Neurons are like Magnets

The earth has magnetic poles just like this magnet

It has long been thought that animals can use the earth's magnetic field to know where they are with respect to the planet itself. Migrating whales and turtles could use this method to determine which direction to swim, and pigeons could use this to navigate over long distances.

Loggerhead hatchling: brain still developing

Recently a paper out of Baylor college of medicine has shown the neural correlates which underlie this magnetic sense. They actually recorded from individual neurons while manipulating the surrounding magnetic field.

Specifically, Wu and Dickman (2012) recorded from electrodes implanted in the pigeon brain stem while the pigeon was sitting in a fully manipulable magnetic field. Wu and Dickman cancelled out the earths magnetic field and then applied a specific magnetic stimulation to the bird's head. After the last test stimulation, the brain stems were stained for c-fos. C-fos is an immediate early gene and is an indicator of activity in a neuron.  In other words the cells that show c-fos after stimulation are cells that were active during the stimulation. (For more on the use of immediate early genes see: erasing memories cell by cell.)

Wu and Dickman, 2012 Figure 2

They show the recording sites (red stars) and the c-fos positive neurons for all the pigeons in C. B is an example (dark dots are c-fos neurons). This brain stem diagram might look familiar, we've talked about the bird brain stem here before regarding sound localization. It seems that birds do a lot of spatial localization with their brain stem.

Wu and Dickman not only found that many neurons in this brain area expressed c-fos after magnetic stimulation, but they also recorded the actual spiking activity of these neurons during the magnetic stimulation.  They stimulated using small magnetic fields in the micro-tesla (uT) range.  For reference a typical MRI machine has a magnetic strength of 3 tesla or so (as in 3,000,000 uT).  They found 53 neurons in the recording area were sensitive to magnetic stimulation (but 276 were not), and that these neurons were sensitive to many types of signal modulation.

"We have shown that single vestibular brainstem neurons encode the direction, intensity, and polarity of an applied magnetic field...Our findings demonstrate that MR neurons are most sensitive within an intensity range that is naturally produced by Earth’s magnetic field, a necessary condition for a magnetoreception system to be useful in the derivation of geopositional information. However, Earth’s magnetic field varies over time (for instance, there has been a 35% decrease in its strength over the past 2000 years), so it would seem likely that magnetoreception systems adapt to the slowly changing fields through evolution and/or developmental plasticity in order to maximize magnetic sense perception." Wu and Dickman, 2012

This is an exciting paper that answers longstanding questions, but also raises new ones (as most exciting science does). For example, how is the magnetic field actually sensed by the pigeon? It is likely that these magnetic response (MR) neurons are receiving input from a sensory organ, and they probably wouldn't be magneto-sensitive on their own. Though an interesting experiment would be to culture these neurons (or the neurons of the putative magneto-sensory organ) and record their sensitivity to direct magnetic stimulation.

Darwin's pigeons

It would also be exciting to test changes in magnetic sensitivity of these neurons based on exposure.  If a pigeon is raised in a completely magnet-free area with the earth's magnetic field actively canceled out, would it lose its ability to detect the magnetic field? Or perhaps in that case, these neurons would become sensitized and respond more strongly to a magnetic stimulation. And how are these magneto-sensitive areas of the brain altered between bird species?  So many exciting questions!



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Wu LQ, & Dickman JD (2012). Neural correlates of a magnetic sense. Science (New York, N.Y.), 336 (6084), 1054-7 PMID: 22539554