X-Message-Number: 26945
From: 
Date: Tue, 6 Sep 2005 14:54:34 EDT
Subject: Uploading technology (1.ii.2) soma processing.

Uploading technology (1.ii.2) soma processing.

The body cell  or soma make only the house keeping work and so is not very 
interesting on the  information side. Post synaptics potential sum up here and 
if there is  sufficient depolarisation, an action potential is produced in the 
axon. this  classical viewpoint is somewhat simplist. First, the action 

potential starts at  different points depending on the membrane depolarisation.

there is not indeed a  single threshold to activate it. Sodium channels 
producing 
it have a deceasing  density from the axon initial part, the hillock, to the 
soma body and even the  trunk part of the dendrites.

If a threshold limit depolarisation  come from the dendrites, that post 

synaptics potential will be insufficient to  start an AP in low density channel

domains. So the depolarisation will pas the  dendrites, the soma and produce its
AP at the hillock. A stronger depolarisation  will be sufficient to start the 
AP at the soma or even the dendrite. This AP  will be amplified by the hillock 
and because of the capacitiv property of the  soma will be longer.The result 
will be a long, strong AP, the one able to  release a second messenger at the 
axon terminal. So a strong AP can generate  another strong AP in the next cell.

There is too a back propagating  AP in the dendrite. It can produce a 

resonance effect with the next pulse and  augment its transmission probability.

Depending where the AP starts, the go and  back time may be more or less long. A
weak postsynaptic signal will produce an  AP at the hillock and so the go and 

back time will be long. If there is a  resonance, the next pulse will be favored
and can be able to produce another AP.  After some time, the neuron link will 
be reinforced and the signal will be  stronger. It will start an AP at the 
soma or dendrite level, the go and back  time will be shorter and the back 

propagating AP, will fall out of sync. This is  a form of habituation effect 
where 
stronger signals are  discarded.

This example show that the back propagation time must be  taken into account 
in the simulation.

Another function to look at  is the GABA-A synapses at the hillock, these can 
short circuit any action  potential produced by the cell.

Metabotrophic or G-protein  receptors are particularly important at the soma, 
because they activate c-AMP  systems regulating genes expression or 

regulation. for example this could agment  or decrease the Na+ channel density. 
This 
would modify the AP threshold. Because  this part is simulated on a general 
purpose computer, the data exchanged is  about the threshold adjustment in the 

different parts: The hillock, soma or  dendrites. Each dendrite trunk must have
its own control, for example, in  pyramidal cells, apical and basal dendrites 
may not have the same Na+ channel  density.

Yvan Bozzonetti.
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