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Nerve lectures

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  • 1. Types of neuronesTypes of neurones Histological ClassificationHistological Classification 1- Unipolar.1- Unipolar. 2- Bipolar2- Bipolar 3- Multipolar3- Multipolar
  • 2. Functional classificationFunctional classification :: 1- Sensory ( afferent )1- Sensory ( afferent ) :: conductconduct impulses from receptors to C.N.S .impulses from receptors to C.N.S . 2- Motor (efferent):2- Motor (efferent): conductconduct impulses from the C.N.S to theimpulses from the C.N.S to the effector organ.effector organ. 3-Interneurons3-Interneurons:: are located insideare located inside the C.N.S connect each other &the C.N.S connect each other & connect sensory & motor neurons.connect sensory & motor neurons.
  • 3. Cross section in spinal cord
  • 4. Structure of neuroneStructure of neurone  Cell body ( soma )Cell body ( soma ) :: The soma also contains :The soma also contains :  Nissil bodiesNissil bodies :: responsiple for proteinresponsiple for protein synthesis.synthesis.  NeurofibrilsNeurofibrils :: extend into cell processes &extend into cell processes & form the main support of the neuron.form the main support of the neuron.  Microtubules & MicrofilamentsMicrotubules & Microfilaments :: theythey extend into the axon & have supportive andextend into the axon & have supportive and transport function ( axoplasmic transporttransport function ( axoplasmic transport ).). NO centrosomeNO centrosome
  • 5. Neuron structure
  • 6. 2- Cell processes:2- Cell processes: a-Dendrites :a-Dendrites : - Multiple short processes- Multiple short processes extend from the cell body .extend from the cell body . - Collect impulses & transmit- Collect impulses & transmit them towards the cell body.them towards the cell body.
  • 7.  b- Axonb- Axon :: -- Single long process that conductSingle long process that conduct impulses away from the cell body.impulses away from the cell body. - Vary in length from few millimeter to- Vary in length from few millimeter to about one meter .about one meter . - Its origin from the cell body is called- Its origin from the cell body is called the axon hillock .the axon hillock . - It branches freely near its terminal .- It branches freely near its terminal . - The terminal branches end by swollen- The terminal branches end by swollen part (synaptic knobs) which containpart (synaptic knobs) which contain chemical transmitter vesicles .chemical transmitter vesicles .
  • 8. Types of axonsTypes of axons Axons are one of two types :Axons are one of two types : 1-1- MyelinatedMyelinated:: covered by myelin sheath&covered by myelin sheath& neurilemmal sheath.neurilemmal sheath. 2-2-UnmyelinatedUnmyelinated :: covered only by neurilemmal sheathcovered only by neurilemmal sheath but no underlined myelin sheath .but no underlined myelin sheath .
  • 9. Myelin sheathMyelin sheath lipoprotein in nature & insulatorlipoprotein in nature & insulator formed in peripheral nervousformed in peripheral nervous system by wrapping of cellsystem by wrapping of cell membrane ofmembrane of Schwan cellsSchwan cells around the axon while in thearound the axon while in the C.N.S. it is formed byC.N.S. it is formed by oligodendroglial cellsoligodendroglial cells ..
  • 10. Myelination of neurones
  • 11. Myelination in PNS & CNS
  • 12.  absent from axon hillock &absent from axon hillock & terminal branches .terminal branches .  interrupted at intervals ofinterrupted at intervals of about 0.1 – 1.0 mm by nodesabout 0.1 – 1.0 mm by nodes of Ranvierof Ranvier  responsiple for the whiteresponsiple for the white colour of myelinated nerve&colour of myelinated nerve& the white matter of C.N.S .the white matter of C.N.S .
  • 13. Resting membrane potentialResting membrane potential ( R.M.P.)( R.M.P.) DefinitionDefinition:: It is theIt is the potential difference betweenpotential difference between inside & outside theinside & outside the membrane at restmembrane at rest ValueValue :: it is – 70 mv.it is – 70 mv.
  • 14. MeasurementMeasurement :: it is measured by specialit is measured by special apparatus withapparatus with microelectrodes , amplifier ,andmicroelectrodes , amplifier ,and sensitive voltmeter .sensitive voltmeter . One microelectrode is put inOne microelectrode is put in the inner side of the membranethe inner side of the membrane & the other microelectrode is& the other microelectrode is put on the outside surface .put on the outside surface .
  • 15. Cathode ray oscilloscope
  • 16. Causes of RMPCauses of RMP 1- Selective1- Selective permeability of thepermeability of the membrane.membrane. 2- Sodium – potassium2- Sodium – potassium pump.pump.
  • 17. 1-Selective permeability of the1-Selective permeability of the membranemembrane POTASSIUM ION DIFFUSIONPOTASSIUM ION DIFFUSION :: The cell membrane is about 100The cell membrane is about 100 times more permeable fortimes more permeable for potassium ions than for sodiumpotassium ions than for sodium ions.ions. Potassium ions are concentratedPotassium ions are concentrated inside 30-40 times more thaninside 30-40 times more than outside.outside.
  • 18. Potassium ions tend to movePotassium ions tend to move from inside the cell to outside.from inside the cell to outside. Potassium ion diffusionPotassium ion diffusion continue untill the positivecontinue untill the positive charge outside the membranecharge outside the membrane reach a level that repells thereach a level that repells the outflow of more potassium ionsoutflow of more potassium ions (( equilibrium ofequilibrium of potassium ionpotassium ion diffusiondiffusion ) .) .
  • 19. SODIUM ION DIFFUSIONSODIUM ION DIFFUSION ::  Sodium ion concentration outsideSodium ion concentration outside the membrane is 10-15 times thethe membrane is 10-15 times the concentration inside.concentration inside. Sodium tends to diffuse to insideSodium tends to diffuse to inside the cell.the cell. Sodium ion diffusion is limited bySodium ion diffusion is limited by the low permeability of the restingthe low permeability of the resting membrane to sodium.membrane to sodium.
  • 20. CHLORIDE ION DIFFUSION :CHLORIDE ION DIFFUSION : Chloride ions tend to diffuse fromChloride ions tend to diffuse from outside to inside the celloutside to inside the cell (concentrated 25 times outside more(concentrated 25 times outside more than inside ) .than inside ) . This diffusion is prevented byThis diffusion is prevented by repulsionrepulsion force caused by theforce caused by the negativitynegativity insideinside & also by the& also by the attractionattraction force between chloride &force between chloride & sodium ionssodium ions outsideoutside the membrane .the membrane .
  • 21. PROTEIN DIFFUSIONPROTEIN DIFFUSION :: The cell membrane offersThe cell membrane offers absolute barrier to the passageabsolute barrier to the passage of organic anions (proteins),of organic anions (proteins), because the protein moleculesbecause the protein molecules are of large size .are of large size . They cannot pass through theThey cannot pass through the cell membrane.cell membrane.
  • 22. movement of ions at rest occurmovement of ions at rest occur through thethrough the leak proteinleak protein channelschannels which are alwayswhich are always opened.opened. the most important ion at restthe most important ion at rest is the potassiumis the potassium ion because ofion because of highest concentration gradienthighest concentration gradient & highest membrane& highest membrane permeabilitypermeability
  • 23. Potassium equilibrium Nernest equation Ek = - 61. log Conc inside / Conc outside. = - 61 .log 140/4.5 = - 91 mv. Electrochemical potential & ionic current
  • 24. 2-Sodium – potassium pump2-Sodium – potassium pump
  • 25. Function of sodium-potassiumFunction of sodium-potassium pumppump 1- It1- It keepskeeps highhigh Na + concentrationNa + concentration outsideoutside the cell & highthe cell & high K+ concentrationK+ concentration inside theinside the cell .cell . 2- It is an2- It is an electrogenic pump.Itelectrogenic pump.It pumps threepumps three positive charges to outside & only twopositive charges to outside & only two positive charges to inside with the result ofpositive charges to inside with the result of one negative charge inside , helpone negative charge inside , help negativity of R.M.P.negativity of R.M.P. 3-3- keeps normal cell volumekeeps normal cell volume by preventingby preventing accumulation of electrolytes inside the cellaccumulation of electrolytes inside the cell
  • 26. Action potentialAction potential  DefinitionDefinition :: it is theit is the rapidrapid propagatedpropagated change in membranechange in membrane potential as a result ofpotential as a result of stimulationstimulation by an effective stimulusby an effective stimulus  MONOPHASIC ACTIONMONOPHASIC ACTION POTENTIALPOTENTIAL ::  ItIt is recorded by putting oneis recorded by putting one microelectrode in the inside surfacemicroelectrode in the inside surface of the membrane & the other on theof the membrane & the other on the outer surface.outer surface.
  • 27. Monophasic action potentialMonophasic action potential - 70 Zero + 35 L.P Depolarization Repolarization After depolarization After hyperpolarization Duration (m.Sec.) Voltage(m.v.)
  • 28. Latent periodLatent period it is theit is the timetime passed sincepassed since application of stimulus until theapplication of stimulus until the start of recording action potential.start of recording action potential. It depends on theIt depends on the velocity ofvelocity of conductionconduction in the nerve & thein the nerve & the distance between the stimulatingdistance between the stimulating and the recording electrode .and the recording electrode .
  • 29. It is inversely proportional to theIt is inversely proportional to the velocity of conduction but directlyvelocity of conduction but directly proportional to the distanceproportional to the distance between the stimulating &between the stimulating & recording electrode .It can be usedrecording electrode .It can be used to calculate the velocity ofto calculate the velocity of conduction as follows :conduction as follows : Velocity =Velocity = Distance / Latent period.Distance / Latent period.
  • 30. Na + & K+ conductance during action potential
  • 31. Na + & K + conductance during Action potential
  • 32. Spike potential :Spike potential :  It is composed of two limbs :It is composed of two limbs : a-a-ascending limbascending limb ( Depolarization wave ) .( Depolarization wave ) . b-b-descending limbdescending limb ( Repolarization wave).( Repolarization wave). Duration of the spike potential isDuration of the spike potential is 0.5 – 1.0 m.sec0.5 – 1.0 m.sec
  • 33. Sodium channels during action potential occurence
  • 34. The ascending limbThe ascending limb ( Depolarization ):( Depolarization ): --It is due to Na+ influx & it is of two phases:It is due to Na+ influx & it is of two phases: a-Slow phase :a-Slow phase : from R.M.P. till firing levelfrom R.M.P. till firing level ( from –70 mv to –55 mv ) it is due to( from –70 mv to –55 mv ) it is due to increase in Na+ permeability by the effectincrease in Na+ permeability by the effect of the stimulusof the stimulus b-Rapid phase :b-Rapid phase : from –55 mv to + 35 mvfrom –55 mv to + 35 mv It is due to rapid Na+ influx caused byIt is due to rapid Na+ influx caused by opening of Na+ voltage gated channels .opening of Na+ voltage gated channels .
  • 35. The descending limbThe descending limb ( repolarization ) :( repolarization ) : It represents the major part of theIt represents the major part of the process of repolarization (about 70process of repolarization (about 70 %).%). It is due to closure of Na+ voltageIt is due to closure of Na+ voltage gated channels & opening of K+gated channels & opening of K+ voltage gated channels resulting involtage gated channels resulting in rapid k+ efflux while Na+ influxrapid k+ efflux while Na+ influx stopsstops
  • 36. Voltage gated Na+ & K+Voltage gated Na+ & K+ channels open by the samechannels open by the same signal ( change of membranesignal ( change of membrane potential from resting state to –potential from resting state to – 55 mv ).55 mv ). Na+ voltage gated channelsNa+ voltage gated channels are rapid to open & rapid toare rapid to open & rapid to close while K+ voltage gatedclose while K+ voltage gated channels are slow .channels are slow .
  • 37. Depolarazation triggers Phases Of A.P Ascend -ing limb Peak Desce -nding Limb Na channel activation Gate open rapidly Positive Feedback loop Na enter cells More depolarization To stop cycle Na inactivation gate close Slow K Channels open Potassium Leaves cells
  • 38. aa-After depolarization :-After depolarization : ( Negative after potential )( Negative after potential ) durationduration 4 m.sec.4 m.sec. it is theit is the slow repolarizationslow repolarization following the descending limb offollowing the descending limb of the spike till R.M.P. is reachedthe spike till R.M.P. is reached it is due toit is due to slow K+ outfluxslow K+ outflux due todue to closure of some voltage gated K+closure of some voltage gated K+ channels. .channels. .
  • 39. b-b-After hyperpolarizationAfter hyperpolarization :: ( Positive after potenial )( Positive after potenial ) durationduration 40 m.sec.40 m.sec. it isit is small hyperpolarizationsmall hyperpolarization followingfollowing the after depolarization till the R.M.P.the after depolarization till the R.M.P. is reached .is reached . it is caused by :it is caused by : a- some K+ voltage gated channels stilla- some K+ voltage gated channels still openedopened b- increased activity of Na+ - K+ pump.b- increased activity of Na+ - K+ pump.
  • 40. Conduction of action potential
  • 41. Saltatory conductionPoint to point conduction
  • 42. This is anThis is an active processactive process that needsthat needs energy .energy . TheThe depolarized point make a potentialdepolarized point make a potential difference with the adjacent point.difference with the adjacent point. A current develop between that point &A current develop between that point & the adjacent pointthe adjacent point which becomewhich become depolarized to the firing level producing adepolarized to the firing level producing a new action potential in the new point.new action potential in the new point. The new action potential producesThe new action potential produces another action potential at the adjacentanother action potential at the adjacent point & so on.point & so on.
  • 43. Differences between propagation inDifferences between propagation in myelinated & unmyelinated nerve fibers :myelinated & unmyelinated nerve fibers : MyelinatedMyelinated nerve fibersnerve fibers UnmyelinatedUnmyelinated nerve fibersnerve fibers Type ofType of conductionconduction SaltatorySaltatory ( jumping )( jumping ) Point to pointPoint to point Velocity ofVelocity of conductionconduction RapidRapid SlowSlow EnergyEnergy consumptionconsumption Consume lessConsume less energyenergy Consume moreConsume more energyenergy
  • 44. Biphasic action potentialBiphasic action potential It is measured by putting theIt is measured by putting the twotwo microelectrodesmicroelectrodes on theon the outer surface.outer surface. As the depolarization wave reaches theAs the depolarization wave reaches the electrode near the stimulator , theelectrode near the stimulator , the electrode becomes negative relative to theelectrode becomes negative relative to the other electrode & a downward deflection isother electrode & a downward deflection is recorded .recorded . When the wave passes to the part of theWhen the wave passes to the part of the nerve fiber between the two electrodes ,nerve fiber between the two electrodes , the potential difference returns to zero .the potential difference returns to zero .
  • 45. When the wave reaches the secondWhen the wave reaches the second electrode , it becomes negativeelectrode , it becomes negative relative to the first electrode & anrelative to the first electrode & an upward deflection is recorded .upward deflection is recorded . When the depolarization wave leavesWhen the depolarization wave leaves the second electrode , the potentialthe second electrode , the potential difference returns again to zero.difference returns again to zero. Therefore , the record shows aTherefore , the record shows a downward deflection followed by andownward deflection followed by an isoelectric interval & then an upwardisoelectric interval & then an upward
  • 46. Biphasic action potential Depolarization In the nearby electrode Depolarization In the distant electrode Voltage Duration
  • 47. EXCITABILITYEXCITABILITY DefinitionDefinition :: It is the ability of nerveIt is the ability of nerve fiber or muscle fiber tofiber or muscle fiber to respond to stimuli .respond to stimuli .
  • 48. Stimulus :Stimulus : It is the change in the surroundingIt is the change in the surrounding environment .environment . Types of stimuli :Types of stimuli : 1-1- ElectricalElectrical :: it is of two types :it is of two types : a-Galvanica-Galvanic :: direct current obtaineddirect current obtained from a battery of low intensity & offrom a battery of low intensity & of long duration.long duration. b-Farradicb-Farradic :: high intensity & of shorthigh intensity & of short duration .duration .
  • 49. Electrical stimuli are commonlyElectrical stimuli are commonly used in experimental workused in experimental work because they are :because they are : 1- easily applied .1- easily applied . 2- accurately adjusted as strength &2- accurately adjusted as strength & duration .duration . 3- similar to physiological process of3- similar to physiological process of excitationexcitation 4- cause no or minimal damage to the4- cause no or minimal damage to the tissuetissue
  • 50. 2 -2 - ChemicalChemical :: Chemical tranasmittersChemical tranasmitters: acetyl: acetyl choline , adrenaline &choline , adrenaline & noradrenaline.noradrenaline. Hormones.Hormones. Drugs.Drugs. IonsIons : Na+,K+& Ca++ .: Na+,K+& Ca++ . GasesGases : O2 & CO2.: O2 & CO2.
  • 51. 3-3- PhysicalPhysical :: a-a-ThermalThermal :: cooling & warmingcooling & warming b-b-MechanicalMechanical : stretch , touch: stretch , touch & pressure .& pressure . c-c-ElectromagneticElectromagnetic :: lightlight stimuli to the retina of the eye.stimuli to the retina of the eye.
  • 52. Factors affectingFactors affecting effectiveness of stimuli :effectiveness of stimuli : 1-Strength (intensity) of1-Strength (intensity) of stimulistimuli.. 2- Duration (time of2- Duration (time of application) of stimuli.application) of stimuli. 3- Rate of rise of intensity.3- Rate of rise of intensity.
  • 53. 1-Strength (intensity) of stimuli1-Strength (intensity) of stimuli ::  According to the stimulus strength theAccording to the stimulus strength the following types of stimuli are known :following types of stimuli are known : Sub minimal stimulusSub minimal stimulus (sub(sub threshold):threshold): it produces no actionit produces no action potential ,but only produces localizedpotential ,but only produces localized change (local excitatory state)change (local excitatory state) Minimal (threshold ) stimulusMinimal (threshold ) stimulus :: it is theit is the weakest stimulus which can produce anweakest stimulus which can produce an action potential.action potential.
  • 54. Supra threshold stimulusSupra threshold stimulus .. Maximal stimulus.Maximal stimulus. Supramaximal stimulus.Supramaximal stimulus.  Threshold ,supra threshold ,Threshold ,supra threshold , maximal & supra maximalmaximal & supra maximal stimuli produce the same actionstimuli produce the same action potential in single nerve fiberpotential in single nerve fiber because it obeys the all or nonbecause it obeys the all or non law.law.
  • 55. Action potential Local excitatory state Threshold stimulus Subthreshold stimuli Suprathreshold stimuli Resting potential Threshold
  • 56. 2-Duration (the time of application)2-Duration (the time of application) of stimulus :of stimulus : The duration of stimulus is closelyThe duration of stimulus is closely related to the strength.related to the strength.  The strength-duration curve can beThe strength-duration curve can be obtained by stimulating the nerve withobtained by stimulating the nerve with electrical stimuli of different intensitieselectrical stimuli of different intensities and recording the time needed byand recording the time needed by each stimulus to start the response.each stimulus to start the response.
  • 57. Strength-Duration curve R 2R Duration Strength C R: Rheobase C: Chronaxie
  • 58. From the curve we observe thatFrom the curve we observe that :: 1-Within limits1-Within limits , the stronger the, the stronger the stimulus , the shorter will be the duration .stimulus , the shorter will be the duration . 22-There is a-There is a minimal timeminimal time , less than this, less than this time , no stimulus whatever its strengthtime , no stimulus whatever its strength can produce response .can produce response . 33-There is a-There is a minimal strengthminimal strength ((Rheobase)Rheobase) , less than this strength , no, less than this strength , no stimulus whatever its duration canstimulus whatever its duration can produce response .produce response .
  • 59. 4-The4-The ChronaxieChronaxie is the timeis the time needed by stimulus strengthneeded by stimulus strength double rheobase to producedouble rheobase to produce response.response. N.B. :N.B. : It is aIt is a measuremeasure of excitability .of excitability . It isIt is inversily proportionalinversily proportional toto excitability .excitability .
  • 60. 3- Rate of rise of intensity3- Rate of rise of intensity :: The stimulus becomes effectiveThe stimulus becomes effective only if the rate of riseonly if the rate of rise exceeds aexceeds a certain limit.certain limit. When the current rises too slowly,When the current rises too slowly, thethe nerve accommodatesnerve accommodates itself toitself to the rise of the current (thethe rise of the current (the threshold of stimulation isthreshold of stimulation is increased).increased).
  • 61. Excitability changesExcitability changes during an action potentialduring an action potential During conduction of a nerveDuring conduction of a nerve impulse , the excitability of the nerveimpulse , the excitability of the nerve fibers varies & it passes in thefibers varies & it passes in the following phases:following phases: 1-Temporal rise of excitability1-Temporal rise of excitability :: it is associated with the localit is associated with the local response (local depolarization ) in theresponse (local depolarization ) in the nerve fiber before the firing level .nerve fiber before the firing level .
  • 62. Excitability changes duringExcitability changes during A.P.A.P. - 70 Zero + 35 Exc . 100% Zero 1 2 3 4 5 1- Initial increase. 2- Absolute refractory period (ARP). 3- Relative refractory period (RRP). 4- Super-normal phase. 5- Subnormal phase
  • 63. Absolute & relative refractory period
  • 64. Absolute refractory period (A.R.P):Absolute refractory period (A.R.P): it corresponds to the ascending limb & theit corresponds to the ascending limb & the first third of the descending limb of thefirst third of the descending limb of the spike potential .The excitability isspike potential .The excitability is completely lost = zero ( no stimuluscompletely lost = zero ( no stimulus whatever its strength can excite ) .whatever its strength can excite ) . Significance :Significance : A.R.P. limits the numberA.R.P. limits the number of impulses that can be produced &of impulses that can be produced & conducted by the nerve fibersconducted by the nerve fibers
  • 65. Mechanism :Mechanism : During the ascending limb theDuring the ascending limb the Na+ voltage gated channels areNa+ voltage gated channels are already opened ( no effect ofalready opened ( no effect of other stimuli).other stimuli). During the first third ofDuring the first third of descending limb Na+ channelsdescending limb Na+ channels are closed by the inactivationare closed by the inactivation gate and can not be opened.gate and can not be opened.
  • 66. Relative refractory periodRelative refractory period ( R.R.P. ):( R.R.P. ): It corresponds to the second two thirds ofIt corresponds to the second two thirds of the descending limb of spike potential .the descending limb of spike potential . The excitability is less than normal (supraThe excitability is less than normal (supra threshold stimulus strength can excite).threshold stimulus strength can excite). MechanismMechanism :: it is due to difficulty to openit is due to difficulty to open the closed voltage gated Na+ channels .the closed voltage gated Na+ channels .
  • 67. 4- Supernormal phase:4- Supernormal phase: it corresponds to the afterit corresponds to the after depolarization ( the membrane isdepolarization ( the membrane is partially depolarized) .partially depolarized) .  The excitability is higher thanThe excitability is higher than normalnormal (sub threshold stimulus can(sub threshold stimulus can excite)excite)
  • 68. 5-5- Subnormal phase :Subnormal phase : It corresponds to the after hyperIt corresponds to the after hyper polarization .polarization . The excitability is lower thanThe excitability is lower than normalnormal Supra threshold stimuli areSupra threshold stimuli are needed to excite .needed to excite .
  • 69. Factors affectingFactors affecting excitability of nerve fibersexcitability of nerve fibers 1- Physical factors1- Physical factors 2- Chemical factors2- Chemical factors 3- Electrical factors3- Electrical factors
  • 70. 1- Physical factors1- Physical factors  ThermalThermal :: Warming increase excitability due toWarming increase excitability due to increase of the metabolic reactions .increase of the metabolic reactions . Cooling decrease excitability due toCooling decrease excitability due to decrease of the metabolic reactions .decrease of the metabolic reactions .  MechanicalMechanical :: deep pressuredeep pressure decrease excitabilitydecrease excitability
  • 71. 2- Chemical factors2- Chemical factors :: A- Local anaestheticsA- Local anaesthetics :: Local anaesthetics asLocal anaesthetics as cocaine , novocaine &cocaine , novocaine & xylocaine decreasexylocaine decrease excitability by decreasing Na+excitability by decreasing Na+ permeability .permeability .
  • 72. B- Ions :B- Ions : -- Calcium :Calcium : Increase E.C.F calcium decreasesIncrease E.C.F calcium decreases excitability by decreasing Na+excitability by decreasing Na+ permeability & decrease E.C.F. calciumpermeability & decrease E.C.F. calcium do the reverse.do the reverse. - Sodium:- Sodium: Increase E.C.F. sodium increaseIncrease E.C.F. sodium increase excitability while its decrease decreasesexcitability while its decrease decreases excitability.excitability.
  • 73. Potassium :Potassium : increase of E.C.F. potassiumincrease of E.C.F. potassium increase excitability while itsincrease excitability while its decrease decreases excitability .decrease decreases excitability . Oxygen lack & carbon dioxideOxygen lack & carbon dioxide excess:excess: decreases excitability .decreases excitability . Acidosis:Acidosis: decreases excitabilitydecreases excitability whilewhile alkalosisalkalosis increases it.increases it.
  • 74. 3-Electrical factors3-Electrical factors ( Electrotonus ) :( Electrotonus ) : These are the changes that occurThese are the changes that occur at the cathode (catelectrotonus) &at the cathode (catelectrotonus) & at the anode (anelectrotonus)at the anode (anelectrotonus) when a subthreshold galvanicwhen a subthreshold galvanic current is applied to a nerve fiber .current is applied to a nerve fiber .
  • 75. CatelectrotonusCatelectrotonus :: slightslight depolarizationdepolarization occurs near theoccurs near the cathodecathode which is accompaniedwhich is accompanied byby increased excitabilityincreased excitability .. AnelectrotonusAnelectrotonus :: slightslight hyperhyper polarizationpolarization occurs near theoccurs near the anodeanode which is accompaniedwhich is accompanied byby decreased excitabilitydecreased excitability..
  • 76. Types of nerve fibersTypes of nerve fibers TypeType AA BB CC DiameterDiameter 3-20 um3-20 um 1-3 um1-3 um 0.5-1.0 um0.5-1.0 um MyelinationMyelination MyelinatedMyelinated MyelinatedMyelinated unmyelinatedunmyelinated Velocity ofVelocity of conductionconduction 10-120 m/sec10-120 m/sec 5-10 m/sec5-10 m/sec 0.5-5.0 m/sec0.5-5.0 m/sec Sensitivity toSensitivity to PressurePressure LocalLocal anestheticanesthetic + + + ++ + + + ++ + ++ + + ++ + ++ + + + ++ + + + ExampleExample Sensory &Sensory & motor somaticmotor somatic PreganglionicPreganglionic autonomicautonomic PostganglionicPostganglionic autonomicautonomic
  • 77. All or non lawAll or non law Action potential either occursAction potential either occurs maximally or it does not occurmaximally or it does not occur at all , provided that all otherat all , provided that all other conditions remain constant .conditions remain constant . It is obeyed by a single nerveIt is obeyed by a single nerve or single muscle fiber .or single muscle fiber .
  • 78. Action potential Local excitatory state Threshold stimulus Subthreshold stimuli Suprathreshold stimuli Resting potential Threshold
  • 79. Sub minimal ( sub threshold )Sub minimal ( sub threshold ) stimulus does not produce anstimulus does not produce an action potential .action potential . Threshold stimulus producesThreshold stimulus produces the maximal action potential .the maximal action potential . Supra threshold , maximal &Supra threshold , maximal & supra maximal stimuli producesupra maximal stimuli produce the same action potential .the same action potential .
  • 80. Differences between action potentialDifferences between action potential & local excitatory state& local excitatory state LocalLocal excitatory stateexcitatory state (L.E.S.)(L.E.S.) ActionAction potentialpotential (A.P.)(A.P.) CauseCause Sub thresholdSub threshold stimulus.stimulus. Threshold & more.Threshold & more. All or non lawAll or non law Doesn’t obey.Doesn’t obey. Obeys.Obeys. GradationGradation Can be graded.Can be graded. No gradation.No gradation. SummationSummation Can be summated.Can be summated. No summation.No summation. ConductionConduction Decreased graduallyDecreased gradually till disappear.till disappear. Conducted with theConducted with the same intensity.same intensity.
  • 81. Action potential in mixed nerveAction potential in mixed nerve (compound action potential)(compound action potential) Mixed nerve contains different types ofMixed nerve contains different types of nerve fibers withnerve fibers with differentdifferent excitabilityexcitability && differentdifferent velocity of conductionvelocity of conduction .. Action potential in mixed nerve:Action potential in mixed nerve: 1- Has multiple peaks1- Has multiple peaks :: the first peak isthe first peak is caused by the most rapid fibers followedcaused by the most rapid fibers followed by the peak of less rapid fibers & lastly theby the peak of less rapid fibers & lastly the peak of the slowest fibers.peak of the slowest fibers.
  • 82. 2- Doesn't obey all or non law2- Doesn't obey all or non law ::  Sub threshold stimulusSub threshold stimulus produces no actionproduces no action potential .potential .  Threshold stimulusThreshold stimulus produces an actionproduces an action potentialpotential  Supra thresholdSupra threshold stimulus produces morestimulus produces more intense action potentialintense action potential  Maximal stimulusMaximal stimulus produces the maximalproduces the maximal action potential due to stimulation of all nerveaction potential due to stimulation of all nerve fibers .fibers .  Supra maximal stimulusSupra maximal stimulus produces the sameproduces the same maximal action potentiamaximal action potentia
  • 83. Action potential in a mixed nerveAction potential in a mixed nerve (Compound action potential)(Compound action potential) DurationDuration Voltage