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 are linked in a multiscale hierarchy of devices ( that interact across scales ) to create nerve signals , called all or none (= binary ) action potentials . We ask , how does biology create the action potential from the structure of sodium channels and axons ? We analyze structure and field equations of interacting electrical , mechanical , chemical , and biological systems to see how they make one biological system that produces the electrical signal of nerves . Many other biological systems have input output relations ; from enzymatic reactions , to pathways of intermediary metabolism , to physiological systems of cells , tissues and organs , although they are rarely called ‘ devices ’. Perhaps they should be , so these systems are analyzed in the best traditions of physical science using the magnificent structural information provided by biologists . �
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