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Magnetoreception is a sense which allows an organism to detect a magnetic field to perceive direction, altitude or location. This sensory modality is used by a range of animals for orientation and navigation,〔 and as a method for animals to develop regional maps. For the purpose of navigation, magnetoreception deals with the detection of the Earth's magnetic field. Magnetoreception is present in bacteria, arthropods, molluscs and members of all major taxonomic groups of vertebrates. Magnetoreception in humans is controversial. ==Proposed mechanisms== An unequivocal demonstration of the use of magnetic fields for orientation within an organism has been in a class of bacteria known as magnetotactic bacteria. These bacteria demonstrate a behavioural phenomenon known as magnetotaxis, in which the bacterium orients itself and migrates in the direction along the Earth's magnetic field lines. The bacteria contain magnetosomes, which are particles of magnetite or iron sulfide enclosed within the bacterial cells. Each bacterium cell essentially acts as a magnetic dipole. They form in chains where the moments of each magnetosome align in parallel, giving the bacteria its permanent-magnet characteristics. These chains are formed symmetrically to preserve the crystalline structure of the cells.〔The Magneto-Lab. "Biochemistry and molecular biology of magnetosome formation in ''Magnetospirillum gryphiswaldense''." Available: http://magnum.mpi-bremen.de/magneto/research/index.html.〕 These bacteria are said to have permanent magnetic sensitivity. For animals the mechanism for magnetoreception is unknown, but there exist two main hypotheses to explain the phenomenon. According to one model, cryptochrome, when exposed to blue light, becomes activated to form a pair of two radicals (molecules with a single unpaired electron) where the spins of the two unpaired electrons are correlated. The surrounding magnetic field affects the dynamics of this correlation (parallel or anti-parallel), and this in turn affects the length of time cryptochrome stays in its activated state. Activation of cryptochrome may affect the light-sensitivity of retinal neurons, with the overall result that the bird can "see" the magnetic field.〔(Cryptochrome and Magnetic Sensing ), ''Theoretical and Computational Biophysics Group'' at the University of Illinois at Urbana-Champaign. Accessed 13 February 2009〕 The Earth's magnetic field is only 0.5 Gauss and so it is difficult to conceive of a mechanism by which such a field could lead to any chemical changes other than those affecting the weak magnetic fields between radical pairs. Cryptochromes are therefore thought to be essential for the light-dependent ability of the fruit fly ''Drosophila melanogaster'' to sense magnetic fields. The second proposed model for magnetoreception relies on Fe3O4, also referred to as iron (II, III) oxide or magnetite, a natural oxide with strong magnetism. Iron (II, III) oxide remains permanently magnetized when its length is larger than 50 nm and becomes magnetized when exposed to a magnetic field if its length is less than 50 nm. In both of these situations the Earth's magnetic field leads to a transducible signal via a physical effect on this magnetically sensitive oxide. Another less general type of magnetic sensing mechanism in animals that has been thoroughly described is the inductive sensing methods used by sharks, stingrays and chimaeras (cartilaginous fish). These species possess a unique electroreceptive organ known as ''ampullae of Lorenzini'' which can detect a slight variation in electric potential. These organs are made up of mucus-filled canals that connect from the skin's pores to small sacs within the animal's flesh that are also filled with mucus. The ampullae of Lorenzini are capable of detecting DC currents and have been proposed to be used in the sensing of the weak electric fields of prey and predators. These organs could also possibly sense magnetic fields, by means of Faraday's law: as a conductor moves through a magnetic field an electric potential is generated. In this case the conductor is the animal moving through a magnetic field, and the potential induced depends on the time varying rate of flux through the conductor according to These organs detect very small fluctuations in the potential difference between the pore and the base of the electroreceptor sack. An increase in potential results in a decrease in the rate of nerve activity, and a decrease in potential results in an increase in the rate of nerve activity. This is analogous to the behavior of a current carrying conductor; with a fixed channel resistance, an increase in potential would decrease the amount of current detected, and vice versa. These receptors are located along the mouth and nose of sharks and stingrays. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Magnetoreception」の詳細全文を読む スポンサード リンク
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