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Chapter 12Chapter 12Nervous TissueNervous Tissue

Controls and integrates all body Controls and integrates all body activities within limits that maintain activities within limits that maintain lifelife

Three basic functionsThree basic functionssensing changes with sensory receptorssensing changes with sensory receptors

fullness of stomach or sun on your facefullness of stomach or sun on your faceinterpreting and remembering those interpreting and remembering those

changeschangesreacting to those changes with effectorsreacting to those changes with effectors

muscular contractionsmuscular contractionsglandular secretionsglandular secretions

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Major Structures of the Nervous Major Structures of the Nervous SystemSystem

Brain, cranial nerves, spinal cord, spinal nerves, Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptorsganglia, enteric plexuses and sensory receptors

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Organization of the Nervous Organization of the Nervous SystemSystem

CNS is brain and spinal cordCNS is brain and spinal cord PNS is everything elsePNS is everything else

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Nervous System DivisionsNervous System Divisions

Central nervous system (CNS) Central nervous system (CNS) consists of the brain and spinal cordconsists of the brain and spinal cord

Peripheral nervous system (PNS)Peripheral nervous system (PNS) consists of cranial and spinal nerves that consists of cranial and spinal nerves that

contain both sensory and motor fiberscontain both sensory and motor fibersconnects CNS to muscles, glands & all connects CNS to muscles, glands & all

sensory receptorssensory receptors

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Subdivisions of the PNSSubdivisions of the PNSSomatic (voluntary) nervous system (SNS)Somatic (voluntary) nervous system (SNS)

neurons from cutaneous and special sensory neurons from cutaneous and special sensory receptors to the CNSreceptors to the CNS

motor neurons to skeletal muscle tissuemotor neurons to skeletal muscle tissueAutonomic (involuntary) nervous systemsAutonomic (involuntary) nervous systems

sensory neurons from visceral organs to CNSsensory neurons from visceral organs to CNSmotor neurons to smooth & cardiacmotor neurons to smooth & cardiac muscle and muscle and

glandsglandssympathetic division (speeds up heart rate)sympathetic division (speeds up heart rate)parasympathetic division (slow down heart rate)parasympathetic division (slow down heart rate)

Enteric nervous system (ENS)Enteric nervous system (ENS)involuntary sensory & motor neurons control GI involuntary sensory & motor neurons control GI

tracttractneurons function independently of ANS & CNSneurons function independently of ANS & CNS

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NeuronsNeuronsFunctional unit of nervous systemFunctional unit of nervous systemHave capacity to produce action Have capacity to produce action

potentialspotentialselectrical excitabilityelectrical excitability

Cell bodyCell bodysingle nucleus with prominent nucleolussingle nucleus with prominent nucleolusNissl bodies (chromatophilic substance) Nissl bodies (chromatophilic substance)

rough ER & free ribosomes for protein synthesisrough ER & free ribosomes for protein synthesisneurofilaments give cell shape and supportneurofilaments give cell shape and supportmicrotubules move material inside cellmicrotubules move material inside celllipofuscin pigment clumps (harmless aging)lipofuscin pigment clumps (harmless aging)

Cell processes = dendrites & axonsCell processes = dendrites & axons

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Nucleus with Nucleolus

Parts of a NeuronParts of a Neuron

Axons or Dendrites

Cell body

Neuroglial cells

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DendritesDendrites

Conducts impulses Conducts impulses towards the cell bodytowards the cell body

Typically short, highly Typically short, highly branched & branched & unmyelinatedunmyelinated

Surfaces specialized for Surfaces specialized for contact with other contact with other neuronsneurons

Contains neurofibrils & Contains neurofibrils & Nissl bodiesNissl bodies

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AxonsAxons Conduct impulses Conduct impulses

away from cell bodyaway from cell body Long, thin cylindrical Long, thin cylindrical

process of cell process of cell Arises at axon hillock Arises at axon hillock Impulses arise from Impulses arise from

initial segment (trigger initial segment (trigger zone)zone)

Swollen tips called Swollen tips called synaptic end bulbs synaptic end bulbs contain vesicles filled contain vesicles filled with neurotransmitterswith neurotransmitters

Synaptic boutons

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Axonal TransportAxonal TransportCell body is location for most protein Cell body is location for most protein

synthesissynthesisneurotransmitters & repair proteins neurotransmitters & repair proteins

Axonal transport system moves substances Axonal transport system moves substances slow axonal flowslow axonal flow

movement at 1-5 mm per day movement at 1-5 mm per day movement in one direction only -- away from cell bodymovement in one direction only -- away from cell body

fast axonal flowfast axonal flow moves organelles & materials along surface of moves organelles & materials along surface of

microtubulesmicrotubules at 200-400 mm per dayat 200-400 mm per daytransports in either directiontransports in either directionfor use or for recycling in cell bodyfor use or for recycling in cell body

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Axonal Transport & DiseaseAxonal Transport & Disease

Fast axonal transport route by which Fast axonal transport route by which toxins or pathogens reach neuron cell toxins or pathogens reach neuron cell bodiesbodiestetanus (Clostridium tetani bacteria)tetanus (Clostridium tetani bacteria)disrupts motor neurons causing painful disrupts motor neurons causing painful

muscle spasmsmuscle spasmsBacteria enter the body through a Bacteria enter the body through a

laceration or puncture injurylaceration or puncture injurymore serious if wound is in head or neck more serious if wound is in head or neck

because of shorter transit timebecause of shorter transit time

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Functional Classification of Functional Classification of NeuronsNeurons

Sensory (afferent) neuronsSensory (afferent) neuronstransport sensory information from skin, transport sensory information from skin,

muscles, joints, sense organs & viscera to muscles, joints, sense organs & viscera to CNSCNS

Motor (efferent) neuronsMotor (efferent) neuronssend motor nerve impulses to muscles & send motor nerve impulses to muscles &

glandsglands Interneurons (association) neuronsInterneurons (association) neurons

connect sensory to motor neuronsconnect sensory to motor neurons90% of neurons in the body90% of neurons in the body

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Structural Classification of Structural Classification of NeuronsNeurons

Based on number of processes found on cell bodyBased on number of processes found on cell body multipolar = several dendrites & one axonmultipolar = several dendrites & one axon

most common cell typemost common cell type bipolar neurons = one main dendrite & one axonbipolar neurons = one main dendrite & one axon

found in retina, inner ear & olfactory found in retina, inner ear & olfactory unipolar neurons = one process only(develops from a unipolar neurons = one process only(develops from a

bipolar)bipolar)are always sensory neuronsare always sensory neurons

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Association or InterneuronsAssociation or Interneurons Named for histologist that first Named for histologist that first

described them or their appearancedescribed them or their appearance

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Half of the volume of the CNS Half of the volume of the CNS Smaller cells than neuronsSmaller cells than neurons50X more numerous50X more numerousCells can divide Cells can divide

rapid mitosis in tumor formation (gliomas)rapid mitosis in tumor formation (gliomas)4 cell types in CNS4 cell types in CNS

astrocytes, oligodendrocytes, microglia & ependymalastrocytes, oligodendrocytes, microglia & ependymal2 cell types in PNS2 cell types in PNS

schwann and satellite cellsschwann and satellite cells

Neuroglial CellsNeuroglial Cells

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AstrocytesAstrocytes

Star-shaped cellsStar-shaped cells Form blood-brain Form blood-brain

barrier by covering barrier by covering blood capillariesblood capillaries

Metabolize Metabolize neurotransmittersneurotransmitters

Regulate K+ balanceRegulate K+ balance Provide structural Provide structural

supportsupport

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OligodendrocytesOligodendrocytes

Most common Most common glial cell typeglial cell type

Each forms Each forms myelin sheath myelin sheath around more than around more than one axons in CNSone axons in CNS

Analogous to Analogous to Schwann cells of Schwann cells of PNSPNS

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MicrogliaMicroglia

Small cells found near blood vesselsSmall cells found near blood vessels Phagocytic role -- clear away dead cellsPhagocytic role -- clear away dead cells Derived from cells that also gave rise to Derived from cells that also gave rise to

macrophages & monocytesmacrophages & monocytes

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Ependymal cellsEpendymal cells

Form epithelial membrane lining cerebral Form epithelial membrane lining cerebral cavities & cavities & central canalcentral canal

Produce cerebrospinal fluid (CSF)Produce cerebrospinal fluid (CSF)

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Satellite CellsSatellite Cells

Flat cells surrounding neuronal cell bodies Flat cells surrounding neuronal cell bodies in peripheral gangliain peripheral ganglia

Support neurons in the PNS gangliaSupport neurons in the PNS ganglia

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Schwann CellSchwann Cell

Cells encircling PNS axonsCells encircling PNS axons Each cell produces part of the myelin Each cell produces part of the myelin

sheath surrounding an axon in the PNSsheath surrounding an axon in the PNS

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Axon Coverings in PNSAxon Coverings in PNS All axons surrounded by a lipid & protein covering All axons surrounded by a lipid & protein covering

(myelin sheath) produced by Schwann cells(myelin sheath) produced by Schwann cells Neurilemma is cytoplasm & nucleusNeurilemma is cytoplasm & nucleus

of Schwann cellof Schwann cell gaps called nodes of Ranviergaps called nodes of Ranvier

Myelinated fibers appear whiteMyelinated fibers appear white jelly-roll like wrappings made of jelly-roll like wrappings made of

lipoprotein = myelinlipoprotein = myelin acts as electrical insulatoracts as electrical insulator speeds conduction of nerve impulsesspeeds conduction of nerve impulses

Unmyelinated fibersUnmyelinated fibers slow, small diameter fibersslow, small diameter fibers only surrounded by neurilemma but no myelin sheath only surrounded by neurilemma but no myelin sheath

wrappingwrapping

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Gray and White MatterGray and White Matter

White matter = myelinated processes (white in color)White matter = myelinated processes (white in color) Gray matter = nerve cell bodies, dendrites, axon Gray matter = nerve cell bodies, dendrites, axon

terminals, bundles of unmyelinated axons and terminals, bundles of unmyelinated axons and neuroglia (gray color)neuroglia (gray color) In the spinal cord = gray matter forms an H-shaped inner In the spinal cord = gray matter forms an H-shaped inner

core surrounded by white mattercore surrounded by white matter In the brain = a thin outer shell of gray matter covers the In the brain = a thin outer shell of gray matter covers the

surface & is found in clusters called nuclei inside the CNSsurface & is found in clusters called nuclei inside the CNS

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Electrical Signals in NeuronsElectrical Signals in Neurons

Neurons are electrically excitable due to the Neurons are electrically excitable due to the voltage difference across their membrane voltage difference across their membrane

Communicate with 2 types of electric signalsCommunicate with 2 types of electric signals action potentials that can travel long distancesaction potentials that can travel long distances graded potentials that are local membrane graded potentials that are local membrane

changes onlychanges only In living cells, a flow of ions occurs through In living cells, a flow of ions occurs through

ion channels in the cell membraneion channels in the cell membrane

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Two Types of Ion ChannelsTwo Types of Ion ChannelsLeakage (nongated) channels are Leakage (nongated) channels are

always openalways openGated channels open and close in Gated channels open and close in

response to a stimulus results in response to a stimulus results in neuron excitabilityneuron excitability

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Gated Ion ChannelsGated Ion Channels

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Local AnestheticsLocal Anesthetics

Prevent opening of voltage-gated Prevent opening of voltage-gated Na+ channelsNa+ channels

Nerve impulses cannot pass the Nerve impulses cannot pass the anesthetized regionanesthetized region

Novocaine and lidocaineNovocaine and lidocaine

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Encoding of Stimulus Encoding of Stimulus IntensityIntensity

How do we differentiate a light touch How do we differentiate a light touch from a firmer touch?from a firmer touch?frequency of impulsesfrequency of impulses

firm pressure generates impulses at a firm pressure generates impulses at a higher frequencyhigher frequency

number of sensory neurons activatednumber of sensory neurons activatedfirm pressure stimulates more neurons than firm pressure stimulates more neurons than

does a light touchdoes a light touch

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Signal Transmission at Signal Transmission at SynapsesSynapses

2 Types of synapses2 Types of synapseselectrical electrical

ionic current spreads to next cell through gap ionic current spreads to next cell through gap junctionsjunctions

faster, two-way transmission & capable of faster, two-way transmission & capable of synchronizing groups of neuronssynchronizing groups of neurons

chemicalchemicalone-way information transfer from a presynaptic one-way information transfer from a presynaptic

neuron to a postsynaptic neuronneuron to a postsynaptic neuron

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Chemical SynapsesChemical Synapses Action potential reaches end Action potential reaches end

bulb and voltage-gated Ca+ 2 bulb and voltage-gated Ca+ 2 channels openchannels open

Ca+2 flows inward triggering Ca+2 flows inward triggering release of neurotransmitterrelease of neurotransmitter

Neurotransmitter crosses Neurotransmitter crosses synaptic cleft & binding to synaptic cleft & binding to ligand-gated receptors ligand-gated receptors the more neurotransmitter the more neurotransmitter

released the greater the change released the greater the change in potential of the postsynaptic in potential of the postsynaptic cellcell

Synaptic delay is 0.5 msecSynaptic delay is 0.5 msec One-way information transferOne-way information transfer

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Removal of Removal of NeurotransmitterNeurotransmitter

DiffusionDiffusion move down concentration move down concentration

gradientgradient Enzymatic degradationEnzymatic degradation

acetylcholinesteraseacetylcholinesterase Uptake by neurons or glia Uptake by neurons or glia

cellscells neurotransmitter transportersneurotransmitter transporters Prozac = serotonin reuptake Prozac = serotonin reuptake

inhibitorinhibitor

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Strychnine PoisoningStrychnine Poisoning

In spinal cord, Renshaw cells normally In spinal cord, Renshaw cells normally release an inhibitory neurotransmitter release an inhibitory neurotransmitter (glycine) onto motor neurons (glycine) onto motor neurons preventing excessive muscle preventing excessive muscle contractioncontraction

Strychnine binds to and blocks glycine Strychnine binds to and blocks glycine receptors in the spinal cordreceptors in the spinal cord

Massive tetanic contractions of all Massive tetanic contractions of all skeletal muscles are producedskeletal muscles are producedwhen the diaphragm contracts & remains when the diaphragm contracts & remains

contracted, breathing can not occurcontracted, breathing can not occur

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Neurotransmitter EffectsNeurotransmitter Effects

Neurotransmitter effects can be modifiedNeurotransmitter effects can be modifiedsynthesis can be stimulated or inhibitedsynthesis can be stimulated or inhibitedrelease can be blocked or enhancedrelease can be blocked or enhancedremoval can be stimulated or blocked removal can be stimulated or blocked receptor site can be blocked or activatedreceptor site can be blocked or activated

AgonistAgonistanything that enhances a transmitters effects anything that enhances a transmitters effects

AntagonistAntagonistanything that blocks the action of a anything that blocks the action of a

neurotranmitterneurotranmitter

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Small-Molecule Small-Molecule NeurotransmittersNeurotransmitters

Acetylcholine (ACh)Acetylcholine (ACh)released by many PNS neurons & some CNSreleased by many PNS neurons & some CNSexcitatory on NMJ but inhibitory at othersexcitatory on NMJ but inhibitory at othersinactivated by acetylcholinesteraseinactivated by acetylcholinesterase

Amino AcidsAmino Acidsglutamate released by nearly all excitatory glutamate released by nearly all excitatory

neurons in the brain ---- inactivated by neurons in the brain ---- inactivated by glutamate specific transportersglutamate specific transporters

GABA is inhibitory neurotransmitter for 1/3 of GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)-- enhancing its inhibitory effect)

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Biogenic AminesBiogenic Aminesmodified amino acids (tyrosine)modified amino acids (tyrosine)

norepinephrine -- regulates mood, dreaming, norepinephrine -- regulates mood, dreaming, awakening from deep sleepawakening from deep sleep

dopamine -- regulating skeletal muscle tonedopamine -- regulating skeletal muscle toneserotonin -- control of mood, temperature serotonin -- control of mood, temperature

regulation, & induction of sleepregulation, & induction of sleepremoved from synapse & recycled or removed from synapse & recycled or

destroyed by enzymes (monoamine destroyed by enzymes (monoamine oxidase or catechol-0-methyltransferase)oxidase or catechol-0-methyltransferase)

Small-Molecule Small-Molecule Neurotransmitters (2)Neurotransmitters (2)

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NeuropeptidesNeuropeptides

3-40 amino acids linked by peptide bonds3-40 amino acids linked by peptide bondsSubstance P -- enhances our perception Substance P -- enhances our perception

of painof painPain reliefPain relief

enkephalins -- pain-relieving effect by enkephalins -- pain-relieving effect by blocking the release of substance P blocking the release of substance P

acupuncture may produce loss of pain acupuncture may produce loss of pain sensation because of release of opioids-like sensation because of release of opioids-like substances such as endorphins or substances such as endorphins or dynorphinsdynorphins

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Regeneration & RepairRegeneration & Repair

Plasticity maintained throughout lifePlasticity maintained throughout lifesprouting of new dendritessprouting of new dendritessynthesis of new proteinssynthesis of new proteinschanges in synaptic contacts with other changes in synaptic contacts with other

neuronsneuronsLimited ability for regeneration (repair)Limited ability for regeneration (repair)

PNS can repair damaged dendrites or PNS can repair damaged dendrites or axonsaxons

CNS no repairs are possibleCNS no repairs are possible

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Neurogenesis in the CNSNeurogenesis in the CNSFormation of new neurons from stem cells Formation of new neurons from stem cells

was not thought to occur in humanswas not thought to occur in humans1992 a growth factor was found that stimulates 1992 a growth factor was found that stimulates

adult mice brain cells to multiplyadult mice brain cells to multiply1998 new neurons found to form within adult 1998 new neurons found to form within adult

human hippocampus (area important for learning)human hippocampus (area important for learning)Factors preventing neurogenesis in CNSFactors preventing neurogenesis in CNS

inhibition by neuroglial cells, absence of growth inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and stimulating factors, lack of neurolemmas, and rapid formation of scar tissuerapid formation of scar tissue

3939

Axons & dendrites may be repaired ifAxons & dendrites may be repaired ifneuron cell body remains intactneuron cell body remains intactschwann cells remain active and form a tube schwann cells remain active and form a tube scar tissue does not form too rapidlyscar tissue does not form too rapidly

ChromatolysisChromatolysis24-48 hours after injury, Nissl bodies break 24-48 hours after injury, Nissl bodies break

up into fine granular massesup into fine granular masses

Repair within the PNSRepair within the PNS

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Multiple Sclerosis (MS)Multiple Sclerosis (MS)Autoimmune disorder causing Autoimmune disorder causing

destruction of myelin sheaths in CNSdestruction of myelin sheaths in CNSsheaths becomes scars or plaquessheaths becomes scars or plaques1/2 million people in the United States1/2 million people in the United Statesappears between ages 20 and 40 appears between ages 20 and 40 females twice as often as malesfemales twice as often as males

Symptoms include muscular weakness, Symptoms include muscular weakness, abnormal sensations or double visionabnormal sensations or double vision

Remissions & relapses result in Remissions & relapses result in progressive, cumulative loss of functionprogressive, cumulative loss of function

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The second most common neurological The second most common neurological disorder disorder affects 1% of populationaffects 1% of population

Characterized by short, recurrent attacks Characterized by short, recurrent attacks initiated by electrical discharges in the braininitiated by electrical discharges in the brainlights, noise, or smells may be sensedlights, noise, or smells may be sensedskeletal muscles may contract involuntarilyskeletal muscles may contract involuntarilyloss of consciousnessloss of consciousness

Epilepsy has many causes, including;Epilepsy has many causes, including;brain damage at birth, metabolic disturbances, brain damage at birth, metabolic disturbances,

infections, toxins, vascular disturbances, head infections, toxins, vascular disturbances, head injuries, and tumors injuries, and tumors

EpilepsyEpilepsy

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Neuronal Structure & Neuronal Structure & FunctionFunction