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Axon collaterals ?

Matthew Kirkcaldie Matthew.Kirkcaldie at removethis.newcastle.edu.au
Thu Jan 1 20:33:47 EST 2004


In article <zrydnVaB5e0JWUKiU-KYiQ at giganews.com>,
 "Mark Zarella" <zarellam at twcnyremove.rr.comspam> wrote:

> > For the GABAergic neurons the picture is complicated by electrotonic
> > connections between large numbers of these cells, which act in a
> > frequency-dependent, weakly excitatory fashion.
> 
> I'm sorry, I don't follow that last line...

Sorry - I will expand a little.  If you're thinking of a cell 
influencing itself, the idea of an autapse is tough enough to swallow.  
John H neatly summed up my own horror at finding out they were quite 
common - I thought I had a bit of a grip on how the cortex would 
organise itself during development, but the autapse slapped a lot of 
those ideas out of the ball park.  The answer is to break down your 
understanding of the dendrite field of a neuron: it's not a passive 
collector which dutifully transmits every postsynaptic potential it 
receives.  Instead it's an active processor and filter of the incoming 
signals, and the effect that a synaptic potential will eventually have 
at the cell body is shaped by temporal dynamics and the presence or 
absence of activity at other synapses.  In this context you can view the 
autapse as a way for the cell to use its own activity in this process of 
stimulus sorting - if you like, it could mean "accept inputs from this 
dendrite if the receiving cell has recently fired" at a cartoon level.

Now, when we talk about GABAergic interneurons in the cortex, these 
kinds of non-linear dynamics are present, but as a complication most of 
them are electrically (as well as synaptically) coupled to the other 
nearby interneurons.  That means, for instance, that one basket cell may 
make inhibitory synapses on another, but be connected to it in an 
excitatory way by electrotonic gap junctions.  So if it fires, it 
excites AND inhibits the other neuron, but not in precisely the same way 
(the excitation only consists of low frequency shifts in membrane 
potential).  So two neurons synapsing with each other, and with 
themselves, and electrically coupled, makes for some challenging 
dynamics.

And yet it's the simplest interaction in the brain.  Multiply it by 
itself around 10 000 or 100 000 times, keeping in mind that it 
continuously shifts its connectional structure, and you start to 
understand why I have no time for people who feel they can understand 
behaviour at the receptor level, or describe cortical "function" in 
anything but the wildest of guesses.

A cheering thought - but the neuroscientists won't be out of a job for a 
while I suppose!

Happy 2004 everyone, here's to the most enjoyable newsgroup (along with 
alt.movies.kubrick).

      Cheers,

         Matthew.



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