basic question

Hannah Dvorak hdvorak at cns.caltech.edu
Mon Sep 30 12:13:36 EST 1996


In article <52kil0$opn at gaia.ns.utk.edu>, veery at utkux1.utcc.utk.edu 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.  Is it actually possible to take any arbitrary segment of 
>neural fiber and determine the directionality of signals flowing along that 
>fiber?  Are there static, physical features that establish a directionality, 
>or is it theoretically possible for impluses to flow in either direction 
along 
>a neural fiber?  Have experiments been done which measure the directionality 
>of fibers, or which test whether impulses do travel in only one direction?  
Is 
>the distinction between axon and dendrite absolute?  Is it a convenient 
>simplification or a well-established neurobiological fact?

Quite a kettle of fish you've opened...  I assume you've done the basic
reading in some standard neurobiology texts; if not, I would suggest
starting there, e.g. with Kandel, Schwartz, and Jessell's "Principles
of Neural Science" or Nicholls. Martin and Wallace's "From Neuron to Brain."

It seems like you're confounding two issues in your query: distinguishing
between axons and dendrites, and determining the direction in which signals
spread in a neuronal fiber.  Axons and dendrites can be distinguished
at the ultrastructural level: in general, axons are thinner and have
presynaptic specializations (terminal boutons containing neurotransmitter
vesicles), while dendrites are thicker, with postsynaptic specializations
such as spines.  Axons may be myelinated, while dendrites never are.
(Somebody correct me if I'm wrong on that one!)  In general, axons
have the ion channels (voltage-dependent Na and K channels) necessary
to conduct regenerating action potentials.  Postsynaptic potentials
in dendrites are generated by the binding of neurotransmitter to 
specialized receptors, and these potentials may spread passively to
the soma, or be amplified by voltage-dependent Na or Ca channels.

As for directionality of signal propagation:  An isolated axon can
conduct action potentials in either direction, as can be easily
demonstrated in the laboratory.  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.)

Hope this starts to answer your question!

- Hannah Dvorak

--
Hannah Dvorak
Division of Biology 216-76
California Institute of Technology, Pasadena, CA 91125




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