X-Message-Number: 26079 From: Date: Sun, 24 Apr 2005 11:04:17 EDT Subject: Uploading (3.ii.0). The synapse domain. Uploading (3.ii.0). The synapse domain. Most of the interneuron communications use the synapse architecture: At the end of an axon, a neurotransmitter is released and it activate some receptors on the facing dendrite spine head. this system works as an amplifier, there is one action potential on the axon side, it releases from one to seven vesicles, each with some thousand molecules and up to one hundred receptor can be activated. Everything start with the action potential, an electrochemical signal with a very peculiar time shape. It starts the release of vesicle storred against the axon membrane. Experiments have demonstrated that a rectangular electric signal produce the same effect. This is very important: It means that the shape of the action potential is imposed by the way axons conduct the signal but has no information meaning. Any digital transmision could do the job. An artificial brain will so use neurons without simulated axon and the axon tips will be simulated at the synapse unit linked to the dendrite receiving neuron. One part of the learning is produced by a facilitation of the vesicle release. There is a number of release sites on the axon terminal, learning don't act on this number, it works on the probability for each one to release at least one vesicle. this seems to be produced by expanding the number of cytofibrils bringing the vesicle from the "factory domain" to the membrane. This has two important consequences: - In simulated synapses the simulated vesicle will be released with a probability P if there is an incoming action potential. Adjusting P will define the learning of the synapse. - With imperfect conservation, the axon terminal may be destroyed in the original brain. Neverthless, we can define its memory value from the concentration of neurofibrils prensent. The number of vesicles released is at most 7, so it may be defined with a 3 bits number. On the dendrite side, there may be up to one hundred receptors, learning don't act on their sensibility or number, so even if the spine head is destroyed and we can't build back individual receptors, we can guess their rough number and this would be sufficient. For a given receptor kind, there could be a limited number of standard models so there is no need to describe individually each dendrite spine with its precise receptor number. From the decay products of the spine, a rough estimate of the number would be deduced and then the nearest standard value would be used. So what would be medelized? - At the presynaptic level, the action potential is simulated by a rectangular electric signal. - The number of vesicle release site is standardized, with may be 8 sizes. - The release probability P is computer controled and adjusted, it define the root memory process for the synapse. P is infered from the density of transport fibrils. - At the post synaptic level, the dendrite spine head is standardized, for a given kind and a given receptor there may be a 4 or 5 digits size definitions. - Because a single spine can have more than one receptor, the binary word used to define it must have a different frame for each receptor species. If for example a neuron of the category xxxxxx can have four receptor kinds on its dendrites, each dendrite spine head would be defined by a 32 bits word. Bits 1 to 4 for receptor of the first kind, bits 5 to 8 for the second kind and so on. The basic work is assumed to be done by some logic cells at FPGA level. This element must have 4 links with the outside: - One programming, for the synapse complex loading from a memory. - One input from the computer level to adjust P. - One output to repport at computer level the number of firings in a given time lapse. This will be used to adjust P. - One input for the action potential. The output goes to the dendrite tree and so don't need an output chanel outside the device. The global output goes to the buffer and memory when the FPGA is rewired. 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=26079