BIOL3833 Week 11b: Dendrites

A little catch-up Today: Dendrites and brief introduction to sensory processing Thursday: Quiz then first part of lecture on hearing

Back when life was easy: Dendrites Soma Axon Synapse

What do dendrites do? They do a lot. From an electrophysiological perspective, they may be the most complex part of a neuron.

Integrate subthreshold synaptic inputs Compartmentalize information processing Determine how and when the neuron will fire

Integration Passive dendritic properties Active dendritic properties

The Passive Dendrite

Rm Ra I

Vrest = -60mV Depolarize to -50 mV

Rm Ra Decreasing Ra allows more current to move toward the soma thus less signal loss with distance What are the limits on this strategy? Diameter must increase by factor of 4 to double the distance efficiency (length constant)

Dendrite Manipulating dendrite diameter only works up to a point. We need something better. Soma

Limitations of the Passive Dendrite How to overcome the limitations?

Add more receptors to the distal dendrite Another solution that dendrites use: Higher receptor density in distal dendrites

But – there is a limit on how big the distal EPSP can be Reversal potential of AMPA is 0 mV so max EPSP is 0 – Vrest!!

Active Dendrites? The assumption until just 10-15 years ago was that dendrites were just passive – basically just tubes of membrane. Indirect evidence started to accumulate that dendrites are almost never just passive membrane.

Dendrites express voltage-gated Na+, K+ and Ca2+ channels The distributions of these channels is very uneven Can enhance or inhibit excitatory input

Supralinear summation suggests the presence of dendritic ion channels Polsky, et al. 2004, Nature Neuroscience

and get backpropagating action potentials! Add ion channels…. and get backpropagating action potentials! Haüsser, et al. 2000, Science

Yes, those really are action potentials. Hippocampus Cerebellum

and get dendritic spikes! Add ion channels…. and get dendritic spikes! Synaptic stim. Haüsser, et al. 2000, Science

Dendrites express voltage-gated Na+, K+ and Ca2+ channels The location and densities of these channels is highly plastic (changes minute-to-minute) These ion currents can enhance or suppress EPSPs Produce nonlinear summation Generate dendritic Na+ or Ca2+ spikes Backpropagate somatic action potentials The distributions of these channels is very uneven Can enhance or inhibit excitatory input

What can dendritic spikes and backpropagated action potentials do? Help to activate NMDA receptors Increase calcium levels in dendrite Activate slow K+ channels in dendrite (this is where the IAHP and SK channels are!)

Compartmentalization Electrical events Biochemical events

Light microscope Electron microscope (serial reconstruction)

This movie shows CaMKII activation during structural plasticity induced by 2-photon glutamate uncaging in the absence of extracellular Mg2+. The white arrowhead indicates the location of 2-photon glutamate uncaging.

One of the places where the role of dendritic processing has been most thoroughly investigated is in direction selectivity in the retina,