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Physiology of Dendrites
• Passive electrical properties• Active properties of dendrites• How dendrites transform their inputs• Dendrites as axon-like output elements• Spines
• Special physiological features• Behavior in plasticity• Changes in disease and aging
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Segev I J Neurophysiol 2006;95:1295-1297
©2006 by American Physiological Society
Wilfrid Rall
Modeling electrotonic properties
http://www.genesis-sim.org/GENESIS/Tutorials/cnslecs/cns2a.html
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length constantmembrane resistance
internal resistance
• thin dendrites have short length constants (large ri)
• leaky dendrites have short length constants (small rm)
The length constant is the distance at which 37% of Vmax has been reached during the fall of voltage
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Length constant is proportional to square root of process diameter
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Dendrites – electrotonic features
• Synaptic potentials passively conducted down a dendrite are
• attenuated• slowed• temporally filtered
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Slowing and attenuation of distal inputs
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Dendrites – electrotonic features
• Temporal summation of synaptic inputs• nearly synchronous inputs summate (but non-
linearly)• inputs widely separated in time do not interact
• Spatial summation of synaptic inputs• nearby inputs summate (but non-linearly)• widely separated input interact only weakly
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Spatial SummationTemporal Summation
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A potential problem:dendritic filtering
Because of the leaky cable structure of dendrites, inputs fade away with distance.
Can distal inputs influence spiking?
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Mitral cells of olfactory bulbdriven effectively by distal input
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Possible solutions:
Passive Properties
• Increase length constant and lower capacitance.
• Increase size of EPSPs distally.
Active Properties
• Voltage-dependent ion-channels could boost the signal along the way.
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Spines – electrotonic featuresIncrease length constant
Increase input resistance Decrease internal resistance
Ri large Ri small
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Problem: changes in morphology are not always practical:
In order for the length constant to double, the diameter of the dendrite has to increase by a factor of four.
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Spines – electrotonic featuresIncrease strength of distal synapses
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Magee and Cook, 2000
Synaptic strength is higher for distal synapsesso soma “sees” similar EPSP amplitudes
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Vm = -60mV
25 mV
7.5 mV
67.5 mV
Vrev = 0 mV
However, distal inputs can only be so big…
So what is a poor dendrite to do?
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Spines – electrotonic features
• Small neck, high input resistance• maximizes synaptic potentials
• Low capacitance• maximizes frequency response
• Impedance mismatch with dendritic trunk results in asymmetric effects
• spine voltage has relatively little effect on dendrite (local action)• dendritic voltage significantly influences spine
Use voltage gated channels to boost distal inputs
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Dendrites can generate spikes
Llinás and Sugimori 1980
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Active properties vary within and between neurons
• Purkinje cells• P-type calcium channels• Few sodium channels• Little backpropagation of spikes
• Cortical pyramidal cells• Calcium and sodium channels• Robust backpropagation of spikes•
• Some neurons have minimal active properties
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Hausser et al., Science 290, 2000
stimulate soma stimulate dendrite
pyramidal cell
Purkinje cell
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Retinal bipolar cells• electrotonically compact
• few active conductances in dendrite or axon
• lack regenerative spikes
• yet effectively communicate synaptic inputs to inner retina
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Grimes et al., Neuron 65, 873, 2010
Dendritic compartments can act independently
A17 amacrine cells
• single vericosities operate independently• efficiency of single shared soma
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Horizontal cells: uncoupled dendritic and axonal compartments
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Non-linear properties of dendrites serve diverse functions
• Boost synaptic responses in graded fashion
• Thresholding (non-linear amplification of stronger
inputs)
• Propagate spikes in anterograde or retrograde
direction•
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Dendritic spikes propagate in both directions
Forward Propagation Backpropagation
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Early evidence for somadendritic spikes
Eccles, 1957
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Hausser et al., Science 290, 2000
Contemporary evidence for dendritic spikes
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Backpropagation – functional roles
• Pyramidal-cells• boost somadendritic spike so it invades the dendritic tree• reset membrane potential for new inputs• depolarize spines
• gate NMDA receptors• coincidence detection for Hebbian increase in synaptic
strength
• Mitral cells and dentate granule cells • trigger release from presynaptic dendrites
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Direction of information flow in dendrites affected by many factors
• Extent and complexity of branching (electrotonic factors)
• Distribution of excitatory and inhibitory synapses• Distribution of voltage gated channels• Interaction among all of these factors
•
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Spines – special features
• Narrow neck high input resistance• maximizes EPSP evoked by synaptic conductance
• Low capacitance • maximizes frequency response
• Impedance mismatch where neck meets shaft• spine has trouble strongly influencing parent dendrites• voltage fluctuations in shaft do influence spine
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Spines – role in plasticity
• Big changes in spine form and motility in development
• Enriched environments increase spine number
• LTP more and bigger spines •
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Abnormal spine morphology in a model of mental retardation
wildtype Fragile-X
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www.neurostructural.org/images/nine.jpg
normal Alzheimer’s
Spine loss in neurodegenerative diseases
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Dendrites as presynaptic elements
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Mitral cell dendrodendritic synapsesself and lateral inhibition
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Lin and Koleske, 2010
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Bhatt et al., 2009
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Bhatt et al., 2009
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Most retinal amacrine cells lack axons
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Gap junction - substrate for electrical synapses
Wagner, C. Kidney International (2008) 73, 547–555
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A-type and B-type horizontal cells in the rabbit retina have different dye-coupling properties.
O'Brien J J et al. J. Neurosci. 2006;26:11624-11636
©2006 by Society for Neuroscience
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Cx50 plaques occur at dendritic crossings in calbindin-labeled A-type horizontal cells.
O'Brien J J et al. J. Neurosci. 2006;26:11624-11636
©2006 by Society for Neuroscience
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Grimes et al., Neuron 65, 873, 2010
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Spines – electrotonic features
• Small neck, high input resistance• maximizes synaptic potentials
• Low capacitance• maximizes frequency response
• Impedance mismatch with dendritic trunk results in asymmetric effects
• spine voltage has relatively little effect on dendrite (local action)• dendritic voltage significantly influences spine
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Spines – role in plasticity
• Spine morphology changes during development• @
• Enriched environments and training increase spine numbers
• @
• Long-term potentiation• increases spine numbers• increase spine volume in single spines monitored over time
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Direction of Dendritic Spikes is Bidirectional
Forward Propagation Backpropagation
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Active Dendritic Properties Summary:
• Active conductances are present in dendrites.• Not uniform expression within dendrites or between neurons.• Boost subthreshold EPSPs.• Generate dendritic spikes.• Lead to non-linear synaptic integration.• Backpropagate somatic action potentials: open NMDAR, increase dendritic Ca++ levels.
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Hausser et al., Science 290, 2000
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Hausser et al., Science 290, 2000