X-Message-Number: 26965 From: Date: Thu, 8 Sep 2005 02:44:56 EDT Subject: Uploading technology (1.iii.2) Dendrite processing. Uploading technology (1.iii.2) Dendrite processing. Dendrites was seen as simple conductors working along the cable theory scheme or at best, the Hodgkin-Huxley model. Two add on have come into play these past years: First, there was the realisation that far more currents are working here with important effects on the firing pathern of neurons. The second is that differential equations, must not be dealt with special, continuous solutions, they must be worked out with general, discontinuous ones. On the practical ground, that translate into signal boosts by potential gated channels and collective synapse spine effects. At first, all of that was seen as a way to amplify a distant signal so that it would be not negligible when coming to the soma. In fact, in the general case, the conductivity and amplification is such that a distant signal comes with the same power as a nearby one. On a simulation ground, that may be approximated at the first level as a lossless transmission. This active transmission open yet a pandora box, there are many possibilities for modulation and control. The dendrite is no more a passive cable device. It is a full processor of its own. The first control is with tension gated channel and gap junctions. Gap junctions can impose a regional depolarisation or hyperpolarisation able to work on a part of the dendritic tree or even a full neuron group. this can hinder or facilitate the postsynaptic potential propagation. This is an area selective effect. Potential gated channels, using Na+ can modulate the conductivity up to zero loss and beyond, that is signal amplification depending on their density. They can work too as stepping stone for Ca++ channels with harder to get thresholds. Ca++ ion can act on the neurofibrils holding the membrane and for example expand or squeeze the dendrite branch diameter. This may hinder of facilitate the transmission at a branching point, giving more or less weight to synapses on the distal part. Back propagating action potentials may help to depolarise the membrane and trigger a postsynaptic potential, particularly on spines. As seen in the soma part, this back propagating AP can induce some resonance effects. The general solutions of differential equations include the possibility of coherent effects between synapses. There are two possibilities: A/ The potential gated channels of a synapse on a spine could fire a postsynaptic potential even without presynaptic signal, particularly if there is a back propagating AP and some gap junction effects. This is a contribution to the lossless transmission/amplification. B/ A dendrite branch could harbor a set of synapses all linked to the same axon and so firing at the same time. This synchronized activity would trigger other synapses not sufficiently excited by themselve to fire independently. If a dendrite tree would then be broken into a number of functional blocks, each under the rulling of a synaptic pool working toghether. This is an alternative to the Hebbian rule of reinforcement for neuron firing in synchrony, here the reinforcing effect would be spatial and under the control of a set of synapses of the same neuron. Synchronised firing and dendrite domains must be taken with a pinch of salt yet. There is fast sync. for synapse firing with the sole power of potential gated and fast chemical channels. This is synchronisation at the some millisecond scale. Another firing simultaneity may be defined for G-protein channels, with simultaneity at the second duration scale. The common dendritic domain may be different here. A third scale is given by second messenger gated channels, here synapses synchronisation may be at the minute or more. This is a collective behaviour extending may be at regional scale, for example a microcolumn, that is, up to 100,000 neurons. Allof that is skewed towards excitatory functions, inhibitory ones are less glorious, they are neverthless a key component of all neuronal function. GABA-A synapse are hyperpolarising near the outside potential, using for that Cl- ions. They work as short-cut throughout the membrane. They cancel everything upstream and are nearly without effect on down stream synapses. When on the axon hillock, they can cancel even a progressing action potential. Put at the root of a dendrite domain, they can control it. They are yes/no gate for a single spine, a dendrite domain or a full neuron. GABA-B inhibitory synapses produce a larger hyperpolarisation and work as excitatory ones, simply the other way. They are threshold enhancer and work both, upstream and downstream. To summarize: The input parameters are so: The post-synaptic potentials, the back propagating action potential and the gap junction potential. The processing elements are: The Na+ and Ca++ channel densities and repartitions. The branching impedance, the spine density working as amplifier and the coherent spine density. The yes/no GABA-A synapses and thrshold controler GABA-B. A dendrite tree is broken into a number of functional domains, the domains limits evolve in time and for different currents, particularly for fast, g-protein and second messenger channel classes. May be ten to fifteen current must be worked out simultaneously. The output parameters are: The computed fast currents at the domain base trunk, the back propagating potentials, the dendrite junction diameter evolution, the changing channel dentity and localisation. This conclude the analysis of the neuron information processing for the uploading purpose. The next step is to turn this wish list into a number of functions with their requested I/O and memory, so that this system can be implemented on an ASIC neuromorphic chip or an existing FPGA. Yvan Bozzonetti. Content-Type: text/html; charset="US-ASCII" [ AUTOMATICALLY SKIPPING HTML ENCODING! ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=26965