Fields Notes 18.1 - Page 20

From Atoms to Axons: Life as a Hierarchy of Devices Course October 10-18, 2017 • Fields Institute Speaker: Bob Eisenberg (Rush Medical Center), 2018 Fields Research Fellow Robert Eisenberg's course looked at life as a hierarchy of systems built by evolution to reproduce from just a genetic blueprint coded in the sequence of nucleobases of DNA. Anything that helps reproduction soon becomes a general property of the population of organisms, which is called process ‘evolution’. Every biologist knows that handfuls of atoms in DNA control the function of organisms 10 9 × larger than the DNA. But every physical scientist wonders, how is that possible? Macroscopic properties are derived by averaging. The atoms of DNA are a negligible fraction of the atoms in an organism. How can their effects survive averaging? The answer is that life imposes structure in a hierarchy of modules, from atoms to animals. Correlations guarantee that modules behave (and interact) predictably. Devices are created by the correlations imposed on averaging by the structure of the module. Outputs and inputs are ‘perfectly’ correlated in devices. Devices have well defined input output relations that respond predictably to external conditions. Without correlations imposed by structure, electric fields extend to infinity. Devices are local because correlations confine electric fields. Outputs are usually on a different scale from the input. A hierarchy of devices propels the effects of a few atoms from gene, to protein, to cell, tissue, organ and organism. Nerve fibers illustrate these principles. Nerves are known to be a multiscale composite of devices. Protein channels (10 nm) in nerve membranes constrain flow of atomic ions Na+, K+ (0.2 nm). Insulating cell membranes (2 nm) create electrical devices. These are transmission lines (10 μmeters × 1m) described by the telegrapher’s partial differential equation. Channels, ions and membrane ɔ)ձѥ͍Ʌɍ䁽٥(ѡЁѕɅЁɽ͍̤́Ѽ)ɕє̰ٔͥ)􁉥䤁ѥѕѥ̸)]ͬ܁́䁍ɕє)ѡѥѕѥɽѡ)Սɔͽմ)ό]锁Սɔ)Յѥ́ѕɅѥ)ɥ)ѕ́Ѽ͕)܁ѡ䁵)ѕѡЁɽՍ́ѡ)ɥ̸ٕͥ5)ѡȁѕ́ٔ)ЁЁɕѥ쁙ɽ)嵅ѥɕѥ̰Ѽѡ݅)ѕɵ䁵хʹѼ)ͥѕ̰́)ѥՕ́ɝ̰ѡ՝ѡ)ɔɅɕ䁍a٥ϊdAɡ)ѡ͡ձͼѡ͔ѕ)ɔ镐ѡЁɅѥ)͍ͥͥѡ)ЁՍɅɵѥ)ɽ٥䁉̸(