In the dark, the cell membrane of the outer segment of a vertebrate rod or cone has a heavy leakage current density of certain positive ions into the cell through gated cation channels embedded in the cell membrane. 80% of the current is carried by Na+, 15% by Ca++, and 5% by Mg++ ions. The inward leakage current is balanced by active outward pumping of Na+, etc., by metabolic pumps in the inner segment of the cell.
When light is absorbed by the photopigment molecules on the disks inside the outer segment, a complex cascade of chemical reactions leads to the closure of the cation channels, allowing the photoreceptor cell to hyperpolarize (Kolb et al., 1999). The important thing in this model is that in the dark there is a radially directed, cation current density Jx crossing the membrane of the outer segment of the photoreceptor cell. If a magnetic field is applied perpendicular to Jx, the resulting Lorentz force, FL, given by the right-hand screw rule, will be perpendicular to both B and Jx, and tangential to the cell membrane. Thus, a moving cation will experience a force perpendicular to its direction of motion through the ion channel. This lateral force could slow or block the passage of individual leakage cations, reducing the dark Jx and causing the cell to hyperpolarize slightly, mimicking low-level light being absorbed by the receptor. Obviously, such dual-use of a rod or cone as a photoreceptor and a magnetoreceptor would only be possible in the absence of light. If such a magnetic field-sensing rod (or cone) in the retina were oriented on the animal's anterior-posterior axis when the animal was aligned with Be, there could be maximum neural output by a special ganglion cell fiber to the CNS.
Semm and Demaine (1986) showed that visual neurons in the pigeon's brain responded to changes in the direction of a magnetic field. Their assumption was that the neural activity of magnetic origin came from the eyes, and was not from magnetite-containing magnetosensors heretofore implicated in pigeon navigation. The responses occurred only in light, which is contrary to the hypothetical scenario above, unless a special magnetosensing rod or cone lacked the normal biochemical machinery to close cation channels in response to light, and responded only to Be.
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