basic question

[Nigel Foden]

Jeroen Schaap wrote:
> 
> In article <52ov40$4q2 at gap.cco.caltech.edu>,
>    hdvorak at cns.caltech.edu (Hannah Dvorak) wrote:
> [snip]

> >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.)
> >{snip some more}
>         Third, why couldn't any action potential be generated on an
> axon, by means of axo-axonic synapses etc. Off course the axons don't
> have the ap generating mechanism, axon hillock, but nevertheless, why
> couldn't the 'axonic synapses' depolarize enough in order to generate an
> action potential.
> What about axon trunks that route together from one area of cortex to the 
next?  If a volley of (classically) efferent information left a nucleus down 
several axons that surrounded a non-firing axon, could not the Na+ influx and 
K+ efflux in the firing cells cause enough local depolarisation to initiate 
an axon potential in a previously non-firing cell?  Then the stimulus would 
travel ortho- and antidromically, presumably reaching the soma, although it 
may not stimulate a second orthodromic ap as the early segment of the soma 
could still be refractory.  If this mechanism works it provides a synapse 
free mechanism for synaptic interaction (due to myelination it would 
presumably be more common in grey tissue, unless the nodes of ranvier line up 
in nerve bundles, which I don't _think_ happens)

Nigel

n.foden at ic.ac.uk