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Dendritic computation

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Passive contributions to computation Active contributions to computation Dendrites as computational elements: Examples Dendritic computation

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r V m = I m R m Current flows uniformly out through the cell: I m = I 0 /4 r 2 Input resistance is defined as R N = V m (t)/I 0 = R m /4 r 2 Injecting current I 0 Geometry matters: the isopotential cell

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r m and r i are the membrane and axial resistances, i.e. the resistances of a thin slice of the cylinder Linear cable theory

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riri rmrm cmcm For a length L of membrane cable: r i r i L r m r m / L c m c m L Axial and membrane resistance

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(1) (2) (1) or where Time constant Space constant The cable equation

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0 Decay of voltage in space for current injection at x=0, T + I ext (x,t)

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Electrotonic length Properties of passive cables

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Johnson and Wu Electrotonic length

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Properties of passive cables Electrotonic length Current can escape through additional pathways: speeds up decay

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Johnson and Wu Voltage rise time Current can escape through additional pathways: speeds up decay

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Koch Impulse response

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General solution as a filter

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Step response

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Electrotonic length Current can escape through additional pathways: speeds up decay Cable diameter affects input resistance Properties of passive cables

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Electrotonic length Current can escape through additional pathways: speeds up decay Cable diameter affects input resistance Cable diameter affects transmission velocity Properties of passive cables

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Step response

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Other factors Finite cables Active channels

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Rall model Impedance matching: If a 3/2 = d 1 3/2 + d 2 3/2 can collapse to an equivalent cylinder with length given by electrotonic length

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Active conductances New cable equation for each dendritic compartment

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Wholl be my Rall model, now that my Rall model is gone, gone Genesis, NEURON

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Passive computations London and Hausser, 2005

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Linear filtering: Inputs from dendrites are broadened and delayed Alters summation properties.. coincidence detection to temporal integration Segregation of inputs Nonlinear interactions within a dendrite -- sublinear summation -- shunting inhibition Delay lines Dendritic inputs labelled Passive computations

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Spain; Scholarpedia Delay lines: the sound localization circuit

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Passive computations London and Hausser, 2005

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Mechanisms to deal with the distance dependence of PSP size Subthreshold boosting: inward currents with reversal near rest Eg persistent Na + Synaptic scaling Dendritic spikes Na +, Ca 2+ and NMDA Dendritic branches as mini computational units backpropagation: feedback circuit Hebbian learning through supralinear interaction of backprop spikes with inputs Active dendrites

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Segregation and amplification

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The single neuron as a neural network

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Currents Potential Distal: integration Proximal: coincidence Magee, 2000 Synaptic scaling

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Synaptic potentialsSomatic action potentials Magee, 2000 Expected distance dependence

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CA1 pyramidal neurons

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Passive properties

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Active properties: voltage-gated channels Na +, Ca 2+ or NDMA receptor block eliminates supralinearity For short intervals (0-5ms), summation is linear or slightly supralinear For longer intervals (5-100ms), summation is sublinear I h and K + block eliminates sublinear temporal summation

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Active properties: voltage-gated channels Major player in synaptic scaling: hyperpolarization activated K current, I h Increases in density down the dendrite Shortens EPSP duration, reduces local summation

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Synaptic properties While active properties contribute to summation, dont explain normalized amplitude Shape of EPSC determines how it is filtered.. Adjust ratio of AMPA/NMDA receptors

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Rall; fig London and Hausser Direction selectivity

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Back-propagating action potentials

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Johnson and Wu, Foundations of Cellular Physiology, Chap 4 Koch, Biophysics of Computation Magee, Dendritic integration of excitatory synaptic input, Nature Reviews Neuroscience, 2000 London and Hausser, Dendritic Computation, Annual Reviews in Neuroscience, 2005 References

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Nucleus Dendrites Collect electrical signals Cell body Integrates incoming signals and generates outgoing signal to axon Axon Passes electrical signals.

Nucleus Dendrites Collect electrical signals Cell body Integrates incoming signals and generates outgoing signal to axon Axon Passes electrical signals.

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