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Bioelectromagnetism

 

FIGURE 4.7 A bird’s eye model used for the calculation of visual modulation patterns via plausible radical pair

(Ritz et al., 2000). (Reproduced with permission from Ritz et al., 2000, Copyright 2000, Elsevier.)

CRY was immuno-chemically identifed in the outer segments of the UV/V cones (SWS1 receptors)

in the retinae of chickens, robins and several other species (Nießner et al., 2011; Bolte et al., 2021). Tis

indicates the eyes as the site of the reception for magnetic directions, even if details are not yet entirely

clear (Wiltschko and Wiltschko, 2019; Bolte et al., 2021).

In the case of the bird’s eye model, the yield of the triplet spin state depends on the angle formed by

the external magnetic feld and the radical pair. Ritz et al. (2000) proposed a bird’s eye model used for

the calculation of visual modulation patterns via plausible radical pair (Figure 4.7).

Rays 1 and 2 enter through an infnitesimal hole at O′ and are projected onto a spherical retina

(Figure 4.7). Te receptor molecules are assumed to be oriented normally to the retina surface (direc­

tions z1 and z2), thus forming diferent angles with the direction of the magnetic feld vector. Tis

angle dependence has important implications. Tis is because, when the radical pair exists perpen­

dicular to the retina sphere, the angle between the radical pair and the external magnetic feld (GMF)

is diferent depending on the retina surface point, and the concentration of the product is diferent

at each point. It is thought that this concentration diference is replaced with a signal transmitted to

nerves, and fnally, the confguration image of the magnetic feld is acquired in the migratory bird’s

brain.

Behavioral studies in European robins (Zapka et al., 2009) strongly suggest that a forebrain region

named “Cluster N” (Mouritsen et al., 2005), which receives input from the eyes via the thalamofugal

visual pathway (Heyers et al., 2007), is involved in processing magnetic compass information. Several

studies on the migratory birds’ brains showed that bilateral lesions of Cluster N disables magnetic ori­

entation, and therefore, Cluster N is assumed to be a light-processing forebrain region (Möller et al.,

2004; Mouritsen et al., 2004; Zapka et al., 2009). Moreover, it is presumed that it is the cryptochrome

(CRY) of the retina that meets the conditions under which the electron transfer reaction occurs at the

photoreceptors on the retina sphere (Ritz et al., 2000). According to the model proposed by Ritz et al.

(2000), this would allow birds to perceive the GMF as a pattern superimposed on the visual image. In a

sense, they could literally “see” the magnetic feld (Ritz et al., 2000).

Te RPM of the avian magnetic compass deals with the quantum evolution of highly non-

equilibrium electron spin states of pairs of transient spin-correlated radicals residing inside a bird’s

retina as illustrated in Figure 4.8 (Pedersen et al., 2016).