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The Journal of Neuroscience, January 1, 1998, 18(1):10-15
Input Summation by Cultured Pyramidal Neurons Is Linear and
Position-Independent
Sydney
Cash and
Rafael
Yuste
Department of Biological Sciences, Columbia University, New York,
New York 10027
The role of dendritic morphology in integration and processing of
neuronal inputs is still unknown. Models based on passive cable theory
suggest that dendrites serve to isolate synapses from one another.
Because of decreases in driving force or resistance, two inputs onto
the same dendrite would diminish their joint effect, resulting in
sublinear summation. When on different dendrites, however, inputs would
not interact and therefore would sum linearly. These predictions have
not been rigorously tested experimentally. In addition, recent results
indicate that dendrites have voltage-sensitive conductances and are not
passive cables. To investigate input integration, we characterized the
effects of dendritic morphology on the summation of subthreshold
excitatory inputs on cultured hippocampal neurons with pyramidal
morphologies. We used microiontophoresis of glutamate to systematically
position inputs throughout the dendritic tree and tested the summation
of two inputs by measuring their individual and joint effects. We find
that summation was surprisingly linear regardless of input position.
For small inputs, this linearity arose because no significant shunts or
changes in driving force occurred and no voltage-dependent channels
were opened. Larger inputs also added linearly, but this linearity was
caused by balanced action of NMDA and IA potassium
conductances. Therefore, active conductances can maintain,
paradoxically, a linear input arithmetic. Furthermore, dendritic
morphology does not interfere with this linearity, which may be
essential for particular neuronal computations.
Key words:
hippocampus; cortex; NMDA; iontophoresis; dendrite; potassium
Copyright © 1998 Society for Neuroscience 0270-6474/98/18110-06$05.00/0
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