From: samodena@csemail.cropsci.ncsu.edu (S. A. Modena)
Newsgroups: bionet.info-theory
Subject: Animate Nature and Noise
Summary: Isothermal-ism is a dead end idea
Keywords: info theory isothermal noise physical limit receptors
Date: 27 Jan 93 06:21:12 GMT
Organization: Crop Science Dept., NCSU, Raleigh, NC 27695-7620

Animate Nature Operates at or below the Ambient Noise Level
 
"Walker and Bitterman's measurement of the threshold sensitivity of 
the bees to a small static anomoly superimposed on the background 
[earth's magnetic] field is the most dramatic experimental result 
to date from their conditioning experiments.  By starting with a 
moderately strong anomoly in a small two-choice training program, 
and by reducing the amplitude of the anomoly in small exponential 
steps, the threshold sensitivity could be determined by the point 
at which bees were no longer able to discriminate correctly.  Of 
nine bees run through the procedure, the median threshold was 250 
nanoTelsa (nT, 0.6% of the background field in Hawaii), whereas 
their best bee lost the ability to discrimiate in fields below 25 
nT (0.06%)."
        ....Discrimination of Low-Frequency Magnetic Fields
            by Honeybees: Biophysics and Experimental Tests
            J. L. Kirschvink et al.

"In Limulus photoreceptors the rate of normal spontaneous waves 
is ~1/s at room temp.  Such events are likely to come from a 
variety of sources including spontaneous rhodopsin 
isomerization, reversion of deactivated meta-rhodopsin back to 
the active state, spontaneous activation of G protein, and 
spontaneous activations of subsequent steps in the cascade.  The 
fraction of spontaneous events that are generated by spontaneous 
activation of the G protein is not clear, but for our purposes we 
can make the worst case assumption that all spontaneous events 
are due to G protein.  This gives an upper limit on the rate 
constant for apparent spontaneous nulceotide exchange on G 
proteins as follows: if there is one spontaneous event per second 
in a cell with 10^8 G proteins--there are 10^9 rhodopsin 
molecules per cell and ~ 0.1 G protein/rhodopsin--the apparent 
rate constant for spontaneous activation of G protein is 10^-
8/sec.  Remarkably this number is more than three orders of 
magnitude lower than the the rate of spontaneous nucleotide 
exchange  measured biochemically for the vertebrate photoreceptor 
G protein (2 x 10^-5/s).  Similar measurements on G proteins from 
other tissues show an even faster rate of exchange.  This large 
discrepancy raises the question, Why do most spontaneously 
activated G proteins fail to produce an electrical response?  
Stated differently, the biochemical evidence would predict that 
~1000 G proteins would undergo nucletide exchange per second and 
that this would generate enormous electrical noise.  We do not 
observe noise of this magnitude......"

    .....Mechanisms of Amplification, Deactivation, and Noise 
         Noise Reduction in Invertabrate Photoreceptors
         J. Lisman et al.

"Hearing Limits

"Hearing is an altogether different situation. The quantum of 
sound is the phonon....Although firm numbers are hard to arrive 
at, basilar membrane displacements at the hearing threshold are 
believed to be in the range of atomic, even subatomic, 
dimensions: ~0.1 to 1 Angstrom.  The motion is even less than the 
thermal excursions of the hair bundle itself.....The astounding 
sensitivity of hearing is evident from recent experiments of Denk 
and Webb showing that hair cells faithfully transduce Brownian 
(thermal) noise at their inputs.  These experiments were made 
possible by the developemt of optical instrumentation that...."

"Thermoreception Limits

"What about thermal signals themselves?.... The record holder in 
the insect world is probably the eyeless cave beetle..., whose 
antennal sensillum shows both hot and cold sensitivity, with a 
threshold for firing near 3 x 10^-3 deg C/s (3 mDeg/s).....The 
California cockroach does just a bit less well: 20 mDeg/s...The 
golden eyelash viper...can sense thermal gradients estimated at 
1-10 mDeg C/s.  Bialek has derived an expression for the noise 
level in a thermoreceptor, considered as a blackbody radiator in 
thermal equilibrium with its environment.  He found that the cave 
beetle, in particular, performs close to the physical limit...."

"APPENDIX: Thermal Fluctuations in Concentration

...  delta-E = 0.5 ( kT/n) delta-n^2  where n = # of molecules


     ...Biophysical Principles of Sensory Transduction
        Steven M. Block

IN "Sensory TRansduction"
   Society of General Physiologists Series Vol. 47 (46th Ann. 
Symp.)
   Edited by D.P. Corey & S.D. Roper
   New York: Rockefeller University Press,  1991
   ISBN 0-87470-051-5

Steve
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