Cryptochromes

One of the candidate mechanisms for how magnetic fields might interact with the body

Cryptochromes are types of proteins (flavoproteins) which are present in plants, insects and mammals. They are blue-light sensitive components of the biological clock and two main forms have been identified, type 1 which are photoreceptive (react directly to light) and type 2 which are important regulatory factors in the biological clock controling daily biological rhythms.

Cryptochromes have been identified as a candidate for the mechanism by which migratory birds use the earth's geomagnetic field to navigate.  Research has shown that the magnetic compass in migratory birds is located in the eye and is light-dependent; the compass is more efficient when blue light is present. Given cryptochromes' sensitivity to blue light and their location in the retina of birds, they have become one of the leading candidates. Humans do not have these type 1 cryptochromes which are photoreceptive, but do have other photoreceptive pigments located in the retina.

Magnetoreception

The mechanism by which small changes in the earth's magnetic field translate into birds sensing these changes (magnetoreception) has been difficult to identify. Cryptochromes offer a potentially highly sensitive mechanism via a chemical reaction, known as the 'free-radical mechanism'.

When cryptochromes are activated by light, a radical pair is formed, which is two molecules each with unpaired electrons (called its active state). These electrons can spin in different directions resulting in different reaction products being formed. It has been hypothesised that magnetic fields may affect this reaction by altering or modulating the relative direction of the electron spins, therefore altering the length of time that cryptochrome remains in this active state. These differences in reaction products and rates are thought to translate to dark and lighter areas in the bird's visual sphere helping it identify inclination changes in the Earth's magnetic field.

Research investigating magnetic field effects on cryptochromes is ongoing. These free radical effects have been observed at high magnetic field strengths (thousands of microtesla), but have not been observed at low levels (tens of microtesla), similar to those that birds would use to navigate.  

See also:

Other particular mechanisms:

Biological clocks

All organisms exhibit rhythms in many things, from sleeping and eating to the timing of hormone release. Many of these rhythms are 24 h or "circadian", controlled by an internal biological clock. This self-sustaining clock is located in the hypothalamus and generates rhythms via oscillating gene and protein levels.