Fundamentals of neuroscience: cells, axons, and synapses

ANAT 3045 Visual NeuroscienceBIOS 3001 Advanced Visual NeuroscienceFundamentals of neuroscience: cells,axons, and synapsesProfessor Tom SaltUCL Institute of Ophthalmologyt.salt@ucl.ac.ukInstitute ofOphthalmologyThe NeuroneThe Nervous system’s wiring can bedescribed in terms of neuronal cell typesin each of its distinct gray matter regionsand their stereotyped pattern of axonalprojections to cell types both locally and inother gray matter regions (or other tissues– eg muscle)Grey Matter – areas ofneuronal cell bodiesWhite Matter – areas ofnerve fibres (axons)VesicleDendritesCell Body (Soma)AxonSynapticCleftAxonNucleusMyelin SheathDirection ofImpulseAxonTerminalsThe Neurone DoctrineFunctional Polarity Rule• Santiago Ramon y Cajal~1890s– Neurone Doctrine: Eachneurone is an individual entity,the basic unit of neural circuitry(cf ‘reticularist view’ of egCamillo Golgi)• Charles Sherrington ~1897– Postulated that neuronesfunctionally contact each otherand other cell types (eg muscle)via a theoretical structure hetermed the “synapse”.ReticularNeurone• Santiago Ramon y Cajal– Functional Polarity: The Dendrites and Cell bodies ofneurones receive information, whereas the single axonwith its collaterals transmits information to other cells.– This rule allows prediction of information flow directionthrough neural circuits based on morphology ofindividual neurones.• Functional Polarity was the cornerstone of CharlesSherrington’s (1906) revolutionary analysis ofmammalian reflex organisation.Applying the Neurone Doctrine and Functional PolarityResting Membrane PotentialmVCajal’s (1909-1911) neural architecture drawing based on the Golgi method.Glass CapillaryElectrodeTo Voltmeter+200-40Extracellular PotentialKCl-80Resting PotentialChamberSaline0- +0- +CellsVVOptic Tectum(superior colliculus)RetinaFor most cells, RestingMembrane Potential is-55mV to -75mVCellCell1

Distribution of Major Ions Across theMembrane of the Squid Giant AxonDistribution of Major Ions Across theMembrane of the Frog MuscleIon Cytoplasm ExtracellularK + 40020Cl - 52 560Na + 50 440Ion Cytoplasm ExtracellularK + 124 2.3Cl - 1.5 78Na + 10 109(mM)(mM)(mM)(mM)Resting potential ca. -60mVResting potential ca. -100mVWhat Forces Govern the Movements of Ions?1. Concentration Gradients. i.e. diffusionfrom high to low concentration areas.2. Electric Charge Separation. i.e. ionstend to move towards regions of oppositeelectric charge.3. Cell Membrane. i.e. ion movement isrestricted by the physical barrier imposedby the cell membrane.Selectively Permeable MembraneIn OutA -A -K +K +A -A - K + K +K +K + A -K + K +A -A -A - K +K +A - K +A - K +K + A -K + A -K +K + A -A - A -A -Selectively Permeable MembraneSelectively Permeable MembraneIn OutIn OutA -A -K + K +A - A -K +KK ++ K +A - A - K + A - A -K +K +K +KK ++K + AĀ- KA -+K +A -A- A - K +A -A -K +A - K +K +K +A -K +KA - +A - K +K +K K + A -K ++A -K + A - A -KK + A - + A -A - K + K +A -A -A - A - A -- - - + + +- - - - + + + +2

Nernst EquationRTE K =__ZFWhereln____ [K + ] o[K + ] iE K = K + Equilibrium Pot’l.R = Gas ConstantT = Absolute TemperatureZ = Valence of K +F = Faraday Constant[K + ] o,i =K + concentrationsSubstituting,E K = 26mVNerve___ 20ln400 = -75mVRelationship of Membrane Potential (V m ) to [K + ] o0V m25mV5075100Nernst1 10 [K + ] 100o mM1. How can concentration gradients forNa + , K + , and Cl - all be maintainedacross the cell membrane?2. How do these gradients interact todetermine the resting membranepotential?IonDistribution of Major Ions Across theMembrane of the Squid Giant AxonCytoplasmExtracellularFluidNernstPotentialPassive and Active Movement of Ions through the MembraneMembraneIntracellularK + K +E K - V m = -10mVExtracellularK + 400 20-75Cl - 52 560 -60Na + 50 440 +55(mM) (mM) (mV)MetabolicEnergy(ATP)Na +Na + -K +PumpK +Na +Resting Pot’l (V m ) = -75mVE Na - V m = 155mVResting potential ca. -60mVGOLDMAN EQUATIONV m =RT __FlnIf P K>> P Naand P Cl,_______________________P K[K + ] o+ P Na[Na + ] o + P Cl[Cl - ] iP K[K + ] + iP Na[Na + ] i+ P Cl[Cl - ] oRT __V m ~FTherefore,The greater the permeability and concentration ofan ion, the greater will be its contribution to V m .ln_____ P K[K + ] oP K[K + ] iwhere P = Permeability Membrane potential (Vm(Vm) ) is determined primarily by K+ andNa+. Membrane potential will be closest to the Nernst(Equilibrium) Potential of the ion with the greatestconcentrations and membrane permeability. At Rest, Membrane Potential is close to the potassiumequilibrium potential (EK+) because the membrane is mostpermeable to K+. At Rest, as EK+ is slightly more negative than Vm, , there is asteady K+ efflux, balanced by a steady Na+ influx. Thesetwo passive fluxes are balanced by active pumping of Na+and K+ in the opposite directions. Note: this is a steadystate, not an equilibrium. Under most physiological conditions the bulk concentrationsof Na+, K+ and Cl- inside and outside of the cell remainconstant.3

Electrotonic Potential SpreadACTION POTENTIALTHRESHOLDCurrentPotential (V)CurrentElectrotonicPotential+20mV0Overshoot-60mVUpstroke0 100 time (ms)-20Threshold-40UpstrokeRepolarisationCurrent-60-80Resting Pot’lThreshold-80Current Pulses2.5 mm0 ms 50 ms 5Voltage-dependent Na + ChannelsSodium Channel StructureDEPOLARISATION“Positive Feedback”or “Regeneration”Na + EntersCellVoltagedependentNa +Channels OPENNote: Voltage-dependent Na + ChannelsINACTIVATEThe Refractory PeriodPERMEABILITY CHANGES DURING THE ACTION POTENTIAL+20+20 V m 0V m0-20Na + K +-20-40mV-60-800 ms 5300MembranePermeabilityorConductancemS/cm 2-40mV-60-80Threshold0Absolute Relative5 msREFRACTORY PERIOD4

PROPAGATION OF THE ACTION POTENTIAL - 1Unmyelinated nerve membranePROPAGATION OF THE ACTION POTENTIAL - 1Unmyelinated nerve membraneExtracellular+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -IntracellularNa +Extracellular+ + - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +- + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - -IntracellularNote• No significant change in ionic concentrations during the action potential.• No change in Na + Pump activity during the action potential.PROPAGATION OF THE ACTION POTENTIAL - 1Unmyelinated nerve membranePROPAGATION OF THE ACTION POTENTIAL - 2Unmyelinated nerve membraneNa +Extracellular+ + - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +- + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Local Circuit CurrentsIntracellularNa +Extracellular+ + + + + + - - + + + + + + + + + + + + + + + + + + + + + + + + + +- - - - - - + + - - - - - - - - - - - - - - - - - - - - - - - -K +IntracellularPROPAGATION OF THE ACTION POTENTIAL - 3Unmyelinated nerve membranePROPAGATION OF THE ACTION POTENTIAL - 3Unmyelinated nerve membraneNa +Extracellular+ + + + + + + + + + + + + - - + + + + + + + + + + + + + + + + + + + + +Na +Extracellular+ + + + + + + + + + + + + + + + + - - + + + + + + + + + + + + + + + +- - - - - - - - - - - - + + - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - -and so forthK + IntracellularK +IntracellularNote• An action potential is triggered in each portion of the membrane• The action potential can travel in all directions from the initial triggering point.• The action potential cannot immediately reinvade a portion of membrane,because of the refractory period.5

The Erlanger / Gasser Classification of Nerve FibresFibreTypeFunction (examples)Avg. Fibre Dia.(m)AAAABCThe Lloyd Hunt Classification of Nerve FibresGroup Function (examples)Avg. Fibre Dia.(m)IIIIIVPrimary Muscle Spindle AfferentsMotor to Skeletal MusclesCutaneous Touch and Pressure AfferentsMotor to Muscle SpindlesCutaneous Temperature and Pain AfferentsSympathetic PreganglionicCutaneous Pain AfferentsSympathetic PostganglionicPrimary Muscle Spindle AfferentsAfferents from Tendon OrgansCutaneous Mechano-receptorsDeep Pressure Sensors in MuscleUnmyelinated Pain Afferents1585< 331(unmyelinated)13931Avg. Cond. Vel.(m/s)100 (70-120)50 (30-70)20 (15-30)15 (12-30)7 (3-15)1 (0.5-2)Avg. Cond. Vel.(m/s)75 (70-120)55 (25-70)11 (10-25)1 (0.5-2)Myelinated Nerve Fibres• Conduction velocity increases with fibre diameter, but the greatest influence iswhether or not an axon is myelinated.• Myelin sheath consists of membranes of Schwann Cells wrapped around theaxons. This wrapping is periodically interrupted at what are termed the Nodes ofRanvier.• The effect of this sheath is to increase the resistance across the cell membranefrom the inside of the axon to the extracellular space, and to concentrate chargedistribution at the Nodes of Ranvier.Action Potential Propagation in Myelinated AxonsAction Potential Propagation in Myelinated Axons+ ++ +Myelinated nervemembrane+ +ExtracellularNa ++ +Myelinated nervemembrane+ +Extracellular- -- -- -- -- -IntracellularIntracellularIncreased chargedistribution at theNodes of RanvierAction Potential Propagation in Myelinated AxonsAction Potential Propagation in Myelinated AxonsNa ++ +Myelinated nervemembrane+ +ExtracellularMyelinated nervemembraneNa ++ +Extracellular- -Local Circuit Currents- -IntracellularK +- -Intracellularlocal circuit current flow is from one Node of Ranvier to the next6

Action Potential Propagation in Myelinated AxonsVoltage ClampTwo-electrode voltage clamp of squid axon, after Hodgkin & HuxleyExtracellular+ +- -IntracellularMyelinated nervemembraneNa +K +and so forth• Effectively the action potential is propagated from one node of ranvierto the next. This is referred to as Saltatory Conduction.• The net result of this is that an action potential can travel morequickly by jumping along a myelinated axon rather than by beingconventionally propagated along an umyelinated axon.The voltage-clamp technique keeps the voltage across themembrane constant so that the amplitude and time courseof of ionic currents can be measuredVoltage Clamp Reveals Ionic CurrentsVoltage Clamp recordings from CellsTwo electrode voltage clamp(large cells eg oocytes)MeasureVoltage-70mVInjectCurrentSingle electrodevoltage clamp(neurones) • Measure voltage• is it what we want ?• Pass current to adjustvoltageMembranecurrents-70mVTEA - TetraEthylAmmonium – blocks K + currentsTTX - TetrodoToXin – blocks Na + currentsWhole Cell (Patch) recordingsExtracellular Recording methodswholecellMembranecurrentsGlycine 50uM100pAMicroelectrodeSingle neurone (unit) spikes500uVFlash1 sGlycine 50uMneuroneField Potential Recordingisolatedpatch5pA1 sFlash7

SynapticCleftActionPotentialCHEMICAL SYNAPSESAxonTerminalPostsynaptic Cell! Action Potential Propagated tothe Nerve Terminal.! Arrival of an Action Potentialleads to release of a chemical(i.e. the Neurotransmitter)from the terminal.! Neurotransmitter diffusses to thePostsynaptic Membrane.! Binding of Transmitter moleculeto Receptors causes apostsynaptic effect (e.g.Membrane Depolarisation).Process of Chemical Neurotransmission Synthesis of neurotransmitter in the presynapticneurone Storage of the neurotransmitter and/or itsprecursor in the presynaptic nerve terminal Release of the neurotransmitter into the synapticcleft Binding and recognition of the neurotransmitterby target receptors Termination of action of the released transmitterLife Cycle of Synaptic VesiclesTYPES OF NEUROTRANSMITTERAminesAcetycholineNoradrenalineDopamineSerotonin (5HT)PeptidesEnkephalinsSubstance PSomatostatinCholecystokininExcit. Amino AcidsL-Glutamic AcidL-Aspartic AcidL-Homocysteic AcidPurinesAdenosineATPInhibitory Amino AcidsGlycineGABAFree RadicalsNitric Oxide (NO)LipidsCannabinoidsVanillinoidsThe “vesicle hypothesis”8