Neural specification?

yan king yin (dont spam) y.k.y at lycos.com
Fri Feb 15 00:19:49 EST 2002


"Larry Lart" <lart at e-merge.ie>
>   I was looking for a complete neuron specification and I wasn't able to
> find answers to a few questions still. I will really appreciate if you can
> help me (with answers or a link where i can read about) to clarify my self.
> You will save me a few nights and headaches ...
>
>   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.

Im 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.

>         - How far do they actually grow and how fast ?

The average dendrite is about several hundred um (micrometer) long
measured from the soma. Speed of growth cone movement (for axons)
ranges from a few to 100s of um/hr.

>         - Do they grow for the entire life time ?

There is some evidence for this, but I couldnt find many studies on this
issue, especially new ones.

>         - the increase in the number of dendrites for the same neuron since
> his birth isn't random or linear or related to the needs ?!!

Obviously dendritic growth is determined by a multitude of factors
including electrical activity, growth factors, and cell type (ie genes).
I think "probabilistic" is a better word for "random", and _maybe_
the branching patterns of dendrites has a probabilistic component.

>   b. Synapses
>         - is it the incoming potential time related ? Considering 2 neurons
> linked by one synapse. When the first one fires for time t1 when the second is
> blocked for time t2. But there is a gap in time d(t) when the second neuron
> might be able to receive a part of the signal ?

The action potential (AP) is an all-or-none event and basically maintains
its shape during propagation. But the postsynaptic potential (PSP) can be
spread temporally along the dendrite. This forms the basis of spatial and
temporal integration of signals.

>         - what is the signal velocity in a chain of neurons? I know that in
> axon is somewhere between 2-100 m/s but how much will be going through
> all those synapses and soma's function?

The chemical synapse is slower than the electrical synapse and so it
results in a detectable delay when signals pass from one neuron to
another. I dont know the exact figures. Anyone?






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