k15 - nervous tissue (fisio).ppt

Physiology:Neural Tissue

Tortora & Grabowski 9/e 2000 JWS

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OUTLINEINTRODUCTION AN OVERVIEW OF THE NERVOUS SYSTEM NEUROGLIANEUROPHYSIOLOGY SYNAPTIC ACTIVITYINFORMATION PROCESSING HIGHER LEVEL OF ORGANIZATION AND PROCESSINGINTEGRATION WITH OTHER SYSTEMS


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ObjectiveList & describe the characteristics the two major anatomical divisions of the nervous system. Sketch and label the structure of a typical neuron, and describe the functions of each component. Explain how the resting potential is created and maintained. Describe the events involved in the generation and propagation of an action potential. Discuss the factors that affect the speed with which action potentials are propagated.


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Objective (count)Describe the structure and explain the mechanism involved in synaptic activity. Describe the major types of neurotransmitters and neuromodulators, and discuss their effects on postsynaptic membranes. Discuss the interactions that make possible the processing of information in neural tissue. Describe the patterns of interaction between neurons that are involved in the processing of information at higher levels.Give examples of interactions between the nervous system and each of the other organ systems.


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12-*Introduction: Nervous SystemControls and integrates all body activities.Three basic functionssensing changes with sensory receptorsfullness of stomach or sun on your faceinterpreting and remembering those changesreacting to those changes with effectorsmuscular contractionsglandular secretions



12-*Overview: Organization of the Nervous SystemCNS is brain and spinal cordPNS is everything else



12-*NeuronsFunctional unit of nervous systemHave capacity to produce action potentialselectrical excitabilityCell bodysingle nucleus with prominent nucleolusNissl bodies (chromatophilic substance) rough ER & free ribosomes for protein synthesisneurofilaments give cell shape and supportmicrotubules move material inside celllipofuscin pigment clumps (harmless aging)Cell processes = dendrites & axons



12-*Nucleus with NucleolusParts of a NeuronAxons or DendritesCell bodyNeuroglial cells



12-*DendritesConducts impulses towards the cell bodyTypically short, highly branched & unmyelinatedSurfaces specialized for contact with other neuronsContains neurofibrils & Nissl bodies



12-*AxonsConduct impulses away from cell bodyLong, thin cylindrical process of cell Arises at axon hillock Impulses arise from initial segment (trigger zone)Side branches (collaterals) end in fine processes called axon terminalsSwollen tips called synaptic end bulbs contain vesicles filled with neurotransmittersSynaptic boutons



12-*Axonal TransportCell body is location for most protein synthesisneurotransmitters & repair proteins Axonal transport system moves substances slow axonal flowmovement in one direction only -- away from cell bodymovement at 1-5 mm per day fast axonal flow moves organelles & materials along surface of microtubules at 200-400 mm per daytransports in either directionfor use or for recycling in cell body



12-*Functional Classification of NeuronsSensory (afferent) neuronstransport sensory information from skin, muscles, joints, sense organs & viscera to CNSMotor (efferent) neuronssend motor nerve impulses to muscles & glandsInterneurons (association) neuronsconnect sensory to motor neurons90% of neurons in the body



12-*Structural Classification of NeuronsBased on number of processes found on cell bodymultipolar = several dendrites & one axonmost common cell typebipolar neurons = one main dendrite & one axonfound in retina, inner ear & olfactory unipolar neurons = one process only(develops from a bipolar)are always sensory neurons



12-*Association or InterneuronsNamed for histologist that first described them or their appearance



12-*Half of the volume of the CNS Smaller cells than neurons50X more numerousCells can divide rapid mitosis in tumor formation (gliomas)4 cell types in CNSastrocytes, oligodendrocytes, microglia & ependymal2 cell types in PNSschwann and satellite cellsNeuroglial Cells



12-*AstrocytesStar-shaped cellsForm blood-brain barrier by covering blood capillariesMetabolize neurotransmittersRegulate K+ balanceProvide structural support



12-*OligodendrocytesMost common glial cell typeEach forms myelin sheath around more than one axons in CNSAnalogous to Schwann cells of PNS



12-*MicrogliaSmall cells found near blood vesselsPhagocytic role -- clear away dead cellsDerived from cells that also gave rise to macrophages & monocytes



12-*Ependymal cellsForm epithelial membrane lining cerebral cavities & central canalProduce cerebrospinal fluid (CSF)



12-*Satellite CellsFlat cells surrounding neuronal cell bodies in peripheral gangliaSupport neurons in the PNS ganglia



12-*Schwann CellCells encircling PNS axonsEach cell produces part of the myelin sheath surrounding an axon in the PNS



12-*Axon Coverings in PNSAll axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cellsNeurilemma is cytoplasm & nucleus of Schwann cellgaps called nodes of RanvierMyelinated fibers appear whitejelly-roll like wrappings made of lipoprotein = myelinacts as electrical insulatorspeeds conduction of nerve impulsesUnmyelinated fibersslow, small diameter fibersonly surrounded by neurilemma but no myelin sheath wrapping



12-*Myelination in PNSSchwann cells myelinate (wrap around) axons in the PNS during fetal developmentSchwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheathTube guides growing axons that are repairing themselves



12-*Myelination in the CNSOligodendrocytes myelinate axons in the CNS Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axonsNo neurilemma is formedLittle regrowth after injury is possible due to the lack of a distinct tube or neurilemma



12-*Gray and White MatterWhite matter = myelinated processes (white in color)Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color)In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matterIn the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS



12-*Electrical Signals in NeuronsNeurons are electrically excitable due to the voltage difference across their membrane Communicate with 2 types of electric signalsaction potentials that can travel long distancesgraded potentials that are local membrane changes onlyIn living cells, a flow of ions occurs through ion channels in the cell membrane



12-*Two Types of Ion ChannelsLeakage (nongated) channels are always opennerve cells have more K+ than Na+ leakage channels as a result, membrane permeability to K+ is higherexplains resting membrane potential of -70mV in nerve tissueGated channels open and close in response to a stimulus results in neuron excitabilityvoltage-gated open in response to change in voltageligand-gated open & close in response to particular chemical stimuli (hormone, neurotransmitter, ion)mechanically-gated open with mechanical stimulation



12-*Gated Ion Channels



12-*Resting Membrane PotentialNegative ions along inside of cell membrane & positive ions along outsidepotential energy difference at rest is -70 mV cell is polarizedResting potential exists becauseconcentration of ions different inside & outsideextracellular fluid rich in Na+ and Clcytosol full of K+, organic phosphate & amino acidsmembrane permeability differs for Na+ and K+50-100 greater permeability for K+inward flow of Na+ cant keep up with outward flow of K+Na+/K+ pump removes Na+ as fast as it leaks in



12-*Graded PotentialsSmall deviations from resting potential of -70mVhyperpolarization = membrane has become more negativedepolarization = membrane has become more positive



12-*How do Graded Potentials Arise?Source of stimulimechanical stimulation of membranes with mechanical gated ion channels (pressure)chemical stimulation of membranes with ligand gated ion channels (neurotransmitter)Graded/postsynaptic/receptor or generator potentialions flow through ion channels and change membrane potential locallyamount of change varies with strength of stimuliFlow of current (ions) is local change only



12-*Action PotentialSeries of rapidly occurring events that change and then restore the membrane potential of a cell to its resting stateIon channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization)All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second)Travels (spreads) over surface of cell without dying out



12-*Depolarizing Phase of Action PotentialChemical or mechanical stimulus caused a graded potential to reach at least (-55mV or threshold)Voltage-gated Na+ channels open & Na+ rushes into cellin resting membrane, inactivation gate of sodium channel is open & activation gate is closed (Na+ can not get in)when threshold (-55mV) is reached, both open & Na+ entersinactivation gate closes again in few ten-thousandths of secondonly a total of 20,000 Na+ actually enter the cell, but they change the membrane potential considerably(up to +30mV)Positive feedback process


Repolarizing Phase of Action Potential


12-*When threshold potential of -55mV is reached, voltage-gated K+ channels openK+ channel opening is much slower than Na+ channel opening which caused depolarizationWhen K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops)K+ outflow returns membrane potential to -70mVIf enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phaseK+ channels close and the membrane potential returns to the resting potential of -70mV



12-*Refractory Period of Action PotentialPeriod of time during which neuron can not generate another action potentialAbsolute refractory periodeven very strong stimulus will not begin another APinactivated Na+ channels must return to the resting state before they can be reopenedlarge fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possibleRelative refractory perioda suprathreshold stimulus will be able to start an APK+ channels are still open, but Na+ channels have closed



12-*The Action Potential: SummarizedResting membrane potential is -70mVDepolarization is the change from -70mV to +30 mVRepolarization is the reversal from +30 mV back to -70 mV)



12-*Propagation of Action PotentialAn action potential spreads (propagates) over the surface of the axon membraneas Na+ flows into the cell during depolarization, the voltage of adjacent areas is effected and their voltage-gated Na+ channels openself-propagating along the membraneThe traveling action potential is called a nerve impulse



12-*Local AnestheticsPrevent opening of voltage-gated Na+ channelsNerve impulses cannot pass the anesthetized regionNovocaine and lidocaine



12-*Continuous versus Saltatory ConductionContinuous conduction (unmyelinated fibers)step-by-step depolarization of each portion of the length of the axolemma Saltatory conductiondepolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channelscurrent carried by ions flows through extracellular fluid from node to node



12-*Saltatory ConductionNerve impulse conduction in which the impulse jumps from node to node



12-*Speed of Impulse PropagationThe propagation speed of a nerve impulse is not related to stimulus strength. larger, myelinated fibers conduct impulses faster due to size & saltatory conduction Fiber types A fibers largest (5-20 microns & 130 m/sec)myelinated somatic sensory & motor to skeletal muscleB fibers medium (2-3 microns & 15 m/sec)myelinated visceral sensory & autonomic preganglionicC fibers smallest (.5-1.5 microns & 2 m/sec)unmyelinated sensory & autonomic motor



12-*Encoding of Stimulus IntensityHow do we differentiate a light touch from a firmer touch?frequency of impulsesfirm pressure generates impulses at a higher frequencynumber of sensory neurons activatedfirm pressure stimulates more neurons than does a light touch



12-*Action Potentials in Nerve and MuscleEntire muscle cell membrane versus only the axon of the neuron is involvedResting membrane potentialnerve is -70mVskeletal & cardiac muscle is closer to -90mVDurationnerve impulse is 1/2 to 2 msecmuscle action potential lasts 1-5 msec for skeletal & 10-300msec for cardiac & smoothFastest nerve conduction velocity is 18 times faster than velocity over skeletal muscle fiber



12-*OriginGPs arise on dendrites and cell bodiesAPs arise only at trigger zone on axon hillockTypes of ChannelsAP is produced by voltage-gated ion channelsGP is produced by ligand or mechanically-gated channels ConductionGPs are localized (not propagated)APs conduct over the surface of the axonComparison of Graded & Action Potentials



12-*Comparison of Graded & Action PotentialsAmplitudeamplitude of the AP is constant (all-or-none)graded potentials vary depending upon stimulusDurationThe duration of the GP is as long as the stimulus lastsRefractory periodThe AP has a refractory period due to the nature of the voltage-gated channels, and the GP has none.



12-*Signal Transmission at Synapses2 Types of synapseselectrical ionic current spreads to next cell through gap junctionsfaster, two-way transmission & capable of synchronizing groups of neuronschemicalone-way information transfer from a presynaptic neuron to a postsynaptic neuronaxodendritic -- from axon to dendriteaxosomatic -- from axon to cell bodyaxoaxonic -- from axon to axon



12-*Chemical SynapsesAction potential reaches end bulb and voltage-gated Ca+ 2 channels openCa+2 flows inward triggering release of neurotransmitterNeurotransmitter crosses synaptic cleft & binding to ligand-gated receptors the more neurotransmitter released the greater the change in potential of the postsynaptic cellSynaptic delay is 0.5 msecOne-way information transfer



12-*Excitatory & Inhibitory PotentialsThe effect of a neurotransmitter can be either excitatory or inhibitorya depolarizing postsynaptic potential is called an EPSPit results from the opening of ligand-gated Na+ channelsthe postsynaptic cell is more likely to reach thresholdan inhibitory postsynaptic potential is called an IPSPit results from the opening of ligand-gated Cl- or K+ channelsit causes the postsynaptic cell to become more negative or hyperpolarizedthe postsynaptic cell is less likely to reach threshold



12-*Removal of NeurotransmitterDiffusionmove down concentration gradientEnzymatic degradationacetylcholinesteraseUptake by neurons or glia cellsneurotransmitter transportersProzac = serotonin reuptake inhibitor



12-*Summation of effects of neurotransmitters released from several end bulbs onto one neuronSpatial Summation



12-*Temporal SummationSummation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron



12-*Three Possible ResponsesSmall EPSP occurspotential reaches -56 mV onlyAn impulse is generatedthreshold was reachedmembrane potential of at least -55 mV IPSP occursmembrane hyperpolarizedpotential drops below -70 mV



12-*Neurotransmitter EffectsNeurotransmitter effects can be modifiedsynthesis can be stimulated or inhibitedrelease can be blocked or enhancedremoval can be stimulated or blocked receptor site can be blocked or activatedAgonistanything that enhances a transmitters effects Antagonistanything that blocks the action of a neurotranmitter



12-*Small-Molecule NeurotransmittersAcetylcholine (ACh)released by many PNS neurons & some CNSexcitatory on NMJ but inhibitory at othersinactivated by acetylcholinesteraseAmino Acidsglutamate released by nearly all excitatory neurons in the brain ---- inactivated by glutamate specific transportersGABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)



12-*Biogenic Aminesmodified amino acids (tyrosine)norepinephrine -- regulates mood, dreaming, awakening from deep sleepdopamine -- regulating skeletal muscle toneserotonin -- control of mood, temperature regulation, & induction of sleepremoved from synapse & recycled or destroyed by enzymes (monoamine oxidase or catechol-0-methyltransferase)

Small-Molecule Neurotransmitters (2)



12-*ATP and other purines (ADP, AMP & adenosine)excitatory in both CNS & PNSreleased with other neurotransmitters (ACh & NE)Gases (nitric oxide or NO)formed from amino acid arginine by an enzymeformed on demand and acts immediatelydiffuses out of cell that produced it to affect neighboring cellsmay play a role in memory & learningfirst recognized as vasodilator that helps lower blood pressure

Small-Molecule Neurotransmitters (3)



12-*Neuropeptides3-40 amino acids linked by peptide bondsSubstance P -- enhances our perception of painPain reliefenkephalins -- pain-relieving effect by blocking the release of substance P acupuncture may produce loss of pain sensation because of release of opioids-like substances such as endorphins or dynorphins



12-*Neuronal CircuitsNeurons in the CNS are organized into neuronal networksA neuronal network may contain thousands or even millions of neurons.Neuronal circuits are involved in many important activitiesbreathing short-term memorywaking up



12-*Neuronal CircuitsDiverging -- single cell stimulates many othersConverging -- one cell stimulated by many othersReverberating -- impulses from later cells repeatedly stimulate early cells in the circuit (short-term memory)Parallel-after-discharge -- single cell stimulates a group of cells that all stimulate a common postsynaptic cell (math problems)



12-*Regeneration & RepairPlasticity maintained throughout lifesprouting of new dendritessynthesis of new proteinschanges in synaptic contacts with other neuronsLimited ability for regeneration (repair)PNS can repair damaged dendrites or axonsCNS no repairs are possible


k15 - nervous tissue (fisio).ppt

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