Neural specification?

yan king yin (dont spam) y.k.y at lycos.com
Sat Feb 23 02:49:54 EST 2002


"Larry Lart" <larry at alphyra.ie>
> >>   a. Dendrites Growth
> >>         - Do they grow orientated under the action of a neighboor firing
> >> axon ?
> >
> > This question and the following ones concerns two phases, namely the
> > developing and mature brain. In the developing brain dendritic growth
> > is not solely determined by electrical activity. In the extreme case, one
> > can observe neurons growing their characteristic dendritic structures in
> > isolated cell cultures! Generally speaking, blocking electrical activity
> > results in diminished dendritic trees.
> >
>   What about trying to give a direction(in space) to dendrites growth
> using space orientated potentials ?

I think the only known mechanism that influences the direction of neurite
growth is that of chemoattractants/repellants such as neurotransmitters and
growth factors. There are receptors for these molecules on the growth
cone and they trigger some changes in the dynamics of microtubules or
microfilaments in the growth cone, thus changing the direction of growth.
The electrical signals at a distance from the growth cone is very unlikely
to have any effect on its growth direction -- that would be rather spooky.

> > I'm not sure what you mean when you say dendritic growth is "oriented"
> > by electrical activity. There is some speculation that after long term
> > potentiation (LTP), some dendritic spines split into two, forming
> > bifurcating spines. This might be one way the dendrites grow.
>
>   What I don't understand/know is having this context in the mature
> brain:You have an axon in the vicinity of a few dendrites - if those
> dendrites grow either randon or by a rule(ie gene) the probability to meet
> in their way one of the the axon terminations and create a synapse could be
> quite low. right ?

That probability is not very low. You should take a look at an electron
microscope picture of some neural tissue and you'll have an appreciation
of how neurites grow in the tissue. It does not occur in a "void". The
neurites usually grow within existing neural tissue.

Second, I think during development the growth of axons and dendrites
must have some degree of probability because the genes simply cannot
completely specify the exquisite branching and synapse patterns. There
is some specificity, though, in that neurites do seem to distinguish between
target cell types when making synapses.

> And what I was wondering if the directions in which
> those dendrites grow is determined by the electric potential fired by the
> neighboor axon in "void" ? like bellow  :
>
>  firing axon [x]->
>
>                        [z]<----dendrites normal grow direction
>
>  firing axon [y]->
>
>   So now the question is: if the default grow path will be parallel with
> those to axons(or terminations of the same firing axon) those dendrites
> will never meet the axons. But if their direction is changed with the
> electric potential z(x,y) will make more sens ?! Like even the tree's roots
> are a bit smart in this point of view ...

As I said, I dont think electric fields have any thing to do with growth
cone guidance. Also, the firing of an axon only influences a dendrite
when there is a synapse between them. The influence of electric
fields at a distance is insignificant to affect signaling in neurons.






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