| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Editorial Focus
Both dendritic structure and the ion channels expressed in dendrites are incredibly diverse. The dendrites of cerebellar Purkinje cells are virtually devoid of sodium channels (Stuart and Häusser 1994
). On its own, this would limit action potential propagation in Purkinje dendrites, but modeling studies suggest that the extensive branching of the Purkinje cell dendritic tree further limits action potential conduction in these fan-like structures (Vettter et al. 2001). By contrast, both the expression of sodium channels and the limited branching of the mitral cell primary dendrites contribute to axon-like action potential conduction in these olfactory neurons (Bischofberger and Jonas 1997; Chen et al. 1997).
The message is clear: differences in structure and ion channel expression both contribute to functional diversity across different classes of neurons. But what about within a single class of neurons; to what extent do dendritic structure and ion channel expression contribute to variability within a population of neurons? A new study, by Schaefer et al. 2003, reported in this issue of the Journal of Neurophysiology (p. 3143), provides strong evidence that subtle differences in dendritic structure may contribute substantially to neuronal function.
The question of how dendritic structure contributes to function is difficult to address, primarily because manipulating dendritic structure is difficult to do, without stepping outside the boundary of normal biological variability. Schaefer and colleagues (2003) solve the problem by studying layer V pyramidal neurons and relating structural and functional variability within this population of neurons. They use a powerful modeling approach to demonstrate that subtle structural differences within the population are sufficient to explain the functional variability they measured in the same neurons they modeled.
The functional property they measured experimentally is called "BAC firing". In layer V neurons, appropriately timed excitatory postsynaptic potentials (EPSPs) and backpropagating action potentials can lead to the initiation of dendritic calcium spikes, or Backpropagation Activated Calcium spikes (Larkum et al. 1999). The degree of coupling is variable. In some neurons the backpropagating action potential produces a large reduction in the threshold for a dendritic spike (strongly coupled). In other cells the backpropagating action potential produces only a modest threshold reduction (weakly coupled).
To determine whether differences in dendritic structure contributed to the variability in BAC firing observed in their experiments, Schaefer and colleagues reconstructed the dendritic morphology of 29 neurons. They then created computer models of the neurons containing a single set of voltage-gated conductances. Having chosen this set of channels for its ability to produce BAC firing, they found that the population of modeled neurons fully captured the range of variability observed in their experimental measurements of BAC firing. Because each of the neurons contained an identical set of channel conductances, they concluded that morphological variability alone was sufficient to reproduce normal functional variability, so they began a search for the relevant morphological features.
Using a procedure they call "dendritic fingerprinting", Schaefer and colleagues (2003) identified oblique dendritic branches as the culprit. Branches emerging from the proximal apical dendrite (<140 µm from the soma) enhanced BAC coupling, whereas branches emerging from more distal regions of the apical dendrite diminished coupling. The branches act as either current sources or current sinks. Proximal branches provide a current source to the main dendrite; though current is initially drawn away from the main apical dendrite, branches close to the axon are depolarized enough to activate more sodium and calcium channels and provide further excitatory current back to the main dendrite. Distal dendrites act as current sinks; depolarized less by the backpropagating action potential, fewer sodium and calcium channels are activated, so distal branches tend to draw current away from the main apical branch. The result of these current sources and sinks are dendritic branches that enhance or diminish BAC coupling, respectively.
These findings unveil a new capacity for dendritic function. Not only are subtle variations in dendritic morphology rendered functionally relevant, but also a new mechanism for plasticity is implied: neurons may fundamentally alter their integrative function simply through growth or retraction of just a few small dendritic branches. With recent advances in long-term structural and functional imaging of dendrites in vivo (Grutzendler et al. 2002; Trachtenberg et al. 2003), it may soon be possible to see (literally) whether such plasticity is part of the nervous system's remarkable capacity for change.
Institute for Neuroscience, Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208
Address reprint requests to Nelson Spruston. (E-mail: spruston{at}northwestern.edu).
REFERENCES
Bischofberger J and Jonas P. Action potential propagation into the presynaptic dendrites of rat mitral cells. J Physiol 504: 359–365, 1997.[ISI][Medline]
Chen WR, Midtgaard J, and Shepherd GM. Forward and backward propagation of dendritic impulses and their synaptic control in mitral cells. Science 278: 463–467, 1997.
Grutzendler J, Kasthuri N, and Gan WB. Long-term dendritic spine stability in the adult cortex. Nature 420: 812–816, 2002.[Medline]
Larkum ME, Zhu JJ, and Sakmann B. A new cellular mechanism for coupling inputs arriving at different cortical layers. Nature 398: 338–341, 1999.[Medline]
Schaefer A, Larkum ME, Sakmann B, and Roth A. Coincidence detection in pyramidal neurons is tuned by their dendritic branching pattern. J Neurophysiol 89: 3143–3154, 2003.
Stuart G and Häusser M. Initiation and spread of sodium action potentials in cerebellar Purkinje cells. Neuron 3: 703–712, 1994.
Trachtenberg JT, Chen BE, Knott GW, Feng G, Sanes JR, Welker E, and Svoboda K. Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex. Nature 420: 788–794, 2003.
Vetter P, Roth A, and Häusser M. Propagation of action potentials in dendrites depends on dendritic morphology. J Neurophysiol 85: 926–937, 2001.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
