Horace Veery wrote: [are you Horace Veery or Rikki Hall?]
>> 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. Is it actually possible to take any arbitrary segment of
> neural fiber and determine the directionality of signals flowing along that
> fiber?
no
> Are there static, physical features that establish a directionality,
not with respect to "an arbitrary segment"... with respect to groups of neurons
there
are "physical features that establish a directionality"... it's a hierarchical
thing... the bigger the picture, the more-rigorously-defined directionalities
become...
> or is it theoretically possible for impluses to flow in either direction along
> a neural fiber?
yes
> Have experiments been done which measure the directionality
> of fibers, or which test whether impulses do travel in only one direction?
yes... in artificial preparations, impulses can flow both ways...
...within intact nervous systems, your question requires long discussion... there
are
axon-dendrite, dendrite-dendrite, axon-axon, dendrite-axon chemical signals,
electrical junction signals, waxing & waning ionic signals, passive spread,
glia-mediated ionic flows & signaling, and neurochemically-directed signalling...
and
so forth...
...this is why one needs to address such matters at the global level... it's easy
to
see, for instance, that there must be a net afferent flow in sensory neurons...
else
the nervous system would be totally-over-ruling the environment... in general,
there
is a lot of feedback, and cross-talk, but such occurs via circuitry that is
"engineered" to mediate such... and not via 2-way signalling in, say, sensory
neurons
themselves...
...here, again, the big picture is our guide... sensory receptors are distributed
in
an orderly fashion on the surfaces of our bodies... the neighborhood
relationships
among the sensory receptors are preserved within the fiber pathways that carry
sensation into our nervous systems... all the way to the cortex... the Geometry
inherent in these neighborhood relationships enters mightily in our nervous
systems'
capabilities for directing our behavior with respect to directions in our
enviornments... if it were the case that there was a lot of 2-way impulse
activity in
axons, it would also be the case that the neighborhood relationships would be
"over-written", and lost, because such 2-way signalling would wreak havoc on the
timing correlations within, say, a physically-correlated sensory signal...
> Is the distinction between axon and dendrite absolute?
neuroanatomically, it's pretty-much-so... neurophysiologically, things are not so
clear cut... except when one considers things =globally=, as above...
> Is it a convenient simplification or a well-established neurobiological fact?
it's as above...
>> References to texts or journals would be appreciated, as would URLs. Please
> post to this group and/or to rikki at zool46.bio.utk.edu
...sorry, I've worked from memory... from the stuff of hundreds of references...
ken collins
_____________________________________________________
People hate because they fear, and they fear because
they do not understand, and they do not understand
because hating is less work than understanding.
