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. bbbbbbbbb Content-Type: text/html; charset="US-ASCII" [ AUTOMATICALLY SKIPPING HTML ENCODING! ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=26945