Wednesday, August 29, 2012

Magnetoreception is Not a Party For a Supervillain

The majority of the more than 650 species of North American birds migrate. In search of food and nesting sites, some birds travel short distances and others (like Arctic terns) travel up to 12,000 miles each way. But all of them have to figure out where they are going, and much of how they do this is still unknown.

This is a magnetite rock. Scientists think
many animals have magnetite in their brains
to detect magnetic fields! For real?! Photo by
Rob Lavinsky at irocks.com and Wikimedia.
Last week, we learned that many birds get disoriented if the magnetic field around them is messed up by sunspots, magnetic rocks, or researchers gluing magnets to their heads. So they must sense magnetic fields, but how?

Sensation, whether by vision, touch, hearing, smell, taste or even magnetoreception (the sensation of magnetic fields) requires a stimulus to be transformed into an electrical signal that then must reach the brain. In the first five senses I listed, which we know considerably more about, this process occurs when sensory neurons (a type of cell in the brain and nervous system) convert the chemical or physical energy of the stimulus into an electrical signal. The sensory neurons then transmit the electrical signal to the brain, where it is processed and interpreted. So it is reasonable to think that magnetoreception works the same way too, right?

Where might such magnetoreception sensory neurons be? Bob Beason at State University of New York and Peter Semm at the Goethe University Frankfurt in Germany conducted a series of experiments to test whether the trigeminal nerve, a major nerve that provides sensation to the face, might play a role in magnetoreception. The trigeminal nerve has three major branches: the ophthalmic nerve, the maxillary nerve and the mandibular nerve. These nerves are each a bundle of sensory neuron fibers that send electrical signals to communicate to the brain what the head and face is sensing… a good place to look.

This illustration, titled Bubbling Bob the Bobolink,
was created by Louis Agassiz Fuentes in 1919.
Image at Wikimedia.
Bob and Peter exposed bobolinks, migratory birds that use magnetic fields to navigate, to a set of coils that could produce both vertical and horizontal magnetic fields. They then recorded the electrical activity of individual sensory neurons in the trigeminal nerve while exposing the birds to different magnetic fields. They found that many of these trigeminal sensory neurons, especially in the ophthalmic nerve branch, responded to the magnetic fields either by increasing or decreasing their electrical activity. Horizontal and vertical magnetic fields elicited different responses from different sensory neurons. Also, some sensory neurons responded to increases in magnetic strength and others responded to decreases in magnetic strength. The pattern of neuron activity could be a way that the nervous system could communicate the direction, and change in direction, of the magnetic field to the brain during navigation.

In order to test whether the ophthalmic nerve branch carries magnetic information to the brain, Bob and Peter tested another group of bobolinks that were preparing for migration. For each bird, they first tested what direction it preferred to go (for a control). Then, they magnetized the birds such that if their beak were iron, the tip of it would attract the south end of a compass. They figured that this process would send a confusing magnetic signal to the birds’ brains. Then they tested the birds’ preferred flying directions again (as expected, they got confused and went the wrong way). Finally, they numbed the ophthalmic nerve by putting a drop of Lidocaine on it and tested their preferred direction again. They found that although magnetizing the birds made them go the wrong direction, when their ophthalmic nerve was numbed, they ignored this incorrect magnetic information and went the right way again. Clearly, the ophthalmic nerve is sending magnetic information to the brain.

But how does a sensory neuron in the ophthalmic nerve respond to a magnetic force? For magnetoreception to work, magnetic forces need to affect receptors of some kind. Gerta Fleissner, Branko Stahl, Peter Thalau, Gerald Falkenberg, and G√ľnther Fleissner at the Goethe University Frankfurt were the first scientists to systematically seek out such magnetoreceptors. They examined the skin lining the upper beak of homing pigeons with fancy microscope and X-ray techniques that could identify iron compounds in the skin.

These researchers found two different types of iron, magnetite and maghemite, in dendrites, the receiving ends of sensory neurons. Not only do these dendrites have both metals, but the metals are arranged in a particular way. This particular arrangement could cause the dendrite to physically respond to a magnetic field that is oriented in a particular way, perhaps by changing shape and stretching the membrane of the cell (remember, a sensory neuron is a cell). This physical pull on the membrane could cause the neuron to send an electric pulse, in much the same way as a hearing cell does.


Researchers discovered two magnetic metals, magnetite and maghemite, in the receiving
ends of sensory neurons. These two metals were arranged in such a way that if a magnetic
force were to align with the neuron in a particular direction, the metals would likely move and
stretch the membrane, which could activate the neuron, sending a signal to the brain. Figure
from Fleissner, Stahl, Thalau, Falkenberg, and Fleissner's 2007 paper in Naturwissenschaften.
Furthermore, the skin lining the pigeon beak has six separate iron-containing patches. In each of these patches, there is a different prevailing direction of how the iron-containing dendrites are aligned. This means that different nerve endings could be activated by different directions of magnetic field, potentially providing the bird with a complex perception of the magnetic field as it turns its head.

Magnetic dendrites (receiving ends of sensory neurons) were aligned in one of three
directions depending on where they were in the beak. This arrangement could allow birds
to know the direction of a magnetic field based on which neurons are activated. Figure from
Fleissner, Stahl, Thalau, Falkenberg, and Fleissner's 2007 paper in Naturwissenschaften.
Now, before you declare, “These birds have iron in them? What, are they some kind of superhero?” remember, we all (pretty much everyone except for our arthropod and mollusc friends) have iron in us… in our respiratory pigments (like hemoglobin). But this (new to us) use of iron does seem to give these birds super-human abilities.

Want to know more? Check these out:

1. Beason RC, & Semm P (1987). Magnetic responses of the trigeminal nerve system of the bobolink (Dolichonyx oryzivorus). Neuroscience letters, 80 (2), 229-34 PMID: 3683981

2. Semm P, & Beason RC (1990). Responses to small magnetic variations by the trigeminal system of the bobolink. Brain research bulletin, 25 (5), 735-40 PMID: 2289162

3. Beason R, & Semm P (1996). Does the avian ophthalmic nerve carry magnetic navigational information? The Journal of experimental biology, 199 (Pt 5), 1241-4 PMID: 9319100

4. Beason R, Dussourd N, & Deutschlander M (1995). Behavioural evidence for the use of magnetic material in magnetoreception by a migratory bird The Journal of experimental biology, 198 (Pt 1), 141-6 PMID: 9317510

5. Fleissner G, Stahl B, Thalau P, Falkenberg G, & Fleissner G (2007). A novel concept of Fe-mineral-based magnetoreception: histological and physicochemical data from the upper beak of homing pigeons. Die Naturwissenschaften, 94 (8), 631-42 PMID: 17361399

6. Cadiou H, & McNaughton PA (2010). Avian magnetite-based magnetoreception: a physiologist's perspective. Journal of the Royal Society, Interface / the Royal Society, 7 Suppl 2 PMID: 20106875

2 comments:

  1. I was well convince of all this until this came up:
    http://www.nature.com/nature/journal/v484/n7394/full/nature11046.html?WT.ec_id=NATURE-20120419

    so even though there seems to be convincing evidence that the trigeminal nerve mediates magnetoreception the sensory cells are no where to be found.

    on top of this, there are other studies suggesting that magnetoreception is mediated by the octaval nerve:
    http://www.sciencemag.org/content/336/6084/991.summary

    and lets not forget that there is good evidence that magnetoreception is mediated by free radical reactions in the retina!!
    my (and others) guess is the answer is all of the above. (e.g. http://www.springerlink.com/content/d33604405403gm88/). it is a really interesting subject though. one that need much more experiments.

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    Replies
    1. Great points! Although I am not convinced that the Fleissner group and the authors of the nature paper you cited were looking at the same thing. In any case, there is MUCH work left to be done on this subject. Scientists are a skeptic bunch until studies are reproducible... and in physiology there are almost always many interrelated factors that we tend to inviestigate one at a time.

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