gated Ca2+ channels open Ca2+ diffuses inside the cellCa2+ triggers exocytosis of neurotransmitter vesiclesNTs diffuses across synaptic cleft bind w/ receptors on postsynaptic cell

  • Postsynaptic Potentials (Fig 12-22)Excitatory NTs cause Na+ and K+ channels to open depolarization excitatory postsynaptic potential (EPSP)

    Inhibitory NTs cause K+ and Cl- channels to open hyperpolarization inhibitory postsynaptic potential (IPSP)

  • SummationFor every postsynaptic cell there are usually 1K-100K synaptic knobsBoth excitatory & inhibitory NTs are releasedSummation of local potentials (EPSP & IPSP) occur at axon hillockEPSP > IPSP reach threshold action potentialEPSP < IPSP threshold not reached no AP

  • NeurotransmittersSmall-Molecule Transmitters:AcetylcholineAminesSerotoninDopamineEpinephrineNorepinephrineAmino AcidsGlutamateGABAGlycineLarge-Molecule Transmitters:NeuropeptideEndorphins

    Subdivided into smaller systems by location

    CNS center of the entire nervous system; integrates incoming sensory information, evaluates information, initiates outgoing responseIncludes only those cells that start and end in the central nervous system (brain and spinal cord)

    PNS Nerve cell extension nerve fibersOrganized according to the types of organs they innervate

    SNS regulate skeletal muscles (ex: touch hot pan)Somatic sensory division conduct information towards integrators of the CNS (afferent) Somatic motor division carry information to the somatic effectors (skeletal muscles) (efferent) Visceral sensory division: carries feedback information to the autonomic integrating centers of the CNSSympathetic division: pathways exit the middle portions of the spinal cord; prepare body to deal with immediate threats to the internal environment; fight or flightParasympathetic division: pathways exit at the brain or lower portions of the spinal cord; coordinates the bodys normal resting activities Random facts:900 billion glia in the human body (9x the number of stars in our galaxy)Glia retain their capacity for cell division throughout adulthood (neuron do not); susceptible to abnormalities in cell division; most tumors of the nervous system (both benign and malignant) originate in glial cells Similar to astrocytes, but w/ fewer branches**Nerves can be mixed meaning they contain both afferent (sensory) and efferent (motor) fibers. If the nerve is predominantly afferent, it is a sensory nerve; if it predominantly efferent, it is a motor nerve.