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The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 3268-3285
Copyright ©1999 by the American Physiological Society
1Division of Life Sciences, University of Texas at San Antonio, San Antonio, Texas 78249; and 2Department of Psychology, Yale University, New Haven, Connecticut 06520
Jaffe, David B. and
Nicholas T. Carnevale.
Passive Normalization of Synaptic Integration Influenced by
Dendritic Architecture. J. Neurophysiol. 82: 3268-3285, 1999. We examined how biophysical properties and
neuronal morphology affect the propagation of individual postsynaptic
potentials (PSPs) from synaptic inputs to the soma. This analysis is
based on evidence that individual synaptic activations do not reduce local driving force significantly in most central neurons, so each
synapse acts approximately as a current source. Therefore the spread of
PSPs throughout a dendritic tree can be described in terms of transfer
impedance (Zc), which reflects how a current applied at one location affects membrane potential at other locations. We addressed this topic through four lines of study and uncovered new
implications of neuronal morphology for synaptic integration. First,
Zc was considered in terms of two-port theory
and contrasted with dendrosomatic voltage transfer. Second, equivalent
cylinder models were used to compare the spatial profiles of
Zc and dendrosomatic voltage transfer. These
simulations showed that Zc is less affected by
dendritic location than voltage transfer is. Third, compartmental models based on morphological reconstructions of five different neuron
types were used to calculate Zc, input impedance
(ZN), and voltage transfer throughout the
dendritic tree. For all neurons, there was no significant variation of
Zc with location within higher-order dendrites.
Furthermore, Zc was relatively independent of
synaptic location throughout the entire cell in three of the five
neuron types (CA3 interneurons, CA3 pyramidal neurons, and dentate
granule cells). This was quite unlike ZN, which
increased with distance from the soma and was responsible for a
parallel decrease of voltage transfer. Fourth, simulations of fast
excitatory PSPs (EPSPs) were consistent with the analysis of
Zc; peak EPSP amplitude varied <20% in the
same three neuron types, a phenomenon that we call "passive synaptic
normalization" to underscore the fact that it does not require active
currents. We conclude that the presence of a long primary dendrite, as
in CA1 or neocortical pyramidal cells, favors substantial
location-dependent variability of somatic PSP amplitude. In neurons
that lack long primary dendrites, however, PSP amplitude at the soma
will be much less dependent on synaptic location.
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