[Neuroscience] Re: Electrical Synapses..

r norman NotMyRealEmail at _comcast.net
Tue Feb 21 09:36:10 EST 2006


On Mon, 13 Feb 2006 10:14:22 +1100, "freenews.iinet.net.au"
<jkrah_dot_geo_ at yahoo_dot.com> wrote:

>Hello,
>I hope this is an ok place to ask this questions (begginer) .. I am 
>studying basic neurobiology and am wondering about axo-axonal electrical 
>synapses..
>
>Is it a safe to assume that the synapse must be close enough to the Axon 
>Hilckock (or AP generating zone) for an Actionm Potential to be 
>generatable.. of can (axo-axonal) electrical synapses have effect 
>anywhare along an axon ??
>
>
>Appols - if this is not the appropriate group  :-(

I know this is very long delayed, but I am on vacation and have not
had a chance to scan the news group before now.

Why are you asking about this very special type of synapse, if it even
exists?  It is a very specialized type of thing.

There definitely are axo-axonal synapses where the terminal of one
axon synapses onto the axon terminal of another neuron.  These modify
or modulate the synaptic potential that the post-synaptic neuron
produces with a third cell.  That is, Cell B has a synapse with cell
C.  Then cell A forms a synapse onto the presynaptic terminal of cell
B and changes the magnitude of the synaptic potential that B produces
in C.  Presynaptic inhibition is the most familiar case but there are
many other examples.

However this interaction is ordinarily a "normal" chemical synapse.  I
don't know of any examples offhand where such an interaction is
electrical, although such a thing is certainly possible.

You are right in thinking that a synapse must ordinarily be within a
short distance (a few space constants, to be precise) of the spike
initiation zone in order to be effective.  That is because a synaptic
potential ordinarily spreads with decay (electrotonically) and
therefore gets weaker and weaker the farther it travels.  However even
relatively small graded potentials do have important effects on the
axon terminal in determining just how much synaptic transmitter to
release so a synapse relatively close to a presynaptic terminal will
have an effect.  Also, it is possible for an axon to start an axon
potential anywhere along its length, as long as that section reaches
threshold for one reason or another.  It is theoretically possible,
though very unlikely, that a very strong electrical synapse from one
axon to the middle of another will cause the postsynaptic cell to
generate an action potential.  In this case, it is as if the cells
were actually continuous so that the action potential just propagates
from one to the other the same way it propagates down both sides of a
branch  point.  There are also a tremendous number of cells with very
short axons, less than a few space constants long, so that a synaptic
potential produced anywhere along the axon will not decay that much as
it spreads down the entire length of axon.

I hope this helps and that it does not come so long after your
question that you have abandoned all hope of getting an answer.




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