Magnetic field in axons
The vector of the magnetic induction B will form closed loops around the axis of
the neuronal projection and the direction will be defined by the right-handed
screw rule (i.e. counterclockwise if the axial current flows toward your face). In
axons the magnetic field is stronger than the magnetic field in dendrites because
of the greater ion currents flowing inside the axoplasm. The nerve action
potential has the form of a moving solitary wave, which can be modeled as two
opposing current dipoles driven by a potential change of the order of 70 mV. The
peak currents range from 5 to 10 μA (Katz, 1966). Axons range in diameter from
less than 1 μm to 25 μm in humans, but reach gigantic size in squid ~1mm.
Calculations of the magnetic flux density in the largest human axons that have
the greatest electric currents give us:
(88)
(89)
B = μeff μ is
πd
1o×4×1o-7H.m-1×1o-6A -7
B =---------------=---------= 1.6 × 10 T
25 × 1o-6m
Although this result is 3 orders of magnitude greater than the experimentally
measured magnetic field in frog sciatic nerve using SQUID magnetometer
(Wikswo et al., 1980) it remains too weak - only ‰o of the Earth’s magnetic field.
The experimentally measured value for the magnetic field using SQUID
magnetometer 1.3mm aside of the frog sciatic nerve was 1.2x10 -10 T with a
signal-to-noise ratio 40 to 1 (Wikswo et al., 1980). The assessed value for the
magnetic field strength at the nerve surface (where it has peak magnitude) using
the Ampere’s law
(90) φ B ∙ d∣ = μ0 i
Γ
was 1.2x10 -10 tesla because of large frog sciatic the nerve diameter (d~0.6mm).
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