Cns2007

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ORGANIZATION ORGANIZATION Summary Summary Central nervous system Central nervous system Brain Brain Spinal cord Spinal cord Peripheral nervous system Peripheral nervous system Sensory division Sensory division Motor division Motor division Somatic nervous system Somatic nervous system Autonomic nervous system Autonomic nervous system Sympathetic division Sympathetic division Parasympathetic division Parasympathetic division

description

 

Transcript of Cns2007

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ORGANIZATIONORGANIZATIONSummarySummary Central nervous systemCentral nervous system

• BrainBrain• Spinal cordSpinal cord

Peripheral nervous systemPeripheral nervous system• Sensory divisionSensory division• Motor divisionMotor division

Somatic nervous systemSomatic nervous system Autonomic nervous systemAutonomic nervous system

• Sympathetic divisionSympathetic division• Parasympathetic divisionParasympathetic division

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HISTOLOGYHISTOLOGY Nervous system consists mainly of nervous Nervous system consists mainly of nervous

tissuetissue Highly cellularHighly cellular

• e.g., <20% extracellular space in CNSe.g., <20% extracellular space in CNS Two principal cell typesTwo principal cell types

• NeuronsNeurons Excitable nerve cells that transmit electrical signalsExcitable nerve cells that transmit electrical signals

• Supporting cellsSupporting cells Smaller cells surrounding and wrapping neuronsSmaller cells surrounding and wrapping neurons ““Neuroglia”Neuroglia”

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NEUROGLIANEUROGLIA ““Nerve glue”Nerve glue” Six types of small cells associated with neuronsSix types of small cells associated with neurons

• 4 in CNS4 in CNS• 2 in PNS2 in PNS

Most have central cell body and branching Most have central cell body and branching processesprocesses

Several functionsSeveral functions• e.g., Supportive scaffolding for neuronse.g., Supportive scaffolding for neurons• e.g., Electrical isolation of neuronse.g., Electrical isolation of neurons• e.g., Neuron health and growthe.g., Neuron health and growth

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CNS NEUROGLIACNS NEUROGLIA AstrocytesAstrocytes MicrogliaMicroglia Ependymal cellsEpendymal cells OligodendrocytesOligodendrocytes

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CNS NEUROGLIACNS NEUROGLIAAstrocytesAstrocytes Most abundant and versatile glial cellsMost abundant and versatile glial cells Numerous processes support branching neuronsNumerous processes support branching neurons

• Anchor neurons to capillary blood supplyAnchor neurons to capillary blood supply Guide migration of young neuronsGuide migration of young neurons Facilitate nutrient delivery to neuronsFacilitate nutrient delivery to neurons

• (blood (blood glial cell glial cell neuron) neuron) Control chemical environment Control chemical environment

around neuronsaround neurons• Uptake of KUptake of K++, neurotransmitters, neurotransmitters

Communicate with astrocytes Communicate with astrocytes & neurons& neurons• Gap junctions, CaGap junctions, Ca2+2+ surges surges

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CNS NEUROGLIACNS NEUROGLIAMicrogliaMicroglia Small ovoid cellsSmall ovoid cells Relatively long “thorny” Relatively long “thorny”

processesprocesses• Processes touch nearby neuronsProcesses touch nearby neurons

““Checking vitals”Checking vitals” Migrate toward injured neuronsMigrate toward injured neurons Transform into macrophageTransform into macrophage

• Phagocytize microorganisms, debrisPhagocytize microorganisms, debris• (Cells of immune system cannot enter the CNS)(Cells of immune system cannot enter the CNS)

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CNS NEUROGLIACNS NEUROGLIAEpendymal CellsEpendymal Cells Line central cavities of brain and spinal cordLine central cavities of brain and spinal cord

• Form permeable barrier between cerebrospinal fluid inside Form permeable barrier between cerebrospinal fluid inside these cavities and tissue fluid of CNS tissuethese cavities and tissue fluid of CNS tissue

Shapes range from squamous to columnarShapes range from squamous to columnar Many are ciliatedMany are ciliated

• Beating helps circulate cerebrospinal fluid cushioning brain Beating helps circulate cerebrospinal fluid cushioning brain and spinal cordand spinal cord

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CNS NEUROGLIACNS NEUROGLIAOligodendrocytesOligodendrocytes Fewer processes than astrocytesFewer processes than astrocytes Wrap processes tightly around thicker neuron Wrap processes tightly around thicker neuron

fibers in CNSfibers in CNS• ““Myelin sheath”Myelin sheath”• Insulating coveringInsulating covering

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PNS NEUROGLIAPNS NEUROGLIA Satellite cellsSatellite cells Schwann cellsSchwann cells

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PNS NEUROGLIAPNS NEUROGLIASatellite cellsSatellite cells Surround neuron cell bodies within gangliaSurround neuron cell bodies within ganglia

• (A ganglion is a collection of nerve cell bodies (A ganglion is a collection of nerve cell bodies outside of the CNS)outside of the CNS)

Function poorly understoodFunction poorly understood

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PNS NEUROGLIAPNS NEUROGLIASchwann cellsSchwann cells a.k.a., “Neurolemmocytes”a.k.a., “Neurolemmocytes” Surround and form myelin sheaths around larger nerve Surround and form myelin sheaths around larger nerve

fibers of PNSfibers of PNS• Functionally similar to oligodendrocytesFunctionally similar to oligodendrocytes

Vital to regeneration of peripheral nerve fibersVital to regeneration of peripheral nerve fibers

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NEURONSNEURONS a.k.a., Nerve cellsa.k.a., Nerve cells Structural units of nervous systemStructural units of nervous system

• Billions are present in nervous systemBillions are present in nervous system Conduct messages throughout bodyConduct messages throughout body

• Nerve impulsesNerve impulses Extreme longevityExtreme longevity

• Can function optimally for entire lifetimeCan function optimally for entire lifetime AmitoticAmitotic

• Ability to divide is lost in mature cellsAbility to divide is lost in mature cells• Cannot be replaced if destroyedCannot be replaced if destroyed

Some (very few) exceptionsSome (very few) exceptions e.g., stem cells present in olfactory epithelium can produce new neuronse.g., stem cells present in olfactory epithelium can produce new neurons

• Stem cell research shows great promise in repairing damaged neuronsStem cell research shows great promise in repairing damaged neurons High metabolic rateHigh metabolic rate

• Require large amounts of oxygen and glucoseRequire large amounts of oxygen and glucose

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NeuronsNeurons

Axon of anotherneuron

Cell BodyDendrites

Axon

MyelinSheath

Dendrites of another neuron

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NEURONSNEURONS Generally large, complex cellsGenerally large, complex cells Structures vary, but all neurons have the same basic Structures vary, but all neurons have the same basic

structurestructure• Cell bodyCell body• Slender processes Slender processes

extending from cell extending from cell bodybody

• Plasma membrane Plasma membrane is site of signalingis site of signaling

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NEURON CELL BODYNEURON CELL BODY Most neuron cell bodies are located in the CNSMost neuron cell bodies are located in the CNS

• Protected by bones of skull or vertebral columnProtected by bones of skull or vertebral column Clusters of cell bodies in the CNS are termed Clusters of cell bodies in the CNS are termed

“nuclei”“nuclei” Clusters of cell Clusters of cell

bodies in the bodies in the PNS are PNS are termed termed “ganglia”“ganglia”

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NEURON CELL BODYNEURON CELL BODY a.k.a., “perikaryon” or “soma”a.k.a., “perikaryon” or “soma” 5 – 140 5 – 140 m in diameterm in diameter Transparent spherical nucleusTransparent spherical nucleus

• Contains Contains conspicuous conspicuous nucleolusnucleolus

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NEURON CELL BODYNEURON CELL BODY Major biosynthetic center of neuronMajor biosynthetic center of neuron Other usual organelles presentOther usual organelles present

• ER & ribosomes most active and best developed in bodyER & ribosomes most active and best developed in body What do they do?What do they do?

• Centrioles absentCentrioles absent What do centrioles What do centrioles

do?do?

• Sometimes contains Sometimes contains pigment inclusionspigment inclusions

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NEURON CELL BODYNEURON CELL BODY Focal point for the outgrowth of neuron processes Focal point for the outgrowth of neuron processes

during embryonic developmentduring embryonic development• Some processes receive signalsSome processes receive signals• Plasma membrane Plasma membrane

generally also acts generally also acts as part of the as part of the receptive surfacereceptive surface

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NEURON PROCESSESNEURON PROCESSES Extend from the neuron’s cell bodyExtend from the neuron’s cell body CNS contains both neuron cell bodies and their CNS contains both neuron cell bodies and their

processesprocesses• Bundles of CNS processes are termed “tracts”Bundles of CNS processes are termed “tracts”

PNS consists mainly of neuronal processesPNS consists mainly of neuronal processes• Bundles of PNS processes are termed “nerves”Bundles of PNS processes are termed “nerves”

Two types of neuron processesTwo types of neuron processes• DendritesDendrites• AxonsAxons

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NEURON PROCESSESNEURON PROCESSESTypical DendriteTypical Dendrite Short, tapering, diffusely branching extensionsShort, tapering, diffusely branching extensions

• Generally hundreds clustering close to cell bodyGenerally hundreds clustering close to cell body• Most cell body organelles also present in dendritesMost cell body organelles also present in dendrites

Main receptive / input regionsMain receptive / input regions• Large surface area for Large surface area for

receiving signals from receiving signals from other neuronsother neurons

• Convey incoming messages Convey incoming messages toward cell bodytoward cell body

• Short-distance signals are Short-distance signals are “graded potentials”“graded potentials”

Not action potentialsNot action potentials

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Single axon per neuronSingle axon per neuron ““Axon hillock” of cell body narrows to form a slender Axon hillock” of cell body narrows to form a slender

process of uniform diameterprocess of uniform diameter Sometimes very shortSometimes very short Sometimes very longSometimes very long

• e.g., axons controlling e.g., axons controlling big toe are 3 – 4 feet big toe are 3 – 4 feet longlong

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Single axon may branch along lengthSingle axon may branch along length ““Axon collaterals” extend from neurons at ~ 90Axon collaterals” extend from neurons at ~ 90oo angles angles Usually branches Usually branches

profusely at endprofusely at end• 10,000 or more 10,000 or more

terminal branches terminal branches is commonis common

• Distal endings Distal endings termed “axonal termed “axonal terminals”terminals”

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Conducting component of neuronConducting component of neuron Generates nerve impulsesGenerates nerve impulses

• Generated at axon hillock / axon junction in motor neuronsGenerated at axon hillock / axon junction in motor neurons• ““Trigger zone”Trigger zone”

Transmits nerve Transmits nerve impulses away from impulses away from cell bodycell body• To axonal terminalsTo axonal terminals

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Axonal terminals are Axonal terminals are secretory componentsecretory component of neuron of neuron Sequence of eventsSequence of events

• Signal reaches terminalsSignal reaches terminals• Membranes of vesicles fuse with Membranes of vesicles fuse with

plasma membraneplasma membrane ““Axolemma”Axolemma”

• Neurotransmitters releasedNeurotransmitters released• Neurotransmitters interact Neurotransmitters interact

with either other neurons with either other neurons or effector cellsor effector cells

Excite or inhibitExcite or inhibit

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Contains most of the same organelles found in dendrites Contains most of the same organelles found in dendrites

and cell bodyand cell body• Lacks ER and Golgi apparatusLacks ER and Golgi apparatus• What do these What do these

organelles do?organelles do?• Must rely on cell Must rely on cell

body to renew what?body to renew what?

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Rely on cell body for some moleculesRely on cell body for some molecules Rely on efficient transport mechanisms for deliveryRely on efficient transport mechanisms for delivery

• AnterogradeAnterograde movement toward axonal terminals movement toward axonal terminals e.g., Mitochondria, e.g., Mitochondria,

membrane components, membrane components, neurotransmitters or neurotransmitters or enzymes required for enzymes required for neurotransmitter neurotransmitter synthesis, etc.synthesis, etc.

• RetrogradeRetrograde movement movement toward cell bodytoward cell body

e.g., Organelles being e.g., Organelles being returned for recyclingreturned for recycling

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NEURON PROCESSESNEURON PROCESSESTypical AxonTypical Axon Some viruses and bacterial toxins use retrograde Some viruses and bacterial toxins use retrograde

transport to reach the cell bodytransport to reach the cell body• e.g., poliovirus, rabies virus, herpes simplex viruses, tetanus e.g., poliovirus, rabies virus, herpes simplex viruses, tetanus

toxin, etc.toxin, etc. Such viruses can be Such viruses can be

used as vehicles for the used as vehicles for the therapeutic delivery of therapeutic delivery of engineered DNAengineered DNA• ““Gene therapy”Gene therapy”

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MYELIN SHEATHMYELIN SHEATH Whitish, fatty covering of many nerve fibers Whitish, fatty covering of many nerve fibers

• Particularly those long are large in diameterParticularly those long are large in diameter Protects and electrically insulates fibersProtects and electrically insulates fibers Increases speed of nerve impulse transmissionIncreases speed of nerve impulse transmission

• Some axons and all dendrites are unmyelinatedSome axons and all dendrites are unmyelinated

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MYELIN SHEATHMYELIN SHEATH In PNS, myelin sheaths formed by Schwann cellsIn PNS, myelin sheaths formed by Schwann cells

• Continually wrap around nerveContinually wrap around nerve• Cytoplasm gradually squeezed from intracellular spaceCytoplasm gradually squeezed from intracellular space• Result is many concentric layers of plasma membrane Result is many concentric layers of plasma membrane

surrounding the axonsurrounding the axon These plasma membranes contain little proteinThese plasma membranes contain little protein

• Some proteins present interlock adjacent membranesSome proteins present interlock adjacent membranes

• Thickness depends on number of wrappingsThickness depends on number of wrappings Nucleus and most of Nucleus and most of

cytoplasm exist as a bulge cytoplasm exist as a bulge external to the myelin sheathexternal to the myelin sheath• ““Neurilemma”Neurilemma”

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MYELIN SHEATHMYELIN SHEATH Adjacent Schwann cells on axon do not touch Adjacent Schwann cells on axon do not touch

each othereach other• Gaps in sheath occur at regular intervals Gaps in sheath occur at regular intervals

““Nodes of Ranvier”Nodes of Ranvier” a.k.a., “Neurofibril a.k.a., “Neurofibril

nodes”nodes”• Axon collaterals Axon collaterals

can emerge at can emerge at these nodesthese nodes

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MYELIN SHEATHMYELIN SHEATH CNS contains both myelinated and unmyelinated CNS contains both myelinated and unmyelinated

axonsaxons• Those long are large in diameter are typically Those long are large in diameter are typically

myelinatedmyelinated Oligodendrocytes, not Schwann cells, form CNS Oligodendrocytes, not Schwann cells, form CNS

myelin sheathsmyelin sheaths• Oligodendrocytes possess numerous processes that Oligodendrocytes possess numerous processes that

can coil around numerous (up to 60) axons at once can coil around numerous (up to 60) axons at once CNS myelin sheaths lack a neurilemmaCNS myelin sheaths lack a neurilemma

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MYELIN SHEATHMYELIN SHEATH White matterWhite matter

• Regions of the brain and spinal cord containing Regions of the brain and spinal cord containing dense collections of myelinated fibersdense collections of myelinated fibers

Gray matterGray matter• Regions of the brain and spinal cord containing Regions of the brain and spinal cord containing

mostly nerve cell bodies and unmyelinated fibersmostly nerve cell bodies and unmyelinated fibers

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NEURON CLASSIFICATIONNEURON CLASSIFICATION Structural classification based upon number of Structural classification based upon number of

processesprocesses• Multipolar neuronsMultipolar neurons• Bipolar neuronsBipolar neurons• Unipolar neuronsUnipolar neurons

Functional classification based upon direction Functional classification based upon direction nerve impulse travelsnerve impulse travels• Sensory (afferent) neuronsSensory (afferent) neurons• Motor (efferent) neuronsMotor (efferent) neurons• Interneurons (association neurons)Interneurons (association neurons)

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NEURON CLASSIFICATIONNEURON CLASSIFICATIONStructural ClassificationStructural Classification Multipolar neuronsMultipolar neurons

• Three or more processesThree or more processes• Most common neuron Most common neuron

type in humanstype in humans (> 99% of neurons)(> 99% of neurons)

Bipolar neuronsBipolar neurons• Two processes – axon and Two processes – axon and

dendritedendrite• Found only in some Found only in some

special sense organs special sense organs e.g., retina of eyee.g., retina of eye

• Act as receptor cellsAct as receptor cells

Unipolar neuronsUnipolar neurons• Single short processSingle short process• ““Pseudounipolar neurons”Pseudounipolar neurons”

Originate as bipolar neuronsOriginate as bipolar neurons Two processes converge and Two processes converge and

fuse fuse • Process divides into proximal Process divides into proximal

and distal branchesand distal branches Distal process often associated Distal process often associated

with a sensory receptorwith a sensory receptor• ““Peripheral process”Peripheral process”

Central process enters CNSCentral process enters CNS• Most are sensory neurons in Most are sensory neurons in

PNSPNS

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Classification of neurons Classification of neurons by shapeby shape

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NEURON CLASSIFICATIONNEURON CLASSIFICATIONFunctional ClassificationFunctional Classification Sensory (afferent) neuronsSensory (afferent) neurons

• Transmit impulses Transmit impulses towardtoward CNS CNS From sensory receptors or internal From sensory receptors or internal

organsorgans• Most are unipolarMost are unipolar• Cell bodies are located outside Cell bodies are located outside

CNSCNS Motor (efferent) neuronsMotor (efferent) neurons

• Carry impulses Carry impulses away fromaway from CNS CNS Toward effector organsToward effector organs

• MultipolarMultipolar• Cell bodies generally located in Cell bodies generally located in

the CNSthe CNS

Interneurons Interneurons • a.k.a., association a.k.a., association

neuronsneurons• Lie between motor and Lie between motor and

sensory neurons in sensory neurons in neural pathwaysneural pathways

• Shuttle signals through Shuttle signals through CNS pathways where CNS pathways where integration occursintegration occurs

• > 99% of neurons in > 99% of neurons in body body

• Most are multipolarMost are multipolar• Most are confined Most are confined

within the CNSwithin the CNS

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NEUROPHYSIOLOGYNEUROPHYSIOLOGY Neurons are highly irritableNeurons are highly irritable

• Responsive to stimuliResponsive to stimuli Response to stimulus is action potentialResponse to stimulus is action potential

• Electrical impulse carried along length of axonElectrical impulse carried along length of axon• Always the same regardless of stimulusAlways the same regardless of stimulus• The underlying functional feature of the nervous The underlying functional feature of the nervous

systemsystem

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ION CHANNELSION CHANNELSPlasma membranes contain various ion channelsPlasma membranes contain various ion channels Passive channels (leakage channels)Passive channels (leakage channels)

• Always openAlways open Active channels (gated channels)Active channels (gated channels)

• Ligand-gated channelsLigand-gated channels Open when specific chemical bindsOpen when specific chemical binds

• Voltage-gated channelsVoltage-gated channels Open and close in response to membrane potentialOpen and close in response to membrane potential

• Mechanically-gated channelsMechanically-gated channels Open in response to physical deformation of receptorOpen in response to physical deformation of receptor

• e.g., touch and pressure receptorse.g., touch and pressure receptors

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ION CHANNELSION CHANNELS Channels are specific as to Channels are specific as to

what type of ions are what type of ions are allowed to passallowed to pass• e.g., Ke.g., K++ channels allow only channels allow only

KK++ to pass to pass Ions moving through open Ions moving through open

channels follow their channels follow their electrochemical gradientselectrochemical gradients• Electrical current is Electrical current is

generatedgenerated• Voltage changes across the Voltage changes across the

membranemembrane

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MEMBRANE POTENTIALSMEMBRANE POTENTIALS A voltage exists across the plasma membraneA voltage exists across the plasma membrane

• Due to separation of oppositely charged ionsDue to separation of oppositely charged ions Potential difference in a resting membrane is Potential difference in a resting membrane is

termed its termed its “resting membrane potential”“resting membrane potential”• ~ -70 mV in a resting ~ -70 mV in a resting

neuronneuron• Membrane is Membrane is

“polarized”“polarized”

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MEMBRANE POTENTIALSMEMBRANE POTENTIALS Resting potential exists across the membraneResting potential exists across the membrane

• Majority of NaMajority of Na++ outside of cell outside of cell• Majority of KMajority of K++ inside of cell inside of cell

Resting membraneResting membrane• Only slightly permeable to NaOnly slightly permeable to Na++

• 75 times more permeable to K75 times more permeable to K++

• How do these ions cross the membrane?How do these ions cross the membrane?

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MEMBRANE POTENTIALSMEMBRANE POTENTIALS Neurons use changes in membrane potentials as Neurons use changes in membrane potentials as

signalssignals• Used to receive, integrate, and send signalsUsed to receive, integrate, and send signals

Changes in membrane potentials produced byChanges in membrane potentials produced by• Anything changing membrane permeability to ionsAnything changing membrane permeability to ions• Anything altering ion concentrationsAnything altering ion concentrations

Two types of signalsTwo types of signals• Graded potentialsGraded potentials

Short-distance signalsShort-distance signals• Action potentialsAction potentials

Long-distance signalsLong-distance signals

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MEMBRANE POTENTIALSMEMBRANE POTENTIALS Changes in membrane potentials are caused by Changes in membrane potentials are caused by

three eventsthree events DepolarizationDepolarization

• Inside of membrane becomes less negativeInside of membrane becomes less negative• Nerve impulses Nerve impulses moremore likely to be produced likely to be produced

RepolarizationRepolarization• Membrane returns to resting membrane potentialMembrane returns to resting membrane potential

HyperpolarizationHyperpolarization• Inside of membrane becomes more negative than the Inside of membrane becomes more negative than the

resting potentialresting potential• Nerve impulses Nerve impulses lessless likely to be produced likely to be produced

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MEMBRANE POTENTIALSMEMBRANE POTENTIALSGraded PotentialsGraded Potentials Short-lived local changes in membrane potentialShort-lived local changes in membrane potential

• Either depolarizations or hyperpolarizationsEither depolarizations or hyperpolarizations Cause current flows that decrease in magnitude Cause current flows that decrease in magnitude

with distancewith distance Magnitude of potential dependent upon stimulus Magnitude of potential dependent upon stimulus

strengthstrength• Stronger stimulus Stronger stimulus larger voltage change larger voltage change• Larger voltage change Larger voltage change farther current flows farther current flows

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MEMBRANE POTENTIALSMEMBRANE POTENTIALSGraded PotentialsGraded Potentials Triggered by change in neuron’s environmentTriggered by change in neuron’s environment

• Change causes gated ion channels to openChange causes gated ion channels to open Small area of neuron’s plasma membrane becomes Small area of neuron’s plasma membrane becomes

depolarized (by this stimulus)depolarized (by this stimulus) Current flows on both sides of the membraneCurrent flows on both sides of the membrane

• + moves toward – and + moves toward – and vise versavise versa

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MEMBRANE POTENTIALSMEMBRANE POTENTIALSGraded PotentialsGraded Potentials Inside cell: + ions move away from depolarized areaInside cell: + ions move away from depolarized area Outside cell: + ions move toward depolarized areaOutside cell: + ions move toward depolarized area

• (+ and – ions switch places)(+ and – ions switch places) Membrane is leakyMembrane is leaky

• Most of the charge is quickly lost through membraneMost of the charge is quickly lost through membrane• Current dies out after traveling a short distanceCurrent dies out after traveling a short distance

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MEMBRANE POTENTIALSMEMBRANE POTENTIALSGraded PotentialsGraded Potentials Act as signals over very short distancesAct as signals over very short distances Important in initiating action potentialsImportant in initiating action potentials

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MEMBRANE POTENTIALSMEMBRANE POTENTIALSAction PotentialsAction Potentials Principal means by which neurons communicatePrincipal means by which neurons communicate

• Brief reversal of membrane potentialBrief reversal of membrane potential Total amplitude of ~ 100 mV (-70 Total amplitude of ~ 100 mV (-70 +30) +30)

• Depolarization followed by repolarization, then brief Depolarization followed by repolarization, then brief period of hyperpolarizationperiod of hyperpolarization

• Time for entire event is only a few millisecondsTime for entire event is only a few milliseconds Events in generation and transmission of an Events in generation and transmission of an

action potential identical between neurons and action potential identical between neurons and skeletal muscle cellsskeletal muscle cells

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ACTION POTENTIALSACTION POTENTIALS

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ACTION POTENTIALSACTION POTENTIALS Not all local depolarizations produce action Not all local depolarizations produce action

potentialspotentials Depolarization must reach threshold valuesDepolarization must reach threshold values

• Brief, weak stimuli produce subthreshold Brief, weak stimuli produce subthreshold depolarizations that are not translated into nerve depolarizations that are not translated into nerve impulsesimpulses

• Stronger threshold stimuli produce depolarizing Stronger threshold stimuli produce depolarizing eventsevents

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ACTION POTENTIALSACTION POTENTIALS Action potential is all-or-nothing phenomenonAction potential is all-or-nothing phenomenon

• Happens completely or doesn’t happenHappens completely or doesn’t happen Independent of stimulus strength once generatedIndependent of stimulus strength once generated

• Strong stimuli generate Strong stimuli generate moremore impulses of the impulses of the samesame strength per unit timestrength per unit time

• Intensity is determined by number of impulses per Intensity is determined by number of impulses per unit timeunit time

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ACTION POTENTIALSACTION POTENTIALSRefractory PeriodsRefractory Periods Neuron cannot respond to a second stimulus Neuron cannot respond to a second stimulus

while the Nawhile the Na++ channels are still open from channels are still open from previous stimulusprevious stimulus• This period of time is termed the This period of time is termed the “absolute “absolute

refractory period”refractory period” ““Relative refractory period”Relative refractory period” follows the follows the

absolute refractory periodabsolute refractory period• Repolarization is occurringRepolarization is occurring• Threshold for impulse generation is elevatedThreshold for impulse generation is elevated

Only strong stimuli can generate impulsesOnly strong stimuli can generate impulses

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ACTION POTENTIALSACTION POTENTIALSConduction VelocitiesConduction Velocities Conduction velocities of neurons vary widelyConduction velocities of neurons vary widely Rate of impulse propagation dependent uponRate of impulse propagation dependent upon

• Axon diameterAxon diameter Larger axons conduct impulses fasterLarger axons conduct impulses faster

• Degree of myelinationDegree of myelination Myelin sheath dramatically increases rate of propagationMyelin sheath dramatically increases rate of propagation

• Myelin acts as an insulator to prevent almost all leakage from Myelin acts as an insulator to prevent almost all leakage from axonaxon

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ACTION POTENTIALSACTION POTENTIALSMultiple Sclerosis (MS)Multiple Sclerosis (MS) Autoimmune disease mainly affecting young Autoimmune disease mainly affecting young

adultsadults Myelin sheaths in CNS are gradually destroyedMyelin sheaths in CNS are gradually destroyed Interferes with impulse conductionInterferes with impulse conduction

• Visual disturbances, muscle control problems, Visual disturbances, muscle control problems, speech disturbances, etc.speech disturbances, etc.

Some modern treatments showing some promise Some modern treatments showing some promise in delaying problemsin delaying problems

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NERVE FIBERSNERVE FIBERS Classified based onClassified based on

• DiameterDiameter• Degree of myelinationDegree of myelination• Conduction speedConduction speed

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NERVE FIBER CLASSIFICATIONNERVE FIBER CLASSIFICATION Group A fibersGroup A fibers

• Largest diameterLargest diameter• Thick myelin sheathsThick myelin sheaths• Conduct impulses at high speeds (> 300 mph)Conduct impulses at high speeds (> 300 mph)• Mostly somatic sensory ad motor fibers serving skin, skeletal Mostly somatic sensory ad motor fibers serving skin, skeletal

muscles, and jointsmuscles, and joints Group B fibersGroup B fibers

• Intermediate diameterIntermediate diameter• Lightly myelinatedLightly myelinated• Transmit impulses at moderate speeds (40 mph)Transmit impulses at moderate speeds (40 mph)

Group C fibersGroup C fibers• Smallest diameterSmallest diameter• UnmyelinatedUnmyelinated• Transmit impulses comparatively slowly (2 mph or less)Transmit impulses comparatively slowly (2 mph or less)

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Nerve Fiber Nerve Fiber ClassificationClassification

General classification scheme (Erlanger-Gasser):General classification scheme (Erlanger-Gasser):• A fibers: MyelinatedA fibers: Myelinated

Subtypes: Subtypes: some overlap in rangessome overlap in ranges Fastest conducting and largest diameter – Fastest conducting and largest diameter – m/sec, m/sec, ““A” often dropped: alpha motor neuronA” often dropped: alpha motor neuron

• B fibers: Slower myelinated (seldom used)B fibers: Slower myelinated (seldom used)• C fibers: UnmyelinatedC fibers: Unmyelinated

Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 ))

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Nerve Fiber Nerve Fiber ClassificationClassification

Sensory nerve classification (Lloyd-Hunt):Sensory nerve classification (Lloyd-Hunt):• I, II, III fibers: MyelinatedI, II, III fibers: Myelinated

Subtypes: Ia, IbSubtypes: Ia, Ib Fastest conducting and largest diameter – IaFastest conducting and largest diameter – Ia

• IV fibers: UnmyelinatedIV fibers: Unmyelinated Slower conducting than IIIs and smallest diameterSlower conducting than IIIs and smallest diameter