1. BIOL3833
Week 11b: Dendrites

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

5. Back when life was easy:
Dendrites
Soma
Axon
Synapse

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

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

9. Integration
Passive dendritic properties
Active dendritic properties

10. The Passive Dendrite

11. Rm Ra I

12. Vrest = -60mV
Depolarize to -50 mV

14. 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)

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

16. Limitations of the Passive Dendrite
How to overcome the limitations?

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

19. 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!!

20. Active Dendrites?
The assumption until just 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.

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

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

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

25. Yes, those really are action potentials.
Hippocampus
Cerebellum

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

27. 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

28. 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!)

29. Compartmentalization
Electrical events
Biochemical events

30. Light microscope
Electron microscope
(serial reconstruction)

34. 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.

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