Magnetic pulse affects a putative magnetoreceptor mechanism - PubMed (original) (raw)

Magnetic pulse affects a putative magnetoreceptor mechanism

Alfonso F Davila et al. Biophys J. 2005 Jul.

Abstract

Clusters of superparamagnetic (SP) magnetite crystals have recently been identified in free nerve endings in the upper-beak skin of homing pigeons and are interpreted as being part of a putative magnetoreceptor system. Motivated by these findings, we developed a physical model that accurately predicts the dynamics of interacting SP clusters in a magnetic field. The main predictions are: 1), under a magnetic field, a group of SP clusters self-assembles into a chain-like structure that behaves like a compass needle under slowly rotating fields; 2), in a frequently changing field as encountered by a moving bird, a stacked chain is a structurally more stable configuration than a single chain; 3), chain-like structures of SP clusters disrupt under strong fields applied at oblique angles; and 4), reassemble on a timescale of hours to days (assuming a viscosity of the cell plasma eta approximately 1 P). Our results offer a novel mechanism for magnetic field perception and are in agreement with the response of birds observed after magnetic-pulse treatments, which have been conducted in the past to specifically test if ferrimagnetic material is involved in magnetoreception, but which have defied explanation so far. Our theoretical results are supported by experiments on a technical SP model system using a high-speed camera. We also offer new predictions that can be tested experimentally.

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Figures

FIGURE 1

FIGURE 1

Comparison between numerical simulations and experiments (insets) on magnetically interacting ferrofluid droplets, which self-assemble into a linear configuration under the influence of a magnetic field of 9 Oe (arrow). The scale bar represents 50 _μ_m.

FIGURE 2

FIGURE 2

Numerical simulation of the effects of a brief magnetic pulse of strength 0.5 T applied for 2 ms to a single chain and a double chain of SP clusters. The trajectories of the clusters are represented by the gray wiggles. The dashed lines depict the membrane of the dendrite containing the clusters (ρ = 5). (A) A pulse applied parallel to the chain axis leaves the chain intact. (B) If applied at an oblique angle, the pulse torques the chain into alignment with the field. (C) This so-called pseudotorque response occurs only until a critical angle, whereas at higher angles the chain breaks up into subchains of variable sizes.

FIGURE 3

FIGURE 3

Experimental verification of the model (movie available as online Supplementary Material). (Top left) Initial configuration of the chain of ferrofluid droplets, aligned in a bias field of 1 Oe. (Top right and middle left) Application of a 0.5-T pulse for 2 ms perpendicular to the chain axis. The droplets elongate into the field direction and subsequently form pairs, thereby disrupting the chain configuration. (Middle right) Arrangement immediately after the pulse treatment and (bottom right) 25 ms thereafter.

FIGURE 4

FIGURE 4

Proposed mechanism of magnetic-field transduction. The clusters of SP magnetite are ensheathed by the membrane of the free nerve ending (FNE). Without the membrane, the chain of clusters would rotate into the field axis, therefore, the membrane experiences a bending torque T (Eq. 13). Because free nerve endings are sensitive to mechanic stimulation, the torque can be transduced into a nervous signal.

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