From: Ted Carnevale (ted.carnevale@yale.edu)
Date: Wed Jan 05 2000 - 11:38:34 MST
The following two articles may be of interest to those
who are interested in synaptic integration, either from
the theoretical or experimental standpoint, and/or
empirically-based modeling of neurons.
The first paper will be particularly relevant to those
who are interested in modeling the roles of biophysical
mechanisms and use-dependent plasticity in the operation
of individual neurons and neural networks.
Carnevale, N.T., and Hines, M.L.
Expanding NEURON^Òs repertoire of mechanisms with NMODL.
Neural Computation 12:839-851, 2000
This is an "executive summary"; for those who need to
know the details, a preprint of this paper that includes
many more figures, examples, and an extensive index is
available from our WWW site at
http://www.neuron.yale.edu/neuron/papers/nc99/nmodl.htm
The second paper shows that many classes of neurons
are fundamentally similar to the processing elements
of artificial neural nets, in the sense that synaptic
impact at the spike trigger zone is determined by the
properties of the synapse itself, and not by the
anatomical location of the synapse. It also challenges
the widely-held notion that active currents are necessary
to overcome location-dependent attenuation of synaptic
inputs (with one important exception, as noted below).
Passive normalization of synaptic integration influenced
by dendritic architecture.
David B. Jaffe and Nicholas T. Carnevale
J. Neurophysiol. 1999 82(6): p. 3268-3285
http://jn.physiology.org/cgi/content/abstract/82/6/3268
The two most significant findings reported in this paper
are:
1. The peak amplitude of individual PSPs as a function
of synaptic location is best predicted by the spatial
profile of transfer impedance (Zc), rather than the more
commonly studied somatopetal voltage transfer ratio
(Vsoma/Vsynapse).
2. Active currents are generally NOT necessary to
overcome location-dependent attenuation of PSP
amplitudes in real neurons. Dendritic fields that
are organized around a central or "primary" dendrite
were the only exception to this rule. In other words,
peak PSP amplitude observed at the spike trigger zone
is very nearly as large as at the synaptic location,
and shows little variation with synaptic location in
cells such as interneurons, granule cells of the
dentate gyrus, and CA3 pyramidal neurons, even when
active currents are NOT present. This also applies
to synapses onto the basilar branches of CA1 pyramidal
cells and deep neocortical pyramids. Since this
reduction of location-dependent PSP amplitude variance
does not require active currents, we call this
phenomenon "passive synaptic normalization."
As noted above, passive synaptic normalization does not
occur in dendritic fields that have terminal branches
organized around a central or "primary" dendrite, e.g.
the apical dendrites of CA1 and deep neocortical
pyramidal cells. In subsequent work that we presented
at the most recent meeting of the Neuroscience Society
Carnevale, N.T. and Jaffe, D.B.: Dendritic architecture
can compensate for synaptic location without active
currents: the importance of input and transfer impedance
for synaptic integration. Neuroscience Society Abstracts
25:1741, 1999.
we found that the lack of passive synaptic normalization in
such dendritic fields is due to the loading effect of terminal
branches, which tend to flatten the spatial profile of input
impedance along the primary dendrite.
--Ted
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