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Neuron and Dendrite

Neuron and Dendrite.  3 parts of Neuron : dendrite(input) : body(processor) : axon (output)  Hillock : initial section of axon : lowest threshold; 8mV : AP is generated. 그림 569. Dendrite Tree.  Input architecture of neuron : branch collects about 100,000 messages

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Neuron and Dendrite

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  1. Neuron and Dendrite 3 parts of Neuron : dendrite(input) : body(processor) : axon (output) Hillock : initial section of axon : lowest threshold; 8mV : AP is generated 그림 569

  2. Dendrite Tree Input architecture of neuron : branch collects about 100,000 messages : coming from preceding neurons : via synaptic & electrical gap junction  Dendrite branches : nonregenerating & unmyelinated : rapid attenuation of signal  dendrite potential Inhibitory inputs : relatively weak & close to neuron body : raise firing threshold of neuron Output resulting from combination of input signals 그림 569

  3. Dendrite Potential Propagates toward neuron body : increase in time width, decrease in voltage : neuron body is excited by simultaneous APs : less attenuation for closer junctions  fewer stimulating neuron Generation ofAction Potential : when net summation exceeds threshold Graded potential generating neuron : proportional potential to net summation of DPs

  4. Four targets for the messages 1. To skeletal muscle : result in contraction 2. To cardiac, smooth muscle & visceral organs : Action Potential & Graded Potential(Sometimes) :release of chemical transmitter stimulated target tissue  Heart muscle - sympathetic nerve stimulation  adrenaline  increase contraction of heart muscle - parasympathetic nerve stimulation acetylcholine  decrease contraction  Pancreas - parasympathetic nerve stimulation acetylcholine  increase production of digestive enzyme - sympathetic nerve stimulation(during stress)  adrenaline  inhibit production of digestive enzyme

  5. 4 Targets for the messages 3. To neurosecretory cells : release of neurohormones  Hypothalamus neuron : send axon to nearby posterior pituitary  secretion of substance  stimulate or inhibit the secretion of hormones of pituitary 4. On the dendrites of other neurons : transmission of message : processing of message

  6. cf) Temporal summation Spatial Summation of Dendrite

  7. Dendrite  Treelike structure : dendritic tree : gather messages : fewer to 100,000 : via synaptic junction : via electrical gap junction : single AP at the periphery 그림 5-1

  8. Synaptic Junction Dimension - diameter; 1m(10,000Å) - synaptic gap; 200Å  Transmission : Action Potential arrival excitatory transmitter (acetylcholine)  diffuse across the junction  stimulate carrier protein  transport Na+ into dendrite  events similar to Action Potential  Postsynaptic Potential(PSP) : Calcium plays a vital role 그림570

  9. Post Synaptic Potential Waveform of PSP : 0.6ms delay for gap transmission : acetylcholine molecules - not march in unison - straggle across, spread out in space & time : 2ms delay for peak appearance : Pre Synaptic Potential(2ms)  Post Synaptic Potential(4ms) : 100mV high, 4ms wide  200Å junction gap : provides access for different chemical messengers(hormone,--) : modify the excitability of the dendritic terminal gap : mediation by alcohol or drug

  10. Chemical Inhibition Chemical messengers decreasing the excitability : inhibition : gamma aminobutylic acid(GABA) - stimulate potassium carrying protein - blocking sodium carrying : negative charges inside the axon  negative spike  PSP is negative : relatively weak; 10mV of PSP( instead of 100mV) GP generating junction gap : as in retina (distances between cells are small) : junction filled with chemical transmitters : chemical messengers carry GP across the gap

  11. Electrical Gap Junction Electrical gap junction : 2ms time delay is intolerable : no need of alternation of excitability : no need of 200 Å : replaced by tight or electrical gap junction - axon terminal and dendrite are tightly interwined - finite but negligibly small gap - electrical signal gets across to the dendrite : transmit APs, GPs, but no inhibition Population of Synaptic Junctions : axon to dendrite & axon to soma junctions  axon to axon junction & dendrite to dendrite junction

  12. Dendrite Tree Model Tree model : levels 4,3,2,1,0 : level 4 (periphery of the tree) : level 0 (neuron body, soma) : signals from left to right (to a lower level) : funnel & modify messages : a single output axon  collateral branches (upto 1000s) : can generate APs or GPs 그림 5-1

  13. Attenuation in Dendrite  Information flood? : PSP is 4ms wide  250 PSPs can flood the neuron : flood  steady & meaningless stream : for 10,000 junctions 250PSPs/s/dendrite = 0.025 spikes/s/junction (40s) : spontaneous activity of 50 spikes/s  five of these input can saturate neuron ?! Attenuation of PSP : dendrite tree - nonregenerating - unmyelinated : shunt leakage conductance G - long nonregenerative fiber - excessive error without G Figure 5-2

  14. RCG Dendrite Cable Model dV dx = - I•R Figure 5-2 dI(s) dx = - (G+sC)V(s) d2V(s) dx2 = R(G+sC)V(s) R : series axoplasm resistance C : shunt membrane capacitance G : shunt leakage conductance

  15. Attenuation through RC Cable d2V(s) dx2  A0x RC x2RC Gt 2t3/2 4t C = R(G+sC)V(s) V = exp(- - ) V(s) = c1e-xR(G+sC)+ c2e x R(G+sC) : c2= 0 (V  0 as x   ) : c1= Vin(V=Vin at x=0) V(s)= Vin (s) e-x R(G+sC) : Vin(s)= A0(Vin impluse of A0 V•s) V(s)= A0e-x R(G+sC)  Gt C A0x RC x2RC 2t3/2 4t = exp(- )•exp(- ) RC Model Solution G effect

  16. Waveform of PSP  Length Constant : associated with DC response  C=O : V(s)= A0 e-x R(G+sC) = A0 e-x RG  0 = 1/  RG  Normalized distance : z = x/ 0 = x RG : x = z/ RG  Membrane time constant : RC= C/G =4(ms) C=4G(ms)  A0=3054mV•ms A0x RC x2RC Gt 2t3/2 4t C V = exp(- - )  A0z z2 t t3/2 t 4 V = exp(- - )  1723z z2 t t3/2 t 4 V = exp(- - ) Peak at tp=  9+4z2 -3 PSP of 4ms wide 100mV high at z=2

  17. Waveform Parameters 1723z z2 t t3/2 t 4 Area of PSP waveform : exponentially decay : due to shunt leakage : Area = A0exp(-z)  Equivalent wave width : tw = Area/Vp V = exp(- - ) Peak of PSP : at tp=  9+4z2 -3 : tp =2ms for z = 2 : Vp =100mV at tp =2ms  PSP at z=2 : 4ms wide : 100mV peak branch length = 0 /2

  18. Modeled Post Synaptic Potential tp Vp Area tw Level z (ms) (mV) (mVms) (ms) V(z,t) (mV, ms unit) 4 2.0 2.00 100 413 4.13 3 2.5 2.83 49.0 251 5.12 2 3.0 3.71 25.3 152 6.01 1 3.5 4.62 13.5 92 6.83 0 4.0 5.54 7.4 56 7.59 3446 4 t t3/2 t 4 V(2,t) = exp(- - ) 4307.5 6.25 t t3/2 t 4 V(2.5,t) = exp(- - ) 5169 9 t t3/2 t 4 V(3,t) = exp(- - ) 6030.5 12.25 t t3/2 t 4 V(3.5,t) = exp(- - ) 6892 16 t t3/2 t 4 V(4,t) = exp(- - ) When 16 Aps are arrived simultaneously at t=0, at the periphery terminal junctions

  19. Dependence on Diameter  Resistance l=125[cm] R= lll/Ac R= 159.2/d2 [/cm]  Leakage Conductance m=5.647109 [cm] G= As/mlm G= 7.41810-4d [1/cm]  Characteristic Resistance R0= (R/G)1/2 R0 = 463.2/d3/2 [, cm unit] d  (R0)-2/3 R0 = 14.65M when d=10um  Length Constant 0 = 1/(RG)1/2 0= 2.910 d [cm unit] 0  (R0)-1/3 0=0.46mm when d=10um

  20. Rall's Assumption Constant characteristic resistance R0 : independent of number of branches Calculation Assumption : values for entire branches rather than /cm : 10um in level 0( z=3.5~ 4) : each branch length = half of length constant : constant ensemble characteristic resistance R0 One level to next higher level : increase of characteristic resistance : doubles : diameter decrease: 22/3 : length decrease: 21/3 : surface area: halved (2-1) : total surface area of each row: 28,900um2(constant)

  21. Calculated Dendrite Model Parameters Number of d 0 R0 Area R C G z Branches(um) (mm) (M) (um2) (M) (pF) (nS) 2 to 2.5 16 2.50 0.230 117.2 1807 58.58 17.06 4.266 2.5 to 3 8 3.97 0.290 58.59 3614 29.29 34.13 8.533 3 to 3.5 4 6.30 0.365 29.30 7227 14.65 68.26 17.07 3.5 to 4 2 10.0 0.460 14.65 14455 7.323 136.5 34.13 soma 1 15.9 - 7.324 - - - - Ratio 2 2-2/3 2-1/3 22-1 22-1 2-1 Level 4 3 2 1 0

  22. Number of Synaptic Junctions On a single Dendrite Tree : total surface area ; 116,000um2 : junction diameter :1um  junction area 1um2  100,000 junctions on a dendrite tree  signals from 10,000 different neurons is possible (10% use) Many junctions on soma directly

  23. Position is everything in life? Importance of Position in dendrite junction : tree is democratic : important neuron should terminate closer to soma - impossible for genetic coding of junction location (10,000 junctions/neuron: beyond capabilities of DNA) : location dominance; gradually modifies with aging Learning and Memory : learning growth and/or movements of junctions : memory  location and strength of junctions (more than one junction on same dendrite)

  24. Human Memory 1 hour from short term memory to long term memory : accident victim; severe blow, electroconvulsive shock - partial or full erase of memories during last one hour - no effect on events experienced, synaptic junctions : plenty of room for new junctions(100,000 junctions) - anatomical evidence is lacking, difficult in vivo (theory) Forgetting:Junction migration(no evidence; theory) : new junction  move of old junction to higher level - if not, tree will be saturated with junctions : forget as much as learn : person who cannot learn recent event, remarkably recall distant events : old dog; cannot teach new trick, don't forget old ones

  25. Message sequences Complicated function : dendritic location : time of occurrence : complicated responses : computer simulation for arbitrary set of inputs Fig. 5-3 z Node or Time PSP peak value branch# in ms in mV 2 06 0.0 100 2 06 7.5 100 2 06 15.0 100 2 06 22.5 100 2 06 30.0 100

  26. Message sequences  Solid Line z Node or Time PSP peak value branch# in ms in mV 2 01~16 0.0 100 2.5 01~08 0.0 100 3 01~04 0.0 100 3.5 01~02 0.0 100 Fig. 5-4  Dotted Line z Node or Time PSP peak value branch# in ms in mV 2 01~16 0.0 100 2.5 01~08 1.0 100 3 01~04 2.5 100 3.5 01~02 4.0 100

  27. Sequence with Inhibitory Inputs  Solid Line z Node or Time PSP peak value branch# in ms in mV 2 01~16 0.0 100 2.5 01~08 0.0 100 3 01~04 0.0 100 3.5 01~02 0.0 -92 Fig. 5-6

  28. Effects of Inhibitory PSP  without inhibition with inhibition : threshold; -82mV : threshold; -72mV : -80mV; 10Hz AP : -80mV; no AP : -61mV; 50Hz AP : -61mV; 16.6Hz Fig. 3.2 Fig.5.6 Inhibition  No AP or reduced rates of APs

  29. Of what use is inhibition? Muscle contraction : muscles come in pairs(flexor,extensor) : doesn't stimulate at the same time : wired to function antagonistically : flexor(excitation) extensor(inhibition) extensor(excitation) flexor(inhibition)

  30. Pattern of Inhibitory Inputs Weaker than excitatory PSP: 1/10 level : to be effective  close to body, on the body : on the body  not raise to 10mV ( because of large volume of body) : simultaneous inhibition  block or reduce rate of APs  Effective Inhibition : inhibitory PSP arrives first : sustained inhibition continuous barrage of inhibitory inputs

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