Bats use magnetite to detect the earth's magnetic field

While the role of magnetic cues for compass orientation has been confirmed in numerous animals, the mechanism of detection is still debated. Two hypotheses have been proposed, one based on a light dependent mechanism, apparently used by birds and another based on a "compass organelle" containing the iron oxide particles magnetite (Fe(3)O(4)). Bats have recently been shown to use magnetic cues for compass orientation but the method by which they detect the Earth's magnetic field remains unknown. Here we use the classic "Kalmijn-Blakemore" pulse re-magnetization experiment, whereby the polarity of cellular magnetite is reversed. The results demonstrate that the big brown bat Eptesicus fuscus uses single domain magnetite to detect the Earths magnetic field and the response indicates a polarity based receptor. Polarity detection is a prerequisite for the use of magnetite as a compass and suggests that big brown bats use magnetite to detect the magnetic field as a compass. Our results indicate the possibility that sensory cells in bats contain freely rotating magnetite particles, which appears not to be the case in birds. It is crucial that the ultrastructure of the magnetite containing magnetoreceptors is described for our understanding of magnetoreception in animals.     

Biogenic magnetite as a basis for magnetic field detection in animals

Bacteria, sharks, honey bees, and homing pigeons as well as other organisms seem to detect the direction of the earth's magnetic field. Indirect but reproducible evidence suggests that the bees and birds can also respond to very minute changes in its intensity. The mechanisms behind this sensitivity are not known. Naturally magnetic, biologically precipitated magnetite (Fe₃O₄) has been found in chitons, magnetotactic bacteria, honey bees, homing pigeons, and dolphins. Its mineralization in localized areas may be associated with the ability of these animals to respond to the direction and intensity of the earth's magnetic field. The presence of large numbers (～10⁸) of superparamagnetic magnetite crystals in honey bees and similar numbers of single-domain magnetite grains in pigeons suggests that there may be at least two basic types of ferrimagnetic magnetoreceptive organelles. Theoretical calculations show that ferrimagnetic organs using either type of grain when integrated by the nervous system are capable of accounting for even the most extreme magnetic field sensitivities reported. Indirect evidence suggests that organic magnetite may be a common biological component, and may account for the results of numerous high field and electromagnetic experiments on animals.     

Behavioural evidence that magnetic field effects in the land snail,Cepaea nemoralis,might not depend on magnetite or induced electric currents

Although extremely low frequency (ELF) magnetic fields (<300 Hz) appear to exert a variety of biological effects, the magnetic field sensing/transduction mechanism(s) remains to be established. Here, using the inhibitory effects of magnetic fields on endogenous opioid peptide-mediated “analgaesic” response of the land snail. Cepaea nemoralis, we addressed the mechanism(s) of action of ELF magnetic fields. Indirect mechanisms involving both induced electric fields and direct magnetic field detection mechanisms (e.g., magnetite, parametric resonance) were evaluated. Snails were exposed to a static magnetic field (B_DC=78±1 μT) and to a 60 Hz magnetic field (B_AC=299±1 μT peak) with the angle between the static and 60 Hz magnetic fields varied in eight steps between 0° and 90°. At 0° and 90°, the magnetic field reduced opioid-induced analgaesia by approximately 20%, and this inhibition was increased to a maximum of 50% when the angle was between 50° and 70°. Because B_AC was fixed in amplitude, direction, and frequency, any induced electric currents would be constant independent of the B_AC/B_DC angle. Also, an energy transduction mechanism involving magnetite should show greatest sensitivity at 90°. Therefore, the energy transduction mechanism probably does not involve induced electric currents or magnetite. Rather, our results suggest a direct magnetic field detection mechanism consistent with the parametric resonance model proposed by Lednev. © 1996 Wiley-Liss, Inc.     

Magnetite and magnetotaxis in microorganisms

Magnetotactic bacteria from freshwater and marine sediments orient and navigate along geomagnetic field lines. Their magnetotactic response is based on intracellular, single magnetic domains of ferrimagnetic magnetite, which impart a permanent magnetic dipole moment to the cell.     

Superparamagnetic Magnetite in the Upper Beak Tissue of Homing Pigeons

Homing pigeons have been subject of various studies trying to detect magnetic material which might be involved in magnetic field perception. Here we focus on the upper-beak skin of homing pigeons, a region that has previously been shown to contain nerves sensitive to changes of the ambient magnetic field. We localized Fe³⁺ concentrations in the subcutis and identified the material by transmission electronmicroscopy (TEM) as aggregates of magnetite nanocrystals (with grain sizes between 1 and 5 nm). The particles form clusters of 1–3 m diameter, which are arranged in distinct coherent elongated structures, associated with nervous tissue and located between fat cells. Complementary low-temperature magnetic measurements confirm the microscopic observations of fine-grained superparamagnetic particles in the tissue. Neither electron-microscopic nor magnetic measurements revealed any single-domain magnetite in the upper-beak skin tissue.     

Obtaining of new magnetic nanocomposites based on modified polysaccharide

The study presents the preparation of some composite materials with magnetic properties by two different encapsulation methods of magnetite (Fe₃O₄) in a polymer matrix based on carboxymethyl starch-g-polylactic acid (CMS-g-PLA). The copolymer matrix used to obtain the magnetic nanocomposites was synthesized by grafting reaction of carboxymethyl starch (CMS) with d,l-lactic acid (DLLA), in the presence of Sn octanoate [Sn(Oct)₂] as catalyst. Magnetite was obtained by co-precipitation from aqueous salt solutions FeCl₂/FeCl₃ (molar ratio 1/2). The magnetic composites were prepared by precipitation method in acetone (non-solvent) of the DMSO solutions of magnetite and copolymer, and synthesis in situ of the nanocomposites. In the first case, the particle size measured by DLS-technique was 168 nm, and the magnetization was 46.82 emu/g, while after in situ synthesis, the composite materials showed smaller size (141 nm), but the magnetization was reduced (3.04 emu/g). The higher magnetization in the first case is due to the great degree of encapsulation of the magnetite, which was about 43.4 wt.%, compared to 4.37 wt.% for the in situ synthesis (determined by thermogravimetry). The CMS-g-PLA copolymer, magnetite, and the nanocomposites were characterized by infrared spectroscopy (FTIR), near infrared chemical imagistic (NIR-CI), dynamic light scattering (DLS) technique, X-ray diffraction (WAXD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and thermal analyses. Since the polymer matrix and magnetite are biodegradable and biocompatible, the magnetic nanocomposites can be used for conjugation of some drugs. The polymer matrix CMS-g-PLA acts as a shell, and vehicle for the active component, whereas magnetite is the component which makes targeting possible by external magnetic field manipulation.     

Mechanisms of magnetic orientation in birds

Behavior and electrophysiological studies have demonstrateda sensitivity to characteristics of the Geomagnetic field thatcan be used for navigation, both for direction finding (compass)and position finding (map). The avian magnetic compass receptorappears to be a light-dependent, wavelength-sensitive systemthat functions as a polarity compass (i.e., it distinguishespoleward from equatorward rather than north from south) andis relatively insensitive to changes in magnetic field intensity.The receptor is within the retina and is based on one or morephotopigments, perhaps cryptochromes. A second receptor systemappears to be based on magnetite and might serve to transducelocation information independent of the compass system. Thisreceptor is associated with the ophthalmic branch of the trigeminalnerve and is sensitive to very small (<50 nanotesla) changesin the intensity of the magnetic field. In neither case hasa neuron that responded to changes in the magnetic field beentraced to a structure that can be identified to be a receptor.Almost nothing is known about how magnetic information is processedwithin the brain or how it is combined with other sensory informationand used for navigation. These remain areas of future research.     

On the significance of the time constants of magnetic field sensitivity in animals

A variety of organisms is known to have the ability to transduce and respond to relatively weak magnetic fields, including the earth's field. Though biogenic magnetite has been identified as the transducer in a number of cases with regards to geomagnetic field sensing, the mechanism underlying neurophysiological responses in human studies is not understood. Here we note that the time constants involved in this latter type of field sensitivity are much longer than those in organisms that make use of the earth's magnetic field for navigation. The purpose of this brief communication is to suggest that the time constants associated with magnetic field sensitivity may be a useful way to distinguish field sensitivity due to magnetite based receptors from sensitivity that may depend on direct (or downstream) biochemical processes.     

Deep-Etching Electron Microscopy of Cells of <Emphasis Type="Italic">Magnetospirillum magnetotacticum</Emphasis>: Evidence for Filamentous Structures Connecting the Magnetosome Chain to the Cell Surface

Magnetospirillum magnetotacticum are magnetotactic bacteria that form a single chain of magnetite magnetosomes within its cytoplasm. Here, we studied the ultrastructure of M. magnetotacticum by freeze-fracture and deep-etching to understand the spatial correlation between the magnetosome chain and the cell envelope and its possible implications for magnetotaxis. Magnetosomes were found mainly near the cell envelope, forming chains that were closely associated with the granular cytoplasmic material. The membrane surrounding the magnetosomes could be visualized in deep-etching preparations. Thin connections between magnetosome chains and the cell envelope were observed in deep-etching images. These results strengthen the hypothesis for the existence of structures that transfer the torque from the magnetosome chains to the whole cell during the orientation of magnetotactic bacteria to a magnetic field lines.     

The osmotic magnetometer: a new model for magnetite-based magnetoreceptors in animals

Homing pigeons and migratory birds are well known examples for animals that use the geomagnetic field for their orientation. Yet, neither the underlying receptor mechanism nor the magnetoreceptor itself is known. Recently, an innervated structure containing clusters of magnetite nanocrystals was identified in the upper beak skin of the homing pigeon. Here we show theoretically that such a cluster has a magnetic-field-dependent shape, even in fields as weak as the Earth's magnetic field; by converting magnetic stimuli into mechanical strain, the clusters can be assumed as primary units of magnetoperception in homing pigeons. Since the orientation of the strain ellipsoid indicates the direction of the external magnetic field, a cluster of magnetite nanocrystals also has the potential to serve as the basis of the so-called inclination compass of migratory birds. It is quantitatively demonstrated that the magnetic-field-induced shape change of a cluster can be amplified as well as counterbalanced by means of osmotic pressure regulation, which offers an elegant possibility to determine the magnetic field strength just by measuring changes in concentration. Received: 18 May 1998 / Revised version: 11 February 1999 / Accepted: 11 February 1999     

Effects of Weak Environmental Magnetic Fields on the Spontaneous Bioelectrical Activity of Snail Neurons

We examined the effects of 50-Hz magnetic fields in the range of flux densities relevant to our current environmental exposures on action potential (AP), after-hyperpolarization potential (AHP) and neuronal excitability in neurons of land snails, Helix aspersa. It was shown that when the neurons were exposed to magnetic field at the various flux densities, marked changes in neuronal excitability, AP firing frequency and AHP amplitude were seen. These effects seemed to be related to the intensity, type (single and continuous or repeated and cumulative) and length of exposure (18 or 20 min). The extremely low-frequency (ELF) magnetic field exposures affect the excitability of F1 neuronal cells in a nonmonotonic manner, disrupting their normal characteristic and synchronized firing patterns by interfering with the cell membrane electrophysiological properties. Our results could explain one of the mechanisms and sites of action of ELF magnetic fields. A possible explanation of the inhibitory effects of magnetic fields could be a decrease in Ca²⁺ influx through inhibition of voltage-gated Ca²⁺ channels. The detailed mechanism of effect, however, needs to be further studied under voltage-clamp conditions.     

Disruption of magnetic orientation in hatchling loggerhead sea turtles by pulsed magnetic fields

Loggerhead sea turtles (Caretta caretta) derive both directional and positional information from the Earths magnetic field, but the mechanism underlying magnetic field detection in turtles has not been determined. One hypothesis is that crystals of biogenic, single-domain magnetite provide the physical basis of the magnetic sense. As a first step toward determining if magnetite is involved in sea turtle magnetoreception, hatchling loggerheads were exposed to pulsed magnetic fields (40 mT, 4 ms rise time) capable of altering the magnetic dipole moment of biogenic magnetite crystals. A control group of turtles was treated identically but not exposed to the pulsed fields. Both groups of turtles subsequently oriented toward a light source, implying that the pulsed fields did not disrupt the motivation to swim or the ability to maintain a consistent heading. However, when swimming in darkness under conditions in which turtles normally orient magnetically, control turtles oriented significantly toward the offshore migratory direction while those that were exposed to the magnetic pulses did not. These results are consistent with the hypothesis that at least part of the sea turtle magnetoreception system is based on magnetite. In principle, a magnetite-based magnetoreception system might be involved in detecting directional information, positional information, or both.     

Phase locking of single neuron activity to theta oscillations during working memory in monkey extrastriate visual cortex

Working memory has been linked to elevated single neuron discharge in monkeys and to oscillatory changes in the human EEG, but the relation between these effects has remained largely unexplored. We addressed this question by measuring local field potentials and single unit activity simultaneously from multiple electrodes placed in extrastriate visual cortex while monkeys were performing a working memory task. We describe a significant enhancement in theta band energy during the delay period. Theta oscillations had a systematic effect on single neuron activity, with neurons emitting more action potentials near their preferred angle of each theta cycle. Sample-selective delay activity was enhanced if only action potentials emitted near the preferred theta angle were considered. Our results suggest that extrastriate visual cortex is involved in short-term maintenance of information and that theta oscillations provide a mechanism for structuring the recurrent interaction between neurons in different brain regions that underlie working memory.     

A model for encoding of magnetic field intensity by magnetite-based magnetoreceptor cells

A conceptual model is proposed for the encoding of magnetic field intensity from the motion of a chain of single-domain magnetite crystals which is located within a receptor cell, connected at one end to the cell membrane, and linked by cytoskeletal filaments to an array of mechanically gated ion channels centred on the end of the chain. In this arrangement, the physical links between the chain and ion channels will restrict the motion of the magnetite chain in response to the external magnetic field to a narrow cone with its axis through the point where the chain is attached to the membrane. The motion of the chain in the presence of an external magnetic field and thermal agitation will open a varying number of channels, causing the membrane potential to oscillate about some mean value that depends on the component of magnetic intensity oriented perpendicular to the cell membrane. The model permits estimation of magnetic intensity by integration of the motion of the magnetite chain over an area of the cell membrane, explains a number of results from physiological recordings in birds and fish, and makes testable predictions for future experimental studies. The model also provides a mechanism at the cellular level for a constant value of the Weber fraction (the ratio of the threshold sensitivity to a stimulus and the magnitude of that stimulus) for the magnetic sense but requires a separate gain control mechanism for modulation of sensitivity over a range of background fields. If magnetic field detection and encoding works as proposed in the model, the magnetoreceptor system may also be able to reconstruct the magnetic field vector using information about the vertical and horizontal axes from the eyes, gravity detectors, or both.     

The magnetic and electric fields induced by superparamagnetic magnetite in honeybees

Hairs on the abdomen of honeybees contain dendrites and a rod and ring structure composed of black particles, presumed to be superparamagnetic (SPM) magnetite. The rod and ring were divided into compartments and each compartment approximated by a dipole. The magnetic fields were calculated at a point P at various locations for a change of the external geomagnetic field from zero to 0.5 G in 0.1 s. The magnetite amplifies the external field at the rod/ring-dendrite interface. The induced electric field and potential difference for a small circular area are in the order of 10^–7 V/m and 10^–13 V respectively. Mechanisms are proposed for amplifying the electric fields in the dendrite and in an integrating nerve fibre. A hypothesis is developed for associative learning of visual and magnetic stimuli. If magnetic and visual inputs are associated in the ganglion and in the brain, very small changes of either magnetic or visual inputs could be perceived. A bee could sequentially follow the images associated with magnetic gradients on a cloudy day and find the food source.This paper is dedicated to the memory of the late Prof. Dr. W. Reichardt, an outstanding scientist and personality, who will be greatly missed by all who knew him and his work     

Theoretical analysis of an iron mineral-based magnetoreceptor model in birds

Sensing the magnetic field has been established as an essential part of navigation and orientation of various animals for many years. Only recently has the first detailed receptor concept for magnetoreception been published based on histological and physical results. The considered mechanism involves two types of iron minerals (magnetite and maghemite) that were found in subcellular compartments within sensory dendrites of the upper beak of several bird species. But so far a quantitative evaluation of the proposed receptor is missing. In this article, we develop a theoretical model to quantitatively and qualitatively describe the magnetic field effects among particles containing iron minerals. The analysis of forces acting between these subcellular compartments shows a particular dependence on the orientation of the external magnetic field. The iron minerals in the beak are found in the form of crystalline maghemite platelets and assemblies of magnetite nanoparticles. We demonstrate that the pull or push to the magnetite assemblies, which are connected to the cell membrane, may reach a value of 0.2 pN -- sufficient to excite specific mechanoreceptive membrane channels in the nerve cell. The theoretical analysis of the assumed magnetoreceptor system in the avian beak skin clearly shows that it might indeed be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.     

Analysis of magnetic material in the human heart, spleen and liver

Isothermal remanent magnetization (IRM) acquisition and alternating field (A.F.) demagnetization analyses were performed on human heart, spleen and liver samples resected from cadavers. The magnetic properties of the samples were measured both at 77K and at 273K. A.F. demagnetization was performed at 273K. Results from the analyses of the tissue indicate the presence of ferromagnetic, fine-grained, magnetically interacting particles which, due primarily to magnetic properties, are thought to be magnetite and/or maghemite. The presence of superparamagnetic particles can be inferred from the increase in saturation IRM values when measured at 77K compared with measurements at 273K and the decay of remanent magnetization upon warming from 77K. The concentration of magnetic material (assuming it is magnetite or maghemite) in the samples varies from 13.7 ng g-1 to 343 ng g-1, with the heart tissue generally having the highest concentration. The presence of magnetic material in these organs may have implications for the function of biogenic magnetite in the human body.     

Population receptive field dynamics in human visual cortex

Seminal work in the early nineties revealed that the visual receptive field of neurons in cat primary visual cortex can change in location and size when artificial scotomas are applied. Recent work now suggests that these single neuron receptive field dynamics also pertain to the neuronal population receptive field (pRF) that can be measured in humans with functional magnetic resonance imaging (fMRI). To examine this further, we estimated the pRF in twelve healthy participants while masking the central portion of the visual field. We found that the pRF changes in location and size for two differently sized artificial scotomas, and that these pRF dynamics are most likely due to a combination of the neuronal receptive field position and size scatter as well as modulatory feedback signals from extrastriate visual areas.     

Influence of weak magnetic fields on fish retina photoreceptors

It has been shown for the first time that retina photoreceptors of the salmon Oncorhynchus masou (masu salmon) fry exposed to a constant magnetic field (80 Gs) in conditions of twilight illumination demonstrated not a twilight reaction, as in the control, but a darkness (scotopic) reaction. The pigment epithelium in this case reacted as to bright light. Under the influence of long wavelength red light, the retinomotor reaction corresponded to partial light adaptation. After the indemnification of the horizontal component (0.1 Gs) of the geomagnetic field, fish retina adapted to complete darkness demonstrated changes that could not be attributed to one of standard conditions of light exposure. Thus, the rods and cones demonstrated a reaction similar to that to twilight, double cones reacted as to complete darkness ("correct" reaction), and PE borrowed a position as at twilight. Apparently, the basic stimuli of the retina reactions were not light but changes of the magnetic field. Based on these results and the results available in literature, the authors offer a new variant of the hypothesis of light-dependent magnetoreception, which is based on the assumption that the liquid crystals of retina pigments are very sensitive receptors of magnetic field and do not require the presence of magnetite crystals. We assume that the layer of retina pigment epithelium participates in the complex process of transformation of a light signal, which takes into account the influence of magnetic field but not simply absorbs light superfluous for photoreceptors. The changes in magnetic field cause a distortion of the liquid crystal line structure of the pigment, which results in the transformation of the light signal acting on the photoreceptors. Monochromatic red light in the same way deforms the response to the native magnetic signal.     

3D Magnetopneumography Magnetic Dipole Model and Its Application Using Fluxgate Gradiometers

Magnetopneumography (MPG) as a non‐invasive method for pneumoconiosis diagnosis has been developed to evaluate the load and spatial distribution of particles within the human lungs through scanning of remanent magnetic fields after magnetization of the subject in a strong direct current field. The measurement of particle spatial distribution is very important for pneumoconiosis diagnosis because localized deposits may be associated with some pathological changes such as pulmonary fibrosis. Previous research found that loads of magnetite particles were proportional to their magnetic dipole moments. The three‐dimensional (3D) MPG magnetic dipole model (MDM) proposed in this paper and based on Biot–Savart Law and matrix manipulation provides a means of precise measurement of the particle distribution and load amount. A styrofoam + magnetite powder phantom with magnetization was laid on a nonmagnetic board. Two first‐order fluxgate gradiometers with 10^–12 T sensitivity were coaxially applied over and under the phantom and used for scanning remanent magnetic fields. This paper provides validation results using 3D MPG MDM through two experiments. The overall error of the simulation results is 0.2–2.7% in the center and 7.28–9.42% in the corners of the subject. Finally, this paper gives clinical experiments with a welder suffering stage‐II pneumoconiosis and states that the 3D MPG MDM shows similar results to X‐ray chest films in pneumoconiosis diagnosis. The results suggest that the 3D MPG MDM is potentially a reasonable and accurate algorithmic model to inversely track the load amount and distribution of magnetite particles within the lungs. Bioelectromagnetics. 2019;40:472–487. © 2019 Wiley Periodicals, Inc