A survey of the literature regarding the effects of weak, ELF (extremely low frequency) magnetic and electric fields on biological systems concentrates on those effects which may give indications for understanding transduction and nervous processing. Effects are biophysical, biochemical, behavioral and the perception of magnetic fields. Lowest intensities having measurable effects are observed in ethological studies. Magnetic field effects are in most cases cumulative and depend on the state of the cell, tissue or animal. Often the effects occur only within narrow windows for frequency as well as for intensity.
In electrophysiological recordings in the second abdominal ganglion of bees neurons were found which respond to modulations of the external magnetic field within an intensity-frequency window. Responses were analyzed by correllelograms. Cross-correlation data confirm the existence of magneto-sensitive neurons in the second abdominal ganglion of honeybees. Response and correlation maxima occur at the flex-points of the stimulus. Effects increase with duration of stimulation. Cross-correlation for different frequency ranges indicate correlations between different frequency ranges of neuronal firing and different frequency components in the stimulus.
Honeybees perceive very small changes of the external magnetic field. About 100 hairs on the anterodorsal abdomen, the region of magnetic sensitivity of honeybees, contain dendrites and granules of the size of SPM magnetite. Hypothesizing the hairs to constitute micro-antennae amplifying external magnetic field modulations, the magnetic field and the induced electric fields, currents and the Lorentz forces were calculated. The magnetic field is strongly amplified but decays rapidly with distance from the magnetite. An integrating fiber receiving simultaneous inputs from all dendrites was assumed. This fiber feeds into spontaneously firing neurons whose instantaneous frequencies of firing are modulated according to the modulations of the external magnetic field. The neurons transmit their outputs to a multiple RCE neural network trained for classification of frequency and intensity classes. The spontaneously firing neurons may be located in the ganglion underlying the hairs where some neurons respond to changes in frequency and intensity of the external magnetic field. These neurons would constitute the input layer of the network while higher network layers would be located in the mushroom bodies. Learning occurs by modification of synaptic membranes which would allow the bee to learn the map of the magnetic environment.
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