Magnetoreception

Introduction:

The magnetic field of the earth is an important tool for navigation among birds. 18 species of birds have magnetic compass to aid them in their seasonal migration and home navigation. The magnetic compass was first described for European robins. Captive individuals of migrants become restless in their cages at the time of the year when their migration usually starts. They also preferred to stay at the side of the cage pointing to their migrating destination. This behavior was used to analyze the orientation of the birds in a laboratory where magnetic fields can be varied in a controllable manner. When the magnetic north was rotated by coil systems, while the fieldÐŽ¦s total intensity and inclination where held constant, the birds altered their directional preferences according to the change in the magnetic north [1]. This behavior clearly indicates that the birds used the magnetic field for direction finding.

As a result of birdÐŽ¦s dependence on the earth magnetic field for navigation, they have been of particular interest for the study of magnetoreception. Species with magnetoreception functions contain internal chains of either single-domain (SD) magnetite or greigite that produce a magnetic moment large enough to rotate the cell into passive alignment with the geomagnetic field. When a magnetic pulse applied antiparallel to the magnetization direction causes the moment to reverse direction, making a bacterium swim south instead of north, which is a unique property of ferromagnetic material. Suggestions also have been made that deposits of super- paramagnetic (SPM) magnetite detected in some animals may be involved in magnetoreception.

Birds appear to use information from the geomagnetic field in two ways; the first is for position determination while the second as a compass for direction finding. These probably involve separate receptor systems as the biophysical constraints of how both may function differ. In view of these findings, researchers found great interest in determining the existence of magnetic biomaterials in animals through behavioral studies.

In birds, magnetite was found in the head, particularly in the ethmoid region above the beak. The ophthalmic nerve, a branch of the nervus trigminus, inverts these parts of the head. Electrophysiological recordings from this nerve and from the trigeminal ganglion of Bobolinks Dolichonyx oryzivours revealed units that responded to changes in the intensity of the magnetic field. In some studies, treatment with a strong magnetic pulse had a considerable effect on the migratory orientation of Australian silvereyes, deflecting the birds headings from their natural migratory direction approximately 90 degrees towards east for two days. Because the treatment with a strong magnetic pulse usually disorients the birds, it is thought that at least one receptor utilizes a magnetizable material such as magnetite. In a recent study, it was found that the effect of the magnetizing treatment can be abolished by blocking the ophthalmic branch of the trigeminal nerve, but the ability of the bird to select and maintain a certain direction was not affected. These results also support the hypothesis that a magnetizable material is a part of the magnetoreception system.

If magnetite-containing cells are used in magnetoreception, it is reasonable to predict that they should be linked to magnetically responsive nerves. However, understanding the genetic basis of magnetite biomineralization will provide the needed molecular tools for testing the hypothesis of common descent, and for testing magnetiteÐŽ¦s role in magnetoreception of all animal groups. The location and structure of the magnetoreceptors that transduce the magnetic information to the nervous system remain unknown mysteries to the interested researcher.

Aim and Problem Statement:

Many studies have been made to determine whether homing pigeons contain magnetic material in their bodies. Most of these studies aimed to prove the existence of such material through behavioral experiments carried on the pigeons. Some studies also proposed the idea of magnetic material influence being integrated by the ophthalmic nerve to guide the birds in their journey. The mechanism of how the pigeons sense the variation in the earthÐŽ¦s magnetic filed and how to use it as a navigational tool is not fully understood.

Models of how the birds can use the earthÐŽ¦s magnetic field have been discussed but werenÐŽ¦t related to the physiological actions. Lengthy researches and studies are the only way to unrevealing the mystery of the neurological mechanisms leading to the precise determination of destination observed in pigeons.

The purpose of this document is to propose an experiment(s) to determine the shape of the action potential under different magnetic variations and conditioning. The experiment proposed will also utilize a tracer to determine the path of an action potential in the nerves system.

A research method and an electrophysiological recording description will be discussed in subsequent sections. As an introductory step to determine how the information in the ophthalmic affect the signals generated in a birds brain, this study aims to introduce ways of measuring the action potential of a silvereye ophthalmic or/and trigenimal nerve under normal conditions and under a variety of conditions including electrical and photo stimulation.

The collected data from the experimental sets could be then compared and further analyzed to determine whether the action potential changes shapes when traveling from a sensory nerve to another in the process of integrating magnetic information. Furthermore, future work may include the cellular level reaction to such fields and conditions. In doing so, this study might be expanded (given the knowledge of how the cells and the action potential varies under certain conditions) to introduce ways of studying the different brain signals generated in the bird as a result of different information sent by the nerve to proper area of the brain.

Background and Significance:

Abstract:

Several factors have made magnetoreception one of the most controversial and interesting subjects in the behavioral and neural sciences. First, human beings are not consciously aware of this ÐŽ§senseЎЁ which makes it difficult to design and derive laboratory experiments or theories. Another factor is the difficulty of reproducing the behavioral evidence for the existence of magnetoreception prior to the 19970ÐŽ¦s. Also, all laboratory attempts to