Otoliths

Otoliths ("ear-stones") are small crystals of calcium carbonate (mostly) that provide the inertial masses within the utricle and saccule, which are called the otolith organs. In mammals, many small (approx size of each otolith?) otoliths are embedded in the jelly like disks oriented veritcally in the saccule? and horzontally in the utricle? Cilia of hair cells project into the jelly disks. Linear acceleration, by movement or titling of the head, causes movement of the jelly due to the otolithic mass, which causes bending of the cilia of hair cells; tilting or acceleration in one direction causes an increase in hair cell impluses, while acceleration in the other direction causes a decrease in hair cell impulses.

The role of the otoliths is therefore analogous to the role of the endolymph, which provides the inertial mass within the semicircular canals to detect rotational acceleration. Treatments which cause or simulate linear acceleration should mostly stimulate the otolithic organs. Thus, off-axis rotation at a constant angular velocity stimulates the otolithic organs due to the apparent centrifugal force ("hypergravity"). To stimulate the semicircular canals, the angular velocity should vary (i.e. sinusoidal rotation) or compound rotation around multiple axes should be applied. On-axis rotation at a constant rate shouls stimulate neither the otolith organs nor the semicircular canals.

The mineral composition of otoliths in mammals can vary. [something on manganese deficiency?]

Fish Otoliths

Unlike the multiple small otolith crystals in mammals, fish have only 3 large otoliths in each ear. (give range of sizes?) Age of fish can be determined by the layers of mineral deposition (like tree rings), and the geographical history of the fish can be determined by comparing the mineral contant across layers with known mineral deposits within the fish's range. There have been occasional reports of fish with otoliths containing a high content of the naturally occuring mineral magnetite. Because their otoliths are magnetized by this mineral, these fish may sensitive to the effects of magnetic fields by direct force on their otoliths.

Magnetic Susceptibility

The magnetic susceptibity is a measure of how much force or torque is experienced by a substance within a magnetic field. Most biological materials are diamagentic, unless they contain metal ions. (Ferromagnetic substances are paramagnetic with very high magnetic susceptibilty). Calcium carbonate is the primary mineral within the otoconia, and it has a magnetic susceptibility of -38.2x10-6 cgs (CRC Handbook?): higher than the susceptibility of calcium hydroxyapatite in bone (0.9x10-6 cgs) [113], but far lower than the susceptibility of ferromagnetic crystals like Fe2O3 (7200x10-6 cgs).

The Hasenstein lab at UL Lafayette has a nice description of how to calculate the force exerted on a diamagnetic substance in a magnetic gradient.