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How many inputs are needed to excite/saturate a neuron

paully mccarr at rs6105.ecs.rpi.edu
Fri Feb 26 13:08:52 EST 1993

There have been numerous postings recently regarding synaptic integration of
the dendrites of typical neurons.  I had thought, until very recently, that the
notion that all the synapses on a given neuron contribute in some way to the
post-neuron's decision to fire/not-fire was quite sound.  Then some five months
ago a researcher from the Wadsworth center in Albany gave a lecture at RPI 
on his work with 3d fluorescent microscopy.  He was impaling and characterizing
neurons and then leaving them over-night to dye.  Then he obtained 3d images
of single neurons clearly identifying the site of impalement. I apologize
that I do not recall the name of the doctor working on the project I am very 
poor with names.

What he found was that if he impalled a neuron's dendritic field anywhere
beyond a third order branching he was unable to induce an action potential.
When he went on to claim that this implied that synapses beyond the third
order branches of any neuron were unable to communicate any information to the
neron my mouth dropped and I questioned his assumption, but he held his ground.
Not too long after this I came across it in the text "Methods in Neuronal 
Modeling" by Koch and Segev.  There they described a multicompartment model
of a neuron in which the same was discovered there is a point, not very far
from the cell body, beyond which it is no longer possible for any excitation
(voltage or current) to arouse the neuron even in synchrony with lots of other
synapses at the same distance.  The reason seems to be that the depolarization
or hyperpolarization causes an exponential gradient twards the cell body.
Such a relationship would dictate a rather short efficacy for synapses with
respect to distance from the neuron.
Is this a relatively novel idea which is just now being considered?
if not, is this notion well accepted by the neuroscience community?
What does this imply regarding synaptic integration?

It seems to me that if we accept the notion that synapses beyond 3rd order 
(axo-dendritic) do not influence cell activity, than there is a great deal of
communication of some sort occuring between the far reaches of the dendrites 
and other dendrites(lots of dendr-dendritic connections) or there is 
some kind of long-term cellular influences at work.

I would be most interested in anyone's corrections to any incorrects I have
made, answers to any of the questions I have posed or further insights into
these questions.

R. Paul McCarty

Biomedical Engineering (undergrad Senior)
Apt. 1 297 Congress St.
Troy, NY 12180
e-mail: mccarr at rpi.edu

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