basic question

Richard Hall rhall at
Mon Sep 30 16:35:32 EST 1996

>In article <52kil0$opn at>, veery at says...
>>I have a basic question regarding neurons.  I know that neural arbors are
>>generally characterized as afferent (dendritic) and efferent (axonal), but I
>>also know that it is difficult to distinguish one from the other in a
>>practical sense.

>Hannah Dvorak replies on 9/30/96:
>Division of Biology 216-76
>California Institute of Technology, Pasadena, CA 91125

> However, in the intact neuron, action potentials are generated near the
>soma >and propagate towards the axon terminals.  I don't believe it would
>be possible >to take an isolated segment of axon and determine which way
>it was originally
>oriented relative to the soma.  Synaptic potentials in dendrites
>will spread in all directions away from the site at which they were
>generated.  Traditionally, the soma has been seen as the site of
>integration of synaptic potentials, but there has been recent interest
>in the propagation of signals, including action potentials, back into
>the dendrites.  (See e.g. the work of B. Sakmann and collaborators.)

In the cns of crustaceans, synaptic integration, generation of action
potentials, and modulation of transmission occur in the neuropile very
distant (in terms of space constants) from the soma.  In such instances,
the soma have virtually no immediate role in processing spike driven
information (which appear in the soma but heavily attenuated.) Soma may
have some role in graded information processing since clamping the soma can
modulate axonal output from the neuropile in strange and wonderous ways.

The site of generator potentials, the direction of propagation, and the
role of the soma may vary with the morphology and location of the neuron in
a particular neuronal circuit. Expecially, if reciprocal synaptic
configurations and electric synapses (rectified and non-rectified) netween
dendrites, dendrites and axon terminals, etc. are added to the mix. These
interactions allows relatively few neurons to shift responsibilities and
duties as complex motor behaviors are generated - check out the STNS
webpage.  There has been a lot of good science on invertebrate ns that
frequently anticipates work in vertebrate preparations.

The soma of afferent neurons in the Dorsal Root Ganglia of vertebrate
spinal cord illustrate functional propagation of action potentials towards
and away from the soma (which again has little to do with the process of
generating action potentials.)

I concur that Rikki Hall, aka Horace Veery asks a very good question which
probably lacks a simple general answer but makes for very good biology and
interesting discussion.


Richard Hall
Comparative Animal Physiologist
Division of Sciences and Mathematics
University of the Virgin Islands
St. Thomas, USVI  00802

rhall at

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