question about potential distribution in axon and synapses

Richard S. Norman rnorman at umich.edu
Thu Jun 13 11:11:53 EST 2002


On Thu, 13 Jun 2002 16:14:00 +0100, "Larry Lart" <larrylart at dol.ie>
wrote:

>
>Hi all!
>
>
>
> How is the axon potential distributed via synapses?
>
>To be more explicit giving Vsoma - the neuron's potential generated in
>soma(?) then:
>
>-         how "powerful" a synapse  can be? Considering that at initial
>level was L and considering also that there is a constant activity the can
>this level double or so?
>
>-         If one or more of the synapses increases their propagation level
>then is that reflected in the others?
>
>-         I was reading somewhere that if two neurons are active at the same
>time the synapses between them are becoming more powerful ?
>
>  -    how much of Vsoma is wasted in dead ended axon termination?. there
>should be a minimal lost in each of them considering the environment
>conductivity?
>

Your questions indicate you inow some things about neurons but perhaps
just a little?  What is your background?  Have you read any books on
basic neurophysiology?  That is, it is hard to answer this type of
question without knowing what level of background knowledge you have.

The voltage in the soma really isn't relevant to anything.  Synaptic
input generates electrical potentials in cells.  Those potentials
spread passively (electrotonically) down dendrites which are
usually electrically inexcitable.  But the axon (and perhaps the
cell body) is often excitable so it can make an action potential.
In that case, the potential propagates actively without decay along
all the electrically excitable axons.  

Activity at synapses can be very variable (plastic).  The "strength"
of a synapse, that is, the amount of transmitter released, is graded
with the amount of depolarization at the synapse.  But usually a 
synapse is at the end of an axon and so is activated by an "all
or nothing" action potential, so the amount of transmitter release is
then "all or nothing".  However, there are many factors that can
modulate the size of "all" to change the strength of the synapse.

In response to your questions:

Constant activity at a synapse can easily double or more than double
the amount of transmitter release (facilitation or potentiation).  But
constant activity can also decrease the amount of release 
(defacilitation, fatigue).    It is possible for changes at one
synapse to influence the activity of others -- this usually happens
through a postsynaptic effect.  The activity at one synapse stimulates
second messenger events inside the postsynaptic cell which causes
changes in the excitability of that cell, possibly including the
response to a different synapse.  It is also possible for simultaneous
activity at two different synapses to produce long-term changes in
the synaptic strength. 

Your question about "dead end" axons doesn't really make much sense.
There is no potential "waste" and I really don't know what you mean
by "environment conductivity".

I would recommend getting a good basic neurophysiology book and
reading the sections on synaptic integration and plasticity.  That
should answer all your questions.




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