Weak Field Effects
Key features of the model for biological detection of small magnetic field differences
Fractional change in the reaction rate of the radical-pair reaction vs the magnitude of the magnetic field.
Understanding exposure thresholds for the response of biological systems
to extremely low frequency (ELF) electric and magnetic fields is a
fundamental problem of long-standing interest. We consider a two-state
model for voltage-gated channels in the membrane of an isolated
elongated cell (L-cell = 1 mm; r(cell) = 25 mu m) and use a previously
described process of ionic and molecular flux rectification to set lower
bounds for a threshold exposure. A key assumption is that it is the ability
of weak physical fields to alter biochemistry that is limiting, not the
ability of a small number of molecules to alter biological systems.
Moreover, molecular shot noise, not thermal voltage noise, is the basis of
threshold estimates. Models with and without stochastic resonance are
used, with a long exposure time, t(exp) = 10(4) s. We also determined
the dependence of the threshold on the basal transport rate. By considering
both spherical and elongated cells, we find that the lowest bound
for the threshold is E-min approximate to 9 x 10(-3) V m(-1) (9 x 10(-5)
V cm(-1)). Using a conservative value for the loop radius r(loop)
= 0.3 m for induced current, the corresponding lower bound in the human
body for a magnetic field exposure is B-min approximate to 6 x 10(-4) T (6 G).
Unless large, organized, and electrically amplifying multicellular systems
such as the ampullae of Lorenzini of elasmobranch fish are involved, these
results strongly suggest that the biophysical mechanism of voltage-gated
macromolecules in the membranes of cells can be ruled out as a basis of
possible effects of weak ELF electric and magnetic fields in humans.
There is evidence that animals can detect small changes in the Earth's
magnetic field by two distinct mechanisms, one using the mineral magnetite
as the primary sensor and one using magnetically sensitive chemical
reactions. Magnetite responds by physically twisting, or even reorienting
the whole organism in the case of some bacteria, but the magnetic dipoles
of individual molecules are too small to respond in the same way. Here we
assess whether reactions whose rates are affected by the orientation of
reactants in magnetic fields could form the basis of a biological compass.
We use a general model, incorporating biological components and design
criteria, to calculate realistic constraints for such a compass. This
model compares a chemical signal produced owing to magnetic field effects
with stochastic noise and with changes due to physiological temperature
variation. Our analysis shows that a chemically based biological compass
is feasible with its size, for any given detection limit, being dependent
on the magnetic sensitivity of the rate constant of the chemical reaction.